= The Tahoe REST-ful Web API =
1. Enabling the web-API port
2. Basic Concepts: GET, PUT, DELETE, POST
3. URLs, Machine-Oriented Interfaces
4. Browser Operations: Human-Oriented Interfaces
5. Welcome / Debug / Status pages
6. Static Files in /public_html
7. Safety and security issues -- names vs. URIs
8. Concurrency Issues
== Enabling the web-API port ==
Every Tahoe node is capable of running a built-in HTTP server. To enable
this, just write a port number into the "[node]web.port" line of your node's
tahoe.cfg file. For example, writing "web.port = 3456" into the "[node]"
section of $NODEDIR/tahoe.cfg will cause the node to run a webserver on port
3456.
This string is actually a Twisted "strports" specification, meaning you can
get more control over the interface to which the server binds by supplying
additional arguments. For more details, see the documentation on
twisted.application.strports:
http://twistedmatrix.com/documents/current/api/twisted.application.strports.html
Writing "tcp:3456:interface=127.0.0.1" into the web.port line does the same
but binds to the loopback interface, ensuring that only the programs on the
local host can connect. Using
"ssl:3456:privateKey=mykey.pem:certKey=cert.pem" runs an SSL server.
This webport can be set when the node is created by passing a --webport
option to the 'tahoe create-client' command. By default, the node listens on
port 3456, on the loopback (127.0.0.1) interface.
== Basic Concepts ==
As described in architecture.txt, each file and directory in a Tahoe virtual
filesystem is referenced by an identifier that combines the designation of
the object with the authority to do something with it (such as read or modify
the contents). This identifier is called a "read-cap" or "write-cap",
depending upon whether it enables read-only or read-write access. These
"caps" are also referred to as URIs.
The Tahoe web-based API is "REST-ful", meaning it implements the concepts of
"REpresentational State Transfer": the original scheme by which the World
Wide Web was intended to work. Each object (file or directory) is referenced
by a URL that includes the read- or write- cap. HTTP methods (GET, PUT, and
DELETE) are used to manipulate these objects. You can think of the URL as a
noun, and the method as a verb.
In REST, the GET method is used to retrieve information about an object, or
to retrieve some representation of the object itself. When the object is a
file, the basic GET method will simply return the contents of that file.
Other variations (generally implemented by adding query parameters to the
URL) will return information about the object, such as metadata. GET
operations are required to have no side-effects.
PUT is used to upload new objects into the filesystem, or to replace an
existing object. DELETE it used to delete objects from the filesystem. Both
PUT and DELETE are required to be idempotent: performing the same operation
multiple times must have the same side-effects as only performing it once.
POST is used for more complicated actions that cannot be expressed as a GET,
PUT, or DELETE. POST operations can be thought of as a method call: sending
some message to the object referenced by the URL. In Tahoe, POST is also used
for operations that must be triggered by an HTML form (including upload and
delete), because otherwise a regular web browser has no way to accomplish
these tasks. In general, everything that can be done with a PUT or DELETE can
also be done with a POST.
Tahoe's web API is designed for two different consumers. The first is a
program that needs to manipulate the virtual file system. Such programs are
expected to use the RESTful interface described above. The second is a human
using a standard web browser to work with the filesystem. This user is given
a series of HTML pages with links to download files, and forms that use POST
actions to upload, rename, and delete files.
== URLs ==
Tahoe uses a variety of read- and write- caps to identify files and
directories. The most common of these is the "immutable file read-cap", which
is used for most uploaded files. These read-caps look like the following:
URI:CHK:ime6pvkaxuetdfah2p2f35pe54:4btz54xk3tew6nd4y2ojpxj4m6wxjqqlwnztgre6gnjgtucd5r4a:3:10:202
The next most common is a "directory write-cap", which provides both read and
write access to a directory, and look like this:
URI:DIR2:djrdkfawoqihigoett4g6auz6a:jx5mplfpwexnoqff7y5e4zjus4lidm76dcuarpct7cckorh2dpgq
There are also "directory read-caps", which start with "URI:DIR2-RO:", and
give read-only access to a directory. Finally there are also mutable file
read- and write- caps, which start with "URI:SSK", and give access to mutable
files.
(later versions of Tahoe will make these strings shorter, and will remove the
unfortunate colons, which must be escaped when these caps are embedded in
URLs).
To refer to any Tahoe object through the web API, you simply need to combine
a prefix (which indicates the HTTP server to use) with the cap (which
indicates which object inside that server to access). Since the default Tahoe
webport is 3456, the most common prefix is one that will use a local node
listening on this port:
http://127.0.0.1:3456/uri/ + $CAP
So, to access the directory named above (which happens to be the
publically-writable sample directory on the Tahoe test grid, described at
http://allmydata.org/trac/tahoe/wiki/TestGrid), the URL would be:
http://127.0.0.1:3456/uri/URI%3ADIR2%3Adjrdkfawoqihigoett4g6auz6a%3Ajx5mplfpwexnoqff7y5e4zjus4lidm76dcuarpct7cckorh2dpgq/
(note that the colons in the directory-cap are url-encoded into "%3A"
sequences).
Likewise, to access the file named above, use:
http://127.0.0.1:3456/uri/URI%3ACHK%3Aime6pvkaxuetdfah2p2f35pe54%3A4btz54xk3tew6nd4y2ojpxj4m6wxjqqlwnztgre6gnjgtucd5r4a%3A3%3A10%3A202
In the rest of this document, we'll use "$DIRCAP" as shorthand for a read-cap
or write-cap that refers to a directory, and "$FILECAP" to abbreviate a cap
that refers to a file (whether mutable or immutable). So those URLs above can
be abbreviated as:
http://127.0.0.1:3456/uri/$DIRCAP/
http://127.0.0.1:3456/uri/$FILECAP
The operation summaries below will abbreviate these further, by eliding the
server prefix. They will be displayed like this:
/uri/$DIRCAP/
/uri/$FILECAP
=== Child Lookup ===
Tahoe directories contain named children, just like directories in a regular
local filesystem. These children can be either files or subdirectories.
If you have a Tahoe URL that refers to a directory, and want to reference a
named child inside it, just append the child name to the URL. For example, if
our sample directory contains a file named "welcome.txt", we can refer to
that file with:
http://127.0.0.1:3456/uri/$DIRCAP/welcome.txt
(or http://127.0.0.1:3456/uri/URI%3ADIR2%3Adjrdkfawoqihigoett4g6auz6a%3Ajx5mplfpwexnoqff7y5e4zjus4lidm76dcuarpct7cckorh2dpgq/welcome.txt)
Multiple levels of subdirectories can be handled this way:
http://127.0.0.1:3456/uri/$DIRCAP/tahoe-source/docs/webapi.txt
In this document, when we need to refer to a URL that references a file using
this child-of-some-directory format, we'll use the following string:
/uri/$DIRCAP/[SUBDIRS../]FILENAME
The "[SUBDIRS../]" part means that there are zero or more (optional)
subdirectory names in the middle of the URL. The "FILENAME" at the end means
that this whole URL refers to a file of some sort, rather than to a
directory.
When we need to refer specifically to a directory in this way, we'll write:
/uri/$DIRCAP/[SUBDIRS../]SUBDIR
Note that all components of pathnames in URLs are required to be UTF-8
encoded, so "resume.doc" (with an acute accent on both E's) would be accessed
with:
http://127.0.0.1:3456/uri/$DIRCAP/r%C3%A9sum%C3%A9.doc
Also note that the filenames inside upload POST forms are interpreted using
whatever character set was provided in the conventional '_charset' field, and
defaults to UTF-8 if not otherwise specified. The JSON representation of each
directory contains native unicode strings. Tahoe directories are specified to
contain unicode filenames, and cannot contain binary strings that are not
representable as such.
All Tahoe operations that refer to existing files or directories must include
a suitable read- or write- cap in the URL: the wapi server won't add one
for you. If you don't know the cap, you can't access the file. This allows
the security properties of Tahoe caps to be extended across the wapi
interface.
== Slow Operations, Progress, and Cancelling ==
Certain operations can be expected to take a long time. The "t=deep-check",
described below, will recursively visit every file and directory reachable
from a given starting point, which can take minutes or even hours for
extremely large directory structures. A single long-running HTTP request is a
fragile thing: proxies, NAT boxes, browsers, and users may all grow impatient
with waiting and give up on the connection.
For this reason, long-running operations have an "operation handle", which
can be used to poll for status/progress messages while the operation
proceeds. This handle can also be used to cancel the operation. These handles
are created by the client, and passed in as a an "ophandle=" query argument
to the POST or PUT request which starts the operation. The following
operations can then be used to retrieve status:
GET /operations/$HANDLE?output=HTML (with or without t=status)
GET /operations/$HANDLE?output=JSON (same)
These two retrieve the current status of the given operation. Each operation
presents a different sort of information, but in general the page retrieved
will indicate:
* whether the operation is complete, or if it is still running
* how much of the operation is complete, and how much is left, if possible
Note that the final status output can be quite large: a deep-manifest of a
directory structure with 300k directories and 200k unique files is about
275MB of JSON, and might take two minutes to generate. For this reason, the
full status is not provided until the operation has completed.
The HTML form will include a meta-refresh tag, which will cause a regular
web browser to reload the status page about 60 seconds later. This tag will
be removed once the operation has completed.
There may be more status information available under
/operations/$HANDLE/$ETC : i.e., the handle forms the root of a URL space.
POST /operations/$HANDLE?t=cancel
This terminates the operation, and returns an HTML page explaining what was
cancelled. If the operation handle has already expired (see below), this
POST will return a 404, which indicates that the operation is no longer
running (either it was completed or terminated). The response body will be
the same as a GET /operations/$HANDLE on this operation handle, and the
handle will be expired immediately afterwards.
The operation handle will eventually expire, to avoid consuming an unbounded
amount of memory. The handle's time-to-live can be reset at any time, by
passing a retain-for= argument (with a count of seconds) to either the
initial POST that starts the operation, or the subsequent GET request which
asks about the operation. For example, if a 'GET
/operations/$HANDLE?output=JSON&retain-for=600' query is performed, the
handle will remain active for 600 seconds (10 minutes) after the GET was
received.
In addition, if the GET includes a release-after-complete=True argument, and
the operation has completed, the operation handle will be released
immediately.
If a retain-for= argument is not used, the default handle lifetimes are:
* handles will remain valid at least until their operation finishes
* uncollected handles for finished operations (i.e. handles for operations
which have finished but for which the GET page has not been accessed since
completion) will remain valid for one hour, or for the total time consumed
by the operation, whichever is greater.
* collected handles (i.e. the GET page has been retrieved at least once
since the operation completed) will remain valid for ten minutes.
Many "slow" operations can begin to use unacceptable amounts of memory when
operation on large directory structures. The memory usage increases when the
ophandle is polled, as the results must be copied into a JSON string, sent
over the wire, then parsed by a client. So, as an alternative, many "slow"
operations have streaming equivalents. These equivalents do not use operation
handles. Instead, they emit line-oriented status results immediately. Client
code can cancel the operation by simply closing the HTTP connection.
== Programmatic Operations ==
Now that we know how to build URLs that refer to files and directories in a
Tahoe virtual filesystem, what sorts of operations can we do with those URLs?
This section contains a catalog of GET, PUT, DELETE, and POST operations that
can be performed on these URLs. This set of operations are aimed at programs
that use HTTP to communicate with a Tahoe node. The next section describes
operations that are intended for web browsers.
=== Reading A File ===
GET /uri/$FILECAP
GET /uri/$DIRCAP/[SUBDIRS../]FILENAME
This will retrieve the contents of the given file. The HTTP response body
will contain the sequence of bytes that make up the file.
To view files in a web browser, you may want more control over the
Content-Type and Content-Disposition headers. Please see the next section
"Browser Operations", for details on how to modify these URLs for that
purpose.
=== Writing/Uploading A File ===
PUT /uri/$FILECAP
PUT /uri/$DIRCAP/[SUBDIRS../]FILENAME
Upload a file, using the data from the HTTP request body, and add whatever
child links and subdirectories are necessary to make the file available at
the given location. Once this operation succeeds, a GET on the same URL will
retrieve the same contents that were just uploaded. This will create any
necessary intermediate subdirectories.
To use the /uri/$FILECAP form, $FILECAP be a write-cap for a mutable file.
In the /uri/$DIRCAP/[SUBDIRS../]FILENAME form, if the target file is a
writable mutable file, that files contents will be overwritten in-place. If
it is a read-cap for a mutable file, an error will occur. If it is an
immutable file, the old file will be discarded, and a new one will be put in
its place.
When creating a new file, if "mutable=true" is in the query arguments, the
operation will create a mutable file instead of an immutable one.
This returns the file-cap of the resulting file. If a new file was created
by this method, the HTTP response code (as dictated by rfc2616) will be set
to 201 CREATED. If an existing file was replaced or modified, the response
code will be 200 OK.
Note that the 'curl -T localfile http://127.0.0.1:3456/uri/$DIRCAP/foo.txt'
command can be used to invoke this operation.
PUT /uri
This uploads a file, and produces a file-cap for the contents, but does not
attach the file into the virtual drive. No directories will be modified by
this operation. The file-cap is returned as the body of the HTTP response.
If "mutable=true" is in the query arguments, the operation will create a
mutable file, and return its write-cap in the HTTP respose. The default is
to create an immutable file, returning the read-cap as a response.
=== Creating A New Directory ===
POST /uri?t=mkdir
PUT /uri?t=mkdir
Create a new empty directory and return its write-cap as the HTTP response
body. This does not make the newly created directory visible from the
virtual drive. The "PUT" operation is provided for backwards compatibility:
new code should use POST.
POST /uri/$DIRCAP/[SUBDIRS../]SUBDIR?t=mkdir
PUT /uri/$DIRCAP/[SUBDIRS../]SUBDIR?t=mkdir
Create new directories as necessary to make sure that the named target
($DIRCAP/SUBDIRS../SUBDIR) is a directory. This will create additional
intermediate directories as necessary. If the named target directory already
exists, this will make no changes to it.
This will return an error if a blocking file is present at any of the parent
names, preventing the server from creating the necessary parent directory.
The write-cap of the new directory will be returned as the HTTP response
body.
POST /uri/$DIRCAP/[SUBDIRS../]?t=mkdir&name=NAME
Create a new empty directory and attach it to the given existing directory.
This will create additional intermediate directories as necessary.
The URL of this form points to the parent of the bottom-most new directory,
whereas the previous form has a URL that points directly to the bottom-most
new directory.
=== Get Information About A File Or Directory (as JSON) ===
GET /uri/$FILECAP?t=json
GET /uri/$DIRCAP?t=json
GET /uri/$DIRCAP/[SUBDIRS../]SUBDIR?t=json
GET /uri/$DIRCAP/[SUBDIRS../]FILENAME?t=json
This returns a machine-parseable JSON-encoded description of the given
object. The JSON always contains a list, and the first element of the list
is always a flag that indicates whether the referenced object is a file or a
directory. If it is a file, then the information includes file size and URI,
like this:
GET /uri/$FILECAP?t=json :
GET /uri/$DIRCAP/[SUBDIRS../]FILENAME?t=json :
[ "filenode", { "ro_uri": file_uri,
"verify_uri": verify_uri,
"size": bytes,
"mutable": false,
"metadata": {"ctime": 1202777696.7564139,
"mtime": 1202777696.7564139
}
} ]
If it is a directory, then it includes information about the children of
this directory, as a mapping from child name to a set of data about the
child (the same data that would appear in a corresponding GET?t=json of the
child itself). The child entries also include metadata about each child,
including creation- and modification- timestamps. The output looks like
this:
GET /uri/$DIRCAP?t=json :
GET /uri/$DIRCAP/[SUBDIRS../]SUBDIR?t=json :
[ "dirnode", { "rw_uri": read_write_uri,
"ro_uri": read_only_uri,
"verify_uri": verify_uri,
"mutable": true,
"children": {
"foo.txt": [ "filenode", { "ro_uri": uri,
"size": bytes,
"metadata": {
"ctime": 1202777696.7564139,
"mtime": 1202777696.7564139
}
} ],
"subdir": [ "dirnode", { "rw_uri": rwuri,
"ro_uri": rouri,
"metadata": {
"ctime": 1202778102.7589991,
"mtime": 1202778111.2160511,
}
} ]
} } ]
In the above example, note how 'children' is a dictionary in which the keys
are child names and the values depend upon whether the child is a file or a
directory. The value is mostly the same as the JSON representation of the
child object (except that directories do not recurse -- the "children"
entry of the child is omitted, and the directory view includes the metadata
that is stored on the directory edge).
Then the rw_uri field will be present in the information about a directory
if and only if you have read-write access to that directory. The verify_uri
field will be presend if and only if the object has a verify-cap
(non-distributed LIT files do not have verify-caps).
=== Attaching an existing File or Directory by its read- or write- cap ===
PUT /uri/$DIRCAP/[SUBDIRS../]CHILDNAME?t=uri
This attaches a child object (either a file or directory) to a specified
location in the virtual filesystem. The child object is referenced by its
read- or write- cap, as provided in the HTTP request body. This will create
intermediate directories as necessary.
This is similar to a UNIX hardlink: by referencing a previously-uploaded
file (or previously-created directory) instead of uploading/creating a new
one, you can create two references to the same object.
The read- or write- cap of the child is provided in the body of the HTTP
request, and this same cap is returned in the response body.
The default behavior is to overwrite any existing object at the same
location. To prevent this (and make the operation return an error instead of
overwriting), add a "replace=false" argument, as "?t=uri&replace=false".
With replace=false, this operation will return an HTTP 409 "Conflict" error
if there is already an object at the given location, rather than overwriting
the existing object. Note that "true", "t", and "1" are all synonyms for
"True", and "false", "f", and "0" are synonyms for "False". the parameter is
case-insensitive.
=== Deleting a File or Directory ===
DELETE /uri/$DIRCAP/[SUBDIRS../]CHILDNAME
This removes the given name from its parent directory. CHILDNAME is the
name to be removed, and $DIRCAP/SUBDIRS.. indicates the directory that will
be modified.
Note that this does not actually delete the file or directory that the name
points to from the tahoe grid -- it only removes the named reference from
this directory. If there are other names in this directory or in other
directories that point to the resource, then it will remain accessible
through those paths. Even if all names pointing to this object are removed
from their parent directories, then someone with possession of its read-cap
can continue to access the object through that cap.
The object will only become completely unreachable once 1: there are no
reachable directories that reference it, and 2: nobody is holding a read-
or write- cap to the object. (This behavior is very similar to the way
hardlinks and anonymous files work in traditional unix filesystems).
This operation will not modify more than a single directory. Intermediate
directories which were implicitly created by PUT or POST methods will *not*
be automatically removed by DELETE.
This method returns the file- or directory- cap of the object that was just
removed.
== Browser Operations ==
This section describes the HTTP operations that provide support for humans
running a web browser. Most of these operations use HTML forms that use POST
to drive the Tahoe node.
Note that for all POST operations, the arguments listed can be provided
either as URL query arguments or as form body fields. URL query arguments are
separated from the main URL by "?", and from each other by "&". For example,
"POST /uri/$DIRCAP?t=upload&mutable=true". Form body fields are usually
specified by using elements. For clarity, the
descriptions below display the most significant arguments as URL query args.
=== Viewing A Directory (as HTML) ===
GET /uri/$DIRCAP/[SUBDIRS../]
This returns an HTML page, intended to be displayed to a human by a web
browser, which contains HREF links to all files and directories reachable
from this directory. These HREF links do not have a t= argument, meaning
that a human who follows them will get pages also meant for a human. It also
contains forms to upload new files, and to delete files and directories.
Those forms use POST methods to do their job.
=== Viewing/Downloading a File ===
GET /uri/$FILECAP
GET /uri/$DIRCAP/[SUBDIRS../]FILENAME
This will retrieve the contents of the given file. The HTTP response body
will contain the sequence of bytes that make up the file.
If you want the HTTP response to include a useful Content-Type header,
either use the second form (which starts with a $DIRCAP), or add a
"filename=foo" query argument, like "GET /uri/$FILECAP?filename=foo.jpg".
The bare "GET /uri/$FILECAP" does not give the Tahoe node enough information
to determine a Content-Type (since Tahoe immutable files are merely
sequences of bytes, not typed+named file objects).
If the URL has both filename= and "save=true" in the query arguments, then
the server to add a "Content-Disposition: attachment" header, along with a
filename= parameter. When a user clicks on such a link, most browsers will
offer to let the user save the file instead of displaying it inline (indeed,
most browsers will refuse to display it inline). "true", "t", "1", and other
case-insensitive equivalents are all treated the same.
Character-set handling in URLs and HTTP headers is a dubious art[1]. For
maximum compatibility, Tahoe simply copies the bytes from the filename=
argument into the Content-Disposition header's filename= parameter, without
trying to interpret them in any particular way.
GET /named/$FILECAP/FILENAME
This is an alternate download form which makes it easier to get the correct
filename. The Tahoe server will provide the contents of the given file, with
a Content-Type header derived from the given filename. This form is used to
get browsers to use the "Save Link As" feature correctly, and also helps
command-line tools like "wget" and "curl" use the right filename. Note that
this form can *only* be used with file caps; it is an error to use a
directory cap after the /named/ prefix.
=== Get Information About A File Or Directory (as HTML) ===
GET /uri/$FILECAP?t=info
GET /uri/$DIRCAP/?t=info
GET /uri/$DIRCAP/[SUBDIRS../]SUBDIR/?t=info
GET /uri/$DIRCAP/[SUBDIRS../]FILENAME?t=info
This returns a human-oriented HTML page with more detail about the selected
file or directory object. This page contains the following items:
object size
storage index
JSON representation
raw contents (text/plain)
access caps (URIs): verify-cap, read-cap, write-cap (for mutable objects)
check/verify/repair form
deep-check/deep-size/deep-stats/manifest (for directories)
replace-conents form (for mutable files)
=== Creating a Directory ===
POST /uri?t=mkdir
This creates a new directory, but does not attach it to the virtual
filesystem.
If a "redirect_to_result=true" argument is provided, then the HTTP response
will cause the web browser to be redirected to a /uri/$DIRCAP page that
gives access to the newly-created directory. If you bookmark this page,
you'll be able to get back to the directory again in the future. This is the
recommended way to start working with a Tahoe server: create a new unlinked
directory (using redirect_to_result=true), then bookmark the resulting
/uri/$DIRCAP page. There is a "Create Directory" button on the Welcome page
to invoke this action.
If "redirect_to_result=true" is not provided (or is given a value of
"false"), then the HTTP response body will simply be the write-cap of the
new directory.
POST /uri/$DIRCAP/[SUBDIRS../]?t=mkdir&name=CHILDNAME
This creates a new directory as a child of the designated SUBDIR. This will
create additional intermediate directories as necessary.
If a "when_done=URL" argument is provided, the HTTP response will cause the
web browser to redirect to the given URL. This provides a convenient way to
return the browser to the directory that was just modified. Without a
when_done= argument, the HTTP response will simply contain the write-cap of
the directory that was just created.
=== Uploading a File ===
POST /uri?t=upload
This uploads a file, and produces a file-cap for the contents, but does not
attach the file into the virtual drive. No directories will be modified by
this operation.
The file must be provided as the "file" field of an HTML encoded form body,
produced in response to an HTML form like this:
If a "when_done=URL" argument is provided, the response body will cause the
browser to redirect to the given URL. If the when_done= URL has the string
"%(uri)s" in it, that string will be replaced by a URL-escaped form of the
newly created file-cap. (Note that without this substitution, there is no
way to access the file that was just uploaded).
The default (in the absence of when_done=) is to return an HTML page that
describes the results of the upload. This page will contain information
about which storage servers were used for the upload, how long each
operation took, etc.
If a "mutable=true" argument is provided, the operation will create a
mutable file, and the response body will contain the write-cap instead of
the upload results page. The default is to create an immutable file,
returning the upload results page as a response.
POST /uri/$DIRCAP/[SUBDIRS../]?t=upload
This uploads a file, and attaches it as a new child of the given directory.
The file must be provided as the "file" field of an HTML encoded form body,
produced in response to an HTML form like this:
A "name=" argument can be provided to specify the new child's name,
otherwise it will be taken from the "filename" field of the upload form
(most web browsers will copy the last component of the original file's
pathname into this field). To avoid confusion, name= is not allowed to
contain a slash.
If there is already a child with that name, and it is a mutable file, then
its contents are replaced with the data being uploaded. If it is not a
mutable file, the default behavior is to remove the existing child before
creating a new one. To prevent this (and make the operation return an error
instead of overwriting the old child), add a "replace=false" argument, as
"?t=upload&replace=false". With replace=false, this operation will return an
HTTP 409 "Conflict" error if there is already an object at the given
location, rather than overwriting the existing object. Note that "true",
"t", and "1" are all synonyms for "True", and "false", "f", and "0" are
synonyms for "False". the parameter is case-insensitive.
This will create additional intermediate directories as necessary, although
since it is expected to be triggered by a form that was retrieved by "GET
/uri/$DIRCAP/[SUBDIRS../]", it is likely that the parent directory will
already exist.
If a "mutable=true" argument is provided, any new file that is created will
be a mutable file instead of an immutable one. will give the user a way to set this option.
If a "when_done=URL" argument is provided, the HTTP response will cause the
web browser to redirect to the given URL. This provides a convenient way to
return the browser to the directory that was just modified. Without a
when_done= argument, the HTTP response will simply contain the file-cap of
the file that was just uploaded (a write-cap for mutable files, or a
read-cap for immutable files).
POST /uri/$DIRCAP/[SUBDIRS../]FILENAME?t=upload
This also uploads a file and attaches it as a new child of the given
directory. It is a slight variant of the previous operation, as the URL
refers to the target file rather than the parent directory. It is otherwise
identical: this accepts mutable= and when_done= arguments too.
POST /uri/$FILECAP?t=upload
This modifies the contents of an existing mutable file in-place. An error is
signalled if $FILECAP does not refer to a mutable file. It behaves just like
the "PUT /uri/$FILECAP" form, but uses a POST for the benefit of HTML forms
in a web browser.
=== Attaching An Existing File Or Directory (by URI) ===
POST /uri/$DIRCAP/[SUBDIRS../]?t=uri&name=CHILDNAME&uri=CHILDCAP
This attaches a given read- or write- cap "CHILDCAP" to the designated
directory, with a specified child name. This behaves much like the PUT t=uri
operation, and is a lot like a UNIX hardlink.
This will create additional intermediate directories as necessary, although
since it is expected to be triggered by a form that was retrieved by "GET
/uri/$DIRCAP/[SUBDIRS../]", it is likely that the parent directory will
already exist.
=== Deleting A Child ===
POST /uri/$DIRCAP/[SUBDIRS../]?t=delete&name=CHILDNAME
This instructs the node to delete a child object (file or subdirectory) from
the given directory. Note that the entire subtree is removed. This is
somewhat like "rm -rf" (from the point of view of the parent), but other
references into the subtree will see that the child subdirectories are not
modified by this operation. Only the link from the given directory to its
child is severed.
=== Renaming A Child ===
POST /uri/$DIRCAP/[SUBDIRS../]?t=rename&from_name=OLD&to_name=NEW
This instructs the node to rename a child of the given directory. This is
exactly the same as removing the child, then adding the same child-cap under
the new name. This operation cannot move the child to a different directory.
This operation will replace any existing child of the new name, making it
behave like the UNIX "mv -f" command.
=== Other Utilities ===
GET /uri?uri=$CAP
This causes a redirect to /uri/$CAP, and retains any additional query
arguments (like filename= or save=). This is for the convenience of web
forms which allow the user to paste in a read- or write- cap (obtained
through some out-of-band channel, like IM or email).
Note that this form merely redirects to the specific file or directory
indicated by the $CAP: unlike the GET /uri/$DIRCAP form, you cannot
traverse to children by appending additional path segments to the URL.
GET /uri/$DIRCAP/[SUBDIRS../]?t=rename-form&name=$CHILDNAME
This provides a useful facility to browser-based user interfaces. It
returns a page containing a form targetting the "POST $DIRCAP t=rename"
functionality described above, with the provided $CHILDNAME present in the
'from_name' field of that form. I.e. this presents a form offering to
rename $CHILDNAME, requesting the new name, and submitting POST rename.
GET /uri/$DIRCAP/[SUBDIRS../]CHILDNAME?t=uri
This returns the file- or directory- cap for the specified object.
GET /uri/$DIRCAP/[SUBDIRS../]CHILDNAME?t=readonly-uri
This returns a read-only file- or directory- cap for the specified object.
If the object is an immutable file, this will return the same value as
t=uri.
=== Debugging and Testing Features ===
These URLs are less-likely to be helpful to the casual Tahoe user, and are
mainly intended for developers.
POST $URL?t=check
This triggers the FileChecker to determine the current "health" of the
given file or directory, by counting how many shares are available. The
page that is returned will display the results. This can be used as a "show
me detailed information about this file" page.
If a verify=true argument is provided, the node will perform a more
intensive check, downloading and verifying every single bit of every share.
If an output=JSON argument is provided, the response will be
machine-readable JSON instead of human-oriented HTML. The data is a
dictionary with the following keys:
storage-index: a base32-encoded string with the objects's storage index,
or an empty string for LIT files
summary: a string, with a one-line summary of the stats of the file
results: a dictionary that describes the state of the file. For LIT files,
this dictionary has only the 'healthy' key, which will always be
True. For distributed files, this dictionary has the following
keys:
count-shares-good: the number of good shares that were found
count-shares-needed: 'k', the number of shares required for recovery
count-shares-expected: 'N', the number of total shares generated
count-good-share-hosts: the number of distinct storage servers with
good shares. If this number is less than
count-shares-good, then some shares are doubled
up, increasing the correlation of failures. This
indicates that one or more shares should be
moved to an otherwise unused server, if one is
available.
count-wrong-shares: for mutable files, the number of shares for
versions other than the 'best' one (highest
sequence number, highest roothash). These are
either old ...
count-recoverable-versions: for mutable files, the number of
recoverable versions of the file. For
a healthy file, this will equal 1.
count-unrecoverable-versions: for mutable files, the number of
unrecoverable versions of the file.
For a healthy file, this will be 0.
count-corrupt-shares: the number of shares with integrity failures
list-corrupt-shares: a list of "share locators", one for each share
that was found to be corrupt. Each share locator
is a list of (serverid, storage_index, sharenum).
needs-rebalancing: (bool) True if there are multiple shares on a single
storage server, indicating a reduction in reliability
that could be resolved by moving shares to new
servers.
servers-responding: list of base32-encoded storage server identifiers,
one for each server which responded to the share
query.
healthy: (bool) True if the file is completely healthy, False otherwise.
Healthy files have at least N good shares. Overlapping shares
(indicated by count-good-share-hosts < count-shares-good) do not
currently cause a file to be marked unhealthy. If there are at
least N good shares, then corrupt shares do not cause the file to
be marked unhealthy, although the corrupt shares will be listed
in the results (list-corrupt-shares) and should be manually
removed to wasting time in subsequent downloads (as the
downloader rediscovers the corruption and uses alternate shares).
sharemap: dict mapping share identifier to list of serverids
(base32-encoded strings). This indicates which servers are
holding which shares. For immutable files, the shareid is
an integer (the share number, from 0 to N-1). For
immutable files, it is a string of the form
'seq%d-%s-sh%d', containing the sequence number, the
roothash, and the share number.
POST $URL?t=start-deep-check (must add &ophandle=XYZ)
This initiates a recursive walk of all files and directories reachable from
the target, performing a check on each one just like t=check. The result
page will contain a summary of the results, including details on any
file/directory that was not fully healthy.
t=start-deep-check can only be invoked on a directory. An error (400
BAD_REQUEST) will be signalled if it is invoked on a file. The recursive
walker will deal with loops safely.
This accepts the same verify= argument as t=check.
Since this operation can take a long time (perhaps a second per object),
the ophandle= argument is required (see "Slow Operations, Progress, and
Cancelling" above). The response to this POST will be a redirect to the
corresponding /operations/$HANDLE page (with output=HTML or output=JSON to
match the output= argument given to the POST). The deep-check operation
will continue to run in the background, and the /operations page should be
used to find out when the operation is done.
Detailed check results for non-healthy files and directories will be
available under /operations/$HANDLE/$STORAGEINDEX, and the HTML status will
contain links to these detailed results.
The HTML /operations/$HANDLE page for incomplete operations will contain a
meta-refresh tag, set to 60 seconds, so that a browser which uses
deep-check will automatically poll until the operation has completed.
The JSON page (/options/$HANDLE?output=JSON) will contain a
machine-readable JSON dictionary with the following keys:
finished: a boolean, True if the operation is complete, else False. Some
of the remaining keys may not be present until the operation
is complete.
root-storage-index: a base32-encoded string with the storage index of the
starting point of the deep-check operation
count-objects-checked: count of how many objects were checked. Note that
non-distributed objects (i.e. small immutable LIT
files) are not checked, since for these objects,
the data is contained entirely in the URI.
count-objects-healthy: how many of those objects were completely healthy
count-objects-unhealthy: how many were damaged in some way
count-corrupt-shares: how many shares were found to have corruption,
summed over all objects examined
list-corrupt-shares: a list of "share identifiers", one for each share
that was found to be corrupt. Each share identifier
is a list of (serverid, storage_index, sharenum).
list-unhealthy-files: a list of (pathname, check-results) tuples, for
each file that was not fully healthy. 'pathname' is
a list of strings (which can be joined by "/"
characters to turn it into a single string),
relative to the directory on which deep-check was
invoked. The 'check-results' field is the same as
that returned by t=check&output=JSON, described
above.
stats: a dictionary with the same keys as the t=start-deep-stats command
(described below)
POST $URL?t=stream-deep-check
This initiates a recursive walk of all files and directories reachable from
the target, performing a check on each one just like t=check. For each
unique object (duplicates are skipped), a single line of JSON is emitted to
the HTTP response channel. When the walk is complete, a final line of JSON
is emitted which contains the accumulated file-size/count "deep-stats" data.
This command takes the same arguments as t=start-deep-check.
A CLI tool can split the response stream on newlines into "response units",
and parse each response unit as JSON. Each such parsed unit will be a
dictionary, and will contain at least the "type" key: a string, one of
"file", "directory", or "stats".
For all units that have a type of "file" or "directory", the dictionary will
contain the following keys:
"path": a list of strings, with the path that is traversed to reach the
object
"cap": a writecap for the file or directory, if available, else a readcap
"verifycap": a verifycap for the file or directory
"repaircap": the weakest cap which can still be used to repair the object
"storage-index": a base32 storage index for the object
"check-results": a copy of the dictionary which would be returned by
t=check&output=json, with three top-level keys:
"storage-index", "summary", and "results", and a variety
of counts and sharemaps in the "results" value.
Note that non-distributed files (i.e. LIT files) will have values of None
for verifycap, repaircap, and storage-index, since these files can neither
be verified nor repaired, and are not stored on the storage servers.
Likewise the check-results dictionary will be limited: an empty string for
storage-index, and a results dictionary with only the "healthy" key.
The last unit in the stream will have a type of "stats", and will contain
the keys described in the "start-deep-stats" operation, below.
POST $URL?t=check&repair=true
This performs a health check of the given file or directory, and if the
checker determines that the object is not healthy (some shares are missing
or corrupted), it will perform a "repair". During repair, any missing
shares will be regenerated and uploaded to new servers.
This accepts the same verify=true argument as t=check. When an output=JSON
argument is provided, the machine-readable JSON response will contain the
following keys:
storage-index: a base32-encoded string with the objects's storage index,
or an empty string for LIT files
repair-attempted: (bool) True if repair was attempted
repair-successful: (bool) True if repair was attempted and the file was
fully healthy afterwards. False if no repair was
attempted, or if a repair attempt failed.
pre-repair-results: a dictionary that describes the state of the file
before any repair was performed. This contains exactly
the same keys as the 'results' value of the t=check
response, described above.
post-repair-results: a dictionary that describes the state of the file
after any repair was performed. If no repair was
performed, post-repair-results and pre-repair-results
will be the same. This contains exactly the same keys
as the 'results' value of the t=check response,
described above.
POST $URL?t=start-deep-check&repair=true (must add &ophandle=XYZ)
This triggers a recursive walk of all files and directories, performing a
t=check&repair=true on each one.
Like t=start-deep-check without the repair= argument, this can only be
invoked on a directory. An error (400 BAD_REQUEST) will be signalled if it
is invoked on a file. The recursive walker will deal with loops safely.
This accepts the same verify=true argument as t=start-deep-check. It uses
the same ophandle= mechanism as start-deep-check. When an output=JSON
argument is provided, the response will contain the following keys:
finished: (bool) True if the operation has completed, else False
root-storage-index: a base32-encoded string with the storage index of the
starting point of the deep-check operation
count-objects-checked: count of how many objects were checked
count-objects-healthy-pre-repair: how many of those objects were completely
healthy, before any repair
count-objects-unhealthy-pre-repair: how many were damaged in some way
count-objects-healthy-post-repair: how many of those objects were completely
healthy, after any repair
count-objects-unhealthy-post-repair: how many were damaged in some way
count-repairs-attempted: repairs were attempted on this many objects.
count-repairs-successful: how many repairs resulted in healthy objects
count-repairs-unsuccessful: how many repairs resulted did not results in
completely healthy objects
count-corrupt-shares-pre-repair: how many shares were found to have
corruption, summed over all objects
examined, before any repair
count-corrupt-shares-post-repair: how many shares were found to have
corruption, summed over all objects
examined, after any repair
list-corrupt-shares: a list of "share identifiers", one for each share
that was found to be corrupt (before any repair).
Each share identifier is a list of (serverid,
storage_index, sharenum).
list-remaining-corrupt-shares: like list-corrupt-shares, but mutable shares
that were successfully repaired are not
included. These are shares that need
manual processing. Since immutable shares
cannot be modified by clients, all corruption
in immutable shares will be listed here.
list-unhealthy-files: a list of (pathname, check-results) tuples, for
each file that was not fully healthy. 'pathname' is
relative to the directory on which deep-check was
invoked. The 'check-results' field is the same as
that returned by t=check&repair=true&output=JSON,
described above.
stats: a dictionary with the same keys as the t=start-deep-stats command
(described below)
POST $URL?t=stream-deep-check&repair=true
This triggers a recursive walk of all files and directories, performing a
t=check&repair=true on each one. For each unique object (duplicates are
skipped), a single line of JSON is emitted to the HTTP response channel.
When the walk is complete, a final line of JSON is emitted which contains
the accumulated file-size/count "deep-stats" data.
This emits the same data as t=stream-deep-check (without the repair=true),
except that the "check-results" field is replaced with a
"check-and-repair-results" field, which contains the keys returned by
t=check&repair=true&output=json (i.e. repair-attempted, repair-successful,
pre-repair-results, and post-repair-results). The output does not contain
the summary dictionary that is provied by t=start-deep-check&repair=true
(the one with count-objects-checked and list-unhealthy-files), since the
receiving client is expected to calculate those values itself from the
stream of per-object check-and-repair-results.
POST $DIRURL?t=start-manifest (must add &ophandle=XYZ)
This operation generates a "manfest" of the given directory tree, mostly
for debugging. This is a table of (path, filecap/dircap), for every object
reachable from the starting directory. The path will be slash-joined, and
the filecap/dircap will contain a link to the object in question. This page
gives immediate access to every object in the virtual filesystem subtree.
This operation uses the same ophandle= mechanism as deep-check. The
corresponding /operations/$HANDLE page has three different forms. The
default is output=HTML.
If output=text is added to the query args, the results will be a text/plain
list. The first line is special: it is either "finished: yes" or "finished:
no"; if the operation is not finished, you must periodically reload the
page until it completes. The rest of the results are a plaintext list, with
one file/dir per line, slash-separated, with the filecap/dircap separated
by a space.
If output=JSON is added to the queryargs, then the results will be a
JSON-formatted dictionary with six keys. Note that because large directory
structures can result in very large JSON results, the full results will not
be available until the operation is complete (i.e. until output["finished"]
is True):
finished (bool): if False then you must reload the page until True
origin_si (base32 str): the storage index of the starting point
manifest: list of (path, cap) tuples, where path is a list of strings.
verifycaps: list of (printable) verify cap strings
storage-index: list of (base32) storage index strings
stats: a dictionary with the same keys as the t=start-deep-stats command
(described below)
POST $DIRURL?t=start-deep-size (must add &ophandle=XYZ)
This operation generates a number (in bytes) containing the sum of the
filesize of all directories and immutable files reachable from the given
directory. This is a rough lower bound of the total space consumed by this
subtree. It does not include space consumed by mutable files, nor does it
take expansion or encoding overhead into account. Later versions of the
code may improve this estimate upwards.
The /operations/$HANDLE status output consists of two lines of text:
finished: yes
size: 1234
POST $DIRURL?t=start-deep-stats (must add &ophandle=XYZ)
This operation performs a recursive walk of all files and directories
reachable from the given directory, and generates a collection of
statistics about those objects.
The result (obtained from the /operations/$OPHANDLE page) is a
JSON-serialized dictionary with the following keys (note that some of these
keys may be missing until 'finished' is True):
finished: (bool) True if the operation has finished, else False
count-immutable-files: count of how many CHK files are in the set
count-mutable-files: same, for mutable files (does not include directories)
count-literal-files: same, for LIT files (data contained inside the URI)
count-files: sum of the above three
count-directories: count of directories
size-immutable-files: total bytes for all CHK files in the set, =deep-size
size-mutable-files (TODO): same, for current version of all mutable files
size-literal-files: same, for LIT files
size-directories: size of directories (includes size-literal-files)
size-files-histogram: list of (minsize, maxsize, count) buckets,
with a histogram of filesizes, 5dB/bucket,
for both literal and immutable files
largest-directory: number of children in the largest directory
largest-immutable-file: number of bytes in the largest CHK file
size-mutable-files is not implemented, because it would require extra
queries to each mutable file to get their size. This may be implemented in
the future.
Assuming no sharing, the basic space consumed by a single root directory is
the sum of size-immutable-files, size-mutable-files, and size-directories.
The actual disk space used by the shares is larger, because of the
following sources of overhead:
integrity data
expansion due to erasure coding
share management data (leases)
backend (ext3) minimum block size
POST $URL?t=stream-manifest
This operation performs a recursive walk of all files and directories
reachable from the given starting point. For each such unique object
(duplicates are skipped), a single line of JSON is emitted to the HTTP
response channel. When the walk is complete, a final line of JSON is emitted
which contains the accumulated file-size/count "deep-stats" data.
A CLI tool can split the response stream on newlines into "response units",
and parse each response unit as JSON. Each such parsed unit will be a
dictionary, and will contain at least the "type" key: a string, one of
"file", "directory", or "stats".
For all units that have a type of "file" or "directory", the dictionary will
contain the following keys:
"path": a list of strings, with the path that is traversed to reach the
object
"cap": a writecap for the file or directory, if available, else a readcap
"verifycap": a verifycap for the file or directory
"repaircap": the weakest cap which can still be used to repair the object
"storage-index": a base32 storage index for the object
Note that non-distributed files (i.e. LIT files) will have values of None
for verifycap, repaircap, and storage-index, since these files can neither
be verified nor repaired, and are not stored on the storage servers.
The last unit in the stream will have a type of "stats", and will contain
the keys described in the "start-deep-stats" operation, below.
== Other Useful Pages ==
The portion of the web namespace that begins with "/uri" (and "/named") is
dedicated to giving users (both humans and programs) access to the Tahoe
virtual filesystem. The rest of the namespace provides status information
about the state of the Tahoe node.
GET / (the root page)
This is the "Welcome Page", and contains a few distinct sections:
Node information: library versions, local nodeid, services being provided.
Filesystem Access Forms: create a new directory, view a file/directory by
URI, upload a file (unlinked), download a file by
URI.
Grid Status: introducer information, helper information, connected storage
servers.
GET /status/
This page lists all active uploads and downloads, and contains a short list
of recent upload/download operations. Each operation has a link to a page
that describes file sizes, servers that were involved, and the time consumed
in each phase of the operation.
A GET of /status/?t=json will contain a machine-readable subset of the same
data. It returns a JSON-encoded dictionary. The only key defined at this
time is "active", with a value that is a list of operation dictionaries, one
for each active operation. Once an operation is completed, it will no longer
appear in data["active"] .
Each op-dict contains a "type" key, one of "upload", "download",
"mapupdate", "publish", or "retrieve" (the first two are for immutable
files, while the latter three are for mutable files and directories).
The "upload" op-dict will contain the following keys:
type (string): "upload"
storage-index-string (string): a base32-encoded storage index
total-size (int): total size of the file
status (string): current status of the operation
progress-hash (float): 1.0 when the file has been hashed
progress-ciphertext (float): 1.0 when the file has been encrypted.
progress-encode-push (float): 1.0 when the file has been encoded and
pushed to the storage servers. For helper
uploads, the ciphertext value climbs to 1.0
first, then encoding starts. For unassisted
uploads, ciphertext and encode-push progress
will climb at the same pace.
The "download" op-dict will contain the following keys:
type (string): "download"
storage-index-string (string): a base32-encoded storage index
total-size (int): total size of the file
status (string): current status of the operation
progress (float): 1.0 when the file has been fully downloaded
Front-ends which want to report progress information are advised to simply
average together all the progress-* indicators. A slightly more accurate
value can be found by ignoring the progress-hash value (since the current
implementation hashes synchronously, so clients will probably never see
progress-hash!=1.0).
GET /provisioning/
This page provides a basic tool to predict the likely storage and bandwidth
requirements of a large Tahoe grid. It provides forms to input things like
total number of users, number of files per user, average file size, number
of servers, expansion ratio, hard drive failure rate, etc. It then provides
numbers like how many disks per server will be needed, how many read
operations per second should be expected, and the likely MTBF for files in
the grid. This information is very preliminary, and the model upon which it
is based still needs a lot of work.
GET /helper_status/
If the node is running a helper (i.e. if [helper]enabled is set to True in
tahoe.cfg), then this page will provide a list of all the helper operations
currently in progress. If "?t=json" is added to the URL, it will return a
JSON-formatted list of helper statistics, which can then be used to produce
graphs to indicate how busy the helper is.
GET /statistics/
This page provides "node statistics", which are collected from a variety of
sources.
load_monitor: every second, the node schedules a timer for one second in
the future, then measures how late the subsequent callback
is. The "load_average" is this tardiness, measured in
seconds, averaged over the last minute. It is an indication
of a busy node, one which is doing more work than can be
completed in a timely fashion. The "max_load" value is the
highest value that has been seen in the last 60 seconds.
cpu_monitor: every minute, the node uses time.clock() to measure how much
CPU time it has used, and it uses this value to produce
1min/5min/15min moving averages. These values range from 0%
(0.0) to 100% (1.0), and indicate what fraction of the CPU
has been used by the Tahoe node. Not all operating systems
provide meaningful data to time.clock(): they may report 100%
CPU usage at all times.
uploader: this counts how many immutable files (and bytes) have been
uploaded since the node was started
downloader: this counts how many immutable files have been downloaded
since the node was started
publishes: this counts how many mutable files (including directories) have
been modified since the node was started
retrieves: this counts how many mutable files (including directories) have
been read since the node was started
There are other statistics that are tracked by the node. The "raw stats"
section shows a formatted dump of all of them.
By adding "?t=json" to the URL, the node will return a JSON-formatted
dictionary of stats values, which can be used by other tools to produce
graphs of node behavior. The misc/munin/ directory in the source
distribution provides some tools to produce these graphs.
GET / (introducer status)
For Introducer nodes, the welcome page displays information about both
clients and servers which are connected to the introducer. Servers make
"service announcements", and these are listed in a table. Clients will
subscribe to hear about service announcements, and these subscriptions are
listed in a separate table. Both tables contain information about what
version of Tahoe is being run by the remote node, their advertised and
outbound IP addresses, their nodeid and nickname, and how long they have
been available.
By adding "?t=json" to the URL, the node will return a JSON-formatted
dictionary of stats values, which can be used to produce graphs of connected
clients over time. This dictionary has the following keys:
["subscription_summary"] : a dictionary mapping service name (like
"storage") to an integer with the number of
clients that have subscribed to hear about that
service
["announcement_summary"] : a dictionary mapping service name to an integer
with the number of servers which are announcing
that service
["announcement_distinct_hosts"] : a dictionary mapping service name to an
integer which represents the number of
distinct hosts that are providing that
service. If two servers have announced
FURLs which use the same hostnames (but
different ports and tubids), they are
considered to be on the same host.
== Static Files in /public_html ==
The wapi server will take any request for a URL that starts with /static
and serve it from a configurable directory which defaults to
$BASEDIR/public_html . This is configured by setting the "[node]web.static"
value in $BASEDIR/tahoe.cfg . If this is left at the default value of
"public_html", then http://localhost:3456/static/subdir/foo.html will be
served with the contents of the file $BASEDIR/public_html/subdir/foo.html .
This can be useful to serve a javascript application which provides a
prettier front-end to the rest of the Tahoe wapi.
== safety and security issues -- names vs. URIs ==
Summary: use explicit file- and dir- caps whenever possible, to reduce the
potential for surprises when the virtual drive is changed while you aren't
looking.
The vdrive provides a mutable filesystem, but the ways that the filesystem
can change are limited. The only thing that can change is that the mapping
from child names to child objects that each directory contains can be changed
by adding a new child name pointing to an object, removing an existing child
name, or changing an existing child name to point to a different object.
Obviously if you query tahoe for information about the filesystem and then
act upon the filesystem (such as by getting a listing of the contents of a
directory and then adding a file to the directory), then the filesystem might
have been changed after you queried it and before you acted upon it.
However, if you use the URI instead of the pathname of an object when you act
upon the object, then the only change that can happen is when the object is a
directory then the set of child names it has might be different. If, on the
other hand, you act upon the object using its pathname, then a different
object might be in that place, which can result in more kinds of surprises.
For example, suppose you are writing code which recursively downloads the
contents of a directory. The first thing your code does is fetch the listing
of the contents of the directory. For each child that it fetched, if that
child is a file then it downloads the file, and if that child is a directory
then it recurses into that directory. Now, if the download and the recurse
actions are performed using the child's name, then the results might be
wrong, because for example a child name that pointed to a sub-directory when
you listed the directory might have been changed to point to a file (in which
case your attempt to recurse into it would result in an error and the file
would be skipped), or a child name that pointed to a file when you listed the
directory might now point to a sub-directory (in which case your attempt to
download the child would result in a file containing HTML text describing the
sub-directory!).
If your recursive algorithm uses the uri of the child instead of the name of
the child, then those kinds of mistakes just can't happen. Note that both the
child's name and the child's URI are included in the results of listing the
parent directory, so it isn't any harder to use the URI for this purpose.
In general, use names if you want "whatever object (whether file or
directory) is found by following this name (or sequence of names) when my
request reaches the server". Use URIs if you want "this particular object".
== Concurrency Issues ==
Tahoe uses both mutable and immutable files. Mutable files can be created
explicitly by doing an upload with ?mutable=true added, or implicitly by
creating a new directory (since a directory is just a special way to
interpret a given mutable file).
Mutable files suffer from the same consistency-vs-availability tradeoff that
all distributed data storage systems face. It is not possible to
simultaneously achieve perfect consistency and perfect availability in the
face of network partitions (servers being unreachable or faulty).
Tahoe tries to achieve a reasonable compromise, but there is a basic rule in
place, known as the Prime Coordination Directive: "Don't Do That". What this
means is that if write-access to a mutable file is available to several
parties, then those parties are responsible for coordinating their activities
to avoid multiple simultaneous updates. This could be achieved by having
these parties talk to each other and using some sort of locking mechanism, or
by serializing all changes through a single writer.
The consequences of performing uncoordinated writes can vary. Some of the
writers may lose their changes, as somebody else wins the race condition. In
many cases the file will be left in an "unhealthy" state, meaning that there
are not as many redundant shares as we would like (reducing the reliability
of the file against server failures). In the worst case, the file can be left
in such an unhealthy state that no version is recoverable, even the old ones.
It is this small possibility of data loss that prompts us to issue the Prime
Coordination Directive.
Tahoe nodes implement internal serialization to make sure that a single Tahoe
node cannot conflict with itself. For example, it is safe to issue two
directory modification requests to a single tahoe node's wapi server at the
same time, because the Tahoe node will internally delay one of them until
after the other has finished being applied. (This feature was introduced in
Tahoe-1.1; back with Tahoe-1.0 the web client was responsible for serializing
web requests themselves).
For more details, please see the "Consistency vs Availability" and "The Prime
Coordination Directive" sections of mutable.txt, in the same directory as
this file.
[1]: URLs and HTTP and UTF-8, Oh My
HTTP does not provide a mechanism to specify the character set used to
encode non-ascii names in URLs (rfc2396#2.1). We prefer the convention that
the filename= argument shall be a URL-encoded UTF-8 encoded unicode object.
For example, suppose we want to provoke the server into using a filename of
"f i a n c e-acute e" (i.e. F I A N C U+00E9 E). The UTF-8 encoding of this
is 0x66 0x69 0x61 0x6e 0x63 0xc3 0xa9 0x65 (or "fianc\xC3\xA9e", as python's
repr() function would show). To encode this into a URL, the non-printable
characters must be escaped with the urlencode '%XX' mechansim, giving us
"fianc%C3%A9e". Thus, the first line of the HTTP request will be "GET
/uri/CAP...?save=true&filename=fianc%C3%A9e HTTP/1.1". Not all browsers
provide this: IE7 uses the Latin-1 encoding, which is fianc%E9e.
The response header will need to indicate a non-ASCII filename. The actual
mechanism to do this is not clear. For ASCII filenames, the response header
would look like:
Content-Disposition: attachment; filename="english.txt"
If Tahoe were to enforce the utf-8 convention, it would need to decode the
URL argument into a unicode string, and then encode it back into a sequence
of bytes when creating the response header. One possibility would be to use
unencoded utf-8. Developers suggest that IE7 might accept this:
#1: Content-Disposition: attachment; filename="fianc\xC3\xA9e"
(note, the last four bytes of that line, not including the newline, are
0xC3 0xA9 0x65 0x22)
RFC2231#4 (dated 1997): suggests that the following might work, and some
developers (http://markmail.org/message/dsjyokgl7hv64ig3) have reported that
it is supported by firefox (but not IE7):
#2: Content-Disposition: attachment; filename*=utf-8''fianc%C3%A9e
My reading of RFC2616#19.5.1 (which defines Content-Disposition) says that
the filename= parameter is defined to be wrapped in quotes (presumeably to
allow spaces without breaking the parsing of subsequent parameters), which
would give us:
#3: Content-Disposition: attachment; filename*=utf-8''"fianc%C3%A9e"
However this is contrary to the examples in the email thread listed above.
Developers report that IE7 (when it is configured for UTF-8 URL encoding,
which is not the default in asian countries), will accept:
#4: Content-Disposition: attachment; filename=fianc%C3%A9e
However, for maximum compatibility, Tahoe simply copies bytes from the URL
into the response header, rather than enforcing the utf-8 convention. This
means it does not try to decode the filename from the URL argument, nor does
it encode the filename into the response header.