mirror of
https://github.com/tahoe-lafs/tahoe-lafs.git
synced 2024-12-22 14:22:25 +00:00
91047bf828
This uses Read-The-Docs (sphinx/docutils) references exclusively, but adds a README.md for GitHub viewers to remind them that the links there won't work (closes ticket:2835). It also fixes all the dangling references and other Sphinx warnings. The "Preparation" section of docs/magic-folder-howto.rst was removed, since this feature has since been merged to trunk.
149 lines
6.4 KiB
ReStructuredText
149 lines
6.4 KiB
ReStructuredText
.. -*- coding: utf-8-with-signature-unix; fill-column: 77 -*-
|
|
|
|
********************************
|
|
Using Tahoe as a key-value store
|
|
********************************
|
|
|
|
There are several ways you could use Tahoe-LAFS as a key-value store.
|
|
|
|
Looking only at things that are *already implemented*, there are three
|
|
options:
|
|
|
|
1. Immutable files
|
|
|
|
API:
|
|
|
|
* key ← put(value)
|
|
|
|
This is spelled "`PUT /uri`_" in the API.
|
|
|
|
Note: the user (client code) of this API does not get to choose the key!
|
|
The key is determined programmatically using secure hash functions and
|
|
encryption of the value and of the optional "added convergence secret".
|
|
|
|
* value ← get(key)
|
|
|
|
This is spelled "`GET /uri/$FILECAP`_" in the API. "$FILECAP" is the
|
|
key.
|
|
|
|
For details, see "immutable files" in :doc:`performance`, but in summary:
|
|
the performance is not great but not bad.
|
|
|
|
That document doesn't mention that if the size of the A-byte mutable file
|
|
is less than or equal to `55 bytes`_ then the performance cost is much
|
|
smaller, because the value gets packed into the key. Added a ticket:
|
|
`#2226`_.
|
|
|
|
2. Mutable files
|
|
|
|
API:
|
|
|
|
* key ← create()
|
|
|
|
This is spelled "`PUT /uri?format=mdmf`_".
|
|
|
|
Note: again, the key cannot be chosen by the user! The key is
|
|
determined programmatically using secure hash functions and RSA public
|
|
key pair generation.
|
|
|
|
* set(key, value)
|
|
|
|
* value ← get(key)
|
|
|
|
This is spelled "`GET /uri/$FILECAP`_". Again, the "$FILECAP" is the
|
|
key. This is the same API as for getting the value from an immutable,
|
|
above. Whether the value you get this way is immutable (i.e. it will
|
|
always be the same value) or mutable (i.e. an authorized person can
|
|
change what value you get when you read) depends on the type of the
|
|
key.
|
|
|
|
Again, for details, see "mutable files" in :doc:`performance` (and
|
|
`these tickets`_ about how that doc is incomplete), but in summary, the
|
|
performance of the create() operation is *terrible*! (It involves
|
|
generating a 2048-bit RSA key pair.) The performance of the set and get
|
|
operations are probably merely not great but not bad.
|
|
|
|
3. Directories
|
|
|
|
API:
|
|
|
|
* directory ← create()
|
|
|
|
This is spelled "`POST /uri?t=mkdir`_".
|
|
|
|
:doc:`performance` does not mention directories (`#2228`_), but in order
|
|
to understand the performance of directories you have to understand how
|
|
they are implemented. Mkdir creates a new mutable file, exactly the
|
|
same, and with exactly the same performance, as the "create() mutable"
|
|
above.
|
|
|
|
* set(directory, key, value)
|
|
|
|
This is spelled "`PUT /uri/$DIRCAP/[SUBDIRS../]FILENAME`_". "$DIRCAP"
|
|
is the directory, "FILENAME" is the key. The value is the body of the
|
|
HTTP PUT request. The part about "[SUBDIRS../]" in there is for
|
|
optional nesting which you can ignore for the purposes of this
|
|
key-value store.
|
|
|
|
This way, you *do* get to choose the key to be whatever you want (an
|
|
arbitrary unicode string).
|
|
|
|
To understand the performance of ``PUT /uri/$directory/$key``,
|
|
understand that this proceeds in two steps: first it uploads the value
|
|
as an immutable file, exactly the same as the "put(value)" API from the
|
|
immutable API above. So right there you've already paid exactly the
|
|
same cost as if you had used that API. Then after it has finished
|
|
uploading that, and it has the immutable file cap from that operation
|
|
in hand, it downloads the entire current directory, changes it to
|
|
include the mapping from key to the immutable file cap, and re-uploads
|
|
the entire directory. So that has a cost which is easy to understand:
|
|
you have to download and re-upload the entire directory, which is the
|
|
entire set of mappings from user-chosen keys (Unicode strings) to
|
|
immutable file caps. Each entry in the directory occupies something on
|
|
the order of 300 bytes.
|
|
|
|
So the "set()" call from this directory-based API has obviously much
|
|
worse performance than the the equivalent "set()" calls from the
|
|
immutable-file-based API or the mutable-file-based API. This is not
|
|
necessarily worse overall than the performance of the
|
|
mutable-file-based API if you take into account the cost of the
|
|
necessary create() calls.
|
|
|
|
* value ← get(directory, key)
|
|
|
|
This is spelled "`GET /uri/$DIRCAP/[SUBDIRS../]FILENAME`_". As above,
|
|
"$DIRCAP" is the directory, "FILENAME" is the key.
|
|
|
|
The performance of this is determined by the fact that it first
|
|
downloads the entire directory, then finds the immutable filecap for
|
|
the given key, then does a GET on that immutable filecap. So again,
|
|
it is strictly worse than using the immutable file API (about twice
|
|
as bad, if the directory size is similar to the value size).
|
|
|
|
What about ways to use LAFS as a key-value store that are not yet
|
|
implemented? Well, Zooko has lots of ideas about ways to extend Tahoe-LAFS to
|
|
support different kinds of storage APIs or better performance. One that he
|
|
thinks is pretty promising is just the Keep It Simple, Stupid idea of "store a
|
|
sqlite db in a Tahoe-LAFS mutable". ☺
|
|
|
|
.. _PUT /uri: https://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/frontends/webapi.rst#writing-uploading-a-file
|
|
|
|
.. _GET /uri/$FILECAP: https://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/frontends/webapi.rst#viewing-downloading-a-file
|
|
|
|
.. _55 bytes: https://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/src/allmydata/immutable/upload.py?rev=196bd583b6c4959c60d3f73cdcefc9edda6a38ae#L1504
|
|
|
|
.. _PUT /uri?format=mdmf: https://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/frontends/webapi.rst#writing-uploading-a-file
|
|
|
|
.. _#2226: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2226
|
|
|
|
.. _these tickets: https://tahoe-lafs.org/trac/tahoe-lafs/query?status=assigned&status=new&status=reopened&keywords=~doc&description=~performance.rst&col=id&col=summary&col=status&col=owner&col=type&col=priority&col=milestone&order=priority
|
|
|
|
.. _POST /uri?t=mkdir: https://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/frontends/webapi.rst#creating-a-new-directory
|
|
|
|
.. _#2228: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2228
|
|
|
|
.. _PUT /uri/$DIRCAP/[SUBDIRS../]FILENAME: https://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/frontends/webapi.rst#creating-a-new-directory
|
|
|
|
.. _GET /uri/$DIRCAP/[SUBDIRS../]FILENAME: https://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/frontends/webapi.rst#reading-a-file
|
|
|