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