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:
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 Performance costs for some common operations, 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.
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.
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 Performance costs for some common operations (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.
directory ← create()
This is spelled “POST /uri?t=mkdir”.
Performance costs for some common operations 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”. ☺