git-annex/doc/design/p2p_protocol.mdwn

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The git-annex P2P protocol is a custom protocol that git-annex uses to
communicate between peers.
There's a common line-based serialization of the protocol, but other
serializations are also possible. The line-based serialization is spoken
by [[git-annex-shell], and by git-annex over tor.
One peer is known as the client, and is the peer that initiates the
connection and sends commands. The other peer is known as the server, and
is the peer that the client connects to. It's possible for two connections
to be run at the same time between the same two peers, in different
directions.
## Errors
Either the client or the server may send an error message at any
time.
When the client sends an ERROR, the server will close the connection.
If the server sends an ERROR in response to the client's
request, the connection will remain open, and the client can make
another request.
ERROR this repository is read-only; write access denied
## Authentication
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The protocol generally starts with authentication. However, if
authentication already occurs on another layer, as is the case with
git-annex-shell, authentication will be skipped.
The client starts by sending an authentication command to the server,
along with its UUID. The AuthToken is some arbitrary token that has been
agreed upon beforehand.
AUTH UUID AuthToken
The server responds with either its own UUID when authentication
is successful. Or, it can fail the authentication, and close the
connection.
AUTH-SUCCESS UUID
AUTH-FAILURE
Note that authentication does not guarantee that the client is talking to
who they expect to be talking to. This, and encryption of the connection,
are handled at a lower level.
## Protocol version
The default protocol version is 0. The client can choose to
negotiate a new version with the server. This must come after
any authentication.
The client sends the highest protocol version it supports:
VERSION 2
The server responds with the highest protocol version it supports
that is less than or equal to the version the client sent:
VERSION 1
Now both client and server should use version 1.
## Cluster cycle prevention
In protocol version 2, immediately after VERSION, the
client can send an additional message that is used to
prevent cycles when accessing clusters.
BYPASS UUID1 UUID2 ...
The UUIDs are cluster gateways to avoid connecting to when
serving a cluster.
The server makes no response to this message.
## Binary data
The protocol allows raw binary data to be sent. This is done
using a DATA message. In the line-based serialization, this comes
on its own line, followed by a newline and the binary data.
The Len value tells how many bytes of data to read.
DATA 3
foo
Note that there is no newline after the binary data; the next protocol
message will come immediately after it.
If the sender finds itself unable to send as many bytes of data as it
promised (perhaps because a file got truncated while it was being sent),
its only option is to close the protocol connection.
Fixed some other potential hangs in the P2P protocol Finishes the start made in 983c9d5a53189f71797591692c0ed675f5bd1c16, by handling the case where `transfer` fails for some other reason, and so the ReadContent callback does not get run. I don't know of a case where `transfer` does fail other than the locking dealt with in that commit, but it's good to have a guarantee. StoreContent and StoreContentTo had a similar problem. Things like `getViaTmp` may decide not to run the transfer action. And `transfer` could certianly fail, if another transfer of the same object was in progress. (Or a different object when annex.pidlock is set.) If the transfer action was not run, the content of the object would not all get consumed, and so would get interpreted as protocol commands, which would not go well. My approach to fixing all of these things is to set a TVar only once all the data in the transfer is known to have been read/written. This way the internals of `transfer`, `getViaTmp` etc don't matter. So in ReadContent, it checks if the transfer completed. If not, as long as it didn't throw an exception, send empty and Invalid data to the callback. On an exception the state of the protocol is unknown so it has to raise ProtoFailureException and close the connection, same as before. In StoreContent, if the transfer did not complete some portion of the DATA has been read, so the protocol is in an unknown state and it has to close the conection as well. (The ProtoFailureMessage used here matches the one in Annex.Transfer, which is the most likely reason. Not ideal to duplicate it..) StoreContent did not ever close the protocol connection before. So this is a protocol change, but only in an exceptional circumstance, and it's not going to break anything, because clients already need to deal with the connection breaking at any point. The way this new behavior looks (here origin has annex.pidlock = true so will only accept one upload to it at a time): git annex copy --to origin -J2 copy x (to origin...) ok copy y (to origin...) Lost connection (fd:25: hGetChar: end of file) This work is supported by the NIH-funded NICEMAN (ReproNim TR&D3) project.
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And if the receiver finds itself unable to receive all the data for some
reason (eg, out of disk space), its only option is to close the protocol
connection.
## Checking if content is present
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To check if a key is currently present on the server, the client sends:
CHECKPRESENT Key
The server responds with either SUCCESS or FAILURE.
## Locking content
To lock content on the server, preventing it from being removed,
the client sends:
LOCKCONTENT Key
The server responds with either SUCCESS or FAILURE.
The former indicates the content is locked. It will remain
locked until the connection is broken, or the client
sends:
UNLOCKCONTENT Key
The server makes no response to that.
## Removing content
To remove a key's content from the server, the client sends:
REMOVE Key
The server responds with either SUCCESS or FAILURE.
In protocol version 2, the server can optionally reply with SUCCESS-PLUS
or FAILURE-PLUS. Each has a subsequent list of UUIDs of repositories
that the content was removed from.
## Storing content on the server
To store content on the server, the client sends:
PUT AssociatedFile Key
Here AssociatedFile may be the name of a file in the git
repository, for information purposes only. Or it can be the
empty string. It will always have unix directory separators.
(Note that in the line-based serialization. AssociatedFile may not contain any
spaces, since it's not the last token in the line. Use '%' to indicate
whitespace.)
The server may respond with ALREADY-HAVE if it already
had the content of that key.
In protocol version 2, the server can optionally reply with
ALREADY-HAVE-PLUS. The subsequent list of UUIDs are additional
UUIDs where the content is stored, in addition to the UUID where
the client was going to send it.
Otherwise, it responds with:
PUT-FROM Offset
Offset is the number of bytes into the file that the server wants
the client to start. This allows resuming transfers.
The client then sends a DATA message with content of the file from
the offset to the end of file.
In protocol version 1, after the data, the client sends an additional
message, to indicate if the content of the file has changed while it
was being sent.
INVALID
VALID
If the server successfully receives the data and stores the content,
it replies with SUCCESS. Otherwise, FAILURE.
In protocol version 2, the server can optionally reply with SUCCESS-PLUS
and a list of UUIDs where the content was stored.
## Getting content from the server
To get content from the server, the client sends:
GET Offset AssociatedFile Key
The Offset is the number of bytes into the file that the client wants
the server to skip, which allows resuming transfers.
See description of AssociatedFile above.
The server then sends a DATA message with the content of the file
from the offset to end of file.
In protocol version 1, after the data, the server sends an additional
message, to indicate if the content of the file has changed while it
was being sent.
INVALID
VALID
The client replies with SUCCESS or FAILURE.
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Note that the client responding with SUCCESS does not indicate to the
server that it has stored the content. It may receive it and throw it away.
## Connection to services
This is used to connect to services like git-upload-pack and
git-receive-pack that speak their own protocol.
The client sends a message to request the connection.
Service is the name of the service, eg "git-upload-pack".
CONNECT Service
Both client and server may now exchange DATA messages in any order,
encapsulating the service's protocol.
When the service exits, the server indicates this by telling the client
its exit code.
CONNECTDONE ExitCode
After that, the server closes the connection.
## Change notification
The client can request to be notified when a ref in
the git repository on the server changes.
NOTIFYCHANGE
The server will block until at least
one of the refs changes, and send a list of changed
refs.
CHANGED ChangedRefs
For example:
CHANGED refs/heads/master refs/heads/git-annex
Some servers may not support this command.