cb6a703660
More complicated, but less asynchronous, which will make it easier for special remote programs to use it, at the expense of some added complexity in git-annex.
260 lines
9 KiB
Markdown
260 lines
9 KiB
Markdown
See [[todo/support_for_writing_external_special_remotes]] for motivation.
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This is a design for a protocol to be used to communicate between git-annex
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and a program implementing an external special remote.
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The external special remote program has a name like
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`git-annex-remote-$bar`. When `git annex initremote foo type=$bar` is run,
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git-annex finds the appropriate program in PATH.
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The program is started by git-annex when it needs to access the special
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remote, and may be left running for a long period of time. This allows
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it to perform expensive setup tasks, etc. Note that git-annex may choose to
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start multiple instances of the program (eg, when multiple git-annex
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commands are run concurrently in a repository).
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## protocol overview
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Communication is via stdin and stdout. Therefore, the external special
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remote must avoid doing any prompting, or outputting anything like eg,
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progress to stdout. (Such stuff can be sent to stderr instead.)
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The protocol is line based. Messages are sent in either direction, from
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git-annex to the special remote, and from the special remote to git-annex.
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## example session
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The special remote is responsible for sending the first message, indicating
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the version of the protocol it is using.
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VERSION 0
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Once it knows the version, git-annex will send a message telling the
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special remote to start up.
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PREPARE
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The special remote can now ask git-annex for its configuration, as needed,
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and check that it's valid. git-annex responds with the configuration values
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GETCONFIG directory
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/media/usbdrive/repo
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GETCONFIG automount
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true
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Once the special remote is satisfied with its configuration and is
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ready to go, it tells git-annex.
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PREPARE-SUCCESS
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Now git-annex will tell the special remote what to do. Let's suppose
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it wants to store a key.
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TRANSFER STORE somekey tmpfile
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The special remote can continue sending messages to git-annex during this
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transfer. It will typically send progress messages, indicating how many
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bytes have been sent:
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PROGRESS STORE somekey 10240
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PROGRESS STORE somekey 20480
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Once the key has been stored, the special remote tells git-annex the result:
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TRANSFER-SUCCESS STORE somekey
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Once git-annex is done with the special remote, it will close its stdin.
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The special remote program can then exit.
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## git-annex request messages
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These are the request messages git-annex may send to the special remote
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program. None of these messages require an immediate reply. The special
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remote can send any messages it likes while handling the requests.
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Once the special remote has finished performing the request, it should
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send one of the corresponding replies listed in the next section.
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* `PREPARE`
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Tells the special remote it's time to prepare itself to be used.
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Only run once, at startup, always immediately after the special remote
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sends VERSION.
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* `INITREMOTE`
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Request that the remote initialized itself. This is where any one-time
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setup tasks can be done, for example creating an Amazon S3 bucket.
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(PREPARE is still sent before this.)
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Note: This may be run repeatedly, as a remote is initialized in
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different repositories, or as the configuration of a remote is changed.
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So any one-time setup tasks should be done idempotently.
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* `GETCOST`
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Requests the remote return a use cost. Higher costs are more expensive.
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(See Config/Cost.hs for some standard costs.)
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* `TRANSFER STORE|RETRIEVE Key File`
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Requests the transfer of a key. For Send, the File is the file to upload;
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for Receive the File is where to store the download. Note that the File
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should not influence the filename used on the remote. The filename used
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should be derived from the Key.
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Multiple transfers might be requested by git-annex, but it's fine for the
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program to serialize them and only do one at a time.
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* `HAS Key`
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Requests the remote check if a key is present in it.
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* `REMOVE Key`
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Requests the remote remove a key's contents.
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## special remote replies
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These should be sent only in response to the git-annex request messages.
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(Any sent unexpectedly will be ignored.)
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They do not have to be sent immediately after the request; the special
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remote can send other messages and queries (listed in sections below)
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as it's performing the request.
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* `PREPARE-SUCCESS`
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Sent as a response to PREPARE once the special remote is ready for use.
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* `TRANSFER-SUCCESS STORE|RETRIEVE Key`
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Indicates the transfer completed successfully.
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* `TRANSFER-FAILURE STORE|RETRIEVE Key ErrorMsg`
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Indicates the transfer failed.
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* `HAS-SUCCESS Key`
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Indicates that a key has been positively verified to be present in the
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remote.
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* `HAS-FAILURE Key`
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Indicates that a key has been positively verified to not be present in the
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remote.
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* `HAS-UNKNOWN Key ErrorMsg`
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Indicates that it is not currently possible to verify if the key is
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present in the remote. (Perhaps the remote cannot be contacted.)
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* `REMOVE-SUCCESS Key`
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Indicates the key has been removed from the remote. May be returned if
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the remote didn't have the key at the point removal was requested.
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* `REMOVE-FAILURE Key`
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Indicates that the key was unable to be removed from the remote.
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* `COST Int`
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Indicates the cost of the remote.
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* `COST-UNKNOWN`
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Indicates the remote has no opinion of its cost.
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* `INITREMOTE-SUCCESS Setting=Value ...`
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Indicates the INITREMOTE succeeded and the remote is ready to use.
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The settings and values can optionally be returned. They will be added
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to the existing configuration of the remote (and may change existing
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values in it).
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* `INITREMOTE-FAILURE ErrorMsg`
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Indicates that INITREMOTE failed.
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## special remote messages
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These are messages the special remote program can send to
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git-annex at any time. It should not expect any response from git-annex.
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* `VERSION Int`
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Supported protocol version. Current version is 0. Must be sent first
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thing at startup, as until it sees this git-annex does not know how to
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talk with the special remote program!
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* `ERROR ErrorMsg`
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Generic error. Can be sent at any time if things get messed up.
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When possible, use a more specific reply from the list above.
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It would be a good idea to send this if git-annex sends a command
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you do not support. The program should exit after sending this, as
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git-annex will not talk to it any further.
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* `PROGRESS STORE|RETRIEVE Key Int`
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Indicates the current progress of the transfer. May be repeated any
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number of times during the transfer process. This is highly recommended
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for STORE. (It is optional but good for RETRIEVE.)
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## special remote queries
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After git-annex has sent the special remote a request, and before the
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special remote sends back a reply, git-annex enters quiet mode. It will
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avoid sending additional messages. While git-annex is in quiet mode,
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the special remote can send queries to it. Queries can not be sent at any
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other time.
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When it sees a query, git-annex will respond a line containing
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*only* the requested data.
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* `DIRHASH Key`
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Gets a two level hash associated with a Key. Something like "abc/def".
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This is always the same for any given Key, so can be used for eg,
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creating hash directory structures to store Keys in.
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* `GETCONFIG Setting`
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Gets one of the special remote's configuration settings.
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* `SETSTATE Key Value`
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git-annex can store state in the git-annex branch on a
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per-special-remote, per-key basis. This sets that state.
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* `GETSTATE Key`
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Gets any state previously stored for the key from the git-annex branch.
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Note that some special remotes may be accessed from multiple
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repositories, and the state is only eventually consistently synced
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between them. If two repositories set different values in the state
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for a key, the one that sets it last wins.
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## Simple shell example
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[[!format sh """
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#!/bin/sh
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set -e
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echo VERSION 0
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while read line; do
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set -- $line
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case "$1" in
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INITREMOTE)
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# XXX do anything necessary to create resources
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# used by the remote. Try to be idempotent.
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# Use GETCONFIG to get any needed configuration
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# settings.
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echo INITREMOTE-SUCCESS
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;;
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GETCOST)
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echo COST-UNKNOWN
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;;
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PREPARE)
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# XXX Use GETCONFIG to get configuration settings,
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# and do anything needed to start using the
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# special remote here.
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echo PREPARE-SUCCESS
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;;
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TRANSFER)
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key="$3"
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file="$4"
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case "$2" in
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STORE)
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# XXX upload file here
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# XXX when possible, send PROGRESS
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echo TRANSFER-SUCCESS STORE "$key"
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;;
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RETRIEVE)
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# XXX download file here
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echo TRANSFER-SUCCESS RETRIEVE "$key"
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;;
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esac
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;;
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HAS)
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key="$2"
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echo HAS-UNKNOWN "$key" "not implemented"
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;;
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REMOVE)
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key="$2"
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# XXX remove key here
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echo REMOVE-SUCCESS "$key"
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;;
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*)
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echo ERROR "unknown command received: $line"
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exit 1
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;;
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esac
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done
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# XXX anything that needs to be done at shutdown can be done here
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"""]]
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## TODO
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* Communicate when the network connection may have changed, so long-running
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remotes can reconnect.
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* uuid discovery during initremote.
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* Support for splitting files into chunks.
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* Use same verbs as used in special remote interface (instead of different
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verbs used in Types.Remote).
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