111 lines
4.5 KiB
Markdown
111 lines
4.5 KiB
Markdown
This special remote type lets you store content in a remote of your own
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devising, configured via some simple hooks.
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It's not recommended to use this remote type when another like [[rsync]]
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or [[directory]] will do. If your hooks are not carefully written, data
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could be lost.
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If you're building a special remote for others to use,
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instead consider building an [[external_special_remote|external]].
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## example
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Here's a simple example that stores content on clay tablets. If you
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implement this example in the real world, I'd appreciate a tour
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next Apert! :) --[[Joey]]
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# git config annex.cuneiform-store-hook 'tocuneiform < "$ANNEX_FILE" | tablet-writer --implement=stylus --title="$ANNEX_KEY" | tablet-proofreader | librarian --shelve --floor=$ANNEX_HASH_1 --shelf=$ANNEX_HASH_2'
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# git config annex.cuneiform-retrieve-hook 'librarian --get --floor=$ANNEX_HASH_1 --shelf=$ANNEX_HASH_2 --title="$ANNEX_KEY" | tablet-reader --implement=coffee --implement=glasses --force-monastic-dedication | fromcuneiform > "$ANNEX_FILE"'
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# git config annex.cuneiform-remove-hook 'librarian --get --floor=$ANNEX_HASH_1 --shelf=$ANNEX_HASH_2 --title="$ANNEX_KEY" | goon --hit-with-hammer'
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# git config annex.cuneiform-checkpresent-hook 'librarian --find --force-distrust-catalog --floor=$ANNEX_HASH_1 --shelf=$ANNEX_HASH_2 --title="$ANNEX_KEY" --shout-title'
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# git annex initremote library type=hook hooktype=cuneiform encryption=none
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# git annex describe library "the reborn Library of Alexandria (upgrade to bronze plates pending)"
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Can you spot the potential data loss bugs in the above simple example?
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(Hint: What happens when the `tablet-proofreader` exits nonzero?)
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## configuration
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These parameters can be passed to `git annex initremote`:
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* `hooktype` - Required. This specifies a collection of hooks to use for
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this remote.
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* `encryption` - One of "none", "hybrid", "shared", or "pubkey".
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See [[encryption]].
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* `keyid` - Specifies the gpg key to use for [[encryption]].
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* `chunk` - Enables [[chunking]] when storing large files.
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## hooks
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Each type of hook remote is specified by a collection of hook commands.
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Each hook command is run as a shell command line, and should return nonzero
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on failure, and zero on success.
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These environment variables are used to communicate with the hook commands:
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* `ANNEX_KEY` - name of a key to store, retrieve, remove, or check.
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* `ANNEX_FILE` - a file containing the key's content
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* `ANNEX_HASH_1` - short stable value, based on the key, can be used for hashing
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into 1024 buckets.
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* `ANNEX_HASH_2` - another hash value, can be used for a second level of hashing
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The settings to use in git config for the hook commands are as follows:
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* `annex.$hooktype-store-hook` - Command run to store a key in the special remote.
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`ANNEX_FILE` contains the content to be stored.
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* `annex.$hooktype-retrieve-hook` - Command run to retrieve a key from the special remote.
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`ANNEX_FILE` is a file that the retrieved content should be written to.
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The file may already exist with a partial
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copy of the content (or possibly just garbage), to allow for resuming
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of partial transfers.
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* `annex.$hooktype-remove-hook` - Command to remove a key from the special remote.
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* `annex.$hooktype-checkpresent-hook` - Command to check if a key is present
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in the special remote. Should output the key name to stdout, on its own line,
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if and only if the key has been actively verified to be present in the
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special remote (caching presence information is a very bad idea);
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all other output to stdout will be ignored.
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## combined hook program
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This interface is deprecated -- it's better, and not much harder,
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to write an [[external_special_remote|external]]!
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Rather than setting all of the above hooks, you can write a single
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program that handles everything, and set a single hook to make it be used.
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# git config annex.demo-hook /usr/local/bin/annexdemo
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# git annex initremote mydemorepo type=hook hooktype=demo encryption=none
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The program just needs to look at the `ANNEX_ACTION` environment variable
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to see what it's being asked to do. For example:
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[[!format sh """
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#!/bin/sh
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set -e
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case "$ANNEX_ACTION" in
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store)
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demo-upload "$ANNEX_HASH_1/$ANNEX_HASH_2/$ANNEX_KEY" < "$ANNEX_FILE"
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;;
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retrieve)
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demo-download "$ANNEX_HASH_1/$ANNEX_HASH_2/$ANNEX_KEY" > "$ANNEX_FILE"
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;;
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remove)
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demo-nuke "$ANNEX_HASH_1/$ANNEX_HASH_2/$ANNEX_KEY"
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;;
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checkpresent)
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if demo-exists "$ANNEX_HASH_1/$ANNEX_HASH_2/$ANNEX_KEY"; then
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echo "$ANNEX_KEY"
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fi
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;;
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*)
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echo "unknown ANNEX_ACTION: $ANNEX_ACTION" >&2
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exit 1
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;;
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esac
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"""]]
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