151 lines
7.2 KiB
Markdown
151 lines
7.2 KiB
Markdown
Once files are added (or removed or moved), need to send those changes to
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all the other git clones, at both the git level and the key/value level.
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## immediate action items
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* At startup, and possibly periodically, or when the network connection
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changes, or some heuristic suggests that a remote was disconnected from
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us for a while, queue remotes for processing by the TransferScanner,
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to queue Transfers of files it or we're missing.
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* After git sync, identify content that we don't have that is now available
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on remotes, and transfer. (Needed when we have a uni-directional connection
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to a remote, so it won't be uploading content to us.)
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But first, need to ensure that when a remote
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receives content, and updates its location log, it syncs that update
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out.
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## TransferScanner
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The TransferScanner thread needs to find keys that need to be Uploaded
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to a remote, or Downloaded from it.
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How to find the keys to transfer? I'd like to avoid potentially
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expensive traversals of the whole git working copy if I can.
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One way would be to do a git diff between the (unmerged) git-annex branches
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of the git repo, and its remote. Parse that for lines that add a key to
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either, and queue transfers. That should work fairly efficiently when the
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remote is a git repository. Indeed, git-annex already does such a diff
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when it's doing a union merge of data into the git-annex branch. It
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might even be possible to have the union merge and scan use the same
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git diff data.
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But that approach has several problems:
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1. The list of keys it would generate wouldn't have associated git
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filenames, so the UI couldn't show the user what files were being
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transferred.
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2. Worse, without filenames, any later features to exclude
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files/directories from being transferred wouldn't work.
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3. Looking at a git diff of the git-annex branches would find keys
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that were added to either side while the two repos were disconnected.
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But if the two repos' keys were not fully in sync before they
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disconnected (which is quite possible; transfers could be incomplete),
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the diff would not show those older out of sync keys.
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The remote could also be a special remote. In this case, I have to either
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traverse the git working copy, or perhaps traverse the whole git-annex
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branch (which would have the same problems with filesnames not being
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available).
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If a traversal is done, should check all remotes, not just
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one. Probably worth handling the case where a remote is connected
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while in the middle of such a scan, so part of the scan needs to be
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redone to check it.
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## longer-term TODO
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* Test MountWatcher on LXDE.
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* git-annex needs a simple speed control knob, which can be plumbed
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through to, at least, rsync. A good job for an hour in an
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airport somewhere.
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* Find a way to probe available outgoing bandwidth, to throttle so
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we don't bufferbloat the network to death.
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* Investigate the XMPP approach like dvcs-autosync does, or other ways of
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signaling a change out of band.
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* Add a hook, so when there's a change to sync, a program can be run
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and do its own signaling.
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* --debug will show often unnecessary work being done. Optimise.
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* This assumes the network is connected. It's often not, so the
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[[cloud]] needs to be used to bridge between LANs.
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* Configurablity, including only enabling git syncing but not data transfer;
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only uploading new files but not downloading, and only downloading
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files in some directories and not others. See for use cases:
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[[forum/Wishlist:_options_for_syncing_meta-data_and_data]]
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* speed up git syncing by using the cached ssh connection for it too
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(will need to use `GIT_SSH`, which needs to point to a command to run,
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not a shell command line)
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## data syncing
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There are two parts to data syncing. First, map the network and second,
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decide what to sync when.
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Mapping the network can reuse code in `git annex map`. Once the map is
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built, we want to find paths through the network that reach all nodes
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eventually, with the least cost. This is a minimum spanning tree problem,
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except with a directed graph, so really a Arborescence problem.
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With the map, we can determine which nodes to push new content to. Then we
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need to control those data transfers, sending to the cheapest nodes first,
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and with appropriate rate limiting and control facilities.
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This probably will need lots of refinements to get working well.
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### first pass: flood syncing
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Before mapping the network, the best we can do is flood all files out to every
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reachable remote. This is worth doing first, since it's the simplest way to
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get the basic functionality of the assistant to work. And we'll need this
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anyway.
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## done
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1. Can use `git annex sync`, which already handles bidirectional syncing.
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When a change is committed, launch the part of `git annex sync` that pushes
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out changes. **done**; changes are pushed out to all remotes in parallel
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1. Watch `.git/refs/remotes/` for changes (which would be pushed in from
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another node via `git annex sync`), and run the part of `git annex sync`
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that merges in received changes, and follow it by the part that pushes out
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changes (sending them to any other remotes).
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[The watching can be done with the existing inotify code! This avoids needing
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any special mechanism to notify a remote that it's been synced to.]
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**done**
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1. Periodically retry pushes that failed. **done** (every half an hour)
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1. Also, detect if a push failed due to not being up-to-date, pull,
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and repush. **done**
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2. Use a git merge driver that adds both conflicting files,
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so conflicts never break a sync. **done**
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* on-disk transfers in progress information files (read/write/enumerate)
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**done**
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* locking for the files, so redundant transfer races can be detected,
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and failed transfers noticed **done**
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* transfer info for git-annex-shell **done**
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* update files as transfers proceed. See [[progressbars]]
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(updating for downloads is easy; for uploads is hard)
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* add Transfer queue TChan **done**
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* add TransferInfo Map to DaemonStatus for tracking transfers in progress.
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**done**
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* Poll transfer in progress info files for changes (use inotify again!
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wow! hammer, meet nail..), and update the TransferInfo Map **done**
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* enqueue Transfers (Uploads) as new files are added to the annex by
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Watcher. **done**
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* enqueue Tranferrs (Downloads) as new dangling symlinks are noticed by
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Watcher. **done**
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(Note: Needs git-annex branch to be merged before the tree is merged,
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so it knows where to download from. Checked and this is the case.)
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* Write basic Transfer handling thread. Multiple such threads need to be
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able to be run at once. Each will need its own independant copy of the
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Annex state monad. **done**
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* Write transfer control thread, which decides when to launch transfers.
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**done**
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* Transfer watching has a race on kqueue systems, which makes finished
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fast transfers not be noticed by the TransferWatcher. Which in turn
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prevents the transfer slot being freed and any further transfers
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from happening. So, this approach is too fragile to rely on for
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maintaining the TransferSlots. Instead, need [[todo/assistant_threaded_runtime]],
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which would allow running something for sure when a transfer thread
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finishes. **done**
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* Test MountWatcher on KDE, and add whatever dbus events KDE emits when
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drives are mounted. **done**
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