git-annex/doc/todo/git-annex_proxies.mdwn

This is a summary todo covering several subprojects, which would extend
git-annex to be able to use proxies which sit in front of a cluster of
repositories.

1. [[design/passthrough_proxy]]
2. [[design/p2p_protocol_over_http]]
3. [[design/balanced_preferred_content]]
4. [[todo/track_free_space_in_repos_via_git-annex_branch]]
5. [[todo/proving_preferred_content_behavior]]

## table of contents

[[!toc ]]

## planned schedule

Joey has received funding to work on this.
Planned schedule of work:

* June: git-annex proxies and clusters
* July: p2p protocol over http
* August, part 1: git-annex proxy support for exporttree
* August, part 2: [[track_free_space_in_repos_via_git-annex_branch]]
* September, part 1: balanced preferred content
* September, part 2: streaming through proxy to special remotes (especially S3)
* October, part 1: streaming through proxy continued
* October, part 2: proving behavior of balanced preferred content with proxies

[[!tag projects/openneuro]]

## work notes

* `git-annex assist --rebalance` of `balanced=foo:2` 
  sometimes needs several runs to stabalize.

  May not be a bug, needs reproducing and analysis.

* Concurrency issues with RepoSizes calculation and balanced content:

  * What if 2 concurrent threads are considering sending two different
    keys to a repo at the same time. It can hold either but not both.
    It should avoid sending both in this situation.

  * There can also be a race with 2 concurrent threads where one just
    finished sending to a repo, but has not yet updated the location log. 
    So the other one won't see an updated repo size.
    
    The fact that location log changes happen in CommandCleanup makes
    this difficult to fix.

    Could provisionally update Annex.reposizes before starting to send a
    key, and roll it back if the send fails. But then Logs.Location
    would update Annex.reposizes redundantly. So would need to remember
    the provisional update was made until that is called.... But what if it
    is never called for some reason?

    Also, in a race between two threads at the checking preferred content
    stage, neither would have started sending yet, and so both would think
    it was ok for them to.

    This race only really matters when the repo becomes full,
    then the second thread will fail to send because it's full. Or will
    send more than the configured maxsize. Still this would be good to
    fix.

  * If all the above thread concurrency problems are fixed, separate
    processes will still have concurrency problems. One case where that is
    bad is a cluster accessed via ssh. Each connection to the cluster is
    a separate process. So each will be unaware of changes made by others.
    When `git-annex copy --to cluster -Jn` is used, this makes a single
    command behave non-ideally, the same as the thread concurrency
    problems.

  * Possible solution: 

    Add to reposizes db a table for live updates.
    Listing process ID, thread ID, UUID, key, addition or removal
    (done)

    Make checking the balanced preferred content limit record a
    live update in the table (done)

    ... and use other live updates in making its decision
    
    Note: This will only work when preferred content is being checked.
    If a git-annex copy without --auto is run, for example, it won't
    tell other processes that it is in the process of filling up a remote.
    That seems ok though, because if the user is running a command like
    that, they are ok with a remote filling up.

    Make sure that two threads don't check balanced preferred content at the
    same time, so each thread always sees a consistent picture of what is
    happening. Use locking as necessary.
    
    In the unlikely event that one thread of a process is storing a key and
    another thread is dropping the same key from the same uuid, at the same
    time, reconcile somehow. How? Or is this perhaps something that cannot
    happen? Could just record the liveupdate for one, and not for the
    other.

    Also keep an in-memory cache of the live updates being performed by
    the current process. For use in location log update as follows..

    Make updating location log for a key that is in the in-memory cache
    of the live update table update the db, removing it from that table,
    and updating the in-memory reposizes. (done)

    Make updading location log have locking to make sure redundant
    information is never visible:
    Take lock, journal update, remove from live update table.

    Detect when an upload (or drop) fails, and remove from the live
    update table and in-memory cache. (done)

    Have a counter in the reposizes table that is updated on write. This
    can be used to quickly determine if it has changed. On every check of
    balanced preferred content, check the counter, and if it's been changed
    by another process, re-run calcRepoSizes. This would be expensive, but
    it would only happen when another process is running at the same time.
    The counter could also be a per-UUID counter, so two processes
    operating on different remotes would not have overhead.

    When loading the live update table, check if PIDs in it are still
    running (and are still git-annex), and if not, remove stale entries
    from it, which can accumulate when processes are interrupted.
    Note that it will be ok for the wrong git-annex process, running again
    at a pid to keep a stale item in the live update table, because that
    is unlikely and exponentially unlikely to happen repeatedly, so stale
    information will only be used for a short time.

    But then, how to check if a PID is git-annex or not? /proc of course,
    but what about other OS's? Windows?

    How? Possibly have a thread that
    waits on an empty MVar. Thread MVar through somehow to location log
    update. (Seems this would need checking preferred content to return
    the MVar? Or alternatively, the MVar could be passed into it, which 
    seems better..) Fill MVar on location log update. If MVar gets
    GCed without being filled, the thread will get an exception and can
    remove from table and cache then. This does rely on GC behavior, but if
    the GC takes some time, it will just cause a failed upload to take
    longer to get removed from the table and cache, which will just prevent
    another upload of a different key from running immediately.
    (Need to check if MVar GC behavior operates like this.
    See https://stackoverflow.com/questions/10871303/killing-a-thread-when-mvar-is-garbage-collected )
    Perhaps stale entries can be found in a different way. Require the live
    update table to be updated with a timestamp every 5 minutes. The thread
    that waits on the MVar can do that, as long as the transfer is running. If
    interrupted, it will become stale in 5 minutes, which is probably good
    enough? Could do it every minute, depending on overhead. This could
    also be done by just repeatedly touching a file named with the processes's
    pid in it, to avoid sqlite overhead.

* Still implementing LiveUpdate. Check for TODO XXX markers

* In the case where a copy to a remote fails (due eg to annex.diskreserve),
  the LiveUpdate thread can not get a chance to catch its exception when
  the LiveUpdate is gced, before git-annex exits. In this case, the
  database is left with some stale entries in the live update table.

  This is not a big problem because the same can happen when the process is
  interrupted. Still it would be cleaner for this not to happen. Is there
  any way to prevent it? Waiting 1 GC tick before exiting would do it,
  I'd think, but I tried manually doing a performGC at git-annex shutdown
  and it didn't help.

* The assistant is using NoLiveUpdate, but it should be posssible to plumb
  a LiveUpdate through it from preferred content checking to location log
  updating.

* `git-annex info` in the limitedcalc path in cachedAllRepoData
  double-counts redundant information from the journal due to using
  overLocationLogs. In the other path it does not, and this should be fixed
  for consistency and correctness.

## completed items for August's work on balanced preferred content

* Balanced preferred content basic implementation, including --rebalance
  option.
* Implemented [[track_free_space_in_repos_via_git-annex_branch]]
* `git-annex maxsize`
* annex.fullybalancedthreshhold

## completed items for August's work on git-annex proxy support for exporttre

* Special remotes configured with exporttree=yes annexobjects=yes
  can store objects in .git/annex/objects, as well as an exported tree.

* Support proxying to special remotes configured with
  exporttree=yes annexobjects=yes.

* post-retrieve: When proxying is enabled for an exporttree=yes
  special remote and the configured remote.name.annex-tracking-branch
  is received, the tree is exported to the special remote.

* When getting from a P2P HTTP remote, prompt for credentials when
  required, instead of failing.

* Prevent `updateproxy` and `updatecluster` from adding
  an exporttree=yes special remote that does not have
  annexobjects=yes, to avoid foot shooting.

* Implement `git-annex export treeish --to=foo --from=bar`, which
  gets from bar as needed to send to foo. Make post-retrieve use
  `--to=r --from=r` to handle the multiple files case.

## items deferred until later for p2p protocol over http

* `git-annex p2phttp` should support serving several repositories at the same
  time (not as proxied remotes), so that eg, every git-annex repository
  on a server can be served on the same port.

* Support proxying to git remotes that use annex+http urls. This needs a
  translation from P2P protocol to servant-client to P2P protocol.

* Should be possible to use a git-remote-annex annex::$uuid url as
  remote.foo.url with remote.foo.annexUrl using annex+http, and so 
  not need a separate web server to serve the git repository. Doesn't work
  currently because git-remote-annex urls only support special remotes.
  It would need a new form of git-remote-annex url, eg:
  annex::$uuid?annex+http://example.com/git-annex/

* `git-annex p2phttp` could support systemd socket activation. This would
  allow making a systemd unit that listens on port 80.

## completed items for July's work on p2p protocol over http

* HTTP P2P protocol design [[design/p2p_protocol_over_http]].

* addressed [[doc/todo/P2P_locking_connection_drop_safety]]

* implemented server and client for HTTP P2P protocol

* added git-annex p2phttp command to serve HTTP P2P protocol

* Make git-annex p2phttp support https.

* Allow using annex+http urls in remote.name.annexUrl

* Make http server support proxying.

* Make http server support serving a cluster.

## items deferred until later for [[design/passthrough_proxy]]

* Check annex.diskreserve when proxying for special remotes
  to avoid the proxy's disk filling up with the temporary object file
  cached there.

* Resuming an interrupted download from proxied special remote makes the proxy
  re-download the whole content. It could instead keep some of the 
  object files around when the client does not send SUCCESS. This would
  use more disk, but without streaming, proxying a special remote already
  needs some disk. And it could minimize to eg, the last 2 or so.
  The design doc has some more thoughts about this.

* Streaming download from proxied special remotes. See design.
  (Planned for September)

* When an upload to a cluster is distributed to multiple special remotes,
  a temporary file is written for each one, which may even happen in
  parallel. This is a lot of extra work and may use excess disk space.
  It should be possible to only write a single temp file.
  (With streaming this won't be an issue.)

* Indirect uploads when proxying for special remote
  (to be considered). See design.

* Getting a key from a cluster currently picks from amoung
  the lowest cost remotes at random. This could be smarter,
  eg prefer to avoid using remotes that are doing other transfers at the
  same time.

* The cost of a proxied node that is accessed via an intermediate gateway
  is currently the same as a node accessed via the cluster gateway.
  To fix this, there needs to be some way to tell how many hops through
  gateways it takes to get to a node. Currently the only way is to
  guess based on number of dashes in the node name, which is not satisfying.

  Even counting hops is not very satisfying, one cluster gateway could
  be much more expensive to traverse than another one.

  If seriously tackling this, it might be worth making enough information
  available to use spanning tree protocol for routing inside clusters.

* Optimise proxy speed. See design for ideas.

* Speed: A proxy to a local git repository spawns git-annex-shell 
  to communicate with it. It would be more efficient to operate
  directly on the Remote. Especially when transferring content to/from it.
  But: When a cluster has several nodes that are local git repositories,
  and is sending data to all of them, this would need an alternate
  interface than `storeKey`, which supports streaming, of chunks
  of a ByteString.

* Use `sendfile()` to avoid data copying overhead when
  `receiveBytes` is being fed right into `sendBytes`.
  Library to use:
  <https://hackage.haskell.org/package/hsyscall-0.4/docs/System-Syscall.html>

* Support using a proxy when its url is a P2P address.
  (Eg tor-annex remotes.)

## completed items for June's work on [[design/passthrough_proxy]]:

* UUID discovery via git-annex branch. Add a log file listing UUIDs
  accessible via proxy UUIDs. It also will contain the names
  of the remotes that the proxy is a proxy for, 
  from the perspective of the proxy. (done)

* Add `git-annex updateproxy` command (done)

* Remote instantiation for proxies. (done)

* Implement git-annex-shell proxying to git remotes. (done)

* Proxy should update location tracking information for proxied remotes,
  so it is available to other users who sync with it. (done)

* Implement `git-annex initcluster` and `git-annex updatecluster` commands (done)

* Implement cluster UUID insertation on location log load, and removal
  on location log store. (done)

* Omit cluster UUIDs when constructing drop proofs, since lockcontent will
  always fail on a cluster. (done)

* Don't count cluster UUID as a copy in numcopies checking etc. (done)

* Tab complete proxied remotes and clusters in eg --from option. (done)

* Getting a key from a cluster should proxy from one of the nodes that has
  it. (done)

* Implement upload with fanout to multiple cluster nodes and reporting back
  additional UUIDs over P2P protocol. (done)

* Implement cluster drops, trying to remove from all nodes, and returning
  which UUIDs it was dropped from. (done)

* `git-annex testremote` works against proxied remote and cluster. (done)

* Avoid `git-annex sync --content` etc from operating on cluster nodes by
  default since syncing with a cluster implicitly syncs with its nodes. (done)

* On upload to cluster, send to nodes where its preferred content, and not
  to other nodes. (done)

* Support annex.jobs for clusters. (done)

* Add `git-annex extendcluster` command and extend `git-annex updatecluster` 
  to support clusters with multiple gateways. (done)

* Support proxying for a remote that is proxied by another gateway of
  a cluster. (done)

* Support distributed clusters: Make a proxy for a cluster repeat
  protocol messages on to any remotes that have the same UUID as
  the cluster. Needs extension to P2P protocol to avoid cycles.
  (done)

* Proxied cluster nodes should have slightly higher cost than the cluster
  gateway. (done)

* Basic support for proxying special remotes. (But not exporttree=yes ones
  yet.) (done)

* Tab complete remotes in all relevant commands (done)

* Display cluster and proxy information in git-annex info (done)