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

[[!tag projects/openneuro]]

## work notes

* http proxying for a local git remote seems to probably not work

* An interrupted `git-annex copy --to` a cluster via the http server,
  when repeated, fails. The http server outputs "transfer already in
  progress, or unable to take transfer lock". Apparently a second
  connection gets opened to the cluster, because the first connection
  never got shut down.

* When part of a file has been sent to a cluster via the http server,
  the transfer interrupted, and another node is added to the cluster,
  and the transfer of the file performed again, there is a failure
  sending to the node that had an incomplete copy. It fails like this:

	  //home/joey/tmp/bench/c3/.git/annex/tmp/SHA256E-s1048576000--09d7e19983a65682138fa5944f135e4fc593330c2693c41d22cc7881443d6060: withBinaryFile: illegal operation
	git-annex: transfer already in progress, or unable to take transfer lock
	p2pstdio: 1 failed

  When using ssh and not the http server, the node that had the incomplete
  copy also doesn't get the file, altough no error is displayed.

## 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 p2p protocol over http

* Support proxying to git remotes that use annex+http urls.

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

## 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.

* 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)