OCaml
Connect processes into powerful data pipelines with a simple git-like filesystem interface

Datakit

Datakit is tool to orchestrate applications using a 9P dataflow. It revisits the UNIX pipeline concept, with a modern twist: streams of tree-structured data instead of raw text. Datakit allows to define complex build pipelines over version-controlled data, using shell scripts interacting with the filesystem. For instance to trigger a build on every changes in the master branch:

# The Git repository to test
DB=/data

# Load the `map` function.
# `map fn branch` calls `fn` on the current head of `branch`, and then again
# each time it changes. `fn` runs in a transaction directory on a new branch.
# The transaction will be committed when `fn` returns.
source ci/map.sh

function build {
  echo 'BUILD FAILED' > msg
  ((docker build rw && echo PASSED > "$TRANS/msg")  || echo '*** BUILD FAILED ***') 2>&1 | tee "rw/log"
}

map build master

Datakit is currently used as the coordination layer for Hyperkit, the hypervisor component of Docker for Mac and Windows.

Quick Start

The easiest way to use Datakit is to start both the server and the client in a container.

To expose a Git repository as a 9p endpoint on port 5640 on a private network, just run:

$ docker network create datakit-net # create a private network
$ docker run -it --net datakit-net --name datakit -v <path/to/git/repo>:/data docker/datakit

Note: The --name datakit option is mandatory, it will allow the client to connect to a known name on the private network.

You can then start a Datakit client, which will mount the 9p endpoint and expose the database as a filesystem API:

# In an other terminal
$ docker run -it --privileged --net datakit-net docker/datakit:client
$ ls /db
branch     remotes    snapshots  trees

Note: the --privileged option is needed because the container will have to mount the 9p endpoint into its local filesystem.

Now you can explore, edit and script /db. See the Filesystem API for more details.

Experimental Github API bindings

To start Datakit with the experimental Github bindings:

$ docker run -it --net datakit-net --name datakit -v <path/to/git/repo>:/data docker/datakit:github
$ docker run -it --privileged --net datakit-net docker/datakit:client
$ ls /db
branch      github.com  remotes     snapshots   trees

Building

The easiest way to build that project is to use docker, (which is what the start-datakit.sh does under the hood):

$ docker build -t datakit .
$ docker run datakit

This will expose the database's 9p endpoint on port 5640.

If you really want to build the project from sources, you will need to install ocaml and opam. Then write:

$ opam pin add datakit . -n -y
$ opam pin add hvsock https://github.com/djs55/ocaml-hvsock.git -n -y
$ opam depext datakit -y
$ opam install alcotest datakit --deps-only -y
$ make && make test

Usage

$ datakit -h

Filesystem API

The /branch directory contains one subdirectory for each branch. Use mkdir to create a new branch and rm to delete one.

Each branch directory contains:

• fast-forward will do a fast-forward merge to any commit ID written to this file.

• head gives the commit ID of the head of the branch when read (or the empty string if the branch is empty).

• head.live is a stream which produces a list of commit IDs, one per line, starting with the current one and returning new ones as the branch is updated. When the branch does not have a commit, this is represented by a blank line.

• reflog is a stream which outputs a new line each time the current HEAD is updated. The line gives the commit hash (or is blank if the branch has been deleted). Unlike head.live, reflog does not start by outputting the current commit and it does not skip commits.

• ro a live read-only view onto the current contents of the head of the branch.

• transactions, which is used to update the branch.

• watch, which can be used to watch specific files or directories for changes.

Note that reading from head.live will skip directly to the latest commit: even if you read continuously from it, you will not necessarily see all intermediate commits.

The root also contains /snapshots, which can be used to explore any commit in the repository, if you know its ID. The directory will always appear empty, but attempting to access a subdirectory named by a commit ID will work.

The /trees directory works in a similar way to /snapshots, but is indexed by directory tree or file hashes (as read from tree.live) rather than by commit hashes.

Transactions

Read/write transactions can be created my making a new directory for the transaction in transactions. The newly created directory will contain:

• rw a directory with the current contents of the transaction. Initially, this is a copy of the branch's ro directory. Modify this as desired.

• msg is the commit message to use.

• parents is the list of commit hashes of the parents, one per line. Initially, this is the single head commit at the time the transaction was created, but it can be modified to produce other effects. Simply appending another branch's 'head' here is equivalent to doing a Git merge with strategy 'ours' (which is not the same as "recursive/ours").

• ctl can be used to commit the transaction (by writing commit to it) or to cancel it (by writing close).

• merge can be used to start a merge (see below).

For example, to create a file somefile:

~/db $ mkdir branch/master/transactions/foo
~/db $ echo somedata > branch/master/transactions/foo/rw/somefile
~/db $ echo commit > branch/master/transactions/foo/ctl

If the branch has been updated since the transaction was created then, when you try to commit, Irmin will try to merge the changes.

If there is a conflict (two edits to the same file) then the commit will fail. Merge errors are reported as 9p error strings. When a commit succeeds the transaction directory is automatically removed.

Each 9p connection has its own set of transactions, and the changes in a transaction cannot be seen by other clients until it is committed.

Merging

Within a transaction, write a commit ID to the merge file to begin a merge. The transaction directory will change slighty:

• ours is a read-only directory, containing whatever was previously in rw
• theirs is the commit being merged
• base is a common ancestor (or empty, if the commits share no history)
• rw contains irmin9p's initial attempt at a merge
• conflicts is a list of files in rw that need to be resolved manually
• parents has the new commit appended to it

Note that, unlike Git, irmin9p does not attempt to merge within files. It simply replaces files with conflicting changes with a message noting the conflict.

For each file in conflicts you should resolve the problem by either deleting the file or doing your own three-way merge using ours, theirs and base. When a file has been edited, it is removed from conflicts. You cannot commit the transaction while conflicts is non-empty.

You may merge several commits in a single transaction, if desired. However, doing multiple non-trivial merges at once will make viewing the resulting merge commit difficult with most tools.

Snapshots

A snapshots for a given commit can be opened by accessing the directory /snapshots/COMMIT_ID, which is created on demand.

~/db $ cd snapshots/4b6557542ec9cc578d5fe09b664110ba3b68e2c2
~/d/s/4b6557542ec9cc578d5fe09b664110ba3b68e2c2 $ ls
hash  ro/
~/d/s/4b6557542ec9cc578d5fe09b664110ba3b68e2c2 $ cat hash
4b6557542ec9cc578d5fe09b664110ba3b68e2c2
~/d/s/4b6557542ec9cc578d5fe09b664110ba3b68e2c2 $ ls ro
somefile
~/d/s/4b6557542ec9cc578d5fe09b664110ba3b68e2c2 $

The contents of a snapshot directory are:

• ro is the read-only snapshot, which will never change.

• hash contains the commit hash.

• parents contains the hashes of the parent commits, one per line.

Watches

To watch for changes affecting a specific file or subdirectory in a branch, use the branch's watch directory.

Each directory under watch contains a tree.live file that outputs the current hash of the object that directory watches. The top watch/tree.live file tracks changes to all files and directories. To watch for changes under src/ui, read the file watch/src.node/ui.node/tree.live. That is, add .node to each path component to get a directory for that node.

Reading from a tree.live file outputs first one line for the current state of the path. This can be:

• A blank line, if the path does not currently exist.
• D-HASH if the path is a directory (the hash is the tree hash).
• F-HASH if the path is a file (the hash is the hash of the blob).

When the branch head changes so that the path has a different output, a new line will be produced, in the same format. As with head.live, watching for changes is triggered by reading on the open file, so if several changes occur between reads then you will only see the latest one.

Note: Listing a watch directory shows .node subdirectories for paths that currently exist. However, these are just suggestions; you can watch any path, whether it currently exists or not.

Fetch

To fetch from a remote repository, use the /remotes root directory. This directory is not persisted so will disapear accross reboots.

Each directory under /remotes/<name> corresponds to the configuration of a remote server called <name>. Create a new directory (with mkdir) to add a new configuration. Every configuration folder contains:

• A writable file: url, which contains the remote url.
• A control file: fetch, which is used to fetch branches from the remote server.
• A read-only stream file: head which contains the last knowns commit ID of the remote. On every fetch, a new line is added with the commit ID of the remote branch.

To fetch https://github.com/docker/datakit's master branch using the git protocol:

~/db $ cd remotes
~/db/remotes $ mkdir origin
~/db/remotes $ echo git://github.com/docker/datakit > origin/url
~/db/remotes $ echo master > origin/fetch
~/db/remotes $ cat origin/head
4b6557542ec9cc578d5fe09b664110ba3b68e2c2

Github PRs

There is a basic support for interacting with Github PRs.

~/db $ ls github.com/docker/datakit
41  42
~/db $ cat github.com/docker/datakit/pr/41/status/default/state
pending
~/db $ echo success > github.com/docker/datakit/pr/41/status/default/state

This first query the status of the pull request on the Github interface, then update the default status to success.

To create a new status and set its description, url and status:

~/db $ PR=github.com/docker/datakit/pr/41
~/db $ mkdir $PR/status/test
~/db $ echo "My status" > $PR/status/test/descr
~/db $ echo "http://example.com" > $PR/status/test/url
~/db $ echo success > $PR/status/test/state

To subscribe to the stream of new PRS:

~/db $ cat github.com/docker/datakit/pr/updates

This is a blocking read, and will produce a new line on every pull request with the number of the PR, so you can do:

while read PR; do
  mkdir github.com/docker/datakit/pr/$PR/status/test
done < github.com/docker/datakit/pr/updates

To subscribe to the stream of build status updates for a given PR:

~mnt $ cat github.com/docker/datakit/pr/${PR}/status/updates

This is a blocking read and will produce a new line on every build status updates for that PR.

How do I...

Create a new branch

mkdir branch/foo

Fork an existing branch

cd branch
mkdir new-branch
cp old-branch/head new-branch/fast-forward

Rename a branch

mv branch/old-name branch/new-name

Delete a branch

rm branch/foo

Merge a branch

cd branch/master/transactions
mkdir my-merge
cd my-merge
cat ../../../feature/head > merge
cat conflicts
meld --auto-merge ours base theirs --output rw
echo commit > ctl

Licensing

Datakit is licensed under the Apache License, Version 2.0. See LICENSE for the full license text.