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Irminsule

Irminsule is a distributed filesystem and block store that follows the same design principles as Git. The design consists of three main components:

  • a low-level immutable and consistent key/value data-store
  • a DAG persisted in that datastore; and
  • a tag store which associate names to keys of the low-level data-store.

Irminsule is written in pure OCaml, and can thus be compiled to a variety of backends (including Javascript, and Mirage microkernels). Unlike the git frontend, applications can directly iterate over the object graph.

The immutability of the low-level block store makes it significantly easier to apply replication and network coding techniques to improve resilience via replication, and to optimise scheduling across many hosts using MPTCP-style congestion control.

Compile

opam install jsonm uri ocamlgraph cohttp cmdliner obuild lwt
make

Usage

  • API.ml describes the API which is now more or less stabilized.

  • memory.ml is a very simple in-memory implementation of the irminsule API.

  • irminsule.ml is a simple command-line tool to drive in-memory irminsule instances. You can start a dummy irminsule instance using:

    irminsule start . -x .git

    This will scan the filesystem from your current working directory, excluding the .git folder and start a daemon holding an in-memory image of that filesystem. You can command the daemon by connecting to http://localhost:8081 (use -p <PORT> to change 8081 to something else). For instance, http://localhost:8081/key will display all the keys in the database, http://localhost:8081/key/<SHA1> will display the value associated to this key.

    You can then try to modify few files in your filesystem and start a new server by doing:

    irminsule start . -x .git -p 8082

    You can now try to pull/push between the two instance:

    irminsule pull -i 8082 -o 8081

    And the diff will be exchanged between the old server and the new one using a proxy.

TODO

  • check if push/pull works
  • improve the exchange of keys using discover
  • implement watch
  • xenstore-like API
  • benchmark using xstest

Issues

SHA1 is not immune to collisions, but Linus answer is still relevant in the case of irminsule. The only problem being that the inadvertent kind needs to be handle properly and reported back to the user.