Module Irmin

Irmin public API.

Irmin is a library to design and use persistent stores with built-in snapshot, branching and reverting mechanisms. Irmin uses concepts similar to Git but it exposes them as a high level library instead of a complex command-line frontend. It features a bidirectional Git backend, where an application can read and persist its state using the Git format, fully-compatible with the usual Git tools and workflows.

Irmin is designed to use a large variety of backends. It is written in pure OCaml and does not depend on external C stubs; it is thus very portable and aims to run everywhere, from Linux to browser and MirageOS unikernels.

Consult the basics and Examples of use for a quick start. See also the documentation for the unix backends.

Release 1.4.0 - %%HOMEPAGE%%

val version : string

The version of the library.


module Type : sig ... end

Dynamic types for Irmin values.

module Info : sig ... end

Commit info are used to keep track of the origin of write operations in the stores. Info model the metadata associated with commit objects in Git.

module Merge : sig ... end

Merge provides functions to build custom 3-way merge operators for various user-defined contents.

module Diff : sig ... end

Differences between values.


type config

The type for backend-specific configuration values.

Every backend has different configuration options, which are kept abstract to the user.

type 'a diff = 'a Diff.t

The type for representing differences betwen values.

module type RO = sig ... end

Read-only backend stores.

module type AO = sig ... end

Append-only backend store.

Immutable Link store.

module type RW = sig ... end

Read-write stores.

User-Defined Contents

module Path : sig ... end

Store paths.

module Hash : sig ... end

Hashing functions.

module Contents : sig ... end

Contents specifies how user-defined contents need to be serializable and mergeable.

module Branch : sig ... end

User-defined branches.

module Metadata : sig ... end

Metadata defines metadata that is attached to contents but stored in nodes. The Git backend uses this to indicate the type of file (normal, executable or symlink).

module Private : sig ... end

Private defines functions only useful for creating new backends. If you are just using the library (and not developing a new backend), you should not use this module.

High-level Stores

An Irmin store is a branch-consistent store where keys are lists of steps.

An example is a Git repository where keys are filenames, i.e. list of '/'-separated strings. More complex examples are structured values, where steps might contain first-class field accessors and array offsets.

Irmin provides the following features:

  • Support for fast clones, branches and merges, in a fashion very similar to Git.
  • Efficient taging areas for fast, transient, in-memory operations.
  • Fast synchronization primitives between remote stores, using native backend protocols (as the Git protocol) when available.
module type S = sig ... end

Irmin stores.

module type S_MAKER = functor (M : Metadata.S) -> functor (C : Contents.S) -> functor (P : Path.S) -> functor (B : Branch.S) -> functor (H : Hash.S) -> S with type key = P.t and type step = P.step and type metadata = M.t and type contents = C.t and type branch = B.t and type Commit.Hash.t = H.t and type Tree.Hash.t = H.t and type Contents.Hash.t = H.t

S_MAKER is the signature exposed by any backend providing S implementations. M is the implementation of user-defined metadata, C is the one for user-defined contents, B is the implementation for branches and H is the implementation for object (blobs, trees, commits) hashes. It does not use any native synchronization primitives.

module type KV = S with type key = string list and type step = string and type branch = string

KV is similar to S but choose sensible implementations for path and branch.

module type KV_MAKER = functor (C : Contents.S) -> KV with type contents = C.t

KV_MAKER is like S_MAKER but where everything except the contents is replaced by sensible default implementations.


type remote

The type for remote stores.

val remote_uri : string -> remote

remote_uri s is the remote store located at uri. Use the optimized native synchronization protocol when available for the given backend.


These examples are in the examples directory of the distribution.


A simple synchronization example, using the Git backend and the Sync helpers. The code clones a fresh repository if the repository does not exist locally, otherwise it performs a fetch: in this case, only the missing contents is downloaded.

open Lwt.Infix

module S = Irmin_unix.Git.FS.KV(Irmin.Contents.String)
module Sync = Irmin.Sync(S)
let config = Irmin_git.config "/tmp/test"

let upstream =
  if Array.length Sys.argv = 2 then (Irmin.remote_uri Sys.argv.(1))
  else (Printf.eprintf "Usage: sync [uri]\n%!"; exit 1)

let test () =
  S.Repo.v config >>= S.master
  >>= fun t  -> Sync.pull_exn t upstream `Set
  >>= fun () -> S.get t [""]
  >|= fun r  -> Printf.printf "%s\n%!" r

let () = (test ())

Mergeable logs

We will demonstrate the use of custom merge operators by defining mergeable debug log files. We first define a log entry as a pair of a timestamp and a message, using the combinator exposed by mirage-tc:

module Entry = sig
  include Irmin.Contents.Conv
  val v: string -> t
  val compare: t -> t -> int
  val timestamp: t -> int
end = struct

  type t = { timestamp: int; message : string; }

  let compare x y = compare x.timestamp y.timestamp

  let v message =
    incr time;
    { timestamp = !time; message }

  let t =
    let open Irmin.Type in
    record "entry" (fun timestamp message -> { timestamp; message })
    |+ field "timestamp" int    (fun t -> t.timestamp)
    |+ field "message"   string (fun t -> t.message)
    |> sealr

  let timestamp t = t.timestamp

  let pp ppf { timestamp; message } = ppf  "%04d: %s\n" timestamp message

  let of_string str =
    match String.cut ~sep:": " str with
    | None -> Error (`Msg ("invalid entry: " ^ str))
    | Some (x, message) ->
      try Ok { timestamp = int_of_string x; message }
      with Failure e -> Error (`Msg e)

A log file is a list of entries (one per line), ordered by decreasing order of timestamps. The 3-way merge operator for log files concatenates and sorts the new entries and prepend them to the common ancestor's ones.

(* A log file *)
module Log: sig
  include Irmin.Contents.S
  val add: t -> Entry.t -> t
  val empty: t
end = struct

  type t = Entry.t list
  let t = Irmin.Type.(list Entry.t)

  let empty = []

  let pp ppf l = List.iter ( ppf "%a\n" Entry.pp ) (List.rev l)

  let of_string str =
    let lines = String.cuts ~sep:"\n" str in
      List.fold_left (fun acc l ->
          match Entry.of_string l with
          | Ok x           -> x :: acc
          | Error (`Msg e) -> failwith e
        ) [] lines
      |> fun l -> Ok l
    with Failure e ->
      Error (`Msg e)

  let timestamp = function
    | [] -> 0
    | e :: _ -> Entry.timestamp e

  let newer_than timestamp file =
    let rec aux acc = function
      | [] -> List.rev acc
      | h:: _ when Entry.timestamp h <= timestamp -> List.rev acc
      | h::t -> aux (h::acc) t
    aux [] file

  let merge ~old t1 t2 =
    let open Irmin.Merge.Infix in
    old () >>=* fun old ->
    let old = match old with None -> [] | Some o -> o in
    let ts = timestamp old in
    let t1 = newer_than ts t1 in
    let t2 = newer_than ts t2 in
    let t3 = List.sort (List.rev_append t1 t2) in
    Irmin.Merge.ok (List.rev_append t3 old)

  let merge = Irmin.Merge.(option (v t merge))

  let add t e = e :: t


Note: The serialisation primitives used in that example are not very efficient in this case as they parse the file every-time. For real usage, you would write buffered versions of Log.pp and Log.of_string.

To persist the log file on disk, we need to choose a backend. We show here how to use the on-disk Git backend on Unix.

(* Build an Irmin store containing log files. *)
module S = Irmin_unix.Git.FS.KV(Log)

(* Set-up the local configuration of the Git repository. *)
let config = Irmin_git.config ~bare:true "/tmp/irmin/test"

(* Set-up the commit info function *)
let info fmt = ~author:"logger" fmt

We can now define a toy example to use our mergeable log files.

open Lwt.Infix

(* Name of the log file. *)
let file = [ "local"; "debug" ]

(* Read the entire log file. *)
let read_file t =
  S.find t file >|= function
  | None   -> []
  | Some l -> l

(* Persist a new entry in the log. *)
let log t fmt =
  Fmt.kstrf (fun message ->
      read_file t >>= fun logs ->
      let logs = Log.add logs (Entry.v message) in
      S.set t (info "Adding a new entry") file logs
    ) fmt

let () = begin
    S.Repo.v config >>= S.master
    >>= fun t  -> log t "Adding a new log entry"
    >>= fun () -> Irmin.clone_force ~src:t ~dst:"x"
    >>= fun x  -> log x "Adding new stuff to x"
    >>= fun () -> log x "Adding more stuff to x"
    >>= fun () -> log x "More. Stuff. To x."
    >>= fun () -> log t "I can add stuff on t also"
    >>= fun () -> log t "Yes. On t!"
    >>= fun () -> S.merge (info "Merging x into t") x ~into:t
    >|= function Ok () -> () | Errror _ -> failwith "merge conflict!"


val remote_store : (module S with type t = 'a) -> 'a -> remote

remote_store t is the remote corresponding to the local store t. Synchronization is done by importing and exporting store slices, so this is usually much slower than native synchronization using remote_uri but it works for all backends.

module type SYNC = sig ... end

SYNC provides functions to synchronization an Irmin store with local and remote Irmin stores.

module Sync : functor (S : S) -> SYNC with type db = S.t and type commit = S.commit

The default Sync implementation.

module Dot : functor (S : S) -> sig ... end

Dot provides functions to export a store to the Graphviz `dot` format.


API to create new Irmin backends. A backend is an implementation exposing either a concrete implementation of S or a functor providing S once applied.

There are two ways to create a concrete Irmin.S implementation:

  • Make creates a store where all the objects are stored in the same store, using the same internal keys format and a custom binary format based on bin_prot, with no native synchronization primitives: it is usually what is needed to quickly create a new backend.
  • Make_ext creates a store with a deep embedding of each of the internal stores into separate store, with a total control over the binary format and using the native synchronization protocols when available.
module type AO_MAKER = functor (K : Hash.S) -> functor (V : Contents.Conv) -> sig ... end

AO_MAKER is the signature exposed by append-only store backends. K is the implementation of keys and V is the implementation of values.

LINK_MAKER is the signature exposed by store which enable adding relation between keys. This is used to decouple the way keys are manipulated by the Irmin runtime and the keys used for storage. This is useful when trying to optimize storage for random-access file operations or for encryption.

module type RW_MAKER = functor (K : Contents.Conv) -> functor (V : Contents.Conv) -> sig ... end

RW_MAKER is the signature exposed by read-write store backends. K is the implementation of keys and V is the implementation of values.

module Make : functor (AO : AO_MAKER) -> functor (RW : RW_MAKER) -> S_MAKER

Simple store creator. Use the same type of all of the internal keys and store all the values in the same store.

module Make_ext : functor (P : Private.S) -> S with type key = P.Node.Path.t and type contents = P.Contents.value and type branch = P.Branch.key and type Commit.Hash.t = P.Commit.key and type Tree.Hash.t = P.Node.key and type Contents.Hash.t = P.Contents.key and type step = P.Node.Path.step and type metadata = P.Node.Val.metadata and type Key.step = P.Node.Path.step and type repo = P.Repo.t

Advanced store creator.