OCaml bindings to MariaDB, supporting the nonblocking API
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OCaml-MariaDB is a library containing Ctypes-based bindings for MariaDB. The library provides access to the traditional MySQL blocking API via the Mariadb.Blocking module, as well as the MariaDB nonblocking API, via Mariadb.Nonblocking, which is designed mainly for use with OCaml's monadic concurrent programming libraries such as Lwt and Async.

Only the prepared-statement APIs are exposed by OCaml-MariaDB, as these functions provide typed query parameters and database field access.


OCaml-MariaDB requires MariaDB's client library version 5.5.21 or greater or the C connector library version 2.1.0 or greater (but version 3.0.0 or greater is recommended). If your distribution has these already packaged those versions, simply install either package. For example, on Debian or Ubuntu, run

# apt-get install libmariadbclient-dev

to use the client library, or

# apt-get install libmariadb-dev

to use the C connector.

In case both are installed, OCaml-MariaDB will link against C connector.

If your distribution doesn't yet package those versions, you can either install them manually, set up a third-party package archive (for example, there's an Ubuntu PPA that provides version 2.3.1 of the C connector), or configure MariaDB's own repositories, from which the client library will be available.

To install OCaml-MariaDB via OPAM simply type

$ opam install mariadb

To install it manually, type

$ ocaml setup.ml -configure
$ ocaml setup.ml -build
$ ocaml setup.ml -install

If you want to build the Lwt and/or Async example programs, pass respectively
`--enable-lwt` and `--enable-async` to the `configure` command above.
OCaml-Mariadb itself has no dependency on either of those libraries.

The blocking API

OCaml-MariaDB's API should be familiar to those who have used other MySQL libraries in OCaml or other languages before. A query must be initially prepared, resulting in a prepared statement, which can then be executed when given an appropriate set of parameters. Statement execution leads to a query result which can then be used to fetch rows, one at a time.

A simple example is given below.

open Printf

module M = Mariadb.Blocking

let or_die = function
  | Ok x -> x
  | Error (num, msg) -> failwith (sprintf "error #%d: %s" num msg)

let main () =
  let mariadb =
      ~pass:"secret" |> or_die in
  let query = "SELECT * FROM mysql.users WHERE Host LIKE ? LIMIT ?" in
  let stmt = M.prepare mariadb query |> or_die in
  let res = M.Stmt.execute stmt [| `String "%"; `Int 10 |] |> or_die in
  printf "number of rows: %d\n%!" (M.Res.num_rows res);
  print_rows res; (* see below *)
  M.Stmt.close stmt |> or_die;
  M.close mariadb;
  (* Call this only once, before you're done using all
     your database handles. *)
  M.library_end ()

let () = main ()

The nonblocking API

Usage of the nonblocking API is very similar to the blocking one, but designed to support OCaml's popular monadic concurrency libraries (though usage of a monadic library is not mandatory -- see the examples directory for an example using Unix.select).

To use the nonblocking API, a module of type Mariadb.Nonblocking.Wait must be provided to the functor Mariadb.Nonblocking.Make. This module must contain an asynchronous I/O type definition along with the usual bind and return monadic operations, as well as a wait function that specifies how to wait for the MariaDB socket to become readable and/or writable.

The signature is as follows.

module type Wait = sig
  module IO : sig
    type 'a future

    val (>>=) : 'a future -> ('a -> 'b future) -> 'b future
    val return : 'a -> 'a future

  val wait : t -> Mariadb.Nonblocking.Status.t
          -> Mariadb.Nonblocking.Status.t IO.future

The wait function receives a status parameter that specifies which socket events are to be waited for (which can be checked via Status.read, Status.write and Status.timeout -- see the ocamldoc for more details). It must then return a new status, specifying which of those events have actually occurred.

A simple example of the nonblocking library usage is given below. Full examples of wait modules for Lwt and Async can be found in the examples directory.

module M = Mariadb.Nonblocking.Make(struct
  module IO = struct
    type 'a future = ...
    let (>>=) m f = ...
    let return x = ...

  let wait mariadb status =

let main () =
    ~pass:"secret" >>= or_die
  >>= fun mariadb ->
  let query = "SELECT * FROM mysql.users WHERE Host LIKE ? LIMIT ?" in
  M.prepare mariadb query >>= or_die
  >>= fun stmt ->
  M.Stmt.execute stmt [| `String "%"; `Int 10 |] >>= or_die
  >>= fun res ->
  print_rows res >>= fun () -> (* see below *)
  M.Stmt.close stmt >>= or_die >>= fun () ->
  M.close mariadb >>= fun () ->
  M.library_end ()

Fetching rows

Rows can be fetched using the Res.fetch function. This function takes a module as its first parameter that defines the data structure in which the row is to be fetched.

For example,

M.Res.fetch (module M.Row.Array) res

returns the row as an array of Field.t values. The following built-in modules are provided with OCaml-MariaDB, but the user is free to implement one if so desired, in which case it must conform to the Row.S module type (see the ocamldoc for details).

  • Row.Array: fetch row as Field.t array;
  • Row.Map: fetch row as a map of column name (string) to Field.t;
  • Row.Hashtbl: fetch row as a (string, Field.t) Hashtbl.t.

The fetch function returns a row option result, where row represents the row type given by the module argument, and result is a wrapper for the Pervasives.result type that carries an error in the Error case. In the Ok case, fetch returns Some row, containing the next available row, or None, in which case no more rows are available in the result.

Reading fields

A database field is represented by the Field.t type, and its value by the Field.value type, which can be obtained by calling the Field.value function.

type value =
  [ `Int of int
  | `Float of float
  | `String of string
  | `Bytes of bytes
  | `Time of Time.t

The Field.value function can also return `Null in case the field is an SQL NULL.

Since the type of a field is in most cases known beforehand, as the user must be aware of the table definition in order to query it, helper functions are provided to extract the OCaml types directly from a field:

val int : Field.t -> int
val float : Field.t -> float
val string : Field.t -> string
val bytes : Field.t -> bytes
val time : Field.t -> Time.t

These functions will raise an exception if the field value is not of the expected type, but as noted above this shouldn't be a problem.

For nullable fields, the following analogous functions are also provided:

val int_opt : Field.t -> int option
val float_opt : Field.t -> float option
val string_opt : Field.t -> string option
val bytes_opt : Field.t -> bytes option
val time_opt : Field.t -> Time.t option

These functions return None if the field value is `Null, or Some v otherwise.