package caqti-lwt

Synopsis

Caqti provides a monadic cooperative-threaded OCaml connector API for relational databases.

The purpose of Caqti is further to help make applications independent of a particular database system. This is achieved by defining a common signature, which is implemented by the database drivers. Connection parameters are specified as an URI, which is typically provided at run-time. Caqti then loads a driver which can handle the URI, and provides a first-class module which implements the driver API and additional convenience functionality.

Caqti does not make assumptions about the structure of the query language, and only provides the type information needed at the edges of communication between the OCaml code and the database; i.e. for encoding parameters and decoding returned tuples. It is hoped that this agnostic choice makes it a suitable target for higher level interfaces and code generators.

Status

Large parts of Caqti was rewritten for 0.8 and 0.9, the latter being the first release targeted for to the official OPAM repository. The old API is still available in findlib libraries under the suffix .v1. I need to keep these for the time being, but do not use them for new code.

Drivers

The following drivers are available.

• MariaDB (mariadb://)

  • Implemented in terms of ocaml-mariadb using asynchronous calls.

  • Supports transactions.

  • Pools connections and caches statements.

• PostgreSQL (postgresql://)

  • Implemented in terms of postgresql-ocaml using asynchronous calls.

  • Supports transactions.

  • Pools connections and caches statements.

• SQLite3 (sqlite3://)

  • Implemented in terms of sqlite3-ocaml using preemtive threading for non-blocking operation.

  • Supports transactions.

  • Does not pool connections or cache statements.

If you link against caqti-dynload, then drivers are loaded dynamically based on the URI. If dynamic loading is unavailable on your platform, you may instead link against the caqti-driver-* libraries which you expect to use.

Documentation

As the main entry point, you would normally use either of

Caqti_lwt : Caqti_connection_sig.S with type 'a io = 'a Lwt.t
Caqti_async : Caqti_connection_sig.S with type 'a io = 'a Deferred.t

which is provided by caqti-lwt or caqti-async, respectively. These provide a connect functions which receives an URI, loads the appropriate driver, and returns a connection as a first-class module containing query functionality for the database.

A good place to start is with the documented example. The API reference is currently not online, but can be generated with topkg doc from the source code or with odig from an OPAM installation.

The most important modules to know about are:

• Caqti_type and Caqti_request for constructing prepared or one-shot queries.

• Caqti_lwt and Caqti_async for connecting to the database and obtaining a first class module implementing Caqti_connection_sig.S.

• Caqti_connection_sig.S and Caqti_response_sig.S for executing queries.

Running under utop

Dynamic linking does not work under utop. The workaround is to link against the needed database driver. E.g.

> #require "caqti-lwt";;
> #require "caqti-driver-postgresql";;
> open Lwt.Infix;;

(* Create a DB handle. *)
> module Db = (val Caqti_lwt.connect (Uri.of_string "postgresql://") >>= Caqti_lwt.or_fail |> Lwt_main.run);;
module Db : Caqti_lwt.CONNECTION

(* Create a request which merely adds two parameters. *)
> let plus = Caqti_request.find Caqti_type.(tup2 int int) Caqti_type.int "SELECT ?::integer + ?::integer";;
val plus : (int * int, int, [< `Many | `One | `Zero > `One ]) Caqti_request.t = <abstr>

(* Run it. *)
> Db.find plus (7, 13);;
- : (int, [> Caqti_error.call_or_retrieve ]) result = Ok 20