Backend.hs

{-# LANGUAGE MultiParamTypeClasses #-}
module Database.Schema.Migrations.Backend
    ( Backend(..)
    )
where

import Database.Schema.Migrations.Migration
    ( Migration(..) )

-- |A Backend represents a database engine backend such as MySQL or
-- SQLite.  A Backend supplies relatively low-level functions for
-- inspecting the backend's state, applying migrations, and reverting
-- migrations.  A Backend also supplies the migration necessary to
-- "bootstrap" a backend so that it can track which migrations are
-- installed.
class (Monad m) => Backend b m where
    -- |The migration necessary to bootstrap a database with this
    -- connection interface.  This might differ slightly from one
    -- backend to another.
    getBootstrapMigration :: b -> m Migration

    -- |Returns whether the backend has been bootstrapped.  A backend
    -- has been bootstrapped if is capable of tracking which
    -- migrations have been installed; the "bootstrap migration"
    -- provided by getBootstrapMigration should suffice to bootstrap
    -- the backend.
    isBootstrapped :: b -> m Bool

    -- |Apply the specified migration on the backend.  applyMigration
    -- does NOT assume control of the transaction, since it expects
    -- the transaction to (possibly) cover more than one
    -- applyMigration operation.  The caller is expected to call
    -- commit at the appropriate time.  If the application fails, the
    -- underlying SqlError is raised and a manual rollback may be
    -- necessary; for this, see withTransaction from HDBC.
    applyMigration :: b -> Migration -> m ()

    -- |Revert the specified migration from the backend and record
    -- this action in the table which tracks installed migrations.
    -- revertMigration does NOT assume control of the transaction,
    -- since it expects the transaction to (possibly) cover more than
    -- one revertMigration operation.  The caller is expected to call
    -- commit at the appropriate time.  If the revert fails, the
    -- underlying SqlError is raised and a manual rollback may be
    -- necessary; for this, see withTransaction from HDBC.  If the
    -- specified migration does not supply a revert instruction, this
    -- has no effect other than bookkeeping.
    revertMigration :: b -> Migration -> m ()

    -- |Returns a list of installed migration names from the backend.
    getMigrations :: b -> m [String]