Portability	GHC
Stability	experimental
Maintainer	bos@serpentine.com

Database.HDBC.MySQL

Description

This module provides a MySQL driver for the HDBC database interface. To use it, invoke the connectMySQL method to create an Database.HDBC.IConnection that you can use to interact with a MySQL database. Use the defaultMySQLConnectInfo, overriding the default values as necessary.

import Control.Monad
import Database.HDBC
import Database.HDBC.MySQL

main = do
  rows <- withRTSSignalsBlocked $ do
    conn <- connectMySQL defaultMySQLConnectInfo {
              mysqlHost     = "db1.example.com",
              mysqlUser     = "scott",
              mysqlPassword = "tiger"
            }
    quickQuery' conn "SELECT 1 + 1" []
  forM_ rows $ \row -> putStrLn $ show row

There are some important caveats to note about this driver.

RTS signals. If you are writing an application that links against GHC's threaded runtime (as most server-side applications do), you must use withRTSSignalsBlocked to defend the mysqlclient library against the signals the RTS uses, or you may experience crashes.
Transaction support. The MySQL server supports a variety of backend "engines", only some of which support transactional access (e.g., InnoDB). This driver will report that the database supports transactions. Should you decide to make use of the transactional support in the HDBC API, it is up to you to make sure that you use a MySQL engine that supports transactions.
Dates and times. MySQL does not store time zone information in DATETIME or TIMESTAMP columns: instead, it assumes that all dates are stored in the "server's time zone". At some point in the future, this driver may query for the server's time zone and apply appropriate time zone conversion to these datatypes. For now, it simply treats all times as UTC; i.e., it assumes the server's time zone is UTC.

Synopsis

Documentation

data MySQLConnectInfo

Connection information to use with connectMySQL.

You must either supply a host and port, or the full path to a Unix socket.

Constructors

MySQLConnectInfo

Fields

mysqlHost :: String: The server's hostname; e.g., "db1.example.com"
mysqlUser :: String: The MySQL username to use for login; e.g., "scott"
mysqlPassword :: String: The MySQL password to use for login; e.g., "tiger"
mysqlDatabase :: String: the "default" database name; e.g., "emp"
mysqlPort :: Int: The port on which to connect to the server; e.g., 3306
mysqlUnixSocket :: String: The absolute path of the server's Unix socket; e.g., "/var/lib/mysql.sock"
mysqlGroup :: Maybe String: The group name in my.cnf from which it reads options; e.g., "test"

data Connection

Instances

IConnection Connection

connectMySQL :: MySQLConnectInfo -> IO Connection

Connects to a MySQL database using the specified connection information.

defaultMySQLConnectInfo :: MySQLConnectInfo

Typical connection information, meant to be overridden partially, for example:

 connectMySQL defaultMySQLConnectInfo { mysqlHost = "db1" }

In particular, the default values are "127.0.0.1" as the host, 3306 as the port, "root" as the user, no password, and "test" as the default database.

withRTSSignalsBlocked :: IO a -> IO a

Execute an IO action with signals used by GHC's runtime signals blocked. The mysqlclient C library does not correctly restart system calls if they are interrupted by signals, so many MySQL API calls can unexpectedly fail when called from a Haskell application. This is most likely to occur if you are linking against GHC's threaded runtime (using the -threaded option).

This function blocks SIGALRM and SIGVTALRM, runs your action, then unblocks those signals. If you have a series of HDBC calls that may block for a period of time, it may be wise to wrap them in this action. Blocking and unblocking signals is cheap, but not free.

Here is an example of an exception that could be avoided by temporarily blocking GHC's runtime signals:

  SqlError {
    seState = "", 
    seNativeError = 2003, 
    seErrorMsg = "Can't connect to MySQL server on 'localhost' (4)"
  }