persistent-mysql-haskell

A pure haskell backend for persistent using the MySQL database server. Internally it uses the mysql-haskell driver in order to access the database.

See example/Main.hs for how this MySQL backend can be used with Persistent.

Motivation

persistent-mysql uses mysql (via mysql-simple) as the database driver. mysql is a haskell FFI wrapper for mysqlclient written in C.

Reasons to use a pure haskell driver:

• mysql has concurrency issues as noted by @feuerbach.

• mysql-haskell, a pure haskell driver by @winterland1989, outperforms mysql-simple in benchmarks (see hackage or project repo).

• better portability and possible static compilation of an entire project that uses persistent-mysql.

Personal experience on replacing mysql-simple with mysql-haskell in a project:

• Performance gains consistent with benchmark.

• Smoother deployment to Amazon AMI/EC2, since mysql appears to have a hard dependency on the oracle version of libmysqlclient that does not work with the open source variant that is available by default on EC2 (and possibly on other cloud providers).

Potential issues moving from persistent-mysql to persistent-mysql-haskell

ConnectInfo and defaultConnectInfo are not the same between mysql and mysql-haskell, therefore this package is not a 100% drop in replacement for persistent-mysql from the connection configuration perspective.

• mysql-haskell does not allow provide an API for the entirety of mysqlclient options. Therefore neither can this package.

• Given the inevitable incompatibility with persistent-mysql, and in the interest of providing a forward-compatible API, ConnectInfo internals and defaultConnectInfo have been deprecated. However the similar utility can be achieved like so:

  import Database.Persist.MySQL
  
  connectInfo :: MySQLConnectInfo
  - connectInfo = defaultConnectInfo
  -             { connectHost     = "localhost"
  -             , connectUser     = "test"
  -             , connectPassword = "test"
  -             , connectDatabase = "test"
  -             }
  + connectInfo = mkMySQLConnectInfo "localhost" "test" "test" "test"
  
  connectInfoNewPort :: MySQLConnectInfo
  - connectInfoNewPort = connectInfo { connectPort = 3307 }
  + connectInfoNewPort = setMySQLConnectInfoPort 3307 connectInfo
  
  connectInfoNewCharSet :: MySQLConnectInfo
  - connectInfoNewCharSet = connectInfo { connectOptions = [CharsetName "utf8"] }
  + connectInfoNewCharSet = setMySQLConnectInfoCharset 33 connectInfo
  
  

Aside from connection configuration, persistent-mysql-haskell is functionally on par with persistent-mysql (as of writing this). This can be seen by comparing persistent-test between this fork and upstream.

FAQs

Why isn’t this part of the main/upstream persistent repo?

• TLDR: Upstream wants to gauge community interest before absorbing this backend into the main repo.
• Long version: See issue yesodweb/persistent/issues/659.

persistent-mysql supports X but persistent-mysql-haskell API doesn’t. Why?

• Internals (getters/setters) of MySQLConnectInfo and defaultConnectInfo are intentionally masked for forward compatibility.

• For all others, feel free to open an issue and/or submit a PR.

Does persistent-mysql-haskell ship with tests?

• It does! :) persistent-test is fully re-used with an additional flag to specifically test persistent-mysql-haskell.

  stack test persistent-test --flag persistent-test:mysql_haskell