Haskell 中的并发数据库连接池

Question

我是一名学习 Haskell 的 Java 程序员。
我正在开发一个小型 Web 应用程序，该应用程序使用 Happstack 并通过 HDBC 与数据库进行通信。

我编写了 select 和 exec 函数，并且像这样使用它们：

module Main where

import Control.Exception (throw)

import Database.HDBC
import Database.HDBC.Sqlite3 -- just for this example, I use MySQL in production

main = do
    exec "CREATE TABLE IF NOT EXISTS users (name VARCHAR(80) NOT NULL)" []

    exec "INSERT INTO users VALUES ('John')" []
    exec "INSERT INTO users VALUES ('Rick')" []

    rows <- select "SELECT name FROM users" []

    let toS x = (fromSql x)::String
    let names = map (toS . head) rows

    print names

如您所见，非常简单。 有查询、参数和结果。
连接创建和提交/回滚内容隐藏在 select 和 exec 中。
这很好，我不想在我的“逻辑”代码中关心它。

exec :: String -> [SqlValue] -> IO Integer
exec query params = withDb $ \c -> run c query params

select :: String -> [SqlValue] -> IO [[SqlValue]]
select query params = withDb $ \c -> quickQuery' c query params

withDb :: (Connection -> IO a) -> IO a
withDb f = do
    conn <- handleSqlError $ connectSqlite3 "users.db"
    catchSql
        (do r <- f conn
            commit conn
            disconnect conn
            return r)
        (\e@(SqlError _ _ m) -> do
            rollback conn
            disconnect conn
            throw e)

缺点：

• 每次调用都会创建一个新连接 - 这会降低重负载时的性能
• DB url“users.db”是硬编码的 - 我无法在其他项目中重用这些函数而无需编辑

问题1：如何引入一个具有一些定义的（最小，最大）并发连接数的连接池，以便在 select/exec 调用之间重用连接？

问题2：如何使“users.db”字符串可配置？ （如何将其移至客户端代码？）

它应该是一个透明的功能：用户代码不应要求显式的连接处理/释放。

Answer 1

resource-pool 包提供了一个高性能的资源池，可用于数据库连接池。 例如：

import Data.Pool (createPool, withResource)

main = do
    pool <- createPool newConn delConn 1 10 5
    withResource pool $ \conn -> doSomething conn

创建一个数据库连接池，有1个子池，最多5个连接。 每个连接在被销毁之前允许空闲 10 秒。

Answer 2

问题 2：我从未使用过 HDBC，但我可能会写这样的东西。

trySql :: Connection -> (Connection -> IO a) -> IO a
trySql conn f = handleSql catcher $ do
    r <- f conn
    commit conn
    return r
  where catcher e = rollback conn >> throw e

在函数外部的某个位置打开Connection，并且不要在函数内断开连接。

问题 1： 嗯，连接池似乎并不难实现...

import Control.Concurrent
import Control.Exception

data Pool a =
    Pool { poolMin :: Int, poolMax :: Int, poolUsed :: Int, poolFree :: [a] }

newConnPool low high newConn delConn = do
    cs <- handleSqlError . sequence . replicate low newConn
    mPool <- newMVar $ Pool low high 0 cs
    return (mPool, newConn, delConn)

delConnPool (mPool, newConn, delConn) = do
    pool <- takeMVar mPool
    if length (poolFree pool) /= poolUsed pool
      then putMVar mPool pool >> fail "pool in use"
      else mapM_ delConn $ poolFree pool

takeConn (mPool, newConn, delConn) = modifyMVar mPool $ \pool ->
    case poolFree pool of
        conn:cs ->
            return (pool { poolUsed = poolUsed pool + 1, poolFree = cs }, conn)
        _ | poolUsed pool < poolMax pool -> do
            conn <- handleSqlError newConn
            return (pool { poolUsed = poolUsed pool + 1 }, conn)
        _ -> fail "pool is exhausted"

putConn (mPool, newConn, delConn) conn = modifyMVar_ mPool $ \pool ->
    let used = poolUsed pool in
    if used > poolMin conn
      then handleSqlError (delConn conn) >> return (pool { poolUsed = used - 1 })
      else return $ pool { poolUsed = used - 1, poolFree = conn : poolFree pool }

withConn connPool = bracket (takeConn connPool) (putConn conPool)

您可能不应该逐字逐句地接受这个，因为我什至还没有对它进行编译测试（并且 失败 这相当不友好），但我们的想法是做一些类似的事情

connPool <- newConnPool 0 50 (connectSqlite3 "user.db") disconnect

，并根据需要传递 connPool 。

Answer 3

我修改了上面的代码，现在至少可以编译了。

module ConnPool ( newConnPool, withConn, delConnPool ) where

import Control.Concurrent
import Control.Exception
import Control.Monad (replicateM)
import Database.HDBC

data Pool a =
    Pool { poolMin :: Int, poolMax :: Int, poolUsed :: Int, poolFree :: [a] }

newConnPool :: Int -> Int -> IO a -> (a -> IO ()) -> IO (MVar (Pool a), IO a, (a -> IO ()))
newConnPool low high newConn delConn = do
--    cs <- handleSqlError . sequence . replicate low newConn
    cs <- replicateM low newConn 
    mPool <- newMVar $ Pool low high 0 cs 
    return (mPool, newConn, delConn)

delConnPool (mPool, newConn, delConn) = do
    pool <- takeMVar mPool
    if length (poolFree pool) /= poolUsed pool
      then putMVar mPool pool >> fail "pool in use"
      else mapM_ delConn $ poolFree pool

takeConn (mPool, newConn, delConn) = modifyMVar mPool $ \pool ->
    case poolFree pool of
        conn:cs ->
            return (pool { poolUsed = poolUsed pool + 1, poolFree = cs }, conn)
        _ | poolUsed pool < poolMax pool -> do
            conn <- handleSqlError newConn
            return (pool { poolUsed = poolUsed pool + 1 }, conn)
        _ -> fail "pool is exhausted"

putConn :: (MVar (Pool a), IO a, (a -> IO b)) -> a -> IO ()
putConn (mPool, newConn, delConn) conn = modifyMVar_ mPool $ \pool ->
    let used = poolUsed pool in
    if used > poolMin pool
    then handleSqlError (delConn conn) >> return (pool { poolUsed = used - 1 })
    else return $ pool { poolUsed = used - 1, poolFree = conn : (poolFree pool) }

withConn connPool = bracket (takeConn connPool) (putConn connPool)