Haskell中的并发数据库连接池

本文介绍了Haskell中的并发数据库连接池的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 我是学习Haskell的Java程序员。 我在使用Happstack并通过HDBC与数据库进行对话的小型网络应用程序。 我写了 select 和 exec 函数，并且像这样使用它们： 模块Main其中 导入Control.Exception（抛出） 导入Database.HDBC 导入Database.HDBC.Sqlite3 - 仅用于这个例子中，我使用MySQL生产 main = do execCREATE TABLE IF NOT EXISTS users（name VARCHAR（80）NOT NULL）[] execINSERT INTO users VALUES（'John'）[] execINSERT INTO users VALUES（'Rick'）[] rows< - selectSELECT name FROM用户[] let toS x =（fromSql x）:: String let names = map（toS。head）rows print names 非常简单，如您所见。有查询， params 和结果。连接创建和提交/回滚的内容隐藏在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 - >运行c查询参数 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 $ connectSqlite3users.db catchSql （do r commit conn disconnect conn return r）（\e @（SqlError _ _ m） - > do 回滚连接断开连接 throw e） 坏点： 每次通话都会创建一个新连接 - 这会导致重负载时的性能 DB urlusers.db是硬编码的 - 我无法重复使用这些连接在没有编辑的情况下跨其他项目的功能 问题1：定义（最小，最大）并发连接数，所以连接将在select / exec调用之间重用？ 问题2：使users.db字符串可配置？ （如何将其移动到客户端代码？） 它应该是一个透明的功能：用户代码不应该要求明确的连接处理/发布。$ b $问题2：我从来没有使用过HDBC，但是我可能会写这样的东西。 b $ b trySql :: Connection - > （连接 - > IO a） - > IO a trySql conn f = handleSql捕获器$ do r 提交conn 返回r 其中catcher e = rollback conn>>抛出e 打开连接函数，并且不要在函数内断开连接。 问题1：嗯，连接池似乎并不那么难执行... $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $' a = Pool {poolMin :: Int，poolMax :: Int，poolUsed :: Int，poolFree :: [a]} newConnPool low high newConn delConn = do cs < - handleSqlError。序列 。复制低newConn mPool 返回（mPool，newConn，delConn） delConnPool（mPool，newConn，delConn）= pool< - takeMVar mPool if length（poolFree pool）/ = poolUsed pool then putMVar mPool pool>>失败pool in use else mapM_ delConn $ poolFree pool takeConn（mPool，newConn，delConn）= modifyMVar mPool $ \pool - > 案例pool $ 的免费池conn：cs - > return（pool {poolUsed = poolUsed pool + 1，poolFree = cs}，conn） _ |池用水池< poolMax池 - > conn< - handleSqlError newConn return（pool {poolUsed = poolUsed pool + 1}，conn） _ - >失败pool is exhausted putConn（mPool，newConn，delConn）conn = modifyMVar_ mPool $ \pool - > 让used = poolUsed池在中（如果使用）> poolMin conn then handleSqlError（delConn conn）>> return（pool {poolUsed = used - 1}） else return $ pool {poolUsed = used - 1，poolFree = conn：poolFree pool} $ b with conn connPool =括号（takeConn connPool）（ putConn conPool） 你可能不应该接受这个逐字，因为我甚至没有对它进行编译测试（和失败有非常不友好的），但想法是做一些像 connPool 和根据需要传递 connPool around。 I am a Java programmer who learns Haskell.I work on a small web-app that uses Happstack and talks to a database via HDBC.I've written select and exec functions and I use them like this:module Main whereimport Control.Exception (throw)import Database.HDBCimport Database.HDBC.Sqlite3 -- just for this example, I use MySQL in productionmain = 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 namesVery simple as you see. There is query, params and result.Connection creation and commit/rollback stuff is hidden inside select and exec.This is good, I don't want to care about it in my "logic" code.exec :: String -> [SqlValue] -> IO Integerexec query params = withDb $ \c -> run c query paramsselect :: String -> [SqlValue] -> IO [[SqlValue]]select query params = withDb $ \c -> quickQuery' c query paramswithDb :: (Connection -> IO a) -> IO awithDb 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)Bad points:a new connection is always created for every call - this kills performance on heavy loadDB url "users.db" is hardcoded - I can't reuse these functions across other projects w/o editingQUESTION 1: how to introduce a pool of connections with some defined (min, max) number of concurrent connections, so the connections will be reused between select/exec calls?QUESTION 2: How to make "users.db" string configurable? (How to move it to client code?)It should be a transparent feature: user code should not require explicit connection handling/release. 解决方案 QUESTION 2: I've never used HDBC, but I'd probably write something like this.trySql :: Connection -> (Connection -> IO a) -> IO atrySql conn f = handleSql catcher $ do r <- f conn commit conn return r where catcher e = rollback conn >> throw eOpen the Connection somewhere outside of the function, and don't disconnect it within the function.QUESTION 1: Hmm, a connection pool doesn't seem that hard to implement...import Control.Concurrentimport Control.Exceptiondata 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 pooltakeConn (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)You probably shouldn't take this verbatim as I haven't even compile-tested it (and fail there is pretty unfriendly), but the idea is to do something likeconnPool <- newConnPool 0 50 (connectSqlite3 "user.db") disconnectand pass connPool around as needed. 这篇关于Haskell中的并发数据库连接池的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！