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<h2>java.jdbc - How to reuse database connections</h2>
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<h2 id="contents">Contents</h2><ul><li><a href="home.html">Overview</a></li><li><a href="using_sql.html">Using SQL</a></li><li><a href="using_ddl.html">Using DDL</a></li><li><a href="reusing_connections.html">Reusing Connections</a></li></ul><h2 id="reusing-connections">Reusing Connections</h2><p>Since you rarely want every database operation to create a new connection,
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there are two ways to reuse connections:</p><ul><li>Grouping Operations using <code>with-db-connection</code>: If you don't want to deal
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with a connection pooling library, you can use this macro to automatically open a
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connection and maintain it for a body of code, with each operation executed in its
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own transaction, then close the connection.</li><li>Grouping Operations using <code>with-db-transaction</code>: If you want to execute multiple
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operations in a single transaction, you can use this macro to automatically open a
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connection, start a transaction, execute multiple operations, commit the transaction,
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and then close the connection.</li><li>Connection Pooling: This is the recommended approach and is fairly
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straightforward, with a number of connection pooling libraries available. See
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<em>How To Use Connection Pooling</em> below for more information.</li></ul><h2 id="using-with-db-connection">Using <code>with-db-connection</code></h2><p>This macro provides the simplest way to reuse connections, without having to
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add a dependency on an external connection pooling library:</p><pre><code class="clojure">(ns dbexample
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(:require [clojure.java.jdbc :as jdbc]))
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(def db-spec ... ) ;; see above
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(jdbc/with-db-connection [db-con db-spec]
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(let [;; fetch some rows using this connection
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rows (jdbc/query db-con ["SELECT * FROM table WHERE id = ?" 42])]
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;; insert a copy of the first row using the same connection
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(jdbc/insert! db-con :table (dissoc (first rows) :id))))
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</code></pre><p>The <code>query</code> and the <code>insert!</code> are each run in their own transaction and committed
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if they succeed. If you want to run multiple operations in a single transaction
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see the next section about <code>with-db-transaction</code>.</p><h2 id="using-with-db-transaction">Using <code>with-db-transaction</code></h2><p>This macro provides a way to reuse connections, committing or rolling back
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multiple operations in a single transaction:</p><pre><code class="clojure">(ns dbexample
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(:require [clojure.java.jdbc :as jdbc]))
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(def db-spec ... ) ;; see above
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(jdbc/with-db-transaction [t-con db-spec]
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(let [;; fetch some rows using this connection
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rows (jdbc/query t-con ["SELECT * FROM table WHERE id = ?" 42])]
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;; insert a copy of the first row using the same connection
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(jdbc/insert! t-con :table (dissoc (first rows) :id))))
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</code></pre><p>If any operation inside <code>with-db-transaction</code> fails (throws an exception), then
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all of the operations performed so far are rolled back. If all the operations
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succeed, then the entire transaction is committed.</p><p>Transactions are not nested (since not all databases support that) so if this is
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used another active transaction, the outer transaction (and connection) are used
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as-is. If the isolation levels of the outer and inner transaction do not match
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you will get an <code>IllegalStateException</code>.</p><p>See also <code>db-set-rollback-only!</code>, <code>db-unset-rollback-only!</code>, and <code>db-is-rollback-only</code>
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for additional control over the commit/rollback behavior of the enclosing transaction.</p><h2 id="using-connection-pooling">Using Connection Pooling</h2><p><code>java.jdbc</code> does not provide connection pooling directly but it is relatively
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easy to add to your project. There are several connection pooling libraries out
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there, but here we will provide instructions for the popular <code>c3p0</code> library.</p><p>The basic idea is to add your chosen connection pooling library to your
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project, import the appropriate class(es), define a function that consumes a
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"database spec" and produces a map containing a <code>:datasource</code> key whose value
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is the constructed pooled <code>DataSource</code> object, then use that hash map in place of your
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bare <code>db-spec</code> variable. You are responsible for creating the pooled data
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source object and passing the map containing it into any functions that need a
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database connection.</p><h3 id="using-the-c3p0-library">Using the c3p0 library</h3><p>For more information on c3p0, consult the <a href="http://www.mchange.com/projects/c3p0/">c3p0
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documentation</a>.</p><p>If you're using Leiningen, you can add the following to your dependencies:</p><pre><code class="clojure">[com.mchange/c3p0 "0.9.5.2"] ;; check the documentation for latest version
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</code></pre><p>For a Maven-based project, you would add:</p><pre><code class="xml"><dependency>
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<groupId>com.mchange</groupId>
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<artifactId>c3p0</artifactId>
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<version>0.9.5.2</version>
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</dependency>
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</code></pre><h3 id="create-the-pooled-datasource-from-your-db-spec">Create the pooled datasource from your db-spec</h3><p>Define your <code>db-spec</code> using the long form, for example (for MySQL):</p><pre><code class="clojure">(def db-spec
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{:classname "com.mysql.jdbc.Driver"
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:subprotocol "mysql"
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:subname "//127.0.0.1:3306/mydb"
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:user "myaccount"
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:password "secret"})
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</code></pre><p>We have to use the long form here because c3p0 operates on the class name, subprotocol,
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and subname elements. Of course, you don't really need to define a <code>db-spec</code> here
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because you're not going to use it with <code>java.jdbc</code> directly, only with c3p0.</p><p>Import the c3p0 class as part of your namespace declaration, for example:</p><pre><code class="clojure">(ns example.db
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(:import (com.mchange.v2.c3p0 ComboPooledDataSource)))
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</code></pre><p>Define a function that creates a pooled datasource:</p><pre><code class="clojure">(defn pool
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[spec]
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(let [cpds (doto (ComboPooledDataSource.)
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(.setDriverClass (:classname spec))
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(.setJdbcUrl (str "jdbc:" (:subprotocol spec) ":" (:subname spec)))
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(.setUser (:user spec))
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(.setPassword (:password spec))
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;; expire excess connections after 30 minutes of inactivity:
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(.setMaxIdleTimeExcessConnections (* 30 60))
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;; expire connections after 3 hours of inactivity:
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(.setMaxIdleTime (* 3 60 60)))]
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{:datasource cpds}))
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</code></pre><p>Now you can create a single connection pool:</p><pre><code class="clojure">(def pooled-db (delay (pool db-spec)))
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(defn db-connection [] @pooled-db)
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</code></pre><p>And then call <code>(db-connection)</code> wherever you need access to it. If you're using
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a <a href="https://github.com/stuartsierra/component">component</a> lifecycle for your
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application, you won't need <code>pooled-db</code> or <code>db-connection</code>. You'll just create
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<code>(pool db-spec)</code> as part of your application's initialization and pass it
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around as part of your system configuration.</p>
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