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Test App: Add Java code for transaction retries
Add example code for transaction retry logic. Compilation and execution instructions are in a comment in the main code file. Fixes #757.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,83 @@ | ||
| import java.sql.*; | ||
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| /* | ||
| You can compile and run this example with a command like: | ||
| javac TxnSample.java && java -cp .:~/path/to/postgresql-9.4.1208.jar TxnSample | ||
| You can download the postgres JDBC driver jar from https://jdbc.postgresql.org. | ||
| */ | ||
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| class InsufficientBalanceException extends Exception {} | ||
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| // A simple interface that provides a retryable lambda expression. | ||
| interface RetryableTransaction { | ||
| public void run(Connection conn) throws Exception; | ||
| } | ||
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| public class TxnSample { | ||
| public static RetryableTransaction transferFunds(int from, int to, int amount) { | ||
| return new RetryableTransaction() { | ||
| public void run(Connection conn) throws Exception { | ||
| ResultSet res = conn.createStatement().executeQuery("SELECT balance FROM accounts WHERE id = " + from); | ||
| res.next(); | ||
| int balance = res.getInt("balance"); | ||
| if(balance < from) { | ||
| throw new InsufficientBalanceException(); | ||
| } | ||
| conn.createStatement().executeUpdate("UPDATE accounts SET balance = balance - " + amount + " where id = " + from); | ||
| conn.createStatement().executeUpdate("UPDATE accounts SET balance = balance + " + amount + " where id = " + to); | ||
| } | ||
| }; | ||
| } | ||
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| public static void retryTransaction(Connection conn, RetryableTransaction tx) throws Exception { | ||
| Savepoint sp = conn.setSavepoint("cockroach_restart"); | ||
| while(true) { | ||
| try { | ||
| // Attempt the transaction. | ||
| tx.run(conn); | ||
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| // If we reach this point, commit the transaction, | ||
| // which implicitly releases the savepoint. | ||
| conn.commit(); | ||
| break; | ||
| } catch(SQLException e) { | ||
| // Check if the error code indicates a SERIALIZATION_FAILURE. | ||
| if(e.getErrorCode() == 40001) { | ||
| // Signal the database that we will attempt a retry. | ||
| conn.rollback(sp); | ||
| } | ||
| // This is a not a serialization failure, pass it up the chain. | ||
| throw e; | ||
| } | ||
| } | ||
| } | ||
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| public static void main(String[] args) throws ClassNotFoundException, SQLException { | ||
| // Load the postgres JDBC driver. | ||
| Class.forName("org.postgresql.Driver"); | ||
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| // Connect to the "bank" database. | ||
| Connection db = DriverManager.getConnection("jdbc:postgresql://127.0.0.1:26257/bank?sslmode=disable", "maxroach", ""); | ||
| try { | ||
| // We need to turn off autocommit mode to allow for | ||
| // multi-statement transactions. | ||
| db.setAutoCommit(false); | ||
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| RetryableTransaction transfer = transferFunds(1, 2, 100); | ||
| retryTransaction(db, transfer); | ||
| ResultSet res = db.createStatement().executeQuery("SELECT id, balance FROM accounts"); | ||
| while (res.next()) { | ||
| System.out.printf("\taccount %s: %s\n", res.getInt("id"), res.getInt("balance")); | ||
| } | ||
| } catch(InsufficientBalanceException ibe) { | ||
| System.out.println("Insufficient balance"); | ||
| } catch(SQLException sqle) { | ||
| System.out.println("SQLException encountered:" + sqle); | ||
| } catch(Exception e) { | ||
| System.out.println("Unknown exception encountered:" + e); | ||
| } finally { | ||
| // Close the database connection. | ||
| db.close(); | ||
| } | ||
| } | ||
| } |
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