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POHow do I transactionally poll a database queue table with rollback?
 

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  1. POHow do I transactionally poll a database queue table with rollback?
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    <p>I wish to setup a database table that will be used as a message queue. The table will have messages inserted into it with a unique id and a timestamp and a status of 'PENDING'.</p> <p>Assuming that the insertion into this table is properly handled, I wish to know what is the best way to transactionally process messages from this table using a simple HSQLDB 2.0 database (although this question should apply to all transaction supporting databases).</p> <p>I wish to read the next message with a status of 'pending' and make sure that no other queue processor can also process the same record then either commit or rollback.</p> <p>I include some code snippets for how I plan to achieve this using plain old JDBC.</p> <ul> <li>Will this work?</li> <li>Are there better alternatives?</li> </ul> <p>DDL:</p> <pre><code>create table message_queue ( qidx integer, message varchar(120), status varchar(20), inserted_date timestamp, inserted_by varchar(20), processed_date timestamp, processed_by varchar(20), ) insert into message_queue values (1,'Important message here','PENDING','2010-08-10 00:01:00', 'BOB', null,null) </code></pre> <p>Here is my queue reading SQL:</p> <pre><code>SET AUTOCOMMIT FALSE SET TRANSACTION ISOLATION LEVEL SERIALIZABLE START TRANSACTION DECLARE nextID INTEGER DEFAULT 0 SET nextID = select max(qidx) from message_queue where status = 'PENDING' update message_queue set status = 'CONSUMED' where QIDX = nextID select * from message_queue where QIDX = nextID ROLLBACK </code></pre> <p>Here is my connection code fragments:</p> <pre><code> conn.setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE); try { String message = getNextMessage(conn); // uses sql in snippet processMessage(message); conn.commit(); // should commit } catch (Exception e) { conn.rollback(); // should rollback update } </code></pre>
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