Hey guys,
I wanted to use a JDO persistence-backed Sequence to provide
monotonically increasing visitor #'s in my webapp. A static variable
doesn't cut it because I want the number to survive redeployments,
upgrades, etc.
I didn't see any info on Sequence in the wiki "Using JDO" section so I
did this. It works but it seemed kinda hacky. Is this the best way to
configure a JDO-backed Sequence in AppEngine?
(FWIW I'm using the eclipse plug-in.)
(1) Make the following file: MyProject/META-INF/package.jdo
* This directory should already contain a "jdoconfig.xml".
* Both files will be autoplaced into MyProject/war/WEB-INF/classes/
META-INF on deploy.
(2) Paste the following XML into package.jdo:
<?xml version="1.0" encoding="UTF-8" ?>
<jdo xmlns="http://java.sun.com/xml/ns/jdo/jdo";
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
xsi:schemaLocation="http://java.sun.com/xml/ns/jdo/jdo
http://java.sun.com/xml/ns/jdo/jdo_2_2.xsd";>
<!-- name the package whatever you want -->
<package name="MyProject">
<sequence name="VisitorSequence"
datastore-
sequence="VISITOR_SEQUENCE"
strategy="contiguous"
/>
</package>
</jdo>
(3) When you need the next value in your application, call:
PersistenceManager pm = //...allocate pm
//the sequence name is <package name>.<sequence name>
//from the corresponding tags in package.jdo
Sequence seq = pm.getSequence("MyProject.VisitorSequence");
//If "strategy" from package.jdo was "contiguous", this has to happen
//in a transaction.
long nextValue = seq.nextValue();
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