FAQ

Q: What is the Concurrency Model?

Single master, multi-slave.

All writes must be performed on the master, reads may be performed on the master, however you will achieve better scalability by performing all reads against slaves.

Replication concurrency is per-Collection hierarchy. There will be a single Topic for each configured Collection.

If a slave has a collection, with a document which is under heavy-read contention, and there is a pending update for that document on the head of the topic-queue, it may be difficult to apply that update until all reads on the document have ceased, during this period replication of updates to other documents in that Collection will be blocked... Put another way: Attempting to update a specific document (via replication) which is locked for reading (on a slave), stops all replication for that collection (on the affected slave).

In future, we could possibly look at providing replication on a per-Document basis as opposed to per-Collection. Effectively, each Collection would have a fixed topic which describes adding and removing document topics, and then a number of dynamic topics for each document as they are required for updated.

Q: I have a AbstractTriggerProxy exception (runtime)

org.exist.EXistException: org.exist.collections.triggers.DocumentTriggerProxies.add(Lorg/exist/collections/triggers/AbstractTriggerProxy;)V
	at org.exist.storage.BrokerPool.initialize(BrokerPool.java:1065)
	at org.exist.storage.BrokerPool.<init>(BrokerPool.java:722)
	at org.exist.storage.BrokerPool.configure(BrokerPool.java:248)
	at org.exist.storage.BrokerPool.configure(BrokerPool.java:224)
	at org.exist.jetty.JettyStart.run(JettyStart.java:163)
	at org.exist.jetty.JettyStart.main(JettyStart.java:71)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.exist.start.Main.invokeMain(Main.java:126)
	at org.exist.start.Main.run(Main.java:448)
	at org.exist.start.Main.main(Main.java:50)
Caused by: java.lang.NoSuchMethodError: org.exist.collections.triggers.DocumentTriggerProxies.add(Lorg/exist/collections/triggers/AbstractTriggerProxy;)V
	at org.exist.storage.BrokerPool.initialiseTriggersForCollections(BrokerPool.java:1175)
	at org.exist.storage.BrokerPool.initialize(BrokerPool.java:981)
	... 12 more

Answer: The internal Trigger interface (API) has been changed during development of eXist-db 2.2. Please update the extension-code to at least commit 15fe9259a8

Q: how to set the logging level for the extension?

See the FAQ.

Q: My own ActiveMQ/JMS application can not parse the messages with XML payload

For efficiency reasons (XML fragments can consume a lot of memory), the XML payload is GZIPped before transport.

To prevent compression, set 'exist.document.compression' to 'none' in the message properties of the data producer:

let $messageProperties :=
    map {
        "exist.document.compression" := "none"
    }

Q: The slave seems to think it's the master ... but why?

The log file contains the following entry:

(ReplicationJmsListener.java) [onMessage]:111 - Incoming JMS message was sent by this instance.  Processing stopped.

When the extension is started for the first time, the file webapp/WEB-INF/data/jms.identity is generated, containing a unique "token". With this token the extension can prevent in the scenario where an existdb instance is configured both as a master node and slave node, that a replication event originated from this master server is re-replicated because of the slave-configuration of the same server.

exist-a is configured as master and slave; a document is updated on exist-a; extension sends change to broker; broker distributes change to exist-a/b/c/d; exist-a receives change as slave too; extension detects origin and does not process change.

note: as a consequence not all files in the directory web apps/WEB-INF/data can be transferred between eXist-db instances. When setting up a new server, please remove this file.

Durable messaging

The replication feature uses a durable subscription, this cannot be changed. The messaging feature can subscribe for both Queues and Topics. For Topics , the durable subscription is optional. It can be switched on by setting the property "subscriber.durable" := true()

Q: My queries stopped working with v0.9.0

With the v0.9.0 pre-release the java-packages have been updated making them more logical and consistent. It makes the code better maintainable. Apologies for any inconvenience.

Replication:

<trigger class="org.exist.jms.replication.publish.ReplicationTrigger">
<trigger class="org.exist.jms.replication.subscribe.ReceiverStartupTrigger">

import module namespace replication="http://exist-db.org/xquery/replication" 
              at "java:org.exist.jms.xquery.ReplicationModule"; 

Messaging:

import module namespace messaging="http://exist-db.org/xquery/messaging" 
              at "java:org.exist.jms.xquery.MessagingModule";

JMS receiver management:

import module namespace jms="http://exist-db.org/xquery/jms" 
              at "java:org.exist.jms.xquery.JmsModule";