Chapter 35. Diverting and Splitting Message Flows

35.1. Exclusive Divert
35.2. Non-exclusive Divert

HornetQ allows you to configure objects called diverts with some simple server configuration.

Diverts allow you to transparently divert messages routed to one address to some other address, without making any changes to any client application logic.

Diverts can be exclusive, meaning that the message is diverted to the new address, and does not go to the old address at all, or they can be non-exclusive which means the message continues to go the old address, and a copy of it is also sent to the new address. Non-exclusive diverts can therefore be used for splitting message flows, e.g. there may be a requirement to monitor every order sent to an order queue.

Diverts can also be configured to have an optional message filter. If specified then only messages that match the filter will be diverted.

Diverts can also be configured to apply a Transformer. If specified, all diverted messages will have the opportunity of being transformed by the Transformer.

A divert will only divert a message to an address on the same server, however, if you want to divert to an address on a different server, a common pattern would be to divert to a local store-and-forward queue, then set up a bridge which consumes from that queue and forwards to an address on a different server.

Diverts are therefore a very sophisticated concept, which when combined with bridges can be used to create interesting and complex routings. The set of diverts on a server can be thought of as a type of routing table for messages. Combining diverts with bridges allows you to create a distributed network of reliable routing connections between multiple geographically distributed servers, creating your global messaging mesh.

Diverts are defined as xml in the hornetq-configuration.xml file. There can be zero or more diverts in the file.

Please see Section 11.1.17, “Divert” for a full working example showing you how to configure and use diverts.

Let's take a look at some divert examples:

35.1. Exclusive Divert

Let's take a look at an exclusive divert. An exclusive divert diverts all matching messages that are routed to the old address to the new address. Matching messages do not get routed to the old address.

Here's some example xml configuration for an exclusive divert, it's taken from the divert example:

<divert name="prices-divert">                  
    <address>jms.topic.priceUpdates</address>
    <forwarding-address>jms.queue.priceForwarding</forwarding-address>    
    <filter string="office='New York'"/>
    <transformer-class-name>
        org.hornetq.jms.example.AddForwardingTimeTransformer
    </transformer-class-name>     
    <exclusive>true</exclusive>
</divert>

We define a divert called 'prices-divert' that will divert any messages sent to the address 'jms.topic.priceUpdates' (this corresponds to any messages sent to a JMS Topic called 'priceUpdates') to another local address 'jms.queue.priceForwarding' (this corresponds to a local JMS queue called 'priceForwarding'

We also specify a message filter string so only messages with the message property office with value New York will get diverted, all other messages will continue to be routed to the normal address. The filter string is optional, if not specified then all messages will be considered matched.

In this example a transformer class is specified. Again this is optional, and if specified the transformer will be executed for each matching message. This allows you to change the messages body or properties before it is diverted. In this example the transformer simply adds a header that records the time the divert happened.

This example is actually diverting messages to a local store and forward queue, which is configured with a bridge which forwards the message to an address on another HornetQ server. Please see the example for more details.

35.2. Non-exclusive Divert

Now we'll take a look at a non-exclusive divert. Non exclusive diverts are the same as exclusive diverts, but they only forward a copy of the message to the new address. The original message continues to the old address

You can therefore think of non-exclusive diverts as splitting a message flow.

Non exclusive diverts can be configured in the same way as exclusive diverts with an optional filter and transformer, here's an example non-exclusive divert, again from the divert example:

<divert name="order-divert">                 
    <address>jms.queue.orders</address>
    <forwarding-address>jms.topic.spyTopic</forwarding-address>         
    <exclusive>false</exclusive>
</divert>

The above divert example takes a copy of every message sent to the address 'jms.queue.orders' (Which corresponds to a JMS Queue called 'orders') and sends it to a local address called 'jms.topic.SpyTopic' (which corresponds to a JMS Topic called 'SpyTopic').