activemq-artemis/docs/user-manual/en/diverts.md

Diverting and Splitting Message Flows

Apache ActiveMQ Artemis 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.

When an address has both exclusive and non-exclusive diverts configured, the exclusive ones are processed first. If any of the exclusive diverts diverted the message, the non-exclusive ones are not processed.

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 apply a particular routing-type to the message, strip the existing routing type, or simply pass the existing routing-type through. This is useful in situations where the message may have its routing-type set but you want to divert it to an address using a different routing-type. It's important to keep in mind that a message with the anycast routing-type will not actually be routed to queues using multicast and vice-versa. By configuring the routing-type of the divert you have the flexibility to deal with any situation. Valid values are ANYCAST, MULTICAST, PASS, & STRIP. The default is STRIP.

Diverts can also be configured to apply a Transformer. If specified, all diverted messages will have the opportunity of being transformed by the Transformer. When an address has multiple diverts configured, all of them receive the same, original message. This means that the results of a transformer on a message are not directly available for other diverts or their filters on the same address.

See the documentation on adding runtime dependencies to understand how to make your transformer available to the broker.

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 broker.xml file at the core attribute level. There can be zero or more diverts in the file.

Diverted message gets a new message ID, and its address is set to a forward address. To access original values, use message properties: original destination is stored in a String property _AMQ_ORIG_ADDRESS (Message.HDR_ORIGINAL_ADDRESS constant from the Core API), and the original message ID in a Long property _AMQ_ORIG_MESSAGE_ID (Message.HDR_ORIG_MESSAGE_ID constant from the Core API).

Please see the examples for a full working example showing you how to configure and use diverts.

Let's take a look at some divert examples:

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>priceUpdates</address>
   <forwarding-address>priceForwarding</forwarding-address>
   <filter string="office='New York'"/>
   <transformer-class-name>
      org.apache.activemq.artemis.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 priceUpdates to another local address 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 without any configuration properties. 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. See the transformer chapter for more details about transformer-specific configuration.

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 ActiveMQ Artemis server. Please see the example for more details.

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>orders</address>
   <forwarding-address>spyTopic</forwarding-address>
   <exclusive>false</exclusive>
</divert>

The above divert example takes a copy of every message sent to the address 'orders' and sends it to a local address called 'spyTopic'.