118 lines
6.5 KiB
Plaintext
118 lines
6.5 KiB
Plaintext
= Diverting and Splitting Message Flows
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Apache ActiveMQ Artemis allows you to configure objects called _diverts_ with some simple server configuration.
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Diverts allow you to transparently divert messages routed to one address to one or more other addresses, without making any changes to any client application logic.
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Diverts can be _exclusive_, meaning that the message is diverted to the new address(es), 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(es).
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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.
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When an address has both exclusive and non-exclusive diverts configured, the exclusive ones are processed first.
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If any of the exclusive diverts diverted the message, the non-exclusive ones are not processed.
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Diverts can also be configured to have an optional message filter.
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If specified then only messages that match the filter will be diverted.
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Diverts can apply a particular routing-type to the message, strip the existing routing type, or simply pass the existing routing-type through.
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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.
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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.
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By configuring the `routing-type` of the divert you have the flexibility to deal with any situation.
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Valid values are `ANYCAST`, `MULTICAST`, `PASS`, & `STRIP`.
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The default is `STRIP`.
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Diverts can also be configured to apply a xref:transformers.adoc#transformers[`Transformer`].
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If specified, all diverted messages will have the opportunity of being transformed by the `Transformer`.
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When an address has multiple diverts configured, all of them receive the same, original message.
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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.
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See the documentation on xref:using-server.adoc#adding-runtime-dependencies[adding runtime dependencies] to understand how to make your transformer available to the broker.
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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.
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Diverts are therefore a very sophisticated concept, which when combined with bridges can be used to create interesting and complex routings.
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The set of diverts on a server can be thought of as a type of routing table for messages.
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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.
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Diverts are defined as xml in the `broker.xml` file at the `core` attribute level.
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There can be zero or more diverts in the file.
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Diverted messages get xref:copied-message-properties.adoc#properties-for-copied-messages[special properties].
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Please see the examples for a full working example showing you how to configure and use diverts.
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Let's take a look at some divert examples:
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== Exclusive Divert
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Let's take a look at an exclusive divert.
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An exclusive divert diverts all matching messages that are routed to the old address to the new address.
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Matching messages do not get routed to the old address.
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Here's some example xml configuration for an exclusive divert, it's taken from the divert example:
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[,xml]
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----
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<divert name="prices-divert">
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<address>priceUpdates</address>
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<forwarding-address>priceForwarding</forwarding-address>
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<filter string="office='New York'"/>
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<transformer-class-name>
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org.apache.activemq.artemis.jms.example.AddForwardingTimeTransformer
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</transformer-class-name>
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<exclusive>true</exclusive>
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</divert>
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----
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We define a divert called `prices-divert` that will divert any messages sent to the address `priceUpdates` to another local address `priceForwarding`.
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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.
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The filter string is optional, if not specified then all messages will be considered matched.
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In this example a transformer class is specified without any configuration properties.
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Again this is optional, and if specified the transformer will be executed for each matching message.
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This allows you to change the messages body or properties before it is diverted.
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In this example the transformer simply adds a header that records the time the divert happened.
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See the xref:transformers.adoc#transformers[transformer chapter] for more details about transformer-specific configuration.
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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.
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Please see the example for more details.
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== Non-exclusive Divert
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Now we'll take a look at a non-exclusive divert.
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Non exclusive diverts are the same as exclusive diverts, but they only forward a _copy_ of the message to the new address.
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The original message continues to the old address
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You can therefore think of non-exclusive diverts as _splitting_ a message flow.
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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:
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[,xml]
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----
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<divert name="order-divert">
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<address>orders</address>
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<forwarding-address>spyTopic</forwarding-address>
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<exclusive>false</exclusive>
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</divert>
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----
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The above divert example takes a copy of every message sent to the address '[.code]``orders``' and sends it to a local address called '[.code]``spyTopic``'.
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== Composite Divert
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A _composite_ divert is one which forwards messages to multiple addresses.
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This pattern is sometimes referred to as _fan-out_.
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Configuration is simple.
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Just use a comma separated list in `forwarding-address`, e.g.:
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[,xml]
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----
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<divert name="shipping-divert">
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<address>shipping</address>
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<forwarding-address>dallas, chicago, denver</forwarding-address>
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<exclusive>false</exclusive>
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</divert>
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----
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