activemq-artemis/docs/user-manual/retroactive-addresses.adoc

= Retroactive Addresses
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A "retroactive" address is an address that will preserve messages sent to it for queues which will be created on it in the future.
This can be useful in, for example, publish-subscribe use cases where clients want to receive the messages sent to the address _before_ they actually connected and created their multicast "subscription" queue.
Typically messages sent to an address before a queue was created on it would simply be unavailable to those queues, but with a retroactive address a fixed number of messages can be preserved by the broker and automatically copied into queues subsequently created on the address.
This works for both anycast and multicast queues.

== Internal Retroactive Resources

To implement this functionality the broker will create 4 internal resources for each retroactive address:

. A non-exclusive xref:diverts.adoc#diverting-and-splitting-message-flows[divert] to grab the messages from the retroactive address.
. An address to receive the messages from the divert.
. *Two* xref:ring-queues.adoc#ring-queue[ring queues] to hold the messages sent to the address by the divert - one for anycast and one for multicast.
The general caveats for ring queues still apply here.
See xref:ring-queues.adoc#ring-queue[the chapter on ring queues] for more details.

These resources are important to be aware of as they will show up in the web console and other management or metric views.
They will be named according to the following pattern:

----
<internal-naming-prefix><delimiter><source-address><delimiter>(divert|address|queue<delimiter>(anycast|multicast))<delimiter>retro
----

For example, if an address named `myAddress` had a `retroactive-message-count` of 10 and the default `internal-naming-prefix` (i.e. `$.artemis.internal.`) and the default delimiter (i.e. `.`) were being used then resources with these names would be created:

. A divert on `myAddress` named `$.artemis.internal.myAddress.divert.retro`
. An address named `$.artemis.internal.myAddress.address.retro`
. A multicast queue on the address from step #2 named `$.artemis.internal.myAddress.queue.multicast.retro` with a `ring-size` of 10.
. An anycast queue on the address from step #2 named `$.artemis.internal.myAddress.queue.anycast.retro` with a `ring-size` of 10.

This pattern is important to note as it allows one to configure address-settings if necessary.
To configure custom address-settings you'd use a match like:

----
*.*.*.<source-address>.*.retro
----

Using the same example as above the `match` would be:

----
*.*.*.myAddress.*.retro
----

NOTE: Changing the broker's `internal-naming-prefix` once these retroactive resources are created will break the retroactive functionality.

== Configuration

To configure an address to be "retroactive" simply configure the `retroactive-message-count` `address-setting` to reflect the number of messages you want the broker to preserve, e.g.:

[,xml]
----
<address-settings>
   <address-setting match="orders">
      <retroactive-message-count>100</retroactive-message-count>
   </address-setting>
</address-settings>
----

The value for `retroactive-message-count` can be updated at runtime either via `broker.xml` or via the management API just like any other address-setting.
However, if you _reduce_ the value of `retroactive-message-count` an additional administrative step will be required since this functionality is implemented via ring queues.
This is because a ring queue whose ring-size is reduced will not automatically delete messages from the queue to meet the new ring-size in order to avoid unintended message loss.
Therefore, administrative action will be required in this case to manually reduce the number of messages in the ring queue via the management API.