activemq-artemis/docs/user-manual/en/retroactive-addresses.md

Retroactive Addresses

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 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:

1. A non-exclusive divert to grab the messages from the retroactive address.
2. An address to receive the messages from the divert.
3. Two 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 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:

1. A divert on myAddress named $.artemis.internal.myAddress.divert.retro
2. An address named $.artemis.internal.myAddress.address.retro
3. A multicast queue on the address from step #2 named $.artemis.internal.myAddress.queue.multicast.retro with a ring-size of 10.
4. 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.:

<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.