89 lines
3.6 KiB
Markdown
89 lines
3.6 KiB
Markdown
# Retroactive Addresses
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A "retroactive" address is an address that will preserve messages sent to it
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for queues which will be created on it in the future. This can be useful in,
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for example, publish-subscribe use cases where clients want to receive the
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messages sent to the address *before* they actually connected and created
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their multicast "subscription" queue. Typically messages sent to an address
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before a queue was created on it would simply be unavailable to those queues,
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but with a retroactive address a fixed number of messages can be preserved by
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the broker and automatically copied into queues subsequently created on the
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address. This works for both anycast and multicast queues.
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## Internal Retroactive Resources
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To implement this functionality the broker will create 4 internal resources for
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each retroactive address:
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1. A non-exclusive [divert](#diverts) to grab the messages from the retroactive
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address.
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2. An address to receive the messages from the divert.
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3. **Two** [ring queues](#ring-queues) to hold the messages sent to the address
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by the divert - one for anycast and one for multicast. The general caveats
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for ring queues still apply here. See [the chapter on ring queues](#ring-queues)
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for more details.
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These resources are important to be aware of as they will show up in the web
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console and other management or metric views. They will be named according to
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the following pattern:
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```
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<internal-naming-prefix><delimiter><source-address><delimiter>(divert|address|queue<delimiter>(anycast|multicast))<delimiter>retro
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```
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For example, if an address named `myAddress` had a `retroactive-message-count`
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of 10 and the default `internal-naming-prefix` (i.e. `$.artemis.internal.`) and
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the default delimiter (i.e. `.`) were being used then resources with these names
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would be created:
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1. A divert on `myAddress` named `$.artemis.internal.myAddress.divert.retro`
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2. An address named `$.artemis.internal.myAddress.address.retro`
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3. A multicast queue on the address from step #2 named
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`$.artemis.internal.myAddress.queue.multicast.retro` with a `ring-size` of 10.
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4. An anycast queue on the address from step #2 named
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`$.artemis.internal.myAddress.queue.anycast.retro` with a `ring-size` of 10.
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This pattern is important to note as it allows one to configure address-settings
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if necessary. To configure custom address-settings you'd use a match like:
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```
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*.*.*.<source-address>.*.retro
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```
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Using the same example as above the `match` would be:
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```
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*.*.*.myAddress.*.retro
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```
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> Note:
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>
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> Changing the broker's `internal-naming-prefix` once these retroactive
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> resources are created will break the retroactive functionality.
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>
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## Configuration
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To configure an address to be "retroactive" simply configure the
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`retroactive-message-count` `address-setting` to reflect the number of messages
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you want the broker to preserve, e.g.:
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```xml
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<address-settings>
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<address-setting match="orders">
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<retroactive-message-count>100</retroactive-message-count>
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</address-setting>
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</address-settings>
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```
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The value for `retroactive-message-count` can be updated at runtime either via
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`broker.xml` or via the management API just like any other address-setting.
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However, if you *reduce* the value of `retroactive-message-count` an additional
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administrative step will be required since this functionality is implemented
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via ring queues. This is because a ring queue whose ring-size is reduced will
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not automatically delete messages from the queue to meet the new ring-size in
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order to avoid unintended message loss. Therefore, administrative action will
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be required in this case to manually reduce the number of messages in the ring
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queue via the management API.
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