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ARTEMIS-1960 clarify scale-down docs

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Justin Bertram 2021-09-24 13:37:05 -05:00
parent 371a7099a6
commit 1992143836
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docs/user-manual/en/clusters.md
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@ -938,27 +938,69 @@ nodes) the messages on those nodes would be lost unless the broker sent
them to another node in the cluster. Apache ActiveMQ Artemis can be configured to do
just that.
The simplest way to enable this behavior is to set `scale-down` to
`true`. If the server is clustered and `scale-down` is `true` then when
the server is shutdown gracefully (i.e. stopped without crashing) it
will find another node in the cluster and send *all* of its messages
(both durable and non-durable) to that node. The messages are processed
in order and go to the *back* of the respective queues on the other node
(just as if the messages were sent from an external client for the first
time).
To enable this behavior configure `scale-down` in the `live-only`
`ha-policy`, e.g.:
If more control over where the messages go is required then specify
`scale-down-group-name`. Messages will only be sent to another node in
the cluster that uses the same `scale-down-group-name` as the server
being shutdown.
```xml
<ha-policy>
<live-only>
<scale-down>
<enabled>true</enabled>
<discovery-group-ref discovery-group-name="my-discovery-group"/>
</scale-down>
</live-only>
</ha-policy>
```
If `scale-down`/`enabled` is `true` then when the server is shutdown
gracefully (i.e. stopped without crashing) it will find another node in
the cluster and send *all* of its messages (both durable and non-durable)
to that node. The messages are processed in order and go to the *back* of
the respective queues on the other node (just as if the messages were
sent from an external client for the first time).
The _target_ of the scale down operation can be configured a few differnt
ways. The above example uses `discovery-group-ref` to reference a
`discovery-group` which will be used to find the target broker. This
should be the same `discovery-group` referenced by your `cluster-connection`.
You can also specify a static list of `connector` elements, e.g.:
```xml
<connectors>
...
<connector name="server0-connector">tcp://server0:61616</connector>
</connectors>
...
<ha-policy>
<live-only>
<scale-down>
<enabled>true</enabled>
<connectors>
<connector-ref>server0-connector</connector-ref>
</connectors>
</scale-down>
</live-only>
</ha-policy>
```
It's also possible to specify `group-name`. If this is specified then
messages will only be sent to another node in the cluster that uses the
same `group-name` as the server being shutdown, e.g.:
```xml
<ha-policy>
<live-only>
<scale-down>
<enabled>true</enabled>
<group-name>my-group</group-name>
<discovery-group-ref discovery-group-name="my-discovery-group"/>
</scale-down>
</live-only>
</ha-policy>
```
> **Warning**
>
> If cluster nodes are grouped together with different
> `scale-down-group-name` values beware. If all the nodes in a single
> `group-name` values beware. If all the nodes in a single
> group are shut down then the messages from that node/group will be
> lost.
If the server is using multiple `cluster-connection` then use
`scale-down-clustername` to identify the name of the
`cluster-connection` which should be used for scaling down.