activemq-artemis/docs/user-manual/en/core-bridges.md

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# Core Bridges
The function of a bridge is to consume messages from a source queue, and
forward them to a target address, typically on a different Apache ActiveMQ
Artemis server.
The source and target servers do not have to be in the same cluster which makes
bridging suitable for reliably sending messages from one cluster to another,
for instance across a WAN, or internet and where the connection may be
unreliable.
The bridge has built in resilience to failure so if the target server
connection is lost, e.g. due to network failure, the bridge will retry
connecting to the target until it comes back online. When it comes back online
it will resume operation as normal.
In summary, bridges are a way to reliably connect two separate Apache ActiveMQ
Artemis servers together. With a core bridge both source and target servers
must be Apache ActiveMQ Artemis servers.
Bridges can be configured to provide *once and only once* delivery guarantees
even in the event of the failure of the source or the target server. They do
this by using duplicate detection (described in [Duplicate
Detection](duplicate-detection.md)).
> **Note:**
>
> Although they have similar function, don't confuse core bridges with JMS
> bridges!
>
> Core bridges are for linking an Apache ActiveMQ Artemis node with another
> Apache ActiveMQ Artemis node and do not use the JMS API. A JMS Bridge is used
> for linking any two JMS 1.1 compliant JMS providers. So, a JMS Bridge could
> be used for bridging to or from different JMS compliant messaging system.
> It's always preferable to use a core bridge if you can. Core bridges use
> duplicate detection to provide *once and only once* guarantees. To provide
> the same guarantee using a JMS bridge you would have to use XA which has a
> higher overhead and is more complex to configure.
## Configuring Bridges
Bridges are configured in `broker.xml`. Let's kick off with an example (this is
actually from the bridge example):
```xml
<bridge name="my-bridge">
<queue-name>sausage-factory</queue-name>
<forwarding-address>mincing-machine</forwarding-address>
<filter string="name='aardvark'"/>
<transformer-class-name>
org.apache.activemq.artemis.jms.example.HatColourChangeTransformer
</transformer-class-name>
<retry-interval>1000</retry-interval>
<ha>true</ha>
<retry-interval-multiplier>1.0</retry-interval-multiplier>
<initial-connect-attempts>-1</initial-connect-attempts>
<reconnect-attempts>-1</reconnect-attempts>
<failover-on-server-shutdown>false</failover-on-server-shutdown>
<use-duplicate-detection>true</use-duplicate-detection>
<confirmation-window-size>10000000</confirmation-window-size>
<user>foouser</user>
<password>foopassword</password>
<routing-type>PASS</routing-type>
<concurrency>1</concurrency>
<static-connectors>
<connector-ref>remote-connector</connector-ref>
</static-connectors>
<!-- alternative to static-connectors
<discovery-group-ref discovery-group-name="bridge-discovery-group"/>
-->
</bridge>
```
In the above example we have shown all the parameters its possible to configure
for a bridge. In practice you might use many of the defaults so it won't be
necessary to specify them all explicitly.
Let's take a look at all the parameters in turn:
- `name` attribute. All bridges must have a unique name in the server.
- `queue-name`. This is the unique name of the local queue that the bridge
consumes from, it's a mandatory parameter.
The queue must already exist by the time the bridge is instantiated at
start-up.
- `forwarding-address`. This is the address on the target server that the
message will be forwarded to. If a forwarding address is not specified, then
the original address of the message will be retained.
- `filter-string`. An optional filter string can be supplied. If specified then
only messages which match the filter expression specified in the filter
string will be forwarded. The filter string follows the ActiveMQ Artemis filter
expression syntax described in [Filter Expressions](filter-expressions.md).
- `transformer-class-name`. An *optional* transformer can be specified. This
gives you the opportunity to transform the message's header or body before
forwarding it. See the [transformer chapter](transformers.md) for more details
about transformer-specific configuration.
- `ha`. This optional parameter determines whether or not this bridge should
support high availability. True means it will connect to any available server
in a cluster and support failover. The default value is `false`.
- `retry-interval`. This optional parameter determines the period in
milliseconds between subsequent reconnection attempts, if the connection to
the target server has failed. The default value is `2000`milliseconds.
- `retry-interval-multiplier`. This optional parameter determines a multiplier
to apply to the time since the last retry to compute the time to the next
retry.
This allows you to implement an *exponential backoff* between retry
attempts.
Let's take an example:
If we set `retry-interval`to `1000` ms and we set `retry-interval-multiplier`
to `2.0`, then, if the first reconnect attempt fails, we will wait `1000` ms
then `2000` ms then `4000` ms between subsequent reconnection attempts.
The default value is `1.0` meaning each reconnect attempt is spaced at equal
intervals.
- `initial-connect-attempts`. This optional parameter determines the total
number of initial connect attempts the bridge will make before giving up and
shutting down. A value of `-1` signifies an unlimited number of attempts. The
default value is `-1`.
- `reconnect-attempts`. This optional parameter determines the total number of
reconnect attempts the bridge will make before giving up and shutting down. A
value of `-1` signifies an unlimited number of attempts. The default value is
`-1`.
- `use-duplicate-detection`. This optional parameter determines whether the
bridge will automatically insert a duplicate id property into each message
that it forwards.
Doing so, allows the target server to perform duplicate detection on messages
it receives from the source server. If the connection fails or server crashes,
then, when the bridge resumes it will resend unacknowledged messages. This
might result in duplicate messages being sent to the target server. By enabling
duplicate detection allows these duplicates to be screened out and ignored.
This allows the bridge to provide a *once and only once* delivery guarantee
without using heavyweight methods such as XA (see [Duplicate
Detection](duplicate-detection.md) for more information).
The default value for this parameter is `true`.
- `confirmation-window-size`. This optional parameter determines the
`confirmation-window-size` to use for the connection used to forward messages
to the target node. This attribute is described in section [Reconnection and
Session Reattachment](client-reconnection.md)
> **Warning**
>
> When using the bridge to forward messages to an address which uses the
> `BLOCK` `address-full-policy` from a queue which has a `max-size-bytes` set
> it's important that `confirmation-window-size` is less than or equal to
> `max-size-bytes` to prevent the flow of messages from ceasing.
- `producer-window-size`. This optional parameter determines the producer flow
control through the bridge. You usually leave this off unless you are dealing
with huge large messages.
Default=-1 (disabled)
- `user`. This optional parameter determines the user name to use when creating
the bridge connection to the remote server. If it is not specified the
default cluster user specified by `cluster-user` in `broker.xml` will be used.
- `password`. This optional parameter determines the password to use when
creating the bridge connection to the remote server. If it is not specified
the default cluster password specified by `cluster-password` in `broker.xml`
will be used.
- `routing-type`. Bridges can apply a particular routing-type to the messages it
forwards, strip the existing routing type, or simply pass the existing
routing-type through. This is useful in situations where the message may have
its routing-type set but you want to bridge it to an address using a different
routing-type. 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. By configuring the `routing-type` of the bridge you have the
flexibility to deal with any situation. Valid values are `ANYCAST`,
`MULTICAST`, `PASS`, & `STRIP`. The default is `PASS`.
- `concurrency`. For bridging high latency networks, and particularly for destinations
with a high throughput, more workers might have to be commited to the bridge. This is
done with the concurrency parameter. Increasing the concurrency will get reflected
by more consumers and producers showing up on the bridged destination, allowing
for increased parallelism across high latency networks.
Default=1
- `static-connectors` or `discovery-group-ref`. Pick either of these options to
connect the bridge to the target server.
The `static-connectors` is a list of `connector-ref` elements pointing to
`connector` elements defined elsewhere. A *connector* encapsulates knowledge of
what transport to use (TCP, SSL, HTTP etc) as well as the server connection
parameters (host, port etc). For more information about what connectors are and
how to configure them, please see [Configuring the
Transport](configuring-transports.md).
The `discovery-group-ref` element has one attribute - `discovery-group-name`.
This attribute points to a `discovery-group` defined elsewhere. For more
information about what discovery-groups are and how to configure them, please
see [Discovery Groups](clusters.md).