# 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>
   <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-class-name can be
  specified. This is the name of a user-defined class which implements the
  `org.apache.activemq.artemis.core.server.transformer.Transformer` interface.

  If this is specified then the transformer's `transform()` method will be
  invoked with the message before it is forwarded. This gives you the opportunity
  to transform the message's header or body before forwarding it.

- `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 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`.

- `failover-on-server-shutdown`. This optional parameter determines whether the
  bridge will attempt to failover onto a backup server (if specified) when the
  target server is cleanly shutdown rather than crashed.

  The bridge connector can specify both a live and a backup server, if it
  specifies a backup server and this parameter is set to `true` then if the
  target server is *cleanly* shutdown the bridge connection will attempt to
  failover onto its backup. If the bridge connector has no backup server
  configured then this parameter has no effect.

  Sometimes you want a bridge configured with a live and a backup target
  server, but you don't want to failover to the backup if the live server is
  simply taken down temporarily for maintenance, this is when this parameter
  comes in handy.

  The default value for this parameter is `false`.

- `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.

- `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).


