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## Chapter 36. 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 HornetQ 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 HornetQ servers together. With a core bridge both source and target servers must be HornetQ 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 Chapter 37, Duplicate Message Detection).

## Note

Although they have similar function, don't confuse core bridges with JMS bridges!

Core bridges are for linking a HornetQ node with another HornetQ 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.

## 36.1. Configuring Bridges

Bridges are configured in `hornetq-configuration.xml`. Let's kick off with an example (this is actually from the bridge example):

    <bridge name="my-bridge">
        <queue-name>jms.queue.sausage-factory</queue-name>
        <forwarding-address>jms.queue.mincing-machine</forwarding-address>
        <filter-string="name='aardvark'"/>
        <transformer-class-name>
            org.hornetq.jms.example.HatColourChangeTransformer
        </transformer-class-name>
        <retry-interval>1000</retry-interval>
        <ha>true</ha>
        <retry-interval-multiplier>1.0</retry-interval-multiplier>
        <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>
        <connector-ref connector-name="remote-connector"
            backup-connector-name="backup-remote-connector"/>
        <user>foouser</user>
        <password>foopassword</password>
    </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.

  ## Note

  If you're using JMS then normally the JMS configuration `hornetq-jms.xml` is loaded after the core configuration file `hornetq-configuration.xml` is loaded. If your bridge is consuming from a JMS queue then you'll need to make sure the JMS queue is also deployed as a core queue in the core configuration. Take a look at the bridge example for an example of how this is done.

- `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 HornetQ filter expression syntax described in Chapter 14, Filter Expressions.

- `transformer-class-name`. An optional transformer-class-name can be specified. This is the name of a user-defined class which implements the `org.hornetq.core.server.cluster.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.

- `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 Chapter 37, Duplicate Message Detection 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 Chapter 34, Client Reconnection and Session Reattachment

  ## Warning

  When using the bridge to forward messages 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.

- `connector-ref`. This mandatory parameter determines which connector pair the bridge will use to actually make the connection to the target server.

  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 Chapter 16, Configuring the Transport.

  The `connector-ref` element can be configured with two attributes:

  - `connector-name`. This references the name of a connector defined in the core configuration file `hornetq-configuration.xml`. The bridge will use this connector to make its connection to the target server. This attribute is mandatory.

  - `backup-connector-name`. This optional parameter also references the name of a connector defined in the core configuration file `hornetq-configuration.xml`. It represents the connector that the bridge will fail-over onto if it detects the live server connection has failed. If this is specified and `failover-on-server-shutdown` is set to `true` then it will also attempt failover onto this connector if the live target server is cleanly shut-down.

- `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 `hornetq-configuration.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 `hornetq-configuration.xml` will be used.

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