An ActiveMQ Artemis broker can initiate connections using the AMQP protocol. This means that the broker can connect to another AMQP server (not necessarily ActiveMQ Artemis) and create elements on that connection.
To define an AMQP broker connection, add an `<amqp-connection>` element within the `<broker-connections` element in the `broker.xml` configuration file. For example:
-`auto-start` : Should the broker connection start automatically with the broker. Default is `true`. If false it is necessary to call a management operation to start it.
*Notice:* If auto-start is disabled on the broker connection, the start of the broker connection will only happen after the management method `startBrokerConnection(connectionName)` is called on the ServerController.
*Important*: The target endpoint needs permission for all operations that configured. Therefore, If a security manager is being used, ensure to perform the configured operations with a user with sufficient permissions.
* The broker creates both senders and receivers on another endpoint that knows how to handle them. This is currently implemented by Apache Qpid Dispatch.
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# Reconnecting and Failover
It is possible to determine how reconnection will happen on a broker connection.
These are the attributes are available on amqp-connection XML element:
- reconnect-attempts: default as -1 (infinite). How many attempts will be done after a failed connection
- retry-interval: default as 5000, in milliseconds, the wait between each retry in connections.
It is also possible to specify alternate hosts on a broker connection by appending a comma separated list after a # at the end of the URI.
The broker connection would keep trying on the alternate list until one of the targets is available to connect. Example:
Figure 1. Broker Connection - Reconnecting and Failover.
The previous example portrays a case of connection failure towards ServerA. The system would try to connect to serverA, backupA, and backupB until it successfully connects to one of these nodes.
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## Mirroring
The idea of mirroring is to send events that happen on a broker towards another broker, without blocking any operations from producers and consumers, allowing them to keep operating as fast as possible.
It can be used for Disaster Recovery, and depending on the requirements even for failing over the data.
The following events are sent through mirroring:
* Message sending
* Messages sent to one broker will be "replicated" to the target broker.
* Message acknowledgement
* Acknowledgements removing messages at one broker will be sent to the target broker.
* Note that if the message is pending for a consumer on the target mirror, the ack will not succeed and the message might be delivered by both brokers.
* Queue and address creation.
* Queue and address deletion.
### Mirror configuration
Add a `<mirror>` element within the `<amqp-connection>` element to configure mirroring to the target broker.
The following optional arguments can be utilized:
*`queue-removal`: Specifies whether a queue- or address-removal event is sent. The default value is `true`.
*`message-acknowledgements`: Specifies whether message acknowledgements are sent. The default value is `true`.
*`queue-creation`: Specifies whether a queue- or address-creation event is sent. The default value is `true`.
*`address-filter`: An optional comma-separated list of inclusion and/or exclusion filter entries used to govern which addresses (and related queues) mirroring events will be created for on this broker-connection. That is, events will only be mirrored to the target broker for addresses that match the filter.
An address is matched when it begins with an inclusion entry specified in this field, unless the address is also explicitly excluded by another entry. An exclusion entry is prefixed with `!` to denote any address beginning with that value does not match.
If no inclusion entry is specified in the list, all addresses not explicitly excluded will match. If the address-filter attribute is not specified, then all addresses (and related queues) will match and be mirrored.
Examples:
- 'eu'
matches all addresses starting with 'eu'
- '!eu'
matches all address except for those starting with 'eu'
- 'eu.uk,eu.de'
matches all addresses starting with either 'eu.uk' or 'eu.de'
- 'eu,!eu.uk'
matches all addresses starting with 'eu' but not those starting with 'eu.uk'
**Note:**
- Address exclusion will always take precedence over address inclusion.
- Address matching on mirror elements is prefix-based and does not support wild-card matching.
Mirror events are always stored on a local queue prefixed as "$ACTIVEMQ_ARTEMIS_MIRROR_" and then concatenated with the broker connection's configured name.
So, in the following configuration mirror events will be stored on a queue named "$ACTIVEMQ_ARTEMIS_MIRROR_brokerB".
These messages are then transferred to brokerB:5672. A producer to the address $ACTIVEMQ_ARTEMIS_MIRROR_brokerB will be created towards brokerB. If there is a security manager configured, security roles must be provided to the user on the broker connection.
Notice the queue $ACTIVEMQ_ARTEMIS_MIRROR_brokerB will not actually exist on brokerB and so it wont be visible on the administration console. The target broker will treat these messages accordingly as mirror events and perform the appropriate operations at the target broker.
### Pre Existing Messages
The broker will only mirror messages arriving from the point in time the mirror was configured. Previously existing messages will not be forwarded to other brokers.
## Dual Mirror (Disaster Recovery)
ActiveMQ Artemis supports automatic fallback mirroring. Every sent message and every acknowledgement is asynchronously replicated to the mirrored broker.
On the following diagram, there will be two servers called DataCenter1, and DataCenter2. In order to have a dual mirror configuration, it is necessary is to add the mirror broker connection on each broker.xml:
The broker connections will replicate sends and acknowledgements to the other broker, no matter where they originated. If messages are sent on DC1 (DataCenter1) these will be automatically transferred to DC2 (DataCenter2). Messages acknowledgements received on DC2 will be automatically related back to DC1.
The only exception to that rule would be if there were already consumers with pending messages on any server, where a mirrored acknowledgement will not prevent the message being consumed by both consumers. It is recommended to not have active consumers on both servers.
## Example
There is an example as part of the distribution showing dual broker configuration (or disaster recovery) under ./examples/features/broker-connection/disaster-recovery.
On the example two brokers are configured to mirror each other and whatever happens in one broker is immediately copied over to the other broker.
Both elements function as a message bridge. However, there is no additional overhead required to process messages. Senders and receivers behave just like any other consumer or producer in ActiveMQ Artemis.
Specific queues can be configured by senders or receivers. Wildcard expressions can be used to match senders and receivers to specific addresses or _sets_ of addresses. When configuring a sender or receiver, the following properties can be set:
*Important:* Receivers can only be matched to a local queue that already exists. Therefore, if receivers are being used, ensure that queues are pre-created locally. Otherwise, the broker cannot match the remote queues and addresses.
The broker can be configured as a peer which connects to the [Apache Qpid Dispatch Router](https://qpid.apache.org/components/dispatch-router/) and instructs it that the broker will act as a store-and-forward queue for a given AMQP waypoint address configured on the router. In this scenario, clients connect to a router to send and receive messages using a waypointed address, and the router routes these messages to or from the queue on the broker.
The peer configuration causes ActiveMQ Artemis to create a sender and receiver pair for each destination matched in the broker-connection configuration, with these carrying special configuration to let Qpid Dispatch know to collaborate with the broker. This replaces the traditional need of a router-initiated connection and auto-links.
Qpid Dispatch Router offers a lot of advanced networking options that be used together with ActiveMQ Artemis.
With a peer configuration, the same properties are present as when there are senders and receivers. For example, a configuration where queues with names beginning "queue." act as storage for the matching router waypoint address would be:
There must be a matching address waypoint configuration on the router instructing it that the particular router addresses the broker attaches to should be treated as waypoints. For example, a similar prefix- based router address configuration would be:
For more information refer to the "brokered messaging" documentation for [Apache Qpid Dispatch Router](https://qpid.apache.org/components/dispatch-router/).
*Important:* Do not use this feature to connect to another broker, otherwise any message sent will be immediately ready to consume creating an infinite echo of sends and receives.
*Important:* It is not necessary to configure the router with a connector or auto-links to communicate with the broker. The brokers peer configuration replaces these aspects of the router waypoint usage.
It is highly recommended that `address name` and `queue name` are the same. When a queue with its distinct name (as in the following example) is used, senders and receivers will always use the `address name` when creating the remote endpoint.
