Apache ActiveMQ Artemis has a unique addressing model that is both powerful and flexible and that offers great performance. The addressing model comprises three main concepts: addresses, queues and routing types.
An address represents a messaging endpoint. Within the configuration, a typical address is given a unique name, 0 or more queues, and a routing type.
A queue is associated with an address. There can be multiple queues per address. Once an incoming message is matched to an address, the message will be sent on to one or more of its queues, depending on the routing type configured. Queues can be configured to be automatically created and deleted.
A routing type determines how messages are sent to the queues associated with an address. An Apache ActiveMQ Artemis address can be configured with two different routing types.
**Note:** It is possible to define more than one routing type per address, but this typically results in an anti-pattern and is therefore not recommended. If an address does use both routing types, however, and the client does not show a preference for either one, the broker typically defaults to the anycast routing type.
The one exception is when the client uses the MQTT protocol. In that case, the default routing type is multicast. |
## Basic Address Configuration
The following examples show how to configure basic point to point and publish subscribe addresses.
### Point-to-Point Messaging
Point-to-point messaging is a common scenario in which a message sent by a producer has only one consumer. AMQP and JMS message producers and consumers can make use of point-to-point messaging queues, for example. Define an anycast routing type for an address so that its queues receive messages in a point-to-point manner.
When a message is received on an address using anycast, Apache ActiveMQ Artemis locates the queue associated with the address and routes the message to it. When consumers request to consume from the address, the broker locates the relevant queue and associates this queue with the appropriate consumers. If multiple consumers are connected to the same queue, messages are distributed amongst each consumer equally, providing the consumers are equally able to handle them.
Figure 1. Point to Point Messaging
#### Configuring an Address to Use the Anycast Routing Type
Open the file <broker-instance>/etc/broker.xml for editing.
Add an address configuration element and its associated queue if they do not exist already.
**Note** For normal Point to Point semantics, the queue name **MUST** match the address name.
```xml
<configuration...>
<core...>
...
<addressname="orders">
<anycast>
<queuename="orders"/>
</anycast>
</address>
</core>
</configuration>
```
### Publish-Subscribe Messaging
In a publish-subscribe scenario, messages are sent to every consumer subscribed to an address. JMS topics and MQTT subscriptions are two examples of publish-subscribe messaging.
When clients connect to an address with the multicast element, a subscription queue for the client will be automatically created for the client. It is also possible to pre-configure subscription queues and connect to them directly using the queue's [Fully Qualified Queue names](#fully-qualified-queue-names).
Optionally add one or more queue elements to the address and wrap the multicast element around them. This step is typically not needed since the broker will automatically create a queue for each subscription requested by a client.
It is actually possible to define more than one queue on an address with an anycast routing type. When messages are received on such an address, they are firstly distributed evenly across all the defined queues. Using [Fully Qualified Queue names](#fully-qualified-queue-names), clients are able to select the queue that they would like to subscribe to. Should more than one consumer connect direct to a single queue, Apache ActiveMQ Artemis will take care of distributing messages between them, as in the example above.
Add an address configuration with Anycast routing type element and its associated queues.
```xml
<configuration...>
<core...>
...
<addressname="address.foo">
<anycast>
<queuename="q1"/>
<queuename="q2"/>
</anycast>
</address>
</core>
</configuration>
```
### Point-to-Point and Publish-Subscribe Addresses
It is possible to define an address with both point-to-point and publish-subscribe semantics enabled. While not typically recommend, this can be useful when you want, for example, a JMS Queue say orders and a JMS Topic named orders. The different routing types make the addresses appear to be distinct.
Using an example of JMS Clients, the messages sent by a JMS queue producer will be routed using the anycast routing type. Messages sent by a JMS topic producer will use the multicast routing type. In addition when a JMS topic consumer attaches, it will be attached to it’s own subscription queue. JMS queue consumer will be attached to the anycast queue.
**Note:** The behavior in this scenario is dependent on the protocol being used. For JMS there is a clear distinction between topic and queue producers and consumers, which make the logic straight forward. Other protocols like AMQP do not make this distinction. A message being sent via AMQP will be routed by both anycast and multicast and consumers will default to anycast. For more information, please check the behavior of each protocol in the sections on protocols.
The XML snippet below is an example of what the configuration for an address using both anycast and multicast would look like in <broker-instance>/etc/broker.xml. Note that subscription queues are typically created on demand, so there is no need to list specific queue elements inside the multicast routing type.
The redConsumer has a filter that only matches "red" messages. The redConsumer will receive 3 messages.
```
red, red, red
```
The resulting queue would now be
```
green, green, green
```
## Creating and Deleting Addresses and Queues Automatically
You can configure Apache ActiveMQ Artemis to automatically create addresses and queues, and then delete them when they are no longer in use. This saves you from having to preconfigure each address and queue before a client can connect to it. Automatic creation and deletion is configured on a per address basis and is controlled by following:
| Parameter | Description |
|-----------|-------------|
| auto-create-addresses | When set to true, the broker will create the address requested by the client if it does not exist already. The default is true.|
| auto-delete-addresses | When set to true, the broker will be delete any **auto-created** adddress once all of it’s queues have been deleted. The default is true |
|default-address-routing-type | The routing type to use if the client does not specify one. Possible values are MULTICAST and ANYCAST. See earlier in this chapter for more information about routing types. The default value is MULTICAST. |
### Configuring an Address to be Automatically Created
Edit the file <broker-instance>/etc/broker.xml and add the auto-create-addresses element to the address-setting you want the broker to automatically create.
(Optional) Add the address-setting if it does not exits. Use the match parameter and the The Apache ActiveMQ Artemis Wildcard Syntax to match more than one specific address.
Set auto-create-addresses to true
(Optional) Assign MULTICAST or ANYCAST as the default routing type for the address.
The example below configures an address-setting to be automatically created by the broker. The default routing type to be used if not specified by the client is MULTICAST. Note that wildcard syntax is used. Any address starting with /news/politics/ will be automatically created by the broker.
### Configuring an Address to be Automatically Deleted
Edit the file <broker-instance>/etc/broker.xml and add the auto-delete-addresses element to the address-setting you want the broker to automatically create.
(Optional) Add the address-setting if it does not exits. Use the match parameter and the The Apache ActiveMQ Artemis Wildcard Syntax to match more than one specific address.
Set auto-delete-addresses to true
The example below configures an address-setting to be automatically deleted by the broker. Note that wildcard syntax is used. Any address request by the client that starts with /news/politics/ is configured to be automatically deleted by the broker.
Internally the broker maps a client’s request for an address to specific queues. The broker decides on behalf of the client which queues to send messages to or from which queue to receive messages. However, more advanced use cases might require that the client specify a queue directly. In these situations the client and use a fully qualified queue name, by specifying both the address name and the queue name, separated by a ::.
In this example, the address foo is configured with two queues q1, q2 as shown in the configuration below.
```xml
<configuration...>
<core...>
...
<addresses>
<addressname="foo">
<anycast>
<queuename="q1"/>
<queuename="q2"/>
</anycast>
</address>
</addresses>
</core>
</configuration>
```
In the client code, use both the address name and the queue name when requesting a connection from the broker. Remember to use two colons, ::, to separate the names, as in the example Java code below.
## Configuring a Prefix to Connect to a Specific Routing Type
Normally, if a Apache ActiveMQ Artemis receivs a message sent to a particular address, that has both anycast and multicast routing types enable, Apache ActiveMQ Artemis will route a copy of the message to **one** of the anycast queues and to **all** of the multicast queues.
However, clients can specify a special prefix when connecting to an address to specify whether to connect using anycast or multicast. The prefixes are custom values that are designated using the anycastPrefix and multicastPrefix parameters within the URL of an acceptor.
### Configuring an Anycast Prefix
In <broker-instance>/etc/broker.xml, add the anycastPrefix to the URL of the desired acceptor. In the example below, the acceptor is configured to use anycast:// for the anycastPrefix. Client code can specify anycast://foo/ if the client needs to send a message to only one of the anycast queues.
In <broker-instance>/etc/broker.xml, add the anycastPrefix to the URL of the desired acceptor. In the example below, the acceptor is configured to use multicast:// for the multicastPrefix. Client code can specify multicast://foo/ if the client needs the message sent to only the multicast queues of the address.
In most cases it’s not necessary to pre-create subscription queues. The relevant protocol managers take care of creating subscription queues when clients request to subscribe to an address. The type of subscription queue created, depends on what properties the client request. E.g. durable, non-shared, shared etc... Protocol managers uses special queue names to identify which queues below to which consumers and users need not worry about the details.
However, there are scenarios where a user may want to use broker side configuration to pre-configure a subscription. And later connect to that queue directly using a [Fully Qualified Queue name](#fully-qualified-queue-names).. The examples below show how to use broker side configuration to pre-configure a queue with publish subscribe behavior for shared, non-shared, durable and non-durable subscription behavior.
#### Configuring a shared durable subscription queue with up to 10 concurrent consumers
The default behavior for queues is to not limit the number connected queue consumers. The **max-consumers** parameter of the queue element can be used to limit the number of connected consumers allowed at any one time.
#### Configuring a non-shared durable subscription
The broker can be configured to prevent more than one consumer from connecting to a queue at any one time. The subscriptions to queues configured this way are therefore "non-shared". To do this simply set the **max-consumers** parameter to "1"
```xml
<configuration...>
<core...>
...
<addressname="durable.foo">
<multicast>
<!-- pre-configured non shared durable subscription queue -->
<queuename="q1"max-consumers="1">
<durable>true</durable>
</queue>
</multicast>
</address>
</core>
</configuration>
```
#### Pre-configuring a queue as a non-durable subscription queue
Non-durable subscriptions are again usually managed by the relevant protocol manager, by creating and deleting temporary queues.
If a user requires to pre-create a queue that behaves like a non-durable subscription queue the **purge-on-no-consumers** flag can be enabled on the queue. When **purge-on-no-consumers** is set to **true**. The queue will not start receiving messages until a consumer is attached. When the last consumer is detached from the queue. The queue is purged (it's messages are removed) and will not receive any more messages until a new consumer is attached.
Open the file <broker-instance>/etc/broker.xml for editing.
A protocol manager maps protocol specific concepts down to the Apache ActiveMQ Artemis core model of addresses, queues and routing types. For example, when a client sends a MQTT subscription packet with the addresses
```
/house/room1/lights
/house/room2/lights
```
The MQTT protocol manager understands that the two addresses require multicast semantics. The protocol manager will therefore first look to ensure that multicast is enabled for both addresses. If not, it will attempt to dynamically create them. If successful, the protocol manager will then create special subscription queues with special names, for each subscription requested by the client.
The special name allows the protocol manager to quickly identify the required client subscription queues should the client disconnect and reconnect at a later date. If the subscription is temporary the protocol manager will delete the queue once the client disconnects.
When a client requests to subscribe to a point to point address. The protocol manager will look up the queue associated with the point to point address. This queue should have the same name as the addresss.
**Note:** If the queue is auto created, it will be auto deleted once there are no consumers and no messages in it. For more information on auto create see the next section [Configuring Addresses and Queues via Address Settings](#Configuring-Addresses-and-Queues-via-Address-Settings)
## Configuring Addresses and Queues via Address Settings
There are some attributes that are defined against an address wildcard
rather than a specific address/queue. Here an example of an `address-setting`
entry that would be found in the `broker.xml` file.
