# Protocols and Interoperability

## Protocols

ActiveMQ Artemis has a plugable protocol architecture. Protocol plugins come in the form of ActiveMQ Artemis protocol modules. Each protocol module should be added to the brokers class path and are loaded by the broker at boot time. ActiveMQ Artemis ships with 5 protocol modules out of the box. The 5 modules offer support for the following protocols:

- AMQP
- OpenWire
- MQTT
- STOMP
- HornetQ

In addition to the protocols above ActiveMQ Artemis also offers support for it's own highly performant native protocol "Core".

## Configuring protocols

In order to make use of a particular protocol, a transport must be configured with the desired protocol enabled. There is a whole section on configuring transports that can be found here.

The default configuration shipped with the ActiveMQ Artemis distribution comes with a number of acceptors already defined, one for each of the above protocols plus a generic acceptor that supports all protocols. To enable a protocol on a particular acceptor simply add a url parameter "protocol=AMQP,STOMP" to the acceptor url. Where the value of the parameter is a comma separated list of protocol names. If the protocol parameter is omitted from the url all protocols are enabled.

    <!-- The following example enables only MQTT on port 1883 -->
    <acceptors>
       <acceptor>tcp://localhost:1883?protocols=MQTT</acceptor>
    </acceptors>

    <!-- The following example enables MQTT and AMQP on port 61617 -->
    <acceptors>
       <acceptor>tcp://localhost:1883?protocols=MQTT,AMQP</acceptor>
    </acceptors>

    <!-- The following example enables all protocols on 61616 -->
    <acceptors>
       <acceptor>tcp://localhost:61616</acceptor>
    </acceptors>

## AMQP

Apache ActiveMQ Artemis supports the AMQP 1.0 specification. To enable AMQP you must configure a Netty Acceptor to receive AMQP clients, like so:

    <acceptor name="amqp-acceptor">tcp://localhost:5672?protocols=AMQP</acceptor>

Apache ActiveMQ Artemis will then accept AMQP 1.0 clients on port 5672 which is the default AMQP port.

There are 2 AMQP examples available see proton-j and proton-ruby which use the qpid Java and Ruby clients respectively.

### AMQP and security

The Apache ActiveMQ Artemis Server accepts AMQP SASL Authentication and will use this to map onto the underlying session created for the connection so you can use the normal Apache ActiveMQ Artemis security configuration.

### AMQP Links

An AMQP Link is a uni directional transport for messages between a source and a target, i.e. a client and the Apache ActiveMQ Artemis Broker. A link will have an endpoint of which there are 2 kinds, a Sender and A Receiver. At the Broker a Sender will have its messages converted into an Apache ActiveMQ Artemis Message and forwarded to its destination or target. A Receiver will map onto an Apache ActiveMQ Artemis Server Consumer and convert Apache ActiveMQ Artemis messages back into AMQP messages before being delivered.

### AMQP and destinations

If an AMQP Link is dynamic then a temporary queue will be created and either the remote source or remote target address will be set to the name of the temporary queue. If the Link is not dynamic then the the address of the remote target or source will used for the queue. If this does not exist then an exception will be sent

> Note

> For the next version we will add a flag to aut create durable queue but for now you will have to add them via the configuration

### AMQP and Topics

Although amqp has no notion of topics it is still possible to treat amqp consumers or receivers as subscriptions rather than just consumers on a queue. By default any receiving link that attaches to an address with the prefix `jms.topic.` will be treated as a subscription and a subscription queue will be created. If the Terminus Durability is either UNSETTLED_STATE or CONFIGURATION then the queue will be made durable, similar to a JMS durable subscription and given a name made up from the container id and the link name, something like `my-container-id:my-link-name`. if the Terminus Durability is configured as NONE then a volatile queue will be created.

The prefix can be changed by configuring the Acceptor and setting the `pubSubPrefix` like so

> tcp://0.0.0.0:5672?protocols=AMQP;pubSubPrefix=foo.bar.

Artemis also supports the qpid-jms client and will respect its use of topics regardless of the prefix used for the address.

### AMQP and Coordinations - Handling Transactions

An AMQP links target can also be a Coordinator, the Coordinator is used to handle transactions. If a coordinator is used the the underlying HormetQ Server session will be transacted and will be either rolled back or committed via the coordinator.

> Note

> AMQP allows the use of multiple transactions per session, `amqp:multi-txns-per-ssn`, however in this version Apache ActiveMQ Artemis will only support single transactions per session

## OpenWire

Apache ActiveMQ Artemis now supports the OpenWire protocol so that an Apache ActiveMQ Artemis JMS client can talk directly to an Apache ActiveMQ Artemis server. To enable OpenWire support you must configure a Netty Acceptor, like so:

    <acceptor name="openwire-acceptor">tcp://localhost:61616?protocols=OPENWIRE</acceptor>

The Apache ActiveMQ Artemis server will then listens on port 61616 for incoming openwire commands. Please note the "protocols" is not mandatory here. The openwire configuration conforms to Apache ActiveMQ Artemis's "Single Port" feature. Please refer to Configuring Single Port for details.

Please refer to the openwire example for more coding details.

Currently we support Apache ActiveMQ Artemis clients that using standard JMS APIs. In the future we will get more supports for some advanced, Apache ActiveMQ Artemis specific features into Apache ActiveMQ Artemis.

### Connection Monitoring

OpenWire has a few parameters to control how each connection is monitored, they are:

- maxInactivityDuration: It specifies the time (milliseconds) after which the connection is closed by the broker if no data was received. Default value is 30000.

- maxInactivityDurationInitalDelay: It specifies the maximum delay (milliseconds) before inactivity monitoring is started on the connection. It can be useful if a broker is under load with many connections being created concurrently. Default value is 10000.

- useInactivityMonitor: A value of false disables the InactivityMonitor completely and connections will never time out. By default it is enabled. On broker side you don't neet set this. Instead you can set the connection-ttl to -1.

- useKeepAlive: Whether or not to send a KeepAliveInfo on an idle connection to prevent it from timing out. Enabled by default. Disabling the keep alive will still make connections time out if no data was received on the connection for the specified amount of time.

Note at the beginning the InactivityMonitor negotiates the appropriate maxInactivityDuration and maxInactivityDurationInitalDelay. The shortest duration is taken for the connection.

More details please see ActiveMQ InactivityMonitor.

## MQTT

MQTT is a light weight, client to server, publish / subscribe messaging protocol. MQTT has been specifically designed to reduce transport overhead (and thus network traffic) and code footprint on client devices. For this reason MQTT is ideally suited to constrained devices such as sensors and actuators and is quickly becoming the defacto standard communication protocol for IoT.

Apache ActiveMQ Artemis supports MQTT v3.1.1 (and also the older v3.1 code message format). To enable MQTT, simply add an appropriate acceptor with the MQTT protocol enabled. For example:

    <acceptor name="mqtt">tcp://localhost:1883?protocols=MQTT</acceptor>

By default the configuration shipped with Apache ActiveMQ Artemis has the above acceptor already defined, MQTT is also active by default on the generic acceptor defined on port 61616 (where all protocols are enabled), in the out of the box configuration.

The best source of information on the MQTT protocol is in the specification. The MQTT v3.1.1 specification can be downloaded from the OASIS website here: http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html

Some note worthy features of MQTT are explained below:

### MQTT Quality of Service

MQTT offers 3 quality of service levels.

Each message (or topic subscription) can define a quality of service that is associated with it. The quality of service level defined on a topic is the maximum level a client is willing to accept. The quality of service level on a message is the desired quality of service level for this message. The broker will attempt to deliver messages to subscribers at the highest quality of service level based on what is defined on the message and topic subscription.

Each quality of service level offers a level of guarantee by which a message is sent or received:

- QoS 0: AT MOST ONCE: Guarantees that a particular message is only ever received by the subscriber a maximum of one time. This does mean that the message may never arrive. The sender and the receiver will attempt to deliver the message, but if something fails and the message does not reach it's destination (say due to a network connection) the message may be lost. This QoS has the least network traffic overhead and the least burden on the client and the broker and is often useful for telemetry data where it doesn't matter if some of the data is lost.

- QoS 1: AT LEAST ONCE: Guarantees that a message will reach it's intended recipient one or more times. The sender will continue to send the message until it receives an acknowledgment from the recipient, confirming it has received the message. The result of this QoS is that the recipient may receive the message multiple times, and also increases the network overhead than QoS 0, (due to acks). In addition more burden is placed on the sender as it needs to store the message and retry should it fail to receive an ack in a reasonable time.

- QoS 2: EXACTLY ONCE: The most costly of the QoS (in terms of network traffic and burden on sender and receiver) this QoS will ensure that the message is received by a recipient exactly one time. This ensures that the receiver never gets any duplicate copies of the message and will eventually get it, but at the extra cost of network overhead and complexity required on the sender and receiver.

### MQTT Retain Messages

MQTT has an interesting feature in which messages can be "retained" for a particular address. This means that once a retain message has been sent to an address, any new subscribers to that address will receive the last sent retain message before any others messages, this happens even if the retained message was sent before a client has connected or subscribed. An example of where this feature might be useful is in environments such as IoT where devices need to quickly get the current state of a system when they are on boarded into a system.

### Will Messages

A will message can be sent when a client initially connects to a broker. Clients are able to set a "will message" as part of the connect packet. If the client abnormally disconnects, say due to a device or network failure the broker will proceed to publish the will message to the specified address (as defined also in the connect packet). Other subscribers to the will topic will receive the will message and can react accordingly. This feature can be useful in an IoT style scenario to detect errors across a potentially large scale deployment of devices.

### Wild card subscriptions

MQTT addresses are hierarchical much like a file system, and use "/" character to separate hierarchical levels. Subscribers are able to subscribe to specific topics or to whole branches of a hierarchy.

To subscribe to branches of an address hierarchy a subscriber can use wild cards.

There are 2 types of wild card in MQTT:

- "#" Multi level wild card. Adding this wild card to an address would match all branches of the address hierarchy under a specified node. For example: /uk/# Would match /uk/cities, /uk/cities/newcastle and also /uk/rivers/tyne. Subscribing to an address "#" would result in subscribing to all topics in the broker. This can be useful, but should be done so with care since it has significant performance implications.

- "+" Single level wild card. Matches a single level in the address hierarchy. For example /uk/+/stores would match /uk/newcastle/stores but not /uk/cities/newcastle/stores.

## Stomp

Stomp is a text-orientated wire protocol that allows Stomp clients to communicate with Stomp Brokers. Apache ActiveMQ Artemis now supports Stomp 1.0, 1.1 and 1.2.

Stomp clients are available for several languages and platforms making it a good choice for interoperability.

## Native Stomp support

Apache ActiveMQ Artemis provides native support for Stomp. To be able to send and receive Stomp messages, you must configure a `NettyAcceptor` with a `protocols` parameter set to have `stomp`:

    <acceptor name="stomp-acceptor">tcp://localhost:61613?protocols=STOMP</acceptor>

With this configuration, Apache ActiveMQ Artemis will accept Stomp connections on the port `61613` (which is the default port of the Stomp brokers).

See the `stomp` example which shows how to configure an Apache ActiveMQ Artemis server with Stomp.

### Limitations

Message acknowledgements are not transactional. The ACK frame can not be part of a transaction (it will be ignored if its `transaction` header is set).

### Stomp 1.1/1.2 Notes

#### Virtual Hosting

Apache ActiveMQ Artemis currently doesn't support virtual hosting, which means the 'host' header in CONNECT fram will be ignored.

### Mapping Stomp destinations to Apache ActiveMQ Artemis addresses and queues

Stomp clients deals with destinations when sending messages and subscribing. Destination names are simply strings which are mapped to some form of destination on the server - how the server translates these is left to the server implementation.

In Apache ActiveMQ Artemis, these destinations are mapped to addresses and queues. When a Stomp client sends a message (using a `SEND` frame), the specified destination is mapped to an address. When a Stomp client subscribes (or unsubscribes) for a destination (using a `SUBSCRIBE` or `UNSUBSCRIBE` frame), the destination is mapped to an Apache ActiveMQ Artemis queue.

### STOMP heart-beating and connection-ttl

Well behaved STOMP clients will always send a DISCONNECT frame before closing their connections. In this case the server will clear up any server side resources such as sessions and consumers synchronously. However if STOMP clients exit without sending a DISCONNECT frame or if they crash the server will have no way of knowing immediately whether the client is still alive or not. STOMP connections therefore default to a connection-ttl value of 1 minute (see chapter on connection-ttl for more information. This value can be overridden using the `connection-ttl-override` property or if you need a specific connectionTtl for your stomp connections without affecting the broker-wide `connection-ttl-override` setting, you can configure your stomp acceptor with the "connectionTtl" property, which is used to set the ttl for connections that are created from that acceptor. For example:

    <acceptor name="stomp-acceptor">tcp://localhost:61613?protocols=STOMP;connectionTtl=20000</acceptor>

The above configuration will make sure that any Stomp connection that is created from that acceptor and does not include a `heart-beat` header or disables client-to-server heart-beats by specifying a `0` value will have its connection-ttl set to 20 seconds. The `connectionTtl` set on an acceptor will take precedence over `connection-ttl-override`. The default `connectionTtl` is 60,000 milliseconds.

Since Stomp 1.0 does not support heart-beating then all connections from Stomp 1.0 clients will have a connection TTL imposed upon them by the broker based on the aforementioned configuration options. Likewise, any Stomp 1.1 or 1.2 clients that don't specify a `heart-beat` header or disable client-to-server heart-beating (e.g. by sending `0,X` in the `heart-beat` header) will have a connection TTL imposed upon them by the broker.

For Stomp 1.1 and 1.2 clients which send a non-zero client-to-server `heart-beat` header value then their connection TTL will be set accordingly. However, the broker will not strictly set the connection TTL to the same value as the specified in the `heart-beat` since even small network delays could then cause spurious disconnects. Instead, the client-to-server value in the `heart-beat` will be multiplied by the `heartBeatConnectionTtlModifer` specified on the acceptor. The `heartBeatConnectionTtlModifer` is a decimal value that defaults to `2.0` so for example, if a client sends a `heart-beat` header of `1000,0` the the connection TTL will be set to `2000` so that the data or ping frames sent every 1000 milliseconds will have a sufficient cushion so as not to be considered late and trigger a disconnect. This is also in accordance with the Stomp 1.1 and 1.2 specifications which both state, "because of timing inaccuracies, the receiver SHOULD be tolerant and take into account an error margin."

The minimum and maximum connection TTL allowed can also be specified on the acceptor via the `connectionTtlMin` and `connectionTtlMax` properties respectively. The default `connectionTtlMin` is 1000 and the default `connectionTtlMax` is Java's `Long.MAX_VALUE` meaning there essentially is no max connection TTL by default. Keep in mind that the `heartBeatConnectionTtlModifer` is relevant here. For example, if a client sends a `heart-beat` header of `20000,0` and the acceptor is using a `connectionTtlMax` of `30000` and a default `heartBeatConnectionTtlModifer` of `2.0` then the connection TTL would be `40000` (i.e. `20000` * `2.0`) which would exceed the `connectionTtlMax`. In this case the server would respond to the client with a `heart-beat` header of `0,15000` (i.e. `30000` / `2.0`). As described previously, this is to make sure there is a sufficient cushion for the client heart-beats in accordance with the Stomp 1.1 and 1.2 specifications. The same kind of calculation is done for `connectionTtlMin`.

The minimum server-to-client heart-beat value is 500ms.

> Note

> Please note that the STOMP protocol version 1.0 does not contain any heart-beat frame. It is therefore the user's responsibility to make sure data is sent within connection-ttl or the server will assume the client is dead and clean up server side resources. With `Stomp 1.1` users can use heart-beats to maintain the life cycle of stomp connections.

### Selector/Filter expressions

Stomp subscribers can specify an expression used to select or filter what the subscriber receives using the `selector` header. The filter expression syntax follows the core filter syntax described in the Filter Expressions documentation.

### Stomp and JMS interoperability

#### Using JMS destinations

As explained in Mapping JMS Concepts to the Core API, JMS destinations are also mapped to Apache ActiveMQ Artemis addresses and queues. If you want to use Stomp to send messages to JMS destinations, the Stomp destinations must follow the same convention:

- send or subscribe to a JMS Queue by prepending the queue name by `jms.queue.`.

  For example, to send a message to the `orders` JMS Queue, the Stomp client must send the frame:

      SEND
      destination:jms.queue.orders

      hello queue orders
      ^@

- send or subscribe to a JMS Topic by prepending the topic name by `jms.topic.`.

  For example to subscribe to the `stocks` JMS Topic, the Stomp client must send the frame:

      SUBSCRIBE
      destination:jms.topic.stocks

      ^@

#### Sending and consuming Stomp message from JMS or Apache ActiveMQ Artemis Core API

Stomp is mainly a text-orientated protocol. To make it simpler to interoperate with JMS and Apache ActiveMQ Artemis Core API, our Stomp implementation checks for presence of the `content-length` header to decide how to map a Stomp 1.0 message to a JMS Message or a Core message.

If the Stomp 1.0 message does not have a `content-length` header, it will be mapped to a JMS TextMessage or a Core message with a single nullable SimpleString in the body buffer.

Alternatively, if the Stomp 1.0 message has a `content-length` header, it will be mapped to a JMS BytesMessage or a Core message with a byte[] in the body buffer.

The same logic applies when mapping a JMS message or a Core message to Stomp. A Stomp 1.0 client can check the presence of the `content-length` header to determine the type of the message body (String or bytes).

#### Durable Subscriptions

The `SUBSCRIBE` and `UNSUBSCRIBE` frames can be augmented with special headers to create and destroy durable subscriptions respectively.

To create a durable subscription the `client-id` header must be set on the `CONNECT` frame and the `durable-subscription-name` must be set on the `SUBSCRIBE` frame. The combination of these two headers will form the identity of the durable subscription.

To delete a durable subscription the `client-id` header must be set on the `CONNECT` frame and the `durable-subscription-name` must be set on the `UNSUBSCRIBE` frame. The values for these headers should match what was set on the `SUBSCRIBE` frame to delete the corresponding durable subscription.

It is possible to pre-configure durable subscriptions since the Stomp implementation creates the queue used for the durable subscription in a deterministic way (i.e. using the format of `client-id`.`subscription-name`). For example, if you wanted to configure a durable subscription on the JMS topic `myTopic` with a client-id of `myclientid` and a subscription name of `mysubscriptionname` then first you'd configure the topic:

       <jms xmlns="urn:activemq:jms">
          ...
          <topic name="myTopic"/>
          ...
       </jms>

Then configure the durable subscription:

       <core xmlns="urn:activemq:core">
          ...
          <queues>
             <queue name="myclientid.mysubscription">
                <address>jms.topic.myTopic</address>
             </queue>
          </queues>
          ...
       </core>

#### Message IDs for Stomp messages

When receiving Stomp messages via a JMS consumer or a QueueBrowser, the messages have no properties like JMSMessageID by default. However this may bring some inconvenience to clients who wants an ID for their purpose. Apache ActiveMQ Artemis Stomp provides a parameter to enable message ID on each incoming Stomp message. If you want each Stomp message to have a unique ID, just set the `stompEnableMessageId` to true. For example:

    <acceptor name="stomp-acceptor">tcp://localhost:61613?protocols=STOMP;stompEnableMessageId=true</acceptor>

When the server starts with the above setting, each stomp message sent through this acceptor will have an extra property added. The property key is `amq-message-id` and the value is a String representation of a long type internal message id prefixed with "`STOMP`", like:

    amq-message-id : STOMP12345

If `stomp-enable-message-id` is not specified in the configuration, default is `false`.

#### Handling of Large Messages with Stomp

Stomp clients may send very large bodys of frames which can exceed the size of Apache ActiveMQ Artemis server's internal buffer, causing unexpected errors. To prevent this situation from happening, Apache ActiveMQ Artemis provides a stomp configuration attribute `stompMinLargeMessageSize`. This attribute can be configured inside a stomp acceptor, as a parameter. For example:

       <acceptor name="stomp-acceptor">tcp://localhost:61613?protocols=STOMP;stompMinLargeMessageSize=10240</acceptor>

The type of this attribute is integer. When this attributed is configured, Apache ActiveMQ Artemis server will check the size of the body of each Stomp frame arrived from connections established with this acceptor. If the size of the body is equal or greater than the value of `stompMinLargeMessageSize`, the message will be persisted as a large message. When a large message is delievered to a stomp consumer, the HorentQ server will automatically handle the conversion from a large message to a normal message, before sending it to the client.

If a large message is compressed, the server will uncompressed it before sending it to stomp clients. The default value of `stompMinLargeMessageSize` is the same as the default value of min-large-message-size.

### Stomp Over Web Sockets

Apache ActiveMQ Artemis also support Stomp over Web Sockets. Modern web browser which support Web Sockets can send and receive Stomp messages from Apache ActiveMQ Artemis.

Stomp over Web Sockets is supported via the normal Stomp acceptor:

    <acceptor name="stomp-ws-acceptor">tcp://localhost:61614?protocols=STOMP</acceptor>

With this configuration, Apache ActiveMQ Artemis will accept Stomp connections over Web Sockets on the port `61614`. Web browser can then connect to `ws://<server>:61614` using a Web Socket to send and receive Stomp messages.

A companion JavaScript library to ease client-side development is available from GitHub (please see its documentation for a complete description).

The `stomp-websockets` example shows how to configure Apache ActiveMQ Artemis server to have web browsers and Java applications exchanges messages on a JMS topic.

## REST

Please see Rest Interface

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