Apollo允许客户端使用MQTT连接，该MQTT是一种开源协议规范，适用于使用发布-订阅域的不可靠网络上的有限资源设备。这些类型的设备通常需要一个小的内存空间，不太适合 text-based 类型的协议，如HTTP或STOMP，甚至是传统的二进制协议，如Openwire或AMQP。MQTT是一种紧凑的二进制协议，针对这些类型的有限设备和不可靠的网络进行优化。
由于MQTT是一个 wire-level 的协议，任何实现该协议的客户端都应该能够连接到Apollo，并与其他 MQTT-compatibe 消息代理进行互操作。
<connector id="tcp" bind="tcp://0.0.0.0:61613" protocol="mqtt"/>
<connector id="tcp" bind="tcp://0.0.0.0:61613"> <detect protocols="mqtt openwire" /> </connector>
mqtt元素支持以下 configuration 属性:
part_pattern:允许您指定约束命名主题的regex。默认值:[a-zA-Z0-9 \ _ \ \ % \ ~ :()]+
Java : mqtt-client, MeQanTT
C : libmosquitto
Erlang : erlmqtt, my-mqtt4erl
.NET : MQTTDotNet, nMQTT
Perl : net-mqtt-perl, [anyevent-mqtt-perl]https://github.com/beanz/anyevent-mqtt-perl()
Python : nyamuk
Ruby : mqtt-ruby, ruby-em-mqtt
Delphi : TMQTTCLient
Device specific: Arduino, mbed, Nanode, Netduino
To see an up-to-date listing of client libraries, please the MQTT website for its softwarelistings
The Apollo distribution ships with an examples
directory where you can find some simple examples of how to use some of those clients to send and receive messages from a broker instance.
The default broker configuration secures access to the broker so that only the admin
user can connect. The default password for the admin
user is password
MQTT clients cannot specify a Virtual Host (see the section on Virtual Hosts in the user guide) so the default virtual host will be used. This is usually the first Virtual Host defined in the Apollo.xml configuration file.
The MQTT protocol is a publish/subscribe protocol. It does not permit true point-to-point messaging achieved using Queues. Therefore Apollo allows only the use of Topics for MQTT messaging. The concept of a subscription and durable subscription to Topics is similar to what you'd find in other protocols and is controlled by the MQTT CONNECT frame's clean session
When a client sends a connect frame with the clean session
flag set to cleared (false), any previously used session with the same client_id will be re-used. This means while the client was away, that subscription could have received messages. This is the equivalent of a durable subscription in Apollo.
If the clean session
flag is set (true), then a new session will be started and any sessions tha may have been lingering would be removed. This is equivalent to a normal topic subscription in Apollo.
Topic Retained Messages
If a message has been published with the retain flag set, then the message will be 'remembered' by the topic so that if a new subscription arrives, the last retained message is sent to the subscription. For example if you're publishing measurements and you want the last mesasurement published to always be available to a client that subscribes to the topic, you can set the retain flag on the PUBLISH frame.
Note: retained messages are not retained between broker restarts for Quality of Service setting of AT MOST ONCE (QoS=0).
Last Will and Testament Message
You can set a will
message and assocaited QoS for the message when a client first connects to Apollo. The will message is basically a message that will only get sent if there is an unexpected error with the connection and it must be dropped. This can be useful in situations where you have devices that could drop but when they do, you want to know. So if a medical sensor client drops from the broker, a will message could be sent to an “alarm” Topic and handled by the system as a high-priority alert.
MQTT allows a client to publish a message with the following Quality of Service parameters (QoS):
At Most Once (QoS=0)
At Least Once (QoS=1)
Exactly Once (QoS=2)
At Most Once
This QoS will attempt to deliver the message to a client, but it will have the lowest reliability of the three options. If you publish with a QoS=0, At Most Once, then the broker will not send back an Ack saying it received the message, nor will it retry if the broker fails. This QoS is most similar to non-persistent messages in, for example, JMS.
At Least Once
This QoS setting will ensure that the message is delivered at least once to clients. When publishing with this setting, Apollo will send back a PUBACK frame which acknowledges that the broker has received the message and has taken “ownership” for delivering the message. If the client that published the message with QoS=1 does not recieve the PUBACK in a specified period of time, the client may wish to re-publish the message again with the DUP flag set on the PUBLISH frame. It's possible the broker received the first attempt to publish the message and subsequently published it to listening clients. So if the PUBACK got lost somehwere and the client sends the message again, there will be no duplicate detection, and the broker will send the message again to the topic's subscribers.
This QoS is the strongest level of reliability afforded by the MQTT protocol. This assures the publisher that its message will not only get to its intended subscribers, but that the message will not be duplicated as it could with QoS=1. This QoS, however, comes with increased network overhead.
When a message is published, the broker will store the message ID and send the message to the Topic where it would be persisted if there are any durable subscriptions. It will then send the PUBREC frame back to the client implying the broker has received the message. At this point the broker will expect the client to send the PUBREL frame to clear the message ID from its session state and complete the send with the broker sending a PUBCOMP.
Wild cards can be used in destination names when subscribing as a consumer. This allows you to subscribe to multiple destinations or hierarchy of destinations.
is used to separate names in a path
is used to match any name in a path
is used to recursively match path names
For example using the above, these subscriptions are possible
: Any price for any product on any exchange
: Any price for a stock on any exchange
: Any stock price on NASDAQ
: Any IBM stock price on any exchange
Apollo will only set a keep-alive/heart-beat monitor if the client has specified a keepAlive
value in the CONNECT frame. If one is specified, the actual value used by Apollo will be 1.5 * the keep alive value. This is in keeping with the MQTT spec.
Destination Name Restrictions
Destination names are restricted to using the characters a-z
, ' ', '(', ')' or .
in addition to composite separator ,
and the wild card *
. Any other characters must be UTF-8 and then URL encoded if you wish to preserve their significance.