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@ -11,51 +11,44 @@ Design Guide
Architecture Architecture
------------ ------------
Concept Model The emqttd broker 1.0 is more like a network Switch or Router, not a traditional enterprise message queue. Compared to a network router that routes packets based on IP or MPLS label, the emqttd broker routes MQTT messages based on topic trie.
-------------
The emqttd broker 1.0 is more like a network Switch or Router, not traditional enterprise message queue. Compared to a network router that routes packets based on IP or MPLS label, the emqttd broker routes MQTT messages based on a topic trie.
.. image:: _static/images/concept.png .. image:: _static/images/concept.png
Design Philosophy Design Philosophy
----------------- -----------------
1. Focus on handling millions of MQTT connections and route MQTT messages between clustered nodes. 1. Focus on handling millions of MQTT connections and routing MQTT messages between clustered nodes.
2. Embrace Erlang/OTP, The Soft-Realtime, Low-Latency, Concurrent and Fault-Tolerant platform. 2. Embrace Erlang/OTP, The Soft-Realtime, Low-Latency, Concurrent and Fault-Tolerant Platform.
3. Connection, Session, PubSub, Router and Distributed Layers. 3. Layered Design: Connection, Session, PubSub and Router Layers.
4. Seperate the Message Flow Plane and Control/Management Plane. 4. Separate the Message Flow Plane and the Control/Management Plane.
5. Stream out the MQTT messages to various backends. 5. Stream MQTT messages to various backends including MQ or databases.
System Layers System Layers
------------- -------------
1. Connection Layer:
Handle TCP and WebSocket connections, encode/decode MQTT packets.
2. Session Layer:
Process MQTT PUBLISH/SUBSCRIBE Packets received from client, and deliver MQTT messages to client.
3. PubSub Layer:
Dispatch MQTT messages to subscribers in a node.
4. Routing(Distributed) Layer:
Route MQTT messages between clustered nodes.
.. code:: .. code::
-------------- ----------- ---------- ---------- -------------- ----------- ---------- ----------
Client --> | Connection | --> | Session | --> | PubSub | --> | Router | Client --> | Connection | --> | Session | --> | PubSub | --> | Router |
-------------- ----------- ---------- ---------- -------------- ----------- ---------- ----------
1. Connection Layer:
Handle TCP and WebSocket connections, encode/decode MQTT packets.
2. Session Layer:
Process MQTT PUBLISH/SUBSCRIBE Packets recevied from client, and deliver MQTT messages to client.
3. PubSub Layer:
Route and dispatch MQTT messages to subscribers in a node
4. Route(Distributed) Layer:
Route MQTT messages between nodes in a cluster
---------------- ----------------
Connection Layer Connection Layer
---------------- ----------------
@ -96,7 +89,7 @@ Main modules of this layer:
Session Layer Session Layer
------------- -------------
The session layer processes MQTT PUBLISH/SUBSCRIBE packets received from client, and deliver PUBLISH packets to client. The session layer processes MQTT packets received from client, and deliver PUBLISH packets to client.
A MQTT session will store the subscriptions and inflight messages in memory: A MQTT session will store the subscriptions and inflight messages in memory:
@ -105,14 +98,12 @@ A MQTT session will store the subscriptions and inflight messages in memory:
2. Inflight qos1/2 messages sent to the client but unacked, QoS 2 messages which 2. Inflight qos1/2 messages sent to the client but unacked, QoS 2 messages which
have been sent to the Client, but have not been completely acknowledged. have been sent to the Client, but have not been completely acknowledged.
3. Inflight qos2 messages received from client and waiting for pubrel. QoS 2 3. Inflight qos2 messages received from client and waiting for PUBREL. QoS 2
messages which have been received from the Client, but have not been messages which have been received from the Client, but have not been
completely acknowledged. completely acknowledged.
4. All qos1, qos2 messages published to when client has been disconnected. 4. All qos1, qos2 messages published to when client has been disconnected.
Main module of this layer is emqttd_session.
MQueue and Inflight Window MQueue and Inflight Window
-------------------------- --------------------------
@ -124,7 +115,7 @@ IN -> | Messages Queue | Inflight Window | -> Out
----------------------------------------------- -----------------------------------------------
|<--- Win Size --->| |<--- Win Size --->|
1. Inflight Window to store the messages delivered and awaiting for puback. 1. Inflight Window to store the messages delivered and awaiting for PUBACK.
2. Enqueue messages when the inflight window is full. 2. Enqueue messages when the inflight window is full.
@ -136,9 +127,9 @@ The larger the inflight window size, the higher the throughput. The smaller the
PacketId and MessageId PacketId and MessageId
---------------------- ----------------------
The 16bits PacketId is defined by MQTT Protocol Specification, used by client/server to PUBLISH/PUBACK packets. A GUID(128bits global unique Id) will be generated by the borker and assigend to a MQTT message. The 16bits PacketId is defined by MQTT Protocol Specification, used by client/server to PUBLISH/PUBACK packets. A GUID(128bits global unique Id) will be generated by the broker and assigned to a MQTT message.
Format of the global unique messsage id:: Format of the global unique message id::
+------------------------+----------------+------------+ +------------------------+----------------+------------+
| Timestamp | NodeID + PID | Sequence | | Timestamp | NodeID + PID | Sequence |
@ -186,7 +177,7 @@ For example, if node1 subscribed 't/+/x' and 't/+/y', node2 subscribed 't/#' and
| t/a -> node3 | | t/a -> node3 |
------------------------- -------------------------
The routing layer would route MQTT messages between clustered nodes by topic trie match and routing table lookup, and follow the ruels below: The routing layer would route MQTT messages between clustered nodes by topic trie match and routing table lookup, and follow the rules below:
1. A message only gets forwarded to other cluster nodes if a cluster node is interested in it. This reduces the network traffic tremendously, because it prevents nodes from forwarding unnecessary messages. 1. A message only gets forwarded to other cluster nodes if a cluster node is interested in it. This reduces the network traffic tremendously, because it prevents nodes from forwarding unnecessary messages.
@ -206,10 +197,10 @@ emqttd_access_control module provides two APIs that help register/unregister aut
register_mod(auth | acl, atom(), list(), non_neg_integer()) -> ok | {error, any()}. register_mod(auth | acl, atom(), list(), non_neg_integer()) -> ok | {error, any()}.
Authentication Bahavihour Authentication Bahaviour
------------------------- -------------------------
The emqttd_auth_mod defines an Erlang behavihour for authentication module:: The emqttd_auth_mod defines an Erlang behaviour for authentication module::
-module(emqttd_auth_mod). -module(emqttd_auth_mod).
@ -337,7 +328,7 @@ Hooks defined by the emqttd 1.0 broker:
+------------------------+------------------------------------------------------+ +------------------------+------------------------------------------------------+
| message.acked | Run when a MQTT message is acked | | message.acked | Run when a MQTT message is acked |
+------------------------+------------------------------------------------------+ +------------------------+------------------------------------------------------+
| client.disconnected | Run when client disconnnected from broker | | client.disconnected | Run when client disconnected from broker |
+------------------------+------------------------------------------------------+ +------------------------+------------------------------------------------------+
The emqttd broker uses the `Chain-of-responsibility_pattern`_ to implement hook mechanism. The callback functions registered to hook will be executed one bye one:: The emqttd broker uses the `Chain-of-responsibility_pattern`_ to implement hook mechanism. The callback functions registered to hook will be executed one bye one::