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Protocol 4.0 (rev. 1)

This protocol is used to communicate between the Moleculer nodes.

Concept

Subscriptions

After the client is connected to the message broker (NATS, Redis, MQTT), it subscribes to the following topics:

Type Topic name
Event MOL.EVENT.<nodeID>
Event (balanced) MOL.EVENTB.<event>
Request MOL.REQ.<nodeID>
Request (balanced) MOL.REQB.<action>
Response MOL.RES.<nodeID>
Discover MOL.DISCOVER
Discover (targeted) MOL.DISCOVER.<nodeID>
Info MOL.INFO
Info (targeted) MOL.INFO.<nodeID>
Heartbeat MOL.HEARTBEAT
Ping MOL.PING
Ping (targeted) MOL.PING.<nodeID>
Pong MOL.PONG.<nodeID>
Disconnect MOL.DISCONNECT

If namespace is defined, the topic prefix is MOL-<namespace> instead of MOL. E.g.: MOL-dev.EVENT when the namespace is dev.

Variables in topic names:

  • <namespace> - Namespace from broker options
  • <nodeID> - Target nodeID
  • <action> - Action name. E.g.: posts.find
  • <group> - Event group name. E.g.: users
  • <event> - Event name. E.g.: user.created

Discovering

After subscribing, the transporter broadcasts a DISCOVER packet. In response to this, all connected nodes send back INFO packet to the sender node. From these responses, the client builds its own service registry. At last, the client broadcasts own INFO packet to all other nodes.

Heartbeat

The client has to broadcast HEARTBEAT packets periodically. The period value comes from broker options (heartbeatInterval). The default value is 5 secs. If the client does not receive HEARTBEAT for heartbeatTimeout seconds from a node, marks it broken and doesn't route requests to this node.

Request-reply

When you call the broker.call method, the broker sends a REQUEST packet to the targeted node. It processes the request and sends back a RESPONSE packet to the requester node.

Event

When you call the broker.emit method, the broker sends an EVENT packet to the subscriber nodes. The broker groups & balances the subscribers, so only one instance per service receives the event. If you call the broker.broadcast method, the broker sends an ĘVENT packet to all subscriber nodes. It doesn't group & balance the subscribers.

Ping-pong

When you call the broker.ping method, the broker sends a PING packet to the targeted node. If node is not defined, it sends to all nodes. If the client receives the PING packet, sends back a PONG response packet. If it receives, broker broadcasts a local $node.pong event to the local services.

Disconnect

When a node is stopping, it broadcasts a DISCONNECT packet to all nodes.

Protocol messages

DISCOVER

When the transporter established the connection with the message broker, it broadcasts a DISCOVER message to all other nodes.

Topic name:

  • MOL.DISCOVER (if broadcasts)
  • MOL.DISCOVER.node-1 (if sent only to node-1)
  • MOL-dev.DISCOVER (if namespace is dev)

Fields:

Field Type Required Description
ver string Protocol version.
sender string Sender nodeID.

Example with JSON serializer

{
  "ver": "4",
  "sender": "node-100"
}

INFO

When the node receives an INFO packet, it sends an INFO packet which contains all mandatory information about the nodes and the loaded services.

Topic name:

  • MOL.INFO (if broadcasts)
  • MOL.INFO.node-1 (if sent only to node-1)
  • MOL-dev.INFO (if namespace is dev)

Fields:

Field Type Required Description
ver string Protocol version.
sender string Sender nodeID.
services object Services list. (*)
config object Client configuration. (*)
instanceID string Instance ID
ipList [string] IP address list of node
hostname string Hostname of node
client object Client information
client.type string Type of client implementation(nodejs, java, go)
client.version string Client (Moleculer) version
client.langVersion string NodeJS/Java/Go version
metadata object Node-specific metadata. (*)

(*) In case of schema-based serializers, the field value is encoded to JSON string.

Example with JSON serializer

{
  "services": [
    {
      "name": "$node",
      "settings": {},
      "metadata": {},
      "actions": [],
      "events": {}
    },
    {
      "name": "greeter",
      "settings": {},
      "metadata": {},
      "actions": [],
      "events": {}
    }
  ],
  "ipList": ["10.35.0.34"],
  "hostname": "moleculer-server",
  "client": {
    "type": "nodejs",
    "version": "0.14.0-beta3",
    "langVersion": "v12.10.0"
  },
  "config": {},
  "instanceID": "ee21e97d-9fd0-4d7e-a303-70b1605f477f",
  "metadata": {},
  "seq": 2,
  "ver": "4",
  "sender": "nodeID-1"
}

HEARTBEAT

Topic name:

  • MOL.HEARTBEAT
  • MOL-dev.HEARTBEAT (if namespace is dev)

Fields:

Field Type Required Description
ver string Protocol version.
sender string Sender nodeID.
cpu double Current CPU utilization (percentage).

Example with JSON serializer

{
  "ver": "4",
  "sender": "node-100",
  "cpu": 13.5
}

REQUEST

Topic name:

  • MOL.REQ.node-2
  • MOL.REQB.<action> (if built-in balancer is disabled)
  • MOL-dev.REQ.node-2 (if namespace is dev)

Fields:

Field Type Required Description
ver string Protocol version.
sender string Sender nodeID.
id string Context ID.
action string Action name. E.g.: posts.find
params object ctx.params object. (**)
paramsType enum Data type of ctx.params. (***)
meta object ctx.meta object. (*)
timeout double Request timeout (distributed) in milliseconds.
level int32 Level of request.
tracing boolean Need to send tracing events.
parentID string Parent context ID.
requestID string Request ID from ctx.requestID.
caller string Action name of the caller.
stream boolean Stream request.
seq int32 Stream sequence number.

(*) In case of schema-based serializers, the field value is encoded to JSON string.

(**) In case of schema-based serializers, the field value is encoded to JSON string and transferred as binary data.

(***) Used only in ProtoBuf, Avro, Thrift or any other schema-based serializer to detect the original type of data.

Example with JSON serializer

{
  "id": "41238213-da6b-4313-9909-e6edd0e40a96",
  "action": "greeter.hello",
  "params": {},
  "meta": {},
  "timeout": 10000,
  "level": 1,
  "tracing": null,
  "parentID": null,
  "requestID": "41238213-da6b-4313-9909-e6edd0e40a96",
  "caller": null,
  "stream": false,
  "ver": "4",
  "sender": "nodeID-1"
}

RESPONSE

Topic name:

  • MOL.RES.node-1
  • MOL-dev.RES.node-1 (if namespace is dev)

Fields:

Field Type Required Description
ver string Protocol version.
sender string Sender nodeID.
id string Context ID (from REQUEST).
success boolean Is it a success response?
data object Response data if success. (**)
dataType enum Data type of ctx.params. (***)
error object Error object if not success. (*)
meta object ctx.meta object. (*)
stream boolean Stream request.
seq int32 Stream sequence number.

(*) In case of schema-based serializers, the field value is encoded to JSON string.

(**) In case of schema-based serializers, the field value is encoded to JSON string and transferred as binary data.

(***) Used only in ProtoBuf, Avro, Thrift or any other schema-based serializer to detect the original type of data.

Example with JSON serializer

{
  "id": "a4dd3ae5-2eff-4924-94bc-4acb8ac034aa",
  "meta": {},
  "success": true,
  "data": {
    "message": "Hello Moleculer"
  },
  "ver": "4",
  "sender": "nodeID-2"
}

EVENT

Topic name:

  • MOL.EVENT.node-1
  • MOL.EVENTB.<group>.<event> (if built-in balancer is disabled)
  • MOL-dev.EVENT.node-1 (if namespace is dev)

Fields:

Field Type Required Description
ver string Protocol version.
sender string Sender nodeID.
id string Context ID.
event string Event name. E.g.: users.created
data object Event payload. (**)
dataType enum Data type of ctx.params. (***)
meta object ctx.meta object. (*)
level int32 Level of event.
tracing boolean Need to send tracing events.
parentID string Parent context ID.
requestID string Request ID from ctx.requestID.
caller string Action/Event name of the caller.
stream boolean Stream request.
seq int32 Stream sequence number.
groups Array<string> Groups for balanced events.
broadcast boolean Broadcast event

(*) In case of schema-based serializers, the field value is encoded to JSON string.

(**) In case of schema-based serializers, the field value is encoded to JSON string and transferred as binary data.

(***) Used only in ProtoBuf, Avro, Thrift or any other schema-based serializer to detect the original type of data.

Example with JSON serializer

{
  "id": "e102630b-c702-4ff9-a0a1-52428395d57a",
  "event": "some.test",
  "data": {
    "name": "John"
  },
  "groups": ["greeter"],
  "broadcast": false,
  "meta": {},
  "level": 1,
  "tracing": null,
  "parentID": null,
  "requestID": "e102630b-c702-4ff9-a0a1-52428395d57a",
  "caller": null,
  "needAck": null,
  "ver": "4",
  "sender": "nodeID-1"
}

EVENTACK

Not implemented yet.

Topic name:

  • MOL.EVENTACK.node-1
  • MOL-dev.EVENTACK (if namespace is dev)

Fields:

Field Type Required Description
ver string Protocol version.
sender string Sender nodeID.
id string Event Context ID.
success boolean Is it successful?
group string Group of event handler.
error object Error object if not success. (*)

(*) In case of schema-based serializers, the field value is encoded to JSON string.

Example with JSON serializer

{
  "ver": "4",
  "sender": "node-100"
}

!!TODO!!

PING

Topic name:

  • MOL.PING (if broadcasts)
  • MOL.PING.node-1 (if sent only to node-1)
  • MOL-dev.PING (if namespace is dev)

Fields:

Field Type Required Description
ver string Protocol version.
sender string Sender nodeID.
id string Message ID.
time int64 Time of sent. (*)

(*) The number of milliseconds between 1 January 1970 00:00:00 UTC and the given date.

Example with JSON serializer

{
  "time": 1567677050576,
  "id": "3e09738f-cedf-4985-85fe-344860c06cfd",
  "ver": "4",
  "sender": "nodeID-2"
}

PONG

Topic name:

  • MOL.PONG.node-1
  • MOL-dev.PONG (if namespace is dev)

Fields:

Field Type Required Description
ver string Protocol version.
sender string Sender nodeID.
id string Message ID.
time int64 Timestamp of sent. (*)
arrived int64 Timestamp of arrived. (*)

(*) The number of milliseconds between 1 January 1970 00:00:00 UTC and the given date.

Example with JSON serializer

{
  "time": 1567677050576,
  "id": "3e09738f-cedf-4985-85fe-344860c06cfd",
  "arrived": 1567677050577,
  "ver": "4",
  "sender": "nodeID-1"
}

DISCONNECT

Topic name:

  • MOL.DISCONNECT
  • MOL-dev.DISCONNECT (if namespace is dev)

Fields:

Field Type Required Description
ver string Protocol version.
sender string Sender nodeID.

Example with JSON serializer

{
  "ver": "4",
  "sender": "node-100"
}

Graceful starting & stopping

Start

The broker starts by establishing connection with the transporter (e.g., NATS server, MQTT broker). After successfully establishing the connection, it starts all services, i.e., calls service started handlers. Once all services have started successfully, broker publishes the local service list to remote nodes. Hence remote nodes will send requests only after all local service have started properly.

Start lifecycle sequence

image

Stop

When broker is stopping, it starts by publishing an empty service list to all remote nodes. This is done to inform all remote nodes that the node and it's service will be shut down. Next, broker starts stopping all local services. After that, the transporter disconnects.

Stop lifecycle sequence

image

Streams

While transferring streams, sequence number (seq) field is used to keep track of the chunks and their order. This is especially important in "multi-threaded systems" that can shuffle the packets before sending them. If packets arrive unordered they are stored in a pool until previous chucks arrive.

Disabled built-in balancer mode

When built-in load balancing mechanisms are disabled, the balancing is done by the brokers (e.g., NATS, RabbitMQ). This means that there are no local calls in actions and events. All requests are transferred to transporter that will be responsible for choosing the destination of the request and delivery of the message.

Changes from version 3

INFO

  • added instanceID: string
  • added metadata: object specific to each nodes

PING / PONG

  • added messaged ID id: string on both message types

REQUEST

  • params now is not required anymore
  • removed metrics property and added tracing: boolean whether tracing events is needed
  • new paramsType: enum needed for schema-based serializers
  • new caller: string property for action name of the caller
  • new seq: int32 to have better tracking of stream request

RESPONSE

  • new dataType: enum needed for schema-based serializers
  • new seq: int32 to have better tracking of stream request

EVENT

  • added context ID id: string in the payload
  • new dataType: enum needed for schema-based serializers
  • new meta: object that represents ctx.meta object
  • added level: int32 as level of the event
  • added tracing: boolean whether tracing events is needed
  • new caller: string property for action/event name of the caller
  • added parent context ID parentID: string
  • added requestID from context in the payload
  • handled stream requests, hence we have new properties stream: boolean whether it's a stream request and seq: int32 to have better tracking