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Protocol v3 (rev. 2)

This documentation describes the communication protocol between Moleculer nodes.

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

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 (targetted) MOL.DISCOVER.<nodeID>
Info MOL.INFO
Info (targetted) MOL.INFO.<nodeID>
Heartbeat MOL.HEARTBEAT
Ping MOL.PING
Ping (targetted) MOL.PING.<nodeID>
Pong MOL.PONG.<nodeID>
Disconnect MOL.DISCONNECT

If namespace is defined, the topic prefix is MOL-namespace instead of MOL. For example: MOL-dev.EVENT if the namespace is dev.

Discovering

After subscriptions, the client 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 targetted 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 targetted 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.

Packets

DISCOVER

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. Current: '3'.
sender string ✔ Sender nodeID.

INFO

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. Current: '3'.
sender string ✔ Sender nodeID.
services object ✔ Services list. (*)
config object ✔ Client configuration. (*)
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

(*) In case of ProtoBuf, Avro or any other schema-based serializer, the field value is encoded to JSON string.

HEARTBEAT

Topic name:

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

Fields:

Field Type Required Description
ver string ✔ Protocol version. Current: '3'.
sender string ✔ Sender nodeID.
cpu double ✔ Current CPU utilization (percentage).

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. Current: '3'.
sender string ✔ Sender nodeID.
id string ✔ Context ID.
action string ✔ Action name. E.g.: posts.find
params object ✔ ctx.params object. (*)
meta object ✔ ctx.meta object. (*)
timeout double ✔ Request timeout (distributed) in milliseconds.
level int32 ✔ Level of request.
metrics boolean ✔ Need to send metrics events.
parentID string Parent context ID.
requestID string Request ID from ctx.requestID.
stream boolean ✔ Stream request.

(*) In case of ProtoBuf, Avro or any other schema-based serializer, the field value is encoded to JSON string.

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. Current: '3'.
sender string ✔ Sender nodeID.
id string ✔ Context ID (from REQUEST).
success boolean ✔ Is it a success response?
data object Response data if success. (*)
error object Error object if not success. (*)
meta object ✔ ctx.meta object. (*)
stream boolean ✔ Stream request.

(*) In case of ProtoBuf, Avro or any other schema-based serializer, the field value is encoded to JSON string.

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. Current: '3'.
sender string ✔ Sender nodeID.
event string ✔ Event name. E.g.: users.created
data object Event payload. (*)
groups Array<string> Groups for balanced events.
broadcast boolean ✔ Broadcast event

(*) In case of ProtoBuf, Avro or any other schema-based serializer, the field value is encoded to JSON string.

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. Current: '3'.
sender string ✔ Sender nodeID.
time int64 ✔ Time of sent. (*)

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

PONG

Topic name:

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

Fields:

Field Type Required Description
ver string ✔ Protocol version. Current: '3'.
sender string ✔ Sender nodeID.
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.

DISCONNECT

Topic name:

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

Fields:

Field Type Required Description
ver string ✔ Protocol version. Current: '3'.
sender string ✔ Sender nodeID.
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