EventMQ Protocol Specification¶
The status of this document is alpha and subject to heavy change
Goals¶
The EventMQ Protocol (eMQP) defines a reliable service-oriented request-reply and pub-sub dialog between a set of clients, a broker, and a set of workers.
The goals are to:
- Specify a protocol to follow when implementing a component to EventMQ.
- Allow requests to be routed to workers by an abstracted service name (named queues).
- Detect disconnected peers through heartbeating.
- Allow for message tracing and debugging.
License¶
This Specification is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 2.1 of the License, or (at your option) any later version.
This Specification is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.
Language¶
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119[[1](http://tools.ietf.org/html/rfc2119)].
Architecture¶
insert pretty picture here
Topology¶
eMQP connects a set of client applications (e.g. web servers), a broker, and a pool of workers. Clients connect to the broker as well as the workers.
‘Clients’ is defined as application issuing requests and ‘workers’ as applications that process these requests. (Workers consist of a JobManager and a pool of Worker resources where the job executes.)
The EventMQ broker handles a set of named queues. The broker SHOULD serve clients on a fair request and MAY deliver requests to workers on any basis, including 0MQ’s built-in round robin or least-recently used.
ROUTER Addressing¶
In the case of request-reply, the broker MUST use a ROUTER socket to accept requests from both clients and workers. The broker MAY use a seperate socket implementing a subset of eMQP, or MAY use a single socket implementing all of eMQP.
From the 0MQ manual[[2](http://api.zeromq.org/master:zmq-socket)] > When receiving messages a ROUTER socket shall prepend a message part containing the identity of the originating peer to the message before passing it to the application. When sending messages a ROUTER socket shall remove the first part of the message and use it to determine the identity of the peer the message shall be routed to.
This extra frame is not shown in the specifications below.
Global Frames¶
An ACK command consists of a 4-frame multipart message, formatted as follows.
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | ACK | command |
| 3 | _MSGID_ | A unique id for the msg |
| 4 | _MSGID_ | The message id of the message this ACK is acknowledging |
A REPLY frame consists of a 5-frame multipart message, formatted as follows.
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | REPLY | command |
| 3 | _MSGID_ | A unique id for the msg |
| 4 | _MSG_ | The reply to respond with |
A HEARTBEAT frame consists of a
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | HEARTBEAT | command |
| 3 | _MSGID_ | A unique id for the msg |
| 4 | _UNIX_TS_ | A unix timestamp |
A DISCONNECT frame consists of
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | DISCONNECT | command |
| 3 | _MSGID_ | A unique id for the msg |
A KBAI frame consists of
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | KBAI | command |
| 3 | _MSGID_ | A unique id for the msg |
eMQP / Client¶
A REQUEST command consists of a 7-frame multipart message, formatted as follows.
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | REQUEST | command |
| 3 | _MSGID_ | A unique id for the msg |
| 4 | _QUEUE_NAME_ | the name of the queue the request should be sent to |
| 5 | _HEADERS_ | dictionary of headers. can be an empty set |
| 6 | _MSG_ | The message to send |
A PUBLISH command consists of a 7-frame multipart messag, formatted as follows.
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | PUBLISH | command |
| 3 | _MSGID_ | A unique id for the msg |
| 4 | _TOPIC_NAME_ | the name of the topic this message should be published across |
| 5 | _HEADERS_ | csv list of headers |
| 6 | _MSG_ | The message to send |
A SCHEDULE command consists of a 7-frame multipart message, formatted as follows.
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | SCHEDULE | command |
| 3 | _MSGID_ | A unique id for the msg |
| 4 | _QUEUE_NAME_ | name of queue that the job should run in |
| 5 | _HEADERS_ | csv list of headers for this message |
| 6 | _MSG_ | The message to send |
An UNSCHEDULE command consists of a 7-frame multipart message, formatted as follows.
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | UNSCHEDULE | command |
| 3 | _MSGID_ | A unique id for the msg |
| 4 | _QUEUE_NAME_ | ignored for this command, broadcasted to all queues |
| 5 | _HEADERS_ | csv list of headers for this message |
| 6 | _MSG_ | The message to send |
eMQP / Scheduler¶
An INFORM command consists of a 6-frame multipart message, formatted as follows.
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | INFORM | command |
| 3 | _MSGID_ | A unique id for the msg |
| 4 | Queues. Unused for scheduler | |
| 5 | scheduler | type of peer connecting |
eMQP / Worker¶
An INFORM command consists of a 5-frame multipart message, formatted as follows.
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | INFORM | command |
| 3 | _MSGID_ | A unique id for the msg |
| 4 | _QUEUES_ | csv seperated arrays containing an int and a string for weight and name. e.g. [40, ‘email’] |
| 5 | worker | type of peer connecting |
A READY frame consists of a 4-frame multipart message, formatted as follows.
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | READY | command |
| 3 | _MSGID_ | A unique id for the msg |
eMQP / Publisher¶
A PUBLISH frame consists of a 6-frame multipart message, formatted as follows.
| FRAME | Value | Description |
|---|---|---|
| 0 | _EMPTY_ | leave empty |
| 1 | eMQP/1.0 | Protocol version |
| 2 | PUBLISH | command |
| 3 | _MSGID_ | A unique id for the msg |
| 4 | TOPIC | A topic to publish to |
| 5 | MSG | Message to be published |
Heartbeating¶
- HEARTBEAT commands are valid at any time after an INFORM command.
- Any command except DISCONNECT act as a heartbeat. Peers SHOULD NOT send HEARTBEAT commands while sending other commands.
- Worker and broker MUST send heartbeats at regular and agreed-upon intervals.
- Scheduler and broker MUST send heartbeats at regular and agreed-upon intervals.
- If the worker detects that the broker disconnected it SHOULD restart the conversation.
- If the broker detects that a worker has disconnected it should stop sending it a message of any type.
- If the scheduler detects that the broker disconnects it SHOULD restart the conversation.
Headers¶
Headers MUST be 0 to many comma seperated values inserted into the header field. If there are no headers required, an empty string MUST be sent where headers are required.
Below is a table which defines and describes the headers.
| Header | REQUEST | PUBLISH | SCHEDULE | Default | Description |
|---|---|---|---|---|---|
| reply-requested | X | False | Once the job is finished, send a reply back with information from the job. If there is no information reply with a True value. | ||
| retry-count:# | X | 0 | Retry a failed job this many times before accepting defeat. | ||
| timeout:# | X | 0 | Kill the job after X seconds, defaults to never timing out (0) | ||
| guarantee | X | False | Ensure the job completes by letting someone else worry about a success reply. | ||
| nohaste | X | False | When scheduling a job, set this to True if you don’t want the job to run immediately as it’s scheduled. Instead, it will run for the first time when the interval has elapsed. |
DISCONNECT and KBAI¶
- When a component receives a DISCONNECT command it:
- MUST send a KBAI command to all connected components.
- MUST stop sending and receiving any messages
- MUST allow any pending messages or jobs to complete.
- When a component receives a KBAI command it:
- MUST stop sending any messages to the disconnecting component.
- SHOULD Clean up references to the disconnecting component.