recodex-wiki/Communication.md

# Communication

This section gives detailed overview about communication in ReCodEx solution. Basic concept is captured on following image:

![Communication Img](https://github.com/ReCodEx/GlobalWiki/raw/master/images/Backend_Connections.png)

Red connections are through ZeroMQ sockets, Blue are through WebSockets and Green are through HTTP. All ZeroMQ messages are sent as multipart with one string (command, option) per part, with no empty frames (unles explicitly specified otherwise).


## Internal worker communication

Communication between the two worker threads is split into two separate parts, 
each one holding dedicated connection line. These internal lines are realized by 
ZeroMQ inproc PAIR sockets. In this section we assume that the thread of the 
worker which communicates with broker is called _listening thread_ and the other 
one, which is evaluating incoming jobs is called _execution thread_. _Listening 
thread_ is a server in both cases (the one who calls the `bind()` method), but 
because of how ZeroMQ works, it's not very important (`connect()` call in 
clients can precede server `bind()` call with no issue).

### Main communication

Main communication is on `inproc://jobs` sockets. _Listening thread_ is waiting 
for any messages (from broker, jobs and progress sockets) and passes incoming 
requests to the _execution thread_, which handles them properly.

Commands from _listening thread_ to _execution thread_:

- **eval** - evaluate a job. Requires 3 message frames:
    - `job_id` - identifier of this job (in ASCII representation -- we avoid endianness issues and also 
  support alphabetic ids)
    - `job_url` - URI location of archive with job configuration and submitted source code
    - `result_url` - remote URI where results will be pushed to

Commands from _execution thread_ to _listening thread_:

- **done** - notifying of finished job. Requires 2 message frames:
    - `job_id` - identifier of finished job
    - `result` - response result, one of "OK" and "ERR"

Note that we will need to store the job ID and the assignment configuration 
somewhere close to the submitted files so it's possible to check how a 
submission was evaluated. The job ID will likely be a part of the submission's 
path. The configuration could be linked there under some well-known name.


### Progress callback

Progress messages are sent through `inproc://progress` sockets. This is only one way communication from _execution thread_ to the _listening thread_.

Commands:

- **progress** - notice about evaluation progress. Requires 2 or 4 arguments:
    - `job_id` - identifier of current job
    - `state` - what is happening now. One of "DOWNLOADED" (submission successfuly fetched), "UPLOADED" (results are uploaded to fileserver), "STARTED" (evaluation started), "ENDED" (evaluation is finnished) and "TASK" (task state changed - see below)
    - `task_id` - only present for "TASK" state - identifier of task in current job
    - `task_state` - only present for "TASK" state - result of task evaluation. One of "COMPLETED" and "FAILED".


## Broker - Worker communication

Broker is server when comminicating with worker. IP address and port are configurable, protocol is TCP. Worker socket is DEALER, broker one is ROUTER type. Because of that, very first part of every (multipart) message from broker to worker must be target worker's socket identity (which is saved on it's **init** command).

Commands from broker to worker:

- **eval** - evaluate a job. See **eval** command in [[Communication#main-communication]]
- **intro** - introduce yourself to the broker (with **init** command) - this is 
  required when the broker loses track of the worker who sent the command. 
  Possible reasons for such event are e.g. that one of the communicating sides 
  shut down and restarted whithout the other side noticing.
- **pong** - reply to **ping** command, no arguments

Commands from worker to broker:

- **init** - introduce yourself to the broker. Useful on startup or after reestablishing lost connection. Requires at least 2 arguments:
    - `hwgroup` - hardware group of this worker
    - `header` - additional header describing worker capabilities. Format must be `header_name=value`, every header shall be in a separate message frame. There is no maximum limit on number of headers.
- **done** - job evaluation finished, see **done** command in [[Communication#main-communication]].
- **progress** - evaluation progress report, see **progress** command in [[Communication#progress-callback]]
- **ping** - tell broker I'm alive, no arguments

### Heartbeating

It is important for the broker and workers to know if the other side is still 
working (and connected). This is achieved with a simple heartbeating protocol.

The protocol requires the workers to send a **ping** command regularly (the 
interval is configurable on both sides - future releases might let the worker 
send its ping interval with the **init** command). Upon receiving a **ping** 
command, the broker responds with **pong**.

Both sides keep track of missing heartbeating messages since the last one was 
received. When this number reaches a threshold (called maximum liveness), the 
other side is considered dead.

When the broker decides a worker died, it tries to reschedule its jobs to other 
workers.

If a worker thinks the broker is dead, it tries to reconnect with a bounded,
exponentially increasing delay.

## Worker - File Server communication

**TODO:**

## Broker - Monitor communication

Broker communicates with monitor also through ZeroMQ over TCP protocol. Type of 
socket is same on both sides, ROUTER. Monitor is set as server in this 
communication, its IP address and port are configurable in monitor's config 
file. ZeroMQ socket ID (set on monitor's side) is "recodex-monitor" and must be 
sent as first frame of every multipart message - see ZeroMQ ROUTER socket 
documentation for more info. 

Note that the monitor is designed so that it can receive data both from the 
broker and workers. The current architecture prefers the broker to do all the 
communication so that the workers don't have to know too many network services.

Monitor is treated as a somewhat optional part of whole solution, so no special 
effort on communication realibility was made.

Commands from monitor to broker:

Because there is no need for the monitor to communicate with the broker, there 
are no commands so far. Any message from monitor to broker is logged and 
discarded.

Commands from broker to monitor:

- **progress** - notification about progress with job evaluation. See [[Communication#progress-callback]] for more info.


## Broker - Frontend communication

Broker communicates with frontend through ZeroMQ connection over TCP. Socket 
type on broker side is ROUTER, on frontend part it's REQ. Broker acts as a 
server, its IP address and port is configurable in frontend.

Commands from frontend to broker:

- **eval** - evaluate a job. Requires at least 4 frames:
    - `job_id` - identifier of this job (in ASCII representation -- we avoid endianness issues and also 
  support alphabetic ids)
    - `header` - additional header describing worker capabilities. Format must be `header_name=value`, every header shall be in a separate message frame. There is no maximum limit on number of headers. There may be also no headers at all.
    - empty frame (with empty string)
    - `job_url` - URI location of archive with job configuration and submitted source code
    - `result_url` - remote URI where results will be pushed to

Commands from broker to frontend (all are responses to **eval** command):

- **accept** - broker is capable of routing request to a worker
- **reject** - broker can't handle this job (for example when the requirements 
  specified by the headers cannot be met). There are (rare) cases when the 
  broker finds that it cannot handle the job after it's been confirmed. In such 
  cases it uses the frontend REST API to mark the job as failed.


## File Server - Frontend communication

**TODO:**

## Monitor - Browser communication

Monitor interacts with browser through WebSocket connection. Monitor acts as server and browsers are connecting to it. IP address and port are also configurable. When client connects to the monitor, it sends a message with string representation of channel id (which messages are interested in, usually id of evaluating job). There can be at most one listener per channel, latter connection replaces previous one. After establishing the connection, the message "Connection established" is sent from monitor to browser.

When monitor receives "progress" message from broker there are two options:

- there is no WebSocket connection for listed channel (job id) - message is dropped
- there is active WebSocket connection for listed channel - message is parsed into JSON format (see below) and send as string to browser. Messages for active connections are queued, so no messages are discarded even on heavy workload.

Message JSON format is dictionary with keys:

- **command** - type of progress. One of "STARTED" (evaluation started), "DOWNLOADED" (submission source downloaded), "TASK" (progress on one of the tasks), "UPLOADED" (results are uploaded), "ENDED" (evaluation ended)
- **task_id** - id of currently evaluated task. Present only if **command** is "TASK".
- **task_state** - state of task with id **task_id**. Present only if **command** is "TASK". Value is one of "COMPLETED" and "FAILED".


## Frontend - Browser communication

**TODO:**