Skeleton of communication description

Communication.md

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+# 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.
+
+## Internal worker communication
+
+## Broker - Worker communication
+
+When a worker is started, it registers itself with the broker by sending the 
+`init` command followed by its hardware group and headers that describe its 
+capabilities (such as the number of threads it can run simultaneously, 
+languages it can work with...). The headers are expected to be in following 
+format: `header_name=value`. Every header shall be in a separate frame.
+
+Whenever the broker receives an assignment suitable for the worker, it just 
+forwards the evaluation request message it originally received from the 
+frontend. The worker has to:
+
+- Download the archive containing the submission and an isoeval configuration 
+  file
+- Download any supplementary files based on the configuration file, such as test 
+  inputs or helper programs (This can be done on demand, using a `fetch` command
+  in the assignment configuration)
+- Download the source codes of the student's submission
+- Evaluate the submission according to the assignment's configuration
+- Upload the results of the evaluation to the file server
+- Notify the broker that the evaluation is finished
+
+Thanks to this message structure, it's possible to cache the configuration file 
+and only download the student's submissions when the same assignment is 
+evaluated repeatedly for different students (a common case for homeworks and 
+classroom assignments).
+
+After finishing the evaluation, worker notifies the broker of this fact by 
+sending:
+
+- The `done` command
+- The job id
+
+This allows the broker to reliably distribute messages - if a worker doesn't 
+succeed in processing a request (it doesn't respond in a time limit), the 
+request can be sent to another worker.
+
+## Worker - File Server communication
+
+## Broker - Monitor communication
+
+## Broker - Frontend communication
+
+The communication between the frontend and the workers is mediated by a broker 
+that passes jobs to workers capable of processing them. 
+
+### Assignment evaluation request
+
+The frontend must send a multipart message that contains the following frames:
+
+- The `eval` command
+- The job id (in ASCII representation -- we avoid endianness issues and also 
+  support alphabetic ids)
+- A frame for each header (e.g. `hwgroup=group_1`)
+- An URL of the archive that contains the submitted files and isoeval 
+  configuration
+- An URL where the worker should store the result of the evaluation
+
+If the broker is capable of routing the request to a worker, it responds with 
+`accept`. Otherwise (for example when the requirements specified by the headers 
+cannot be met), it responds with `reject`.
+
+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.
+
+### Notifying the frontend about evaluation progress
+
+The script that requested the evaluation will have exited by the time a worker 
+processes the request. This issue remains to be resolved.
+
+## File Server - Frontend communication
+
+## Monitor - Browser communication
+
+## Frontend - Browser communication