Monitor is part of ReCodEx solution for reporting progress of job evaluation back to user in the real time. It gets progress notifications from broker and sends them through WebSockets to clients' browsers. For now, it is meant as an optional part of whole solution, but for full experince it is recommended to use one.
Monitor is needed one per broker, that is one per separate ReCodEx instance. Also, monitor has to be publicly visible (has to have public IP address or be behind public proxy server) and also needs a connection to the broker. If the web application is using HTTPS, it is required to use a proxy for monitor to provide encryption over WebSockets. If this is not done, browsers of the users will block unencrypted connection and will not show the progress to the users.
Monitor is written in Python, tested versions are 3.4 and 3.5. This language was chosen because it is already in project requirements (fileserver) and there are great libraries for ZeroMQ, WebSockets and asynchronous operations. This library saves system resources and provides us great amount of processed messages. Also, coding in Python was pretty simple and saves us time for improving the other parts of ReCodEx.
Monitor runs in 2 threads. _Thread 1_ is the main thread, which initializes all components (logger for example), starts the other thread and runs the ZeroMQ part of the application. This thread receives and parses incomming messages from broker and forwards them to _thread 2_ sending logic.
_Thread 2_ is responsible for managing all of WebSocket connections asynchronously. Whole thread is one big _asyncio_ event loop through which all actions are processed. None of custom data types in Python are thread-safe, so all events from other threads (actually only `send_message` method invocation) must be called within the event loop (via `asyncio.loop.call_soon_threadsafe` function). Please note, that most of the Python interpreters use [Global Interpreter Lock](https://wiki.python.org/moin/GlobalInterpreterLock), so there is actualy no parallelism in the performance point of view, but proper synchronization is still required!
Incomming ZeroMQ progress message is received and parsed to JSON format (same as our WebSocket communication format). JSON string is then passed to _thread 2_ for asynchronous sending. Each message has an identifier of channel where to send it to.
There can be multiple receivers to one channel id. Each one has separate _asyncio.Queue_ instance where new messages are added. In addition to that, there is one list of all messages per channel. If a client connects a bit later than the point when monitor starts to receive messages, it will receive all messages from the beginning. Messages are stored 5 minutes after last progress command (normally FINISHED) is received, then are permanently deleted.
Messages from client's queue are sent through corresponding WebSocket connection via main event loop as soon as possible. This approach with separate queue per connection is easy to implement and guarantees reliability and order of message delivery.
