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Back-end Developer Challenge: Follower Maze

Thanks for trying our development challenge!

With this document you should have received two other files:

  • followermaze.sh, an executable bash script
  • FollowerMaze-assembly-2.0.jar, a JAR file to be executed on a JDK 7 JVM

If you haven't received any of these, or if you think there are any problems with the files, please contact us immediately and we will re-send you the missing pieces.

IMPORTANT: we have anonymised code reviews so please ensure you omit any personal details from your challenge response which note your gender, age, ethnicity, etc.

The Challenge

The challenge proposed here is to build a system which acts as a socket server, reading events from an event source and forwarding them when appropriate to user clients.

Clients will connect through TCP and use the simple protocol described in a section below. There will be two types of clients connecting to your server:

  • One event source: It will send you a stream of events which may or may not require clients to be notified
  • Many user clients: Each one representing a specific user, these wait for notifications for events which would be relevant to the user they represent

The Protocol

The protocol used by the clients is string-based (i.e. a CRLF control character terminates each message). All strings are encoded in UTF-8.

The event source connects on port 9090 and will start sending events as soon as the connection is accepted.

The many user clients will connect on port 9099. As soon as the connection is accepted, they will send to the server the ID of the represented user, so that the server knows which events to inform them of. For example, once connected a user client may send down: 2932\n, indicating that they are representing user 2932.

After the identification is sent, the user client starts waiting for events to be sent to them. Events coming from event source should be sent to relevant user clients exactly like read, no modification is required or allowed.

The Events

There are five possible events. The table below describe payloads sent by the event source and what they represent:

Payload Sequence # Type From User Id To User Id
666|F|60|50 666 Follow 60 50
1|U|12|9 1 Unfollow 12 9
542532|B 542532 Broadcast - -
43|P|32|56 43 Private Msg 32 56
634|S|32 634 Status Update 32 -

Using the verification program supplied, you will receive exactly 10000000 events, with sequence number from 1 to 10000000. The events will arrive out of order.

Note: Please do not assume that your code would only handle a finite sequence of events, we expect your server to handle an arbitrarily large events stream (i.e. you would not be able to keep all events in memory or any other storage)

Events may generate notifications for user clients. **If there is a user client ** connected for them, these are the users to be informed for different event types:

  • Follow: Only the To User Id should be notified
  • Unfollow: No clients should be notified
  • Broadcast: All connected user clients should be notified
  • Private Message: Only the To User Id should be notified
  • Status Update: All current followers of the From User ID should be notified

If there are no user client connected for a user, any notifications for them must be silently ignored. user clients expect to be notified of events in the correct order, regardless of the order in which the event source sent them.

The Configuration

During development, it is possible to modify the test program behavior using the following environment variables:

  1. logLevel - Default: info

    Modify to "debug" to print debug messages.

  2. eventListenerPort - Default: 9090

    The port used by the event source.

  3. clientListenerPort - Default: 9099

    The port used to register clients.

  4. totalEvents - Default: 10000000

    Number of messages to send.

  5. concurrencyLevel - Default: 100

    Number of conected users.

  6. numberOfUsers Default: concurrencyLevel * 10

    Total number of users (connected or not)

  7. randomSeed - Default: 666

    The seed to generate random values

  8. timeout - Default: 20000

    Timeout in milliseconds for clients while waiting for new messages

  9. maxEventSourceBatchSize - Default: 100

    The event source flushes messages in random batch sizes and ramdomize the messages order for each batch. For example, if this configuration is "1" the event source will send only ordered messages flushing the connection for each message.

  10. logInterval - Default: 1000

The interval in milliseconds used to log the sent messages counter.

Your Solution

We expect you to send us the source code of a fully functional server for the proposed challenge using the default configurations. You still might want to stress-test your code with different configuration parameters to make sure it is not too tailored to our test-suite, and is generic enough.

Your code should build and run on a Mac or GNU/Linux machine running a recent OS release.

As a non-exhaustive example, we have received successful applications developed on: Node.js, Ruby, JRuby, Haskell, Clojure, Scala, Go, Python, Java, and C/C++.

If your language does not provide what you need in its standard library, you may use 3rd party libraries. Our goal is to review your work and understand your design choices, so please include a description of the 3rd party libraries used in your solution and a short rationale explaining your choices.

Before submitting your code

With this document you received a jar file and a shell script. These contain one possible implementation of the event source and user client described previously.

IMPORTANT: we have anonymised code reviews so please ensure you omit any personal details from your challenge response which note your gender, age, ethnicity, etc.

We expect you to make sure that your solution works with the supplied clients before sending it to us. The first thing we will do with your code is to run it agains these clients, so you can have very early feedback by treating it as a test suite.

To run the clients, first make sure you have the server you wrote running and listening to ports 9090 and 9099, then run:

$ ./followermaze.sh

This will start the clients, which will immediately start sending message to your server. You know it finished without errors when it outputs:

 [INFO] ==================================
 [INFO] \o/ ALL NOTIFICATIONS RECEIVED \o/
 [INFO] ==================================

Assesment Criteria

We expect you to write code you would consider production-ready. This means we want your code to be well-factored, without needless duplication, follow good practices and be automatically verified.

What we will look at:

  • If your code fulfils the requirement, and runs against the supplied example server
  • How clean is your design and implementation, how easy it is to understand and maintain your code
  • How you verified your software, if by automated tests or some other way
  • What kind of documentation you ship with your code