Back-end Developer Challenge: Follower Maze

This Solution

I used node.js to implement this solution. I didn't use any external libraries for the server code. The testing code uses Mocha and Istanbul (for test coverage).

To Run

This solution requires node.js. I used node version v0.12.0 with npm version 2.1.4.

• Clone my repository
• From console at repository root, type node lib/main.js
• To run tests, from console at repository root type npm test

Thank you!

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\r\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
1	U	12	9	1
542532	B	542532	Broadcast	-
43	P	32	56	43
634	S	32	634	Status Update

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.

The challenge was designed so that a candidate can implement the solution using just the standard library of most programming languages and environments (i.e. no external gems/JARs/libs/modules/etc.). Feel free to use your preferred build and testing libraries, but the production code should have very minimal dependencies on third-party code, preferably none at all.

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 you absolutely think you need some sort of third-party library, please write a paragraph to help us better understand your choice.

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.

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