Using Node.js, implement a simple HTTP API service for tracking user activity for a mobile app.

The basic mechanics for user activity are already handled by the app:

• Each user has its own globally unique user identifier assigned by the app;

• Every time a user opens the app on his device, a new user session is started;

• Each new session is assigned a session identifier (generated by the app) when it starts;

• Each session identifier is guaranteed to be unique for that user, but may not be unique across different users.

Immediately after a session is started and its identifier generated, the app sends an "activity" event (i.e. an HTTP request)

to the API you are implementing through the following endpoint:

POST /activity

• The request body is a JSON-encoded object with the following keys:

  • user_id (string): the globally unique identifier of the user that opened the app;

  • session_id (string): the identifier of the user's new session;

Your API should also be able to calculate some meaningful statistics for a specified date range:

• the number of unique users seen in that time period. Unique means if a user opens the app more than once in that time period, it is only counted once.

• the number of sessions seen in that time period;

• the average number of sessions per user in time period.

The endpoint for this is:

GET /stats

• Requests must include the start_date (inclusive) and end_date (inclusive) query parameters to filter the data:

  • dates are specified in the YYYY-MM-DD format (e.g. January 23rd 2016 is written as 2016-01-23);

• Requests may also include an optional user_id query parameter to filter the response to a single user identifier;

• The response body should be a JSON-encoded string with the following format:

  • unique_users (integer): number of unique users in the given time period;

  • num_sessions (integer): number of sessions in the given time period (or all time if no period is passed);

Key requirements:

• Your code should start an HTTP web server that exposes the two endpoints described above and handles the requests accordingly;

• Your code must be written in either JavaScript or CoffeeScript and should compile and run on a standard Node.js environment;

• This API must be built without using any pre-existing database management systems:

  • That means no SQL, SQLite, MongoDB, Elastic, Redis, Memcache, etc.;

• Your system should be fault-tolerant:

  • If the process is killed it can be restarted with no (or minimal) loss of data;

• Your system should be able to store and query the user activity events for the next 5 years;

• The mobile app has about 1 million active daily users, that number is not expected to change in the next 5 years;

• Each user opens the app twice a day on average, generating two new sessions and the corresponding activity events;

Example requests (notice the timestamps):

[2015-10-04T16:33:44] POST /activity { user_id: 4, session_id: 1 } -> 200 OK

[2015-10-05T02:42:18] POST /activity { user_id: 4, session_id: 2 } -> 200 OK

[2015-10-05T02:48:18] POST /activity { user_id: 4, session_id: 2 } -> 200 OK

[2015-10-05T10:37:29] POST /activity { user_id: 4, session_id: 3 } -> 200 OK

[2015-10-05T19:03:14] POST /activity { user_id: 5, session_id: 1 } -> 200 OK

[2015-10-06T23:33:20] POST /activity { user_id: 6, session_id: 1 } -> 200 OK

[2015-10-07T01:24:52] POST /activity { user_id: 4, session_id: 6 } -> 200 OK

[2015-10-08T00:00:01] GET /stats?start_date=2015-10-05&end_date=2015-10-06 ->

  {

    num_sessions: 4,

    unique_users: 3,

    avg_sessions_per_user: 1.33

  }

[2015-10-09T00:00:01] GET /stats?start_date=2015-01-01&end_date=2015-12-31 ->

  {

    num_sessions: 6,

    unique_users: 3,

    avg_sessions_per_user: 2

  }