Coding challenge

The main use case for our API is to calculate real time statistic from the last 60 seconds. There will be two APIs, one of them is called every time a transaction is made. It is also the sole input of this rest API. The other one returns the statistic based of the transactions of the last 60.

Transactions

Every Time a new transaction happened, this endpoint will be called.
Where: amount is double specifying transaction amount timestamp is epoch in millis in UTC time zone specifying transaction time

• Request sample:

POST /api/transactions HTTP/1.1
Content-Type: application/json

{
    "amount": 12.3,
    "timestamp": 1478192204000
}

• Success response sample:

HTTP/1.1 201 Created

• Error response sample - timestamp is older than 60 seconds

HTTP/1.1 204 No Content

Statistics

This is the main endpoint of this task, this endpoint have to execute in constant time and memory (O(1)). It returns the statistic based on the transactions which happened in the last 60 seconds.

• Request sample:

GET /api/statistics HTTP/1.1
Accept: application/json

• Response sample:

HTTP/1.1 200 OK
Content-Type: application/json

{
    "sum": 1000,
    "avg": 100,
    "max": 200,
    "min": 50,
    "count": 10
}

Where: sum is a double specifying the total sum of transaction value in the last 60 seconds avg is a double specifying the average amount of transaction value in the last 60 seconds max is a double specifying single highest transaction value in the last 60 seconds min is a double specifying single lowest transaction value in the last 60 seconds * count is a long specifying the total number of transactions happened in the last 60 seconds

Requirements:

For the rest api, the biggest and maybe hardest requirement is to make the GET /statistics execute in constant time and space. The best solution would be O(1). It is very recommended to tackle the O(1) requirement as the last thing to do as it is not the only thing which will be rated in the code challenge.

Other requirements, which are obvious, but also listed here explicitly: The API have to be threadsafe with concurrent requests The API have to function properly, with proper result The project should be buildable, and tests should also complete successfully. e.g. If maven is used, then mvn clean install should complete successfully. The API should be able to deal with time discrepancy, which means, at any point of time, we could receive a transaction which have a timestamp of the past Make sure to send the case in memory solution without database (including in-memory database) Endpoints have to execute in constant time and memory (O(1))