Transactions statistics

We would like to have a RESTful API for our statistics. The main use case for the API is to calculate realtime statistics for the last 60 seconds of transactions.

The API needs the following endpoints:

POST /transactions – called every time a transaction is made. It is also the sole input of this rest API.
GET /statistics – returns the statistic based of the transactions of the last 60 seconds.
DELETE /transactions – deletes all transactions.

Specs

`POST /transactions`

This endpoint is called to create a new transaction.

Body:

{
  "amount": "12.3343",
  "timestamp": "2018-07-17T09:59:51.312Z"
}

Where:

amount – transaction amount; a string of arbitrary length that is parsable as a BigDecimal
timestamp – transaction time in the ISO 8601 format YYYY-MM-DDThh:mm:ss.sssZ in the UTC timezone (this is not the current timestamp)

Returns: Empty body with one of the following:

201 – in case of success
204 – if the transaction is older than 60 seconds
400 – if the JSON is invalid
422 – if any of the fields are not parsable or the transaction date is in the future

`GET /statistics`

This endpoint returns the statistics based on the transactions that happened in the last 60 seconds. It MUST execute in constant time and memory (O(1)).

Returns:

{
  "sum": "1000.00",
  "avg": "100.53",
  "max": "200000.49",
  "min": "50.23",
  "count": 10
}

Where:

sum – a BigDecimal specifying the total sum of transaction value in the last 60 seconds
avg – a BigDecimal specifying the average amount of transaction value in the last 60 seconds
max – a BigDecimal specifying single highest transaction value in the last 60 seconds
min – a BigDecimal specifying single lowest transaction value in the last 60 seconds
count – a long specifying the total number of transactions that happened in the last 60 seconds All BigDecimal values always contain exactly two decimal places and use HALF_ROUND_UP rounding. eg: 10.345 is returned as 10.35 10.8 is returned as 10.80

`DELETE /transactions`

This endpoint causes all existing transactions to be deleted

The endpoint should accept an empty request body and return a 204 status code.

Requirements

These are the additional requirements for the solution:

You are free to choose any JVM language to complete the challenge in, but your application has to run in Maven.
The API has to be threadsafe with concurrent requests.
The GET /statistics MUST execute in constant time and memory ie O(1). Scheduled cleanup is not sufficient
The solution has to work without a database (this also applies to in-memory databases).
Unit tests are mandatory.
mvn clean install and mvn clean integration-test must complete successfully.
Please ensure that no changes are made to the src/it folder since they contain automated tests that will be used to evaluate the solution.

xCASx / transactions-statistics