dehasi/n26

Solutions description

There are two branches now:

• master (with naive solution)
• remove_stat (with multithreading approach)

There is also a general assumption that there are no transactions from the future.

Naive solution

It uses a fact that we don't need to keep transactions to calculate statistics. It updates min, max, sum and count every transaction. Then, when a statistic requested, counts avg as sum / count. There is also a scheduled job, which resets statistics every minute. That's why we have O(1) time and memory.

Multithreading approach

The scheduled the job, which was described above, removes all statistics, that's why it's not a proper solution. Instead of job, I create a thread for every request, which updates statistic, then waits a counted time (see below) and the updates the statistics again.

We have timestamp it's possible to find how long have a transaction been living. now - timestamp. There is no need to keep the transaction more than minute. That's why the thread has to wait minute - (now - timestamp).

It's easy to update sum and count, we add value to sum and increment count. Then the thread will subtract sum and decrement count. For min and max we need to know the previous values, that's why MinMaxPriorityQueue from Google Guava was used. By this approach the statistics are always up to date, we still count avg as sum / count. This approach gives as O(1) time but not the memory because queues were used. But technically, we can predefine queues size and it gives as O(1) on average.

How to

Build

mvn clean install

Run

mvn spring-boot:run -DskipTests

The app will start on port: 8080 by default

Used technologies

Spring boot

as a general framework to build the web application.

Spring Cloud Contract

to generate stubs and API unit tests. See test/resources/contracts. To turn the contracts generation off, set false to turn-contract-on maven property.

Spring Rest Docs

to generate up to date documentation form unit-tests. All generated documentation is packed to the jar and can be found on /docs/index.html URL.

Maven dependency plugin

to avoid transitive dependencies.

Maven sort-pom plugin

to keep a standard pom structure.