How To

Run

./mvnw install && ./mvnw spring-boot:run

By default it will run at port 8088

Test

./mvnw clean test

Note on Solution

Implementation

• This application provide two endpoints POST /transactions and GET /statistics
• Application maintain a running in memory statistics
• In memory statistics is implemented as thread-safe singleton with ConcurrentHashMap as field to store running statistics
• When new Transaction arrived, it will be validated. Invalid transaction will be ignored
• Each valid Transaction received will update running statistics as describe below:
  • Find existing RunningSummary in ConcurrentHashMap. It use Transaction#epochSecond as key for lookup
  • If summary is already exist, proceed with update
  • Otherwise create RunningSummary for new transaction and put it in ConcurrentHashMap
• Request for latest statistics will be simple read from in memory statistics
  • Get current second since epoch as references
  • Loop trough 60 keys to read RunningSummary of last 60 seconds statistics, then combine them into single statistics

Complexity

• GET /statistics will have constant time and memory since it does not depend on number of transaction.
• It will involve ConcurrentHashMap#get for 60 times. ConcurrentHashMap#get is O(1). So time complexity will be O(1)
• Regardless of the number of transactions, at worst case we will only have to store 60 pairs of key–value to maintain our statistics. So space complexity will also be O(1)

Misc

• This implementation ignored old (stale) statistics. In the real world we should have a way to clean entry older than 60 seconds. Something like a simple TimerTask