[System Design] Transfer Sum

4 min readSep 11, 2023
Photo by Aditya Vyas on Unsplash

Problem Description

We’d like to put up a counter of how much money has been transferred through the money-exchange APP in the past week. The purpose is to help customers trust us by showing how much volume we’re moving successfully.

Step 1: Gather Requirements

First, we need to ask the interviewer some questions to clarify the requirements. Lets imagine the answers to those questions are in italics.


  • What currency should the volume be expressed in? It should be in the local currency of the user. If they’re from a US IP address, let’s give them that value in USD. If they are from France, display in Euros.
  • What exchange rate to use? During time of transaction or when the APP page is loaded? Up to you.
  • What is the data source? There is a Transfer Service which will produce an event whenever a transfer has been sent out, and how mich and in what currency. There is a rate service which you can use.

Non Functional

  • How long should we store the data for? Can we purge data that exist for more than 1 week? Yes, don’t need to store data for more than 1 week.
  • What is the expected writes and reads? More writes than read.
  • How accurate does the data need to be? What if there is some time lag? Does not need to be accurate to the second. Can have some lag.

Step 2: Storage schema

The Transfer Service will emit an event for each transfer.

"currency": "SGD",
"amount": "488",
"timestamp" "21st Jan 2023",

More on how we will track the sum later below.

Step 3: High level design


Each Transfer Service Event is sent to Kafka for durability and reliability of handling many Transfer events.

Service for computing the sum of transfers

We will have a service that will consume messages from Kafka and add the value of the transfer to the current sum, which can be a value stored in cache (for…




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