This essay is a follow up introduction on IM Architecutre . We will discuss some common issues we will face in a multi-datacenter architecture of the IM system. These issues involves consistency, avilability and partition tolerance issues in the distributed systems. We will also discuss how we tackle with thess issues in the service implementation to acieve final consistency with a key component called Kafka. It will take some time to read and understand however it is followed by figures step by step. Do leave comments if u have any queires.

We assume u would love to know about IM that serves for millions of people around the globe but not for a single country.

Multi DataCenter, Multi Master and Multiple Sync Issues…

Why do we need more then one datacenter? A multi-data center architecture (lets say DC-A and DC-B)is necessary as in if one datacenter (DC-A) becomes unavailiable, we can redirect to the users from DC-A to DC-B. However, this is a master(main)-salve(follower) mode that all users are still using a single datacenter where as the other one acts as the backup. This mode is simple as we can constantly sync data from master to slave just based on all request sequence. However When a robust IM-System would love to provide service for global users with great user experiences, every user should be served by the data center that is closer to him. This is to reduce latency as for some region that is remote. These people has week internet connection and it is unstable for them to request from datacenters located at the other side of the ocean and await for loading ring spinning for seconds on their screen. Therefore, our master-slave architecture is not very suitable in this situation.

We need a multi-master architecure, which means we support data writes and retrives from different datacenters at the same time. For example, if a user from SG would love to start a chatting with a user from the US. Your server should be able to handle this. Apprantly, if u have any experience in synchronisation in distributed systems, u know how troublesome this is to achieve some level of consistency.

Draft Design

There should be two sets of your microservices and each of them can operate independently as a complete IM system and here comes the question, how should the SG use sends messages to US if he is using DC-A and the US user is using DC-B? We need a system that does data synchronisation between the two datacenter. And during the synchronisation we need to achieve the following goals:


  1. Support SG-UserA to communicate with US-UserB in one App that communicates with their own DataCenter lets say A would send and retrieve messages from DC A and B would write and accepts responses from DC-B
  2. There shouldnt be any IM message gets lost when A is chatting with B although they are from different DC
  3. A can smoothly change to another DC incase of DC outrage happens (assume DC switch can be done by APP client using different domain)
  4. Messages must be in order and cannot be out of order during data synchronisation or DC switch.