Say, we are building a Social Network and anytime someone reacts to your post, you need to be notified. So, how should the Reaction service talk to the Notification service to send out a notification?

The communication would be much simpler and reliable, just a function call if it was a monolith; but things become tricky as we go distributed.

Microservices need to talk to each other to exchange information and get things done; and there are two categories of communication patterns - Synchronous and Asynchronous.

Synchronous Communication

Communication is synchronous when one service sends a request to another service and waits for the response before proceeding further.

The most common implementation of Sync communication is over HTTP using protocols like REST, GraphQL, and gRPC.

Advantages of Synchronous Communication

• It is simple and intuitive
• Communication happens in realtime

Disadvantages of Synchronous Communication

• Caller is blocked until the response is received
• Servers need to be pro-actively provisioned for peaks
• There is a risk of cascading failures
• The participating services are strongly coupled

When to use Synchronous Communication

• When you cannot proceed without a response from the other service
• When you want real-time responses
• When it takes less time to compute and respond

Asynchronous Communication

The communication is asynchronous when the one service sends a request to another service and does NOT wait for the response; instead, it continues with its own execution.

Async communication is most commonly implemented using a message broker like RabbitMQ, SQS, Kafka, Kinesis, etc.

Advantages of Asynchronous Communication

• Services do not need to wait for the response and can move on
• Services can handle surges and spikes better
• Servers do not need to be proactively provisioned
• No extra network hop due to Load Balancer
• No request drop due to target service being overwhelmed
• Better control over failures and retires is possible
• Services are truly decoupled

Disadvantages of Asynchronous Communication

• Eventual consistency
• Broker could become a SPoF
• It is harder to track the flow of the message between services

When to use Asynchronous Communication

• When delay in processing is okay
• When the job at hand is long-running and takes time to execute
• When multiple services need to react to the same event
• When it is okay for the processing to fail and you are allowed to retry