Microservices Architecture (Part 5): Synchronous Communication

In this series, we’ve explored various aspects of microservices, including service-to-service communication, and now it’s time to dive deeper into a specific type of communication, Synchronous Communication. This method is widely used and important for microservices that need to interact in real-time. Let’s take a closer look at what it is, when to use it, and some best practices.

What is Synchronous Communication?

Synchronous communication in a microservices architecture means that when one service sends a request to another, it waits for a response before continuing. It’s like making a phone call: you ask a question and pause while waiting for an answer. This is different from asynchronous communication, where the service sends the request and moves on without waiting for a reply.

The most common way to implement synchronous communication is via HTTP/REST APIs or gRPC calls between services. For example, a service might request user information from an Account Service and wait for that information before proceeding with its task.

When Should You Use Synchronous Communication?

Synchronous communication is best suited for scenarios where real-time responses are crucial, such as:

• Immediate Data Needs: When a service needs specific data right away to complete its operation, such as fetching user authentication details.
• Transactional Operations: In workflows that depend on the success of multiple steps in real-time, such as placing an order, confirming payment, and reserving inventory.
• Client-Driven Interactions: When user actions require immediate feedback, like submitting a form or retrieving search results, synchronous communication ensures that the client receives an instant response.

Pros and Cons of Synchronous Communication

Pros:

1. Real-Time Interaction: Synchronous communication provides immediate feedback, which is ideal for scenarios where waiting for data is not an option.
2. Simple Implementation: It’s easier to understand and implement since it behaves like a traditional client-server model, where you send a request and wait for a response.
3. Data Consistency: Synchronous interactions ensure that data is consistent across services during a single operation, as all parts of the transaction are executed in a tightly coupled manner.

Cons:

1. Increased Dependency: If one service goes down, any service that depends on it for synchronous communication could also experience issues, leading to a potential cascade of failures.
2. Performance Bottlenecks: The waiting involved in synchronous communication can introduce delays, especially if services have to wait for multiple requests to complete before proceeding.
3. Scalability Challenges: As the number of services grows, synchronous communication can lead to scaling issues since every interaction requires waiting for a response.

Best Practices for Synchronous Communication

Use Timeouts:

Always set reasonable timeouts when making synchronous requests to prevent services from waiting indefinitely. If the response doesn’t arrive within the set time, the calling service can retry, use a fallback method, or fail gracefully.

Implement Circuit Breakers:

Circuit breakers help prevent cascading failures. If a service consistently fails to respond within a given timeframe, the circuit breaker will stop further requests to that service, allowing other parts of the system to keep functioning.

Load Balancing:

Use load balancers to distribute requests evenly across multiple instances of a service. This ensures that no single service instance gets overwhelmed and reduces response time.

Caching for Frequently Accessed Data:

If a service frequently requests the same data from another service, consider caching the data. This minimizes the number of synchronous calls and reduces load on services.

Graceful Fallbacks:

Implement fallback mechanisms when a service doesn’t respond in time. This could be returning a cached result or a default response to keep the system functioning even when the full data is unavailable.

Case Study

Imagine an e-commerce app where the Order Service needs to communicate with the Inventory Service before confirming an order. The Order Service sends a synchronous request to check if the items in the cart are available in stock. This is crucial because the order cannot proceed until the availability of each item is confirmed in real-time.

If the Inventory Service confirms that the items are available, the order is processed. If the Inventory Service is slow or fails to respond, the user may experience delays in placing the order or might be unable to complete the purchase. In such cases, implementing timeouts and circuit breakers helps maintain the app's stability and ensures a better user experience.

Handling Failures in Synchronous Communication

Failures are inevitable, especially in large systems. Here’s how to handle them:

• Retries: If a request to another service fails due to a temporary issue, retrying the request can often resolve the problem. However, be careful not to overwhelm the system with retries.
• Graceful Degradation: If a service can’t provide real-time data, consider offering a degraded experience instead of failing entirely. For instance, in a recommendation engine, if the recommendation service fails, you could show the user default popular products instead of personalized recommendations.
• Monitoring and Logging: Continuously monitor synchronous calls to ensure that performance is within acceptable limits. Logs help detect bottlenecks, and monitoring helps alert teams when response times are getting slow.

Final Thoughts

Synchronous communication is an essential part of microservices architecture, particularly when real-time data is needed. However, it comes with its own set of challenges related to latency, failure handling, and scalability. The key is to implement it thoughtfully, using best practices like timeouts, circuit breakers, and caching to ensure that your system remains resilient and performant.

?? P.S. In the next part of the series, we’ll dive into Event-Driven Microservices and explore how asynchronous communication can help make your system more flexible and fault-tolerant. Don’t miss it! ??