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Synchronous Communication

Basic System Design
≈ 2 min read

The Calling Service Waits for a Response

With synchronous communication, service A sends a request to service B and cannot finish the current operation until it receives a response or an error. This model is natural when the result is needed right now. 

Client -> Service A -> Service B
          waits...      |
             <-----------+

For example, an order service may request the current product price from a pricing service if it cannot form a response to the user without that value.

The Advantage Is a Simple Execution Flow

The code is easy to read: send a request, receive a response, continue the computation. An error can be tied directly to the current user operation, and the client gets the result within one request. 

HttpRequest request = HttpRequest.newBuilder(priceUri)
        .timeout(Duration.ofSeconds(2))
        .GET()
        .build();

HttpResponse<String> response = httpClient.send(
        request,
        HttpResponse.BodyHandlers.ofString()
);

The Cost Is Time Coupling

While B responds slowly, A also keeps its request open longer. If A calls several services sequentially, delays add up, and unavailability of one participant can break the entire chain.

That is why synchronous calls need bounded timeouts and clear error behavior. A remote service must not be waited on forever: a stuck call gradually consumes resources on the calling side.

Synchronous communication is a good fit when the answer is truly needed now. If the current operation can finish without the result of remote work, an asynchronous model often reduces time coupling.
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