# When to use SQS between services instead of direct integration
There are a few scenarios where using SQS as a message queue between services is preferable to direct integration:
- Decoupling services – SQS allows services to communicate without having a direct dependency on each other. If Service A connects directly to Service B, then any downtime or change in Service B will affect Service A. By using SQS as an intermediary, Service A only depends on the queue, not Service B. This decouples the services and makes them more resilient.
- Handling load spikes – SQS can act as a buffer, absorbing load spikes from one service and delivering the messages to the other service at a steady rate. If Service A experiences a sudden increase in load, it can place messages in the queue. Service B can then poll the queue and process messages at a steady rate, without being overwhelmed.
- Rate limiting – SQS allows the consuming service to control the rate at which it receives and processes messages. This can help limit the load on downstream databases or APIs.
- Back pressure – If the consuming service is unable to keep up with the producer, messages will accumulate in the queue. This provides “back pressure” and signals to the producer to slow down.
- Retrying messages – SQS provides features for retrying failed messages, including exponential backoff and dead letter queues. This allows messages to eventually be processed, even after transient failures.
- Asynchronous communication – SQS enables asynchronous communication between services. The producer can continue its work after placing a message in the queue, without waiting for an acknowledgement from the consumer.
In summary, using SQS as a message queue between services provides many benefits like decoupling, load management, rate limiting, retrying, and asynchronous communication. The key is that SQS acts as a buffer, absorbing differences in load and processing rates between the producer and consumer services.
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