Glossary

Queue

A queue is a mechanism for processing tasks asynchronously, outside the main HTTP request. Instead of sending an email or generating a report synchronously (making the user wait), the task is placed in a queue and executed by a worker in the background.

How it works

The task is serialised into a queue broker (Redis, RabbitMQ, SQS, database). A separate worker process continuously reads tasks from the queue and executes them. If a task fails — it is automatically retried N times before moving to the dead letter queue.

Typical tasks

Sending emails and SMS
Generating PDFs and reports
Image resizing
Calling external APIs
Indexing into a search engine

Queue brokers

Redis — the simplest option for small projects. RabbitMQ — an AMQP broker with flexible routing. Amazon SQS — a cloud solution with no infrastructure to manage. Apache Kafka — for high volume and event streaming.