Async (@Async & messaging)

Async programming

Example: suppose prepare the home page, we need 4 independent calls - userInfo(2s), AccountInfo(4s), BillingInfo(1s), UsageInfo(5s). If we execute synchoronously, we have to wait 12s, If we use Async, we only need to wait 5s.

@Async

@Async annotation is used to mark that method should run in a separate thread async. To get start, need to add @EnableAsync to either @Configuration class or @SpringBootApplication class.

By default, Spring uses SimpleAsyncTaskExecutor to run methods async. We can override the default at:
Application level: choose different TaskExecutor or create a custom one.
Method level: use @Async("customExecutor") to specify the executor to use.

ExecutorService

ExecutorService is an interface used to deal with the threads. It can run multiple tasks and manage a thread pool, we don’t need to manually create a thread.

Besides taking Runnable and callable interface, it can also accept CompletableFuture to run asynchronous tasks.

Message Queue (RabbitMQ, Kafka)

when a user submits a time-consuming task (like generating a report or processing an image), the web application can send a message to RabbitMQ to handle the task asynchronously. This way, the user doesn’t have to wait for the task to complete

RabbitMQ

RabbitMQ is a message broker: it accepts and forwards messages. sender can send message to the queue, and receiver can get message from the queue. It’s a middleman between sender and receiver.

Three components of RabbitMQ

• Exchange: a message router, it receives messages from producer and pushes them to queues based on the Binding.

• Queue: place that stores messages, it follows FIFO.

• Binding: a link between exchange and queue. An exchange will use these bindings to route messages to queues

Four types of exchange

• Direct exchange: a message goes to the queues whose binding key exactly matches the routing key of the message.

• Fanout exchange: a message goes to all queues bound to the exchange.

• Topic exchange: a message goes to the queues whose binding key matches the routing key of the message. The binding key can use wildcard.

• Headers exchange: a message goes to the queues whose headers matches the headers of the message.

dead letter exchange (DLX)

When a message is rejected or expired, it will be sent to a dead letter exchange. The dead letter exchange will route the message to a dead letter queue. The dead letter queue can be used to store the messages that can’t be processed.