KV Queue ​

Queues are a powerful design pattern that help you deal with common application scaling and performance challenges. Some examples of problems that Queues can help you solve are:

Smooth out processing peaks. For example, if users can initiate resource-intensive tasks at arbitrary times, you can add these tasks to a queue instead of performing them synchronously. Then you can have worker processes pull tasks from the queue in a controlled manner. You can easily add new Queue consumers to scale up the back-end task handling as the application scales up. Break up monolithic tasks that may otherwise block the V8 (what Deno and Node use internally) event loop. For example, if a user request requires CPU intensive work like audio transcoding, you can delegate this task to other processes, freeing up user-facing processes to remain responsive. Provide a reliable communication channel across various services. For example, you can queue tasks (jobs) in one process or service, and consume them in another.

KvQueue package provides a simple abstraction, allowing you to fire and consume message.

Getting started ​

No need to install an extra module, you can import it from danet/mod.ts

import { KvQueueModule, KvQueue } from `danet/mod.ts`

Import the KvQueueModule into the root AppModule as following:

import { Module, KvQueueModule } from 'danet/mod.ts';

@Module({
  import: [KvQueueModule.configure()],
})
export class AppModule {}

Send message to the queue ​

To send a message, first inject KvQueue using standard constructor injection:

constructor(private queue: KvQueue) {}

::: Hint 💡 Import the KvQueue from the danet/mod.ts package. :::

Then use it in a class as follows:

this.queue.sendMessage('order.created', new OrderCreatedMessage(
  {
    orderId: 1,
    payload: {},
  }
))

Consuming message ​

To declare a message consumer, decorate a method with the @OnEvent() decorator preceding the method definition containing the code to be executed, as follows:

class OrderMessageHandler {
    @OnQueueMessage ('order.created')
    handleOrderCreatedEvent(payload: OrderCreatedMessage) {
      // handle and process "OrderCreatedMessage" event
    }
}

Sending message from a non Danet application ​

To make DX better and comply with SRP , @OnQueueMessage decorator takes a "message type" argument, so one method handles only one type of message.

In order to keep it consistent, KvQueue.sendMessage also takes a message type as first argument, and then your payload.

However, Deno.kv does not have any message discrimination or channel. Which means that when we call kv.listenQueue under the hood, we have to consume all incoming messages.

To make everything work, we are actually sending a message constructed as follows:

{
  type:"order.created",
  data: //object your provided
}

On the listener, we are doing the following:

this.kv.listenQueue((msg: QueueEvent) => {
			const type = msg.type;
			const callback = this.listenersMap.get(type);
			if (callback) {
				return callback(msg.data);
			}
			throw Error('Unhandled message type');
		});

So if you want to send message without using KvQueue, or from a non Danet app, please keep that in mind and format your message as shown above.