LavinMQ Remote procedure call

In an RPC (Remote Procedure Call), a client sends a request to run a function and any related arguments to a program located on another server. The remote server runs the task and sends back the response. The compiled code includes a stub that acts as a representative of the remote procedure code. The stub receives the request while the program is running and the procedure call has been issued. It forwards it to a client program in the local computer, while the initiating program waits until it receives a response.

The RPC process works somewhat differently in LavinMQ. There is no stub available. The program will not stall if you use a non-blocking consumer. Instead, your client sends a message to the broker containing the reply-to header. The target server processes the request and sends data back to the server. Then, in regular pub/sub fashion, your consumer receives the response.

What is LavinMQ RPC?

The reply-to header in LavinMQ lets you create applications that communicate with one another over a special form of RPC.

How does RPC work in LavinMQ?

After a client sends the request to a server, the server gives a response message in return. The client sends a callback queue address with its request in order to receive a response. One queue receives the response while another consumer handles the reply.

Another way to handle RPC in LavinMQ is by using direct reply-to, which passes a response directly to the client, avoiding having to create a response queue to wait on.

When to Use RPC in LavinMQ?

RPC using the LavinMQ direct reply-to header allows you to create responsive applications since you can perform tasks remotely from your mobile and desktop applications.

LavinMQ RPC

There are different RPC options in LavinMQ. One is to let the client send a request message and let a server reply with a response message, sent into a specified queue. This queue is called a callback or response queue. The name of this queue must be sent as an address in the request message sent from the client:

channel.basic_publish(exchange='',
routing_key='rpc_queue',
properties=pika.BasicProperties(
reply_to = response_queue,
),
body=request)

In the code shown above is a callback queue added for every RPC request, which might be confusing since it’s not clear to which request the response belongs. A correlation_id (a unique value) can therefore be sent with every request.

channel.basic_publish(exchange='',
routing_key='rpc_queue',
properties=pika.BasicProperties(
reply_to = response_queue,
correlation_id = correlation_id),
body=request)

Workflow of RPC callback queues in LavinMQ:

The client creates an exclusive callback_queue.
The client sends a message containing the reply queue, the callback queue along with the correlation_id. The value of the correlation_id is set to a unique value at every request.
Next, the request is sent to rpc_queue.
The RPC worker/consumer, i.e. the remote server, is waiting for requests on that queue (rpc_queue) and handles the request once received.
The result is sent back to the broker by the remote server using the reply_to field to know which queue it belongs in. The response goes to the response_queue.
The callback queue is where the client/consumer waits for data.

callback_queue = channel.queue_declare(queue='', exclusive=True)
channel.basic_publish(exchange='',
routing_key='rpc_queue',
properties=pika.BasicProperties(
reply_to = callback_queue,
correlation_id = correlation_id
),
body=request)

As mentioned earlier, you can also handle RPC in LavinMQ by using direct reply-to.

What is Direct Reply-to?

LavinMQ allows you to pass a response directly to the client, avoiding creating a response queue to wait on. To do so, set reply-to to amq.direct.reply-to. Send a message in no-ack mode. LavinMQ generates a special name that the target server sees as the routing key. The remote machine then publishes the result to the default exchange. No queue is created in the process. A direct reply allows you to avoid maintaining a long-lived queue. It also lets you avoid creating short-lived queues that utilize memory.

Using Direct Reply-to in LavinMQ

To use the direct reply feature, use the reply-to header with the amq.direct.reply-to queue. In this RPC example, we send a simple ping request to a remote machine.

Start by consuming from the amq.direct.reply-to queue on your client to avoid missing the response:

def client_consumer_callback(ch, method, properties, body):
msg = body.decode('utf-8')
if msg in "Hello from Consumer":
print("TARGET MACHINE IS ACTIVE")
global RECEIVED_HELLO
RECEIVED_HELLO = True
else:
print("RECEIVED UNEXPECTED MESSAGE")
ch.close()
channel.consume("amq.direct.reply-to", on_message_callback=client_consumer_callback)

The consumer checks that the response message contains a greeting from the server.

Next, create another consumer to mimic a remote computer:

def consumer_callback(ch, method, properties, body):
msg = body.decode('utf-8')
if msg in "Hello World":
basic_props = BasicProperties()
ch.basic_publish(exchange='', routing_key=properties.reply_to, properties=basic_props, body="Hello from Consumer")
ch.basic_ack(delivery_tag=method.delivery_tag)
else:
print("RECEIVED UNEXPECTED MESSAGE")
channel.consume("amq.direct.reply-to", on_message_callback=consumer_callback)

This consumer receives the request, checks that the message contains Hello World, and sends back the greeting. Notice that both consumers subscribe to amq.direct.reply-to.

Finally, send a message to the queue using the reply-to header:

basic_props = BasicProperties(
reply_to="amq.direct.reply-to"
)

channel.basic_publish('', routing_key="", properties=basic_props, body="Hello World")

You can use this method to fetch data or perform tasks such as registering users on a mobile application. We created a simple health check.

RPC differs somewhat from the traditional publisher-subscriber model LavinMQ is known for. No queues are created and the process sends information directly to the client using a direct reply.

When using RPC this way:

Try to establish a connection to the client using the generated name on a disposal channel to see if the client still exists
Set the immediate flag to false when publishing
Start consuming from the amq.lavinmq.reply-to before publishing your message
Set the mandatory flag if using amq.lavinmq.reply-to to create error logs
Do not set the mandatory flag when using a direct-reply if using amq.lavinmq.reply-to.* as your queue

These tips allow you to know when something goes wrong. They also help you handle issues without losing messages.

NOTE: LavinMQ supports the use of amq.rabbitmq.reply-to to allow for compatibility with other brokers.

Wrap up

In conclusion, LavinMQ provides efficient support for Remote Procedure Calls (RPC), allowing applications to communicate with one another seamlessly. Unlike traditional RPC systems that use stubs, LavinMQ simplifies the process by eliminating the need for stubs and providing alternative mechanisms for RPC.

When using LavinMQ for RPC, clients can send requests to servers, and the servers respond with the desired data or results. The reply-to header plays a crucial role in this process, enabling clients to specify where they expect the response. The response can be received through a callback queue or, alternatively, by utilizing the direct reply-to feature. Direct reply-to streamlines the process by directly passing the response to the client, eliminating the need for maintaining long-lived or short-lived queues.

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