Concept of Event/Message flow and configuration details of Queue and Webhook in Event Mesh

Introduction: This blog explains the concept of Event and Message flow and how to create a Queue and Webhook in Event Mesh as per your requirement.

First, let’s understand that how a Publisher or Producer can send events or messages to Event Mesh.

S/4HANA system sends event by event bus created on BTP, where Event Mesh instance is created, other systems can use rest-based Api to send events and messages.

Producer or Publisher sends event to Topic

Event or Message will be sent to Topic and then it will be sent to Queue

Producer or Publisher sends event to Queue

Event or Message will be sent to Queue.

From Queue, there are two ways to consume event or message.

1. Push Based Model->Event or Message would be pushed to Consumer or Receiver System via webhook.
2. Pull based Model->Event or Message would be pulled from queue by Consumer or Receiver System via REST based Api.
   If the message client calls with QoS 1, then, the service needs an acknowledgment request from the message client with the message-id. The message is deleted from the queue only after the acknowledgment is consumed. When QoS 0 is used, the message is deleted from the queue without an acknowledgment from the client.

Let’s understand the parameters used to create Queue.

First i will talk about Queue Name and Access Type

Queue name can be anything as per your naming conventions.

Access Type: Exclusive/ Non-Exclusive

Exclusive: If more then one consumers are connected to the same queue, then message will always go to the first consumer, if first consumer is not available then it will go to next consumer

Non-exclusive: If more then one consumers are connected to the same queue, then messages will be delivered to all consumers in round robin way

Next…..

Max Queue size and Max Message size comes by default, you can reduce it if required.

Max Unacknowledged Messages per Consumer:

We can set a number for unacknowledged messages due to connectivity issue

If the unacknowledged message limit has reached, then the new messages can only be delivered after the successful delivery of unacknowledged messages.

Set the value 1 to maintain sequence.

EM retries sending unacknowledged messages in every 15 mins.

Next…..

Max Redelivery Count:

Default and Min Value is 0 and max value is 255

It is optional but required to set if we are setting dead message queue for the main queue. If the delivery count limit is reached, then message would be considered dead.

Dead Message Queue:

To store dead messages

Respect time to live:

A lifetime can be set for a message, if the message is not delivered with in the defined time, then it would be expired and move to Dead message queue.

If event or message will come from Topic to Queue then after Queue needs to be subscribed to Topic after creation.

A Queue can be subscribed from REST Api as well, click here to know more about APIs

Let’s talk about the Webhook 

Webhook subscription can also be done via REST Api, click here to check details.

Click here to enable S/4HANA public cloud events with Event Mesh