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ADR-2-event-driven-broker.md

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Title:

ADR-2: Use message queues with guaranteed delivery for ticket workflow

Status:

Proposed

Context:

As mentioned in ADR-1, customer-facing services will require special considerations on availability and scalability with respect to the rest of the system. Similar requirements toward the expert-facing software, they need their sub-system to be highly available and provide a better response time than the rest of the system. This makes an implication for decoupling these areas from one another.

Decision:

Using an asynchronous message queues between customer and Sysops services enables the following:

  • enables fire and forget ticket submission for the customer, so that customer does not have to wait for any ticket processing; hence we get very low response time;
  • same for Sysops Experts, they don't have to wait until the system finishes with ticket processing or report generation; while working on the field they just need to search for a ticket or a kb article;
  • enables independent scalability and availability of the corresponding quanta;

If look at the ticket workflow between domains, it looks naturally pretty much as messaging rather than query-response behavior as illustrated in this BPMN diagram (circles are messages):

Ticket workflow

Note that we should guarantee message delivery to the exact destination so this peer-to-peer queue-based messaging, not a topic-based publisher-subscriber communication.

Consequences:

Let's consider the trade-offs:

  • added complexity;
  • message queues can introduce a single point of failure so has to be load-balanced;
  • ticket workflow becomes more difficult for testing and debugging.

Since aforementioned qualities are critical to the business we accept these trade-offs.