Nothing imports the producer

The event producer is a process of its own: it drains the outbox and publishes to the broker. Code that imports it publishes from the request path instead, so a request now fails when the broker is down, and an event can be published for a change that is later rolled back. The answer is a rule that no module outside src/producer/ imports anything inside it.

• Plan: plan 0004, Step 5
• Source: the third of the design's three cookbook questions, design § The developer relations hat
• Rule language: rules.md § The four families
• Requirement: FR-REACH-01

The rule

from.pathNot leaves the producer's own modules out, so src/producer/main.ts may still start src/producer/producer.ts. Everything else, including modules added later, is held to it.

$schema: https://benbahrenburg.github.io/rulebearing/schema/config-v1.json
rules:
  dependencies:
    forbidden:
      - name: nothing-imports-the-producer
        comment: "The producer is its own process that drains the outbox; code that imports it publishes from the request path and fails when the broker is down. plan:0004-wave-4-reach"
        fix: "Write the event to the outbox (src/outbox/outbox.ts) and let the producer process publish it."
        severity: error
        from: { pathNot: "^src/producer/" }
        to: { path: "^src/producer/" }
        examples:
          forbidden:
            - "src/orders/cancel-order.ts -> src/producer/producer.ts"
          allowed:
            - "src/orders/place-order.ts -> src/outbox/outbox.ts"
            - "src/producer/main.ts -> src/producer/producer.ts"
            - "src/producer/producer.ts -> src/outbox/outbox.ts"

The file is rulebearing.yaml. Its examples are the edges rulebearing test checks the rule against.

The same rule for dependency-cruiser

rulebearing config convert rulebearing.yaml --to dependency-cruiser writes the rule in dependency-cruiser's format and names what it dropped: the $schema, the fix and the examples.

{
  "forbidden": [
    {
      "name": "nothing-imports-the-producer",
      "comment": "The producer is its own process that drains the outbox; code that imports it publishes from the request path and fails when the broker is down. plan:0004-wave-4-reach",
      "to": {
        "path": "^src/producer/"
      },
      "severity": "error",
      "from": {
        "pathNot": "^src/producer/"
      }
    }
  ]
}

dropped 3 item(s) dependency-cruiser cannot hold:
  $schema: points at the native schema
  forbidden[nothing-imports-the-producer].fix: a Rulebearing rule field
  forbidden[nothing-imports-the-producer].examples: a Rulebearing rule field

The files are converted/.dependency-cruiser.json and converted/dropped.txt.

The failing edge

In repo/, src/orders/place-order.ts writes its event to the outbox, as it should. src/orders/cancel-order.ts writes to the outbox and then publishes straight away through the producer:

• nothing-imports-the-producer fix: Write the event to the outbox (src/outbox/outbox.ts) and let the producer process publish it.

The fix

Write the event to the outbox and stop there; the producer process publishes it on its next pass:

// src/orders/cancel-order.ts
import { enqueue } from '../outbox/outbox';

export function cancelOrder(id: string): void {
  enqueue({ topic: 'order-cancelled', payload: JSON.stringify({ id }) });
}

If an event must be published sooner, shorten the producer's interval; do not publish from the request.

Try it

The playground holds this page's repository and rule. Run cruises the files in this browser, as rulebearing cruise does in repo/; change the rule and run it again, or pick another output type.