State and Workflow Patterns

Event Queue Case

An event queue can be summarized by topic while preserving typed event fields.

queue summary Counting selected events shows how a typed queue can feed a dashboard or worker.

Event Queue Case

selectedTopic
events.ts
Replay: real traced execution (multi-file project)
type QueueEvent = {
    topic: "build" | "deploy";
    priority: number;
};

function topicCount(events: QueueEvent[], topic: QueueEvent["topic"]): number {
    let count = 0;

    for (const event of events) {
        if (event.topic === topic) {
            count += 1;
        }
    }

    return count;
}

const selectedTopic: QueueEvent["topic"] = "build";
const events: QueueEvent[] = [
    { topic: "build", priority: 2 },
    { topic: "deploy", priority: 1 },
    { topic: "build", priority: 3 }
];

const count: number = topicCount(events, selectedTopic);

console.log(`topic=${selectedTopic};count=${count}`);

type QueueEvent = {
    topic: "build" | "deploy";
    priority: number;
};

function topicCount(events: QueueEvent[], topic: QueueEvent["topic"]): number {
    let count = 0;

    for (const event of events) {
        if (event.topic === topic) {
            count += 1;
        }
    }

    return count;
}

const selectedTopic: QueueEvent["topic"] = "deploy";
const events: QueueEvent[] = [
    { topic: "build", priority: 2 },
    { topic: "deploy", priority: 1 },
    { topic: "build", priority: 3 }
];

const count: number = topicCount(events, selectedTopic);

console.log(`topic=${selectedTopic};count=${count}`);

  1. selectedTopic ← build, events ← [object Object],[object Object],[object Object]

    15    return count;16}1718const selectedTopic→ build: QueueEvent["topic"] = "build";  //@selectedTopic="deploy"19const events→ [object Object],[object Object],[object Object]: QueueEvent[] = [20    { topic: "build", priority: 2 },21    { topic: "deploy", priority: 1 },22    { topic: "build", priority: 3 }23];2425const count: number = topicCount(events[object Object],[object Object],[object Object], selectedTopicbuild);

  2. count ← 0

    3    priority: number;4};56function topicCount(events[object Object],[object Object],[object Object]: QueueEvent[], topicbuild: QueueEvent["topic"]): number {7    let count→ 0 = 0;

  3. if (event.topic === topic)

    pass 1 of 2
    9for (const event of events) {10    if (event.topicbuild === topicbuild) {11        count += 1;12    }

  4. if (event.topic === topic)

    pass 2 of 2
    9for (const event of events) {10    if (event.topicbuild === topicbuild) {11        count += 1;12    }

  5. return count;

    12        }13    }1415    return count2;16}

  6. count ← 2

    22    { topic: "build", priority: 3 }23];2425const count→ 2: number = topicCount(events[object Object],[object Object],[object Object], selectedTopicbuild);2627console.log(`topic=${selectedTopicbuild};count=${count2}`);
    outputtopic=build;count=2

  1. selectedTopic ← deploy, events ← [object Object],[object Object],[object Object]

    15    return count;16}1718const selectedTopic→ deploy: QueueEvent["topic"] = "deploy";19const events→ [object Object],[object Object],[object Object]: QueueEvent[] = [20    { topic: "build", priority: 2 },21    { topic: "deploy", priority: 1 },22    { topic: "build", priority: 3 }23];2425const count: number = topicCount(events[object Object],[object Object],[object Object], selectedTopicdeploy);

  2. count ← 0

    3    priority: number;4};56function topicCount(events[object Object],[object Object],[object Object]: QueueEvent[], topicdeploy: QueueEvent["topic"]): number {7    let count→ 0 = 0;

  3. if (event.topic === topic)

    9for (const event of events) {10    if (event.topicdeploy === topicdeploy) {11        count += 1;12    }

  4. return count;

    12        }13    }1415    return count1;16}

  5. count ← 1

    22    { topic: "build", priority: 3 }23];2425const count→ 1: number = topicCount(events[object Object],[object Object],[object Object], selectedTopicdeploy);2627console.log(`topic=${selectedTopicdeploy};count=${count1}`);
    outputtopic=deploy;count=1