09 — Message Queues

flowchart LR
    subgraph Producers["Producers"]
        P1["Web App"]
        P2["Mobile App"]
        P3["Cron Job"]
    end

    subgraph Queue["Message Queue"]
        Q["Queue / Topic / Stream"]
    end

    subgraph Consumers["Consumers"]
        C1["Email Service"]
        C2["Notification Service"]
        C3["Analytics Pipeline"]
        C4["Search Index"]
    end

    P1 & P2 & P3 --> Queue
    Queue --> C1 & C2 & C3 & C4

    style Producers fill:#3b82f6,color:#fff
    style Queue fill:#7c3aed,color:#fff
    style Consumers fill:#059669,color:#fff

flowchart TB
    MSG["Message Systems"] --> Queue["Queue<br/>SQS, RabbitMQ<br/>Point-to-point<br/>Message deleted after consume"]
    MSG --> PubSub["Pub/Sub<br/>SNS, RabbitMQ<br/>Broadcast to all<br/>subscribers"]
    MSG --> Stream["Stream<br/>Kafka, Kinesis<br/>Ordered log<br/>Replayable"]
    MSG --> DLQ["Dead Letter Queue<br/>Failed messages<br/>Retry policies"]

    style MSG fill:#7c3aed,color:#fff
    style Queue fill:#3b82f6,color:#fff
    style PubSub fill:#059669,color:#fff
    style Stream fill:#f59e0b,color:#fff
    style DLQ fill:#ef4444,color:#fff

sequenceDiagram
    participant User as User
    participant App as Web App
    participant Queue as Message Queue
    participant Worker as Worker (Consumer)
    participant DB as Database
    participant Email as Email Service

    User->>App: Place order
    App->>DB: Save order (status: processing)
    App->>Queue: SendMessage({ orderId: 123 })
    App-->>User: 202 Accepted (order received)

    Note over Worker: Worker polls queue

    Worker->>Queue: ReceiveMessage
    Queue-->>Worker: { orderId: 123 }

    Worker->>DB: Update order (processing)
    Worker->>Worker: Validate payment
    Worker->>Email: Send confirmation email
    Worker->>DB: Update order (confirmed)
    Worker->>Queue: DeleteMessage (done)

    alt Failed after retries
        Queue->>DLQ["Dead Letter Queue"]: Move message
    end

flowchart TB
    subgraph Producers["Producers"]
        Prod1["App Events"]
        Prod2["DB Changes"]
        Prod3["Metrics"]
    end

    subgraph Kafka["Apache Kafka Cluster"]
        Topic1["Topic: user-events<br/>Partition 0 | 1 | 2"]
        Topic2["Topic: page-views<br/>Partition 0 | 1"]
    end

    subgraph Consumers["Consumer Groups"]
        CG1["Analytics Group"]
        CG2["Notification Group"]
    end

    Prod1 & Prod2 --> Topic1
    Prod3 --> Topic2
    Topic1 --> CG1 & CG2
    Topic2 --> CG1

    style Producers fill:#3b82f6,color:#fff
    style Kafka fill:#7c3aed,color:#fff
    style Consumers fill:#059669,color:#fff