Context Diagram: Notification Worker (C4 Level 1)

This is the system boundary the postmortem refers to throughout: the Notification Worker, the people it affects, and the external systems it depends on at the time of the 2026-04-02 incident. Every node below is named in the postmortem's own timeline and impact sections, and every system or actor the postmortem names appears here.

%% Color Palette: Blue #0173B2, Orange #DE8F05, Teal #029E73, Purple #CC78BC, Brown #CA9161
%% C4 Level 1 -- System Context: Notification Worker, at the time of the 2026-04-02 incident
graph TD
    Customer["Customer<br/>#40;person#41;"]:::purple
    OnCall["On-call engineer<br/>#40;person#41;"]:::purple
    ShipmentAPI["Shipment API<br/>#40;external system, event producer#41;"]:::brown
    EventBus["Event Bus #40;Kafka#41;<br/>#40;external, at-least-once delivery#41;"]:::brown
    NotifyGate["NotifyGate<br/>#40;external SMS#47;email provider#41;"]:::brown
    Worker["Notification Worker<br/>#40;this system#41;"]:::blue
 
    ShipmentAPI -->|"publishes shipment events"| EventBus
    EventBus -->|"delivers events, at-least-once"| Worker
    Worker -->|"sends notification"| NotifyGate
    NotifyGate -->|"delivers SMS#47;email to"| Customer
    Customer -->|"reports duplicate via support ticket"| OnCall
    OnCall -->|"investigates send logs of"| Worker
 
    classDef blue fill:#0173B2,stroke:#000000,color:#FFFFFF,stroke-width:2px
    classDef purple fill:#CC78BC,stroke:#000000,color:#FFFFFF,stroke-width:2px
    classDef brown fill:#CA9161,stroke:#000000,color:#FFFFFF,stroke-width:2px

Diagram: two people (Customer, On-call engineer) and three external systems (Shipment API, the Kafka-based Event Bus, NotifyGate) around the Notification Worker, the system this incident concerns. The Event Bus is labeled with the exact property (at-least-once delivery) that this incident's root cause depends on -- the same property named in the postmortem's root-cause section and in the RFC's Context section.

Reading this diagram against the postmortem

• Shipment API is the upstream system whose events, once duplicated by a rebalance, is what the Notification Worker reprocessed -- named in the postmortem's root-cause step 1.
• Event Bus (Kafka) is where the consumer-group rebalance that caused the replay actually happened -- named in the postmortem's timeline (14:02 entries) and root-cause steps 1 and 3.
• Notification Worker is the system whose missing idempotency check is the postmortem's systemic root cause, and the system this capstone's ADR and PR change.
• NotifyGate is the external provider that actually delivered the duplicate SMS/email messages to customers -- the postmortem's Impact section describes what customers received through this system.
• Customer and On-call engineer are the two people named in the postmortem's timeline: the customers who received duplicates and filed support tickets, and the on-call engineer who correlated those tickets into a confirmed pattern.

No node in this diagram is absent from the postmortem's prose, and no system or actor named in the postmortem is missing from this diagram.

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