Email / SMS Delivery Service — Detailed

flowchart TB
  subgraph Customer
    APP([Customer apps])
  end

  subgraph Edge
    GW[API Gateway]
    AUTH[API key auth]
    IDEM[Idempotency key]
  end

  subgraph Pipeline
    Q[[Kafka per priority]]
    TMPL[Template rendering]
    LANG[Localization]
    SUPP[Suppression list - unsub / bounce]
    DEDUP[Dedup window]
  end

  subgraph Channels
    EMAIL([Email workers + MTA])
    SMS([SMS workers + carrier APIs])
    WHATSAPP[WhatsApp Business API]
    VOICE[Voice]
  end

  subgraph Providers
    SES((SES / Sendgrid / Mailgun))
    TWILIO((Twilio / MSG91 / carriers))
    METAB[Meta Business APIs]
  end

  subgraph Reliability
    RT[Routing per region/carrier]
    FALL[Failover provider]
    BR[Bounce reasons handler]
    REC[Delivery receipts]
    DLR([DLR ingest])
  end

  subgraph Data
    CONT[(Contacts)]
    HIST[(Send history + events)]
    SUPP_DB[(Suppression DB)]
  end

  Customer --> Edge --> Pipeline
  Pipeline --> Channels --> Providers
  Providers --> Reliability --> Data

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class APP client;
    class GW edge;
    class AUTH,IDEM,TMPL,LANG,SUPP,DEDUP,WHATSAPP,VOICE,METAB,RT,FALL,BR,REC service;
    class CONT,HIST,SUPP_DB datastore;
    class Q queue;
    class EMAIL,SMS,DLR compute;
    class SES,TWILIO external;

Glossary & fundamentals

Concepts referenced in this design. Each row links to its canonical page; the tag column shows whether it is a high-level (HLD) or low-level (LLD) concept.

Tag	Concept	What it is	Page
HLD	API gateway / BFF	single ingress, auth, rate limit, routing	api-gateway
HLD	Pub/Sub & message brokers	topics, consumer groups, delivery semantics	pub-sub-pattern
HLD	Idempotency & retries	safe re-execution, backoff + jitter	idempotency-retries
LLD	REST API design	verbs, statuses, pagination, errors	rest-api-design
LLD	Async models	futures / async-await / coroutines / actors	async-models