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Email Delivery Workflow

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Email Delivery Workflow

EmailDeliveryWorkflow is a Cloudflare Workflow (WorkflowEntrypoint) that delivers transactional email with step-level checkpointing, automatic retry, and dead-lettering.

Why a Workflow instead of a plain queue consumer?

A plain queue consumer sends the email inline: if the Worker restarts mid-send, the message is retried but the in-flight HTTP call is lost — the email might be sent twice or not at all.

A Cloudflare Workflow checkpoints each named step in durable storage. If the Worker restarts between steps, the Workflow resumes from the last completed checkpoint — not from the beginning. This means:

  • Step 1 (validation) never re-runs after it passes.
  • Step 2 (delivery) retries only delivery, not validation.
  • Step 3 (receipt) can fail without affecting the delivery status.

This is why EmailDeliveryWorkflow exists: no duplicate sends on retry, even across Worker restarts.

Step diagram

stateDiagram-v2
    [*] --> Validate : Workflow created
    Validate --> Deliver : payload valid
    Validate --> [*] : ZodError → workflow failed (no retry)

    Deliver --> WriteReceipt : delivery succeeded
    Deliver --> Deliver : transient error → backoff retry
    Deliver --> DeadLetter : maxRetries exceeded

    WriteReceipt --> [*] : done (non-fatal if D1/KV write fails)
    DeadLetter --> [*] : workflow failed

Steps

Step 1 — Validate payload

  • Zod-validates the EmailDeliveryParams received from the queue or direct trigger.
  • Runs with retries: 0 — a malformed payload is a programming error, not a transient failure.
  • If validation fails, the workflow is immediately marked failed. No retries.

Step 2 — Deliver

  • Calls createEmailService(env, { useQueue: false }).sendEmail(payload).
  • { useQueue: false } selects the best non-queue provider (CfEmailWorkerService) — see below.
  • Default retry configuration:
    • limit: 3
    • delay: '10 seconds'
    • backoff: 'exponential' (10 s → 20 s → 40 s)
  • After all retries exhausted, the workflow is marked failed and the message enters the dead-letter queue (EMAIL_DLQ).

Step 3 — Write delivery receipt

  • Writes a row to email_log_edge (D1) and the email_idempotency_keys D1 table, and stores a receipt object in KV (METRICS) with a 7-day TTL.
  • Runs with retries: 1 and is non-fatal: a failure here does not mark the workflow as failed. The email was already sent in Step 2.

Configuration

The retry configuration is defined as constants in worker/workflows/EmailDeliveryWorkflow.ts:

ParameterDefaultDescription
limit3Number of retry attempts for Step 2 (delivery)
delay'10 seconds'Initial back-off wait after first failure
backoff'exponential'Multiplier strategy applied to the interval on each retry

Why createEmailService(env, { useQueue: false }) not createEmailService(env)?

createEmailService(env) selects QueuedEmailService as the highest-priority provider when EMAIL_QUEUE is configured. If EmailDeliveryWorkflow called createEmailService(env), it would enqueue the message back onto EMAIL_QUEUE, which would trigger another EmailDeliveryWorkflow instance — infinite recursion.

Passing { useQueue: false } skips the queue and selects CfEmailWorkerService directly, breaking the cycle.

// Inside EmailDeliveryWorkflow.run():
const mailer = createEmailService(this.env, { useQueue: false });
// NOT: createEmailService(this.env)  ← would cause queue→workflow→queue recursion

See also