Twilio SendGrid Event Webhooks and a Custom Status Consumer: Reconciling Duplicate and Out-of-Order Events

Question

A service consumes Twilio SendGrid Event Webhook payloads to track per-message delivery status across events such as processed, delivered, opened, and clicked. SendGrid does not guarantee exactly-once delivery: the same event can be POSTed more than once, and status events may arrive out of order or delayed. The endpoint must return a 2xx within a short documented timeout, and failed deliveries are retried only a bounded number of times. No API exists to request retransmission of events missed while an endpoint is unavailable.

The unresolved decision sits at the boundary between the webhook producer and the consumer's state model. Payload event and message identifiers could support deduplication, but the consumer still needs a reconciliation rule for cases such as an opened event being processed before its corresponding delivered event.

What deduplication strategy prevents duplicate POSTs from advancing state incorrectly? How should status precedence be modeled so out-of-order events converge to a consistent final state per message? Given the absence of a retransmission mechanism, what fallback, if any, is appropriate for events lost during endpoint downtime?

Tasadduq Burney · Answer

To handle Twilio SendGrid webhooks effectively, the consumer must shift the logic from event-driven updates to a state-based reconciliation model. Because SendGrid does not guarantee exactly-once delivery or chronological ordering, the system must implement idempotency and state-aware transitions.
Deduplication Strategy
To prevent duplicate POSTs from advancing state incorrectly, use the sg_id (the SendGrid event ID) as a unique idempotency key.

Store and Check: Upon receiving a payload, check a fast-access store (like Redis) or a unique database index for the sg_id.
Atomic Operations: If the ID has already been processed, return a 2xx OK immediately without re-processing the logic. This prevents SendGrid from retrying the request indefinitely.
TTL Policy: Since SendGrid only retries for a bounded window, you can store these IDs with a TTL (Time-to-Live) of 24 hours to keep the storage overhead manageable.

Status Precedence and Out-of-Order Events
To handle events arriving out of order (e.g., an opened event arriving before delivered), implement a state-based priority model rather than a simple sequential update. Assign a numerical weight to each status:

Status
Weight

processed
1

delivered
2

opened
3

clicked
4

The Reconciliation Rule: When an event arrives, compare its weight against the current state of the message in your database. Only update the record if the incoming event's weight is greater than the existing weight. If the incoming event is lower (e.g., a delivered event arriving after an opened event), discard the state change but return a 2xx to acknowledge the webhook.

Fallback for Lost Events
Since there is no retransmission mechanism for events missed during downtime, a proactive fallback is required. 
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Polling Fallback: Implement a background worker that queries the SendGrid Messages API for messages created in the last hour that have not yet reached a terminal state (e.g., delivered/clicked or bounced).
Gap Analysis: This worker should fill the gaps left by missed webhooks, ensuring your local state eventually converges with the actual status in SendGrid.

Assumption: This strategy assumes your system tracks the message_id provided by SendGrid to link multiple sg_id events together.

Twilio SendGrid Event Webhooks and a Custom Status Consumer: Reconciling Duplicate and Out-of-Order Events

1 answer

To handle Twilio SendGrid webhooks effectively, the consumer must shift the logic from event-driven updates to a state-based reconciliation model. Because SendGrid does not guarantee exactly-once delivery or chronological ordering, the system must implement idempotency and state-aware transitions.

Deduplication Strategy

Status Precedence and Out-of-Order Events

Fallback for Lost Events

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