SNS limitations

Every way Overcast's SNS diverges from AWS: subscription confirmation, FIFO topics, delivery failure semantics, and what CloudFormation does with a topic.

Divergences from AWS, in full. The summary is on the SNS page.

Subscriptions

Area	On AWS	Overcast
`ConfirmSubscription`	A token is emailed or POSTed and must be echoed back	Every subscription is confirmed on creation; any token is accepted
`application` (mobile push)	Requires a platform application and device endpoint	`Subscribe` returns `400 InvalidParameter`
`firehose`	Streams to a Kinesis Data Firehose delivery stream	`Subscribe` returns `400 InvalidParameter`
`http` / `https`	The notification is POSTed to the endpoint	Captured in the Inbox as a webhook delivery; nothing is dialled
`sms`	A real message is sent	Captured in the Inbox; the endpoint must be E.164, e.g. `+12125551234`
`TargetArn` on `Publish`	Publishes to a platform endpoint	`400 InvalidParameter` — there is no `CreatePlatformEndpoint`, so no `TargetArn` is ever real

FIFO topics

A topic named *.fifo round-trips its FIFO attributes, and Publish enforces the FIFO-only parameters against it: MessageGroupId is required, and so is MessageDeduplicationId unless the topic has ContentBasedDeduplication set. FIFO-ness is read off the .fifo suffix, the convention AWS enforces at CreateTopic.

Ordering, deduplication and SequenceNumber generation are not implemented — messages fan out exactly as they do on a standard topic. See #183.

Delivery to Lambda

A lambda subscription invokes the function asynchronously with AWS’s own event shape: Records[0].EventSource is aws:sns and the notification sits under Records[0].Sns. RawMessageDelivery has no effect, matching AWS.

“Delivered” means Lambda accepted the event, exactly as an InvocationType=Event invoke returns 202 before the handler runs. A throttled function — including one reserved to zero concurrency — is retried inside Lambda and is not a delivery failure. Whether the handler then succeeded is reported against the function, not the subscription.

These are delivery failures: the function does not exist, is not in an invokable state, is missing a layer version, or has a runtime this emulator cannot execute.

What happens to a failed delivery

Nothing is silently discarded. A failed delivery is logged, published on the event stream as sns:DeliveryFailed, counted as NumberOfNotificationsFailed, and — when the subscription’s RedrivePolicy names a deadLetterTargetArn — written to that SQS queue.

A protocol whose dependency was never wired into the running instance fails the same way, with a one-time warning per topic and protocol saying what is missing.

CloudFormation

AWS::SNS::Topic attributes are applied through SNS on both create and update. A topic’s inline Subscription list creates real SNS subscriptions during creation.
Updating that inline list, or removing an SNS attribute that was previously configured, fails the stack update rather than leaving stale configuration behind.
Standalone AWS::SNS::Subscription attributes are applied through SNS after subscribing.
Cross-region AWS::SNS::Subscription Region is not implemented, and fails the stack rather than being quietly ignored.

SNS — quick start and what works
SNS operations — per-operation status
SQS — the most common subscription target
SES — where email subscriptions land