SNS — Simple Notification Service

AWS docs: https://docs.aws.amazon.com/sns/latest/api/welcome.html

SNS uses a query-string or JSON API. Topics are identified by ARN: arn:aws:sns:us-east-1:000000000000:<topic-name>

Subscription delivery is asynchronous — the HTTP response is returned before delivery completes to subscribers, matching the behaviour of real SNS.

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Known limitations

• Subscription confirmation (ConfirmSubscription token flow) is simplified: the emulator auto-confirms all subscriptions without requiring a token round-trip.
• HTTP/HTTPS subscriptions require a reachable URL inside the Docker network.
• email and email-json subscriptions are captured in the Inbox (/_overcast/ses/inbox), viewable in the web UI.
• sms subscriptions are captured in the same Inbox with kind=sms, viewable in the web UI. No real SMS is sent. The endpoint must be a phone number in E.164 format (e.g. +12125551234).
• Mobile push (application protocol) and Kinesis Firehose (firehose protocol) are not supported and return 400 InvalidParameter on Subscribe.
• lambda subscriptions invoke the function asynchronously with AWS’s SNS event — Records[0].EventSource is aws:sns and the notification sits under Records[0].Sns. As on AWS, RawMessageDelivery has no effect on a lambda subscription: the function always receives the full event.
• Delivery to lambda means Lambda accepted the event, exactly as an InvocationType=Event invoke returns 202 before the handler runs. A function that is throttled — including one reserved to zero concurrency — is retried inside Lambda and is not a delivery failure, matching AWS. Whether the handler then succeeded is reported against the function, not the subscription.
• A delivery that fails is not silently discarded. It is logged, published on the event stream as sns:DeliveryFailed, and — when the subscription’s RedrivePolicy names a deadLetterTargetArn — written to that SQS queue. For lambda that covers a function that does not exist, one that is not in an invokable state, a missing layer version, and a runtime the emulator cannot execute.
• CloudFormation applies AWS::SNS::Topic attributes through SNS on both create and update, and applies standalone AWS::SNS::Subscription attributes through SNS after subscribing. The topic’s inline Subscription list creates the listed SNS subscriptions during creation. Updating that list or removing a previously configured SNS attribute fails the stack update rather than leaving stale SNS configuration. Cross-region AWS::SNS::Subscription Region is not implemented and fails the stack rather than being ignored. FIFO topic attributes round-trip, and Publish validates the FIFO-only MessageGroupId/MessageDeduplicationId parameters against a topic name ending in .fifo, but actual FIFO ordering, deduplication, and SequenceNumber generation remain unimplemented (#183).
• Publish accepts exactly one of TopicArn, PhoneNumber, or TargetArn as the destination, matching AWS. PhoneNumber delivers directly to the Inbox as kind=sms with no topic involved. TargetArn (publishing to a mobile platform endpoint) returns 400 InvalidParameter — Overcast has no CreatePlatformEndpoint, so no TargetArn is ever a real endpoint.
• Publish and PublishBatch validate Message (≤ 256 KB, except SMS) and Subject (< 100 characters, no line breaks or control characters), and support MessageStructure=json: the Message value must be a JSON object with a string default key, and each subscriber receives the entry keyed by its own protocol name (falling back to default) — including under RawMessageDelivery, which strips the notification envelope but not the per-protocol selection underneath it.

Summary

Category	✅ Supported	⚠️ Partial	❌ Unsupported
Topics	8
Subscriptions	7
Publishing	5	1
Platform applications (mobile push)			5
SMS	1		3

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Endpoints

Topics

Operation	Status	Notes	AWS Docs
CreateTopic	✅ Supported	Idempotent; attributes stored; inline Tags applied at creation and left untouched by a repeat call	docs
DeleteTopic	✅ Supported		docs
GetTopicAttributes	✅ Supported		docs
SetTopicAttributes	✅ Supported		docs
ListTopics	✅ Supported		docs
TagResource	✅ Supported	Tags are stored on the topic; member-indexed form encoding	docs
UntagResource	✅ Supported		docs
ListTagsForResource	✅ Supported		docs

Subscriptions

Operation	Status	Notes	AWS Docs
Subscribe	✅ Supported	Protocols: sqs, sms, email, email-json, http, https, lambda. application and firehose return 400.	docs
ConfirmSubscription	✅ Supported	Emulator auto-confirms; any token accepted	docs
Unsubscribe	✅ Supported		docs
ListSubscriptions	✅ Supported		docs
ListSubscriptionsByTopic	✅ Supported		docs
GetSubscriptionAttributes	✅ Supported	Returns SubscriptionArn, TopicArn, Protocol, Endpoint, Owner + custom attributes	docs
SetSubscriptionAttributes	✅ Supported	Stores any attribute; FilterPolicy drives message filtering, RedrivePolicy the subscription dead-letter queue	docs

Publishing

Operation	Status	Notes	AWS Docs
Publish	✅ Supported	TopicArn/PhoneNumber/TargetArn destination selection (exactly one required); MessageStructure=json per-protocol payload selection; Message (256 KB) and Subject (<100 chars, no control/line-break chars) validation; async fan-out to sqs, lambda, email, email-json, http, https, and sms subscribers; records AWS/SNS CloudWatch metrics NumberOfMessagesPublished and PublishSize on accept, and NumberOfNotificationsDelivered/NumberOfNotificationsFailed per subscription delivery attempt (service-metrics-platform.md phase 2) — a delivery whose protocol dependency was never wired into this instance (nil enqueuer/mailer/smsSender/outbound) is treated as a delivery failure like any other protocol’s: failDelivery runs (recording NumberOfNotificationsFailed, redirecting to the subscription’s DLQ when configured), with a one-time Warn per (topic, protocol) telling the operator what to wire (#1306)	docs
PublishBatch	✅ Supported	Up to 10 messages; each entry independently validated (Message/Subject/MessageStructure=json), a bad entry fails without aborting the batch; records NumberOfMessagesPublished/PublishSize per successful entry	docs
Message filtering (subscription filter policy)	✅ Supported	String/Number attribute value matching via FilterPolicy on SetSubscriptionAttributes	docs
Lambda subscription delivery	✅ Supported	Publish invokes the subscribed function asynchronously with the AWS Records[].Sns event; RawMessageDelivery does not apply to lambda, matching AWS	docs
Subscription dead-letter queue (RedrivePolicy)	✅ Supported	A delivery that fails is redirected to the SQS queue named by the subscription’s RedrivePolicy	docs
FIFO topic Publish parameters	⚠️ Partial	MessageGroupId/MessageDeduplicationId are required and validated for a topic named *.fifo (or ContentBasedDeduplication substitutes for the dedup ID); actual FIFO ordering, deduplication, and SequenceNumber generation remain unimplemented	docs

Platform applications (mobile push)

Operation	Status	Notes	AWS Docs
CreatePlatformApplication	❌ Unsupported	APNs, GCM/FCM, ADM	docs
DeletePlatformApplication	❌ Unsupported		docs
ListPlatformApplications	❌ Unsupported		docs
CreatePlatformEndpoint	❌ Unsupported	Device registration	docs
PublishToEndpoint	❌ Unsupported	via Publish with TargetArn; returns 400 InvalidParameter explicitly rather than a generic missing-TopicArn error	docs

SMS

Operation	Status	Notes	AWS Docs
SMS publish	✅ Supported	via sms subscription on Publish, or directly via Publish with PhoneNumber (no topic involved); captured in the Inbox with kind=sms	docs
CheckIfPhoneNumberIsOptedOut	❌ Unsupported		docs
ListPhoneNumbersOptedOut	❌ Unsupported		docs
OptInPhoneNumber	❌ Unsupported		docs