AppSync

AWS AppSync is a managed API service that connects client applications to data and events. It appears in this section because it is where server-side messaging meets the client: a browser or mobile app cannot subscribe to an SNS topic or poll a Kinesis stream, and AppSync is the AWS-managed way to push events to it over a WebSocket.

Two kinds of API

GraphQL APIs. A single endpoint resolves a query against one or more data sources — DynamoDB tables, Lambda functions, Aurora, OpenSearch, HTTP endpoints and other GraphQL APIs merged into one schema. Clients ask for exactly the fields they need in one request, rather than assembling several REST calls. Resolvers are written in JavaScript or TypeScript, or in VTL for older APIs. Subscriptions push updates to connected clients over WebSockets when the data behind them changes.

Events APIs. Since March 2025 AppSync also offers a pure publish/subscribe API over WebSockets, without a GraphQL schema: publish to a channel, and every subscribed client receives the message. This is the simplest managed route for broadcasting to browsers and mobile apps — live dashboards, presence, chat, notifications.

Why it is used

• One endpoint over several back-end data sources
• Real-time delivery to clients without running a WebSocket fleet
• Authorisation built in: API keys, IAM, Amazon Cognito user pools, OpenID Connect providers and Lambda authorisers
• Optional server-side caching for low-latency reads
• Private APIs and AWS WAF integration where the API must not be publicly reachable
• Billed per request and per real-time message delivered
• Client libraries for JavaScript, React, React Native, iOS and Android, and integration with AWS Amplify

A typical pattern in an event-driven system is for back-end events — an SNS message, an EventBridge rule, a Kinesis consumer — to write to a data source or call an AppSync mutation, and for AppSync to push the change to whichever clients are subscribed.