AWS AppSync

Core concept: AppSync is a fully managed GraphQL API service — ideal for real-time, multi-data-source, and mobile/web apps.

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AppSync vs API Gateway

Feature	AppSync	API Gateway
Protocol	GraphQL	REST / HTTP / WebSocket
Real-time	✅ Native subscriptions (WebSocket)	WebSocket API only
Multiple data sources	✅ Single query, multiple sources	❌ One integration per endpoint
Offline sync	✅ Conflict resolution built-in	❌
Caching	✅ Server-side per-resolver	✅ Stage-level
Use case	Mobile/web apps, real-time dashboards	REST APIs, microservices

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Data Sources

Source	Description
DynamoDB	Direct DynamoDB access via VTL mapping
Lambda	Any custom logic, any backend
RDS / Aurora Serverless	SQL queries via Data API
OpenSearch	Full-text search
HTTP	Any REST API
None	Local resolvers (computed values)

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Resolvers

Resolvers map GraphQL operations to data source operations using VTL (Velocity Template Language) or JavaScript (APPSYNC_JS):

// JavaScript resolver — DynamoDB GetItem
export function request(ctx) {
    return {
        operation: 'GetItem',
        key: util.dynamodb.toMapValues({ id: ctx.args.id })
    };
}

export function response(ctx) {
    return ctx.result;
}

Pipeline Resolvers

Chain multiple data sources in one resolver:

Query: getOrderWithProducts
  → Step 1: DynamoDB (get order)
  → Step 2: DynamoDB (get product details)
  → Step 3: Lambda (calculate price with tax)
  → Return merged result

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Real-Time Subscriptions

# Schema
type Subscription {
    onOrderUpdated(customerId: ID!): Order
        @aws_subscribe(mutations: ["updateOrder"])
}

# Client subscribes via WebSocket
subscription {
    onOrderUpdated(customerId: "CUST-123") {
        orderId
        status
        updatedAt
    }
}

• Client connects over WebSocket (MQTT over WebSocket)
• AppSync pushes updates automatically when mutation fires

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Authorization Modes

Mode	Use Case
API Key	Public APIs, development
AWS IAM	Server-to-server, IAM roles
Cognito User Pools	User authentication (most common)
OIDC	Third-party identity providers
Lambda Authorizer	Custom auth logic

Multiple modes can be configured simultaneously, with one as default.

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Caching

Per-resolver TTL: 0 – 3600 seconds
Cache key: resolved query arguments

Enables: Cache frequently-requested data
Reduces: Lambda invocations, DynamoDB reads

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🎯 DVA-C02 Exam Tips

Quick Exam Rules

• AppSync vs API Gateway: If you need real-time WebSocket Subscriptions natively tied to data changes, choose AppSync. If you need simple REST API microservices, choose API Gateway.
• Data Source unification: AppSync allows a single GraphQL endpoint to aggregate data from DynamoDB, RDS, and Lambda simultaneously.
• Resolvers: Use Pipeline Resolvers to sequence operations across multiple data sources.

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Practice Questions

Q1. A mobile app needs real-time order status updates pushed to connected clients whenever an order is updated. Which service is BEST suited?

A) API Gateway WebSocket API
B) SQS + client polling
C) AppSync with GraphQL Subscriptions
D) SNS mobile push notifications

✅ Answer & Explanation

C — AppSync Subscriptions automatically push mutation events to connected WebSocket clients — purpose-built for this pattern. API Gateway WebSocket requires managing connection IDs and routing manually.

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Q2. A single GraphQL query needs to aggregate data from DynamoDB and call a Lambda for business logic. Which AppSync feature enables this without client-side orchestration?

A) Multiple AppSync APIs
B) Pipeline Resolver chaining DynamoDB and Lambda functions
C) Lambda resolver that calls DynamoDB
D) REST API Gateway passthrough

✅ Answer & Explanation

B — Pipeline Resolvers chain multiple resolver functions (each connecting to a different data source) into a single GraphQL operation. The client makes one request; AppSync orchestrates the steps.

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🔗 Resources

• AppSync Developer Guide
• AppSync JavaScript Resolvers
• AppSync Real-time Data
• AppSync with Spring (community)