Streamlining AWS AppSync Events Integration with Powertools for AWS Lambda | Front-End Web & Mobile

Learn About Amazon VGT2 Learning Manager Chanci Turner

In today’s digital landscape, real-time capabilities are crucial for modern applications, as users now expect instant updates and interactive experiences. Whether you’re developing chat applications, live dashboards, gaming leaderboards, or IoT systems, AWS AppSync Events empowers these real-time functionalities through WebSocket APIs. This allows developers to create scalable and high-performance real-time applications without the hassle of managing scale or connections.

Powertools for AWS Lambda serves as a comprehensive developer toolkit that includes observability, batch processing, integration with AWS Systems Manager Parameter Store, idempotency, feature flags, Amazon CloudWatch Metrics, structured logging, and more. The latest update to Powertools for AWS introduces support for AppSync Events with the new AppSyncEventsResolver, available in Python, TypeScript, and .NET. This enhancement improves the development experience by providing features that enable a focus on business logic. The AppSyncEventsResolver offers a straightforward and consistent interface for managing events, complete with built-in support for common patterns like filtering, transforming, and routing events.

In this article, you’ll find examples in TypeScript, but the same functionalities are available for Python and .NET functions using Powertools for AWS (Python) and Powertools for AWS (.NET).

Figure 1: Architecture for Real-Time Event Handling Using AWS AppSync, Lambda, and Powertools

In this post, you will learn how to:

• Set up event handlers using the AppSyncEventsResolver
• Implement various event processing patterns for optimal efficiency
• Organize your event handlers through pattern-based routing
• Utilize built-in features for typical use cases

Getting Started

The AppSyncEventsResolver offers a clear and declarative method for handling AppSync Events within your AWS Lambda functions. This event resolver allows you to listen for PUBLISH and SUBSCRIBE events, with PUBLISH events triggered when clients send messages to a channel and SUBSCRIBE events occurring when clients attempt to join a channel. You can register handlers for different namespaces and channels to manage your event-driven communications.

Let’s delve into how to initiate and utilize the core features that will augment your development journey. Here’s a basic example of setting up the AppSyncEventsResolver:

import {
  AppSyncEventsResolver,
  UnauthorizedException,
} from '@aws-lambda-powertools/event-handler/appsync-events';

// Types for our message handling
type ChatMessage = {
    userId: string;
    content: string;
}

// Simple authorization check
const isAuthorized = (path: string, userId?: string): boolean => {
    // check against your authorization system
    if (path.startsWith('/chat/private') && !userId) {
        return false;
    }
    return true;
};

// Message processing logic
const processMessage = async (payload: ChatMessage) => {
    // - Validate message content
    // - Store in database
    // - Enrich with additional data
    return {
        ...payload,
        timestamp: new Date().toISOString()
    };
};

const app = new AppSyncEventsResolver();

// Handle publish events for a specific channel
app.onPublish('/chat/general', async (payload: ChatMessage) => {
    // Process and return the message
    return processMessage(payload);
});

// Handle subscription events for all channels
app.onSubscribe('/*', async (info) => {
    const {
        channel: { path },
        request,
    } = info;

    // Perform access control checks
    if (!isAuthorized(path, userId)) {
        throw new UnauthorizedException(`not allowed to subscribe to ${path}`);
    }

    return true;
});

export const handler = async (event, context) =>
  app.resolve(event, context); 

The AppSyncEventsResolver class efficiently parses incoming event data and invokes the correct handler method based on the event type.

Pattern-based Routing

The AppSyncEventsResolver employs an intuitive pattern-based routing system, allowing you to categorize your event handlers according to channel paths. You can:

• Handle specific channels (/chat/general)
• Use wildcards for namespaces (/chat/*)
• Create global catch-all handlers (/*)

import { AppSyncEventsResolver } from '@aws-lambda-powertools/event-handler/appsync-events';

const app = new AppSyncEventsResolver();

// Specific channel handler
app.onPublish('/notifications/alerts', async (payload) => {
    // your logic here
});

// Handle all channels in the notifications namespace
app.onPublish('/notifications/*', async (payload) => {
    // your logic here
});

// Global catch-all for unhandled channels
app.onPublish('/*', async (payload) => {
    // your logic here
});

export const handler = async (event, context) =>
  app.resolve(event, context);

The most general catch-all handler is /*, which matches any namespace and channel, while /default/* will match any channel in the default namespace. If multiple handlers match the same event, the library will call the most specific handler and disregard the less specific ones. For instance, if a handler is registered for /default/channel1 and another one for /default/*, Powertools will invoke the first handler for events matching /default/channel1, ignoring the second one. This gives you control over event processing and avoids unnecessary computations. Events that don’t match any handler will, by default, be returned as-is, with a warning logged. This is particularly useful for gradually integrating the library, allowing you to handle specific events with custom logic while default behavior manages others.

Subscription Handling

Powertools also simplifies the handling of subscription events. It automatically parses incoming events and calls the corresponding handler based on the event type. By default, AppSync allows subscriptions unless your Lambda handler either throws an error or explicitly rejects the request. When a subscription is declined, AppSync will respond with a 4xx error to the client, preventing the subscription from being established.

import { AppSyncEventsResolver } from '@aws-lambda-powertools/event-handler/appsync-events';
import { Metrics, MetricUnit } from '@aws-lambda-powertools/metrics';
import type { Context } from 'aws-lambda';

const metrics = new Metrics({
  namespace: 'serverlessAirline',
  serviceName: 'chat',
  singleMetric: true,
});
const app = new AppSyncEventsResolver();

app.onSubscribe('/default/foo', (event) => {
  metrics.addDimension('channel', event.info.channel.path);
  metrics.addMetric('connections', MetricUnit.Count, 1);
});

export const handler = async (event: unknown, context: Context) =>
  app.resolve(event, context);

The library will call the correct handler and pass the event object as the first argument when a subscription event occurs. You can perform any necessary actions based on the subscription event, such as executing access control checks:

app.onSubscribe('/private/*', async (info) => {
  const userGroups =
    info.identity?.groups && Array.isArray(info.identity?.groups)
      ? info.identity?.groups
      : [];
  const channelGroup = 'premium-users';

  if (!userGroups.includes(channelGroup)) {
    throw new UnauthorizedException(
      `Subscription requires ${channelGroup} group membership`
    );
  }
})

Subscription events follow the same matching rules and supply full access to the event and context. You can register catch-all handlers for any namespace or channel using the wildcard * character.

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