aws-patterns

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• SKILL.md

SKILL.md (first lines)

# AWS Patterns

Serverless and managed service patterns for AWS production workloads.

## Lambda Best Practices

```typescript
// Cold start optimization: keep handler thin, initialize outside handler
import { DynamoDBClient } from '@aws-sdk/client-dynamodb'
import { DynamoDBDocumentClient, GetCommand } from '@aws-sdk/lib-dynamodb'

// Initialized ONCE per container (reused across invocations)
const client = DynamoDBDocumentClient.from(new DynamoDBClient({}))

export const handler = async (event: APIGatewayProxyEvent) => {
  try {
    const userId = event.pathParameters?.id
    if (!userId) {
      return { statusCode: 400, body: JSON.stringify({ error: 'Missing user ID' }) }
    }

    const result = await client.send(new GetCommand({
      TableName: process.env.USERS_TABLE!,
      Key: { pk: `USER#${userId}`, sk: `PROFILE` }
    }))

    if (!result.Item) {
      return { statusCode: 404, body: JSON.stringify({ error: 'User not found' }) }
    }

    return {
      statusCode: 200,
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify(result.Item)
    }
  } catch (err) {
    console.error('Handler error:', err)
    return { statusCode: 500, body: JSON.stringify({ error: 'Internal server error' }) }
  }
}
```

## S3 Event Processing

```typescript
// S3 → Lambda: process uploaded files
import { S3Client, GetObjectCommand } from '@aws-sdk/client-s3'

const s3 = new S3Client({})

export const handler = async (event: S3Event) => {
  for (const record of event.Records) {
    const bucket = record.s3.bucket.name
    const key = decodeURIComponent(record.s3.object.key.replace(/\+/g, ' '))
    const size = record.s3.object.size

    // Guard: skip non-image files or oversized uploads
    if (size > 10_000_000) {
      console.warn(`Skipping oversized file: ${key} (${size} bytes)`)
      continue
    }

    const response = await s3.send(new GetObjectCommand({ Bucket: bucket, Key: key }))
    const body = await response.Body!.transformToByteArray()

    await processImage(body, key)

    console.log(`Processed ${key} (${size} bytes)`)
  }
}
```

## SQS/SNS Fanout Pattern

```typescript
// SNS → multiple SQS queues (fanout to parallel consumers)

// Publisher: send to SNS topic
import { SNSClient, PublishCommand } from '@aws-sdk/client-sns'

const sns = new SNSClient({})

async function publishOrderEvent(order: Order): Promise<void> {
  await sns.send(new PublishCommand({
    TopicArn: process.env.ORDER_EVENTS_TOPIC!,
    Message: JSON.stringify(order),
    MessageAttributes: {
      eventType: { DataType: 'String', StringValue: 'order.created' },
      region: { DataType: 'String', StringValue: order.region }
    }
  }))
}

// Consumer: SQS Lambda (one per subscriber: email, analytics, inventory)
export const emailHandler = async (event: SQSEvent) => {
  for (const record of event.Records) {
    const order = JSON.parse(record.body) as Order

    try {
      await sendOrderConfirmation(order)
    } catch (err) {
      console.error(`Failed to send email for order ${order.id}:`, err)
      throw err  // Message returns to queue for retry (DLQ after maxReceiveCount)
    }
  }
}
```

## DynamoDB Single-Table Design

```typescript
// Access patterns drive table design, not entity relationships

// Table: pk (partition key) + sk (sort key) + GSI1PK + GSI1SK
// Entities: User, Order, OrderItem - all in one table

const AccessPatterns = {
  // Get user profile
  getUserProfile: (userId: string) => ({

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