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AWS Service Selection Guide

Quick reference for choosing the right AWS service based on requirements.

Table of Contents

• Compute Services
• Database Services
• Storage Services
• Messaging and Events
• API and Integration
• Networking
• Security and Identity

Compute Services

Decision Matrix

Lambda

Best for: Event-driven functions, API backends, scheduled tasks

Limits:
- Execution: 15 minutes max
- Memory: 128 MB - 10 GB
- Package: 50 MB (zip), 10 GB (container)
- Concurrency: 1000 default (soft limit)

Pricing: $0.20 per 1M requests + compute time

Use when:

• Variable/unpredictable traffic
• Pay-per-use is important
• No server management desired
• Short-duration operations

Avoid when:

• Long-running processes (>15 min)
• Low-latency requirements (<50ms)
• Heavy compute (consider Fargate)

ECS Fargate

Best for: Containerized applications, microservices

Limits:
- vCPU: 0.25 - 16
- Memory: 0.5 GB - 120 GB
- Storage: 20 GB - 200 GB ephemeral

Pricing: Per vCPU-hour + GB-hour

Use when:

• Containerized applications
• Predictable traffic patterns
• Long-running processes
• Need more control than Lambda

EC2

Best for: Custom configurations, specialized hardware

Instance Types:
- General: t3, m6i
- Compute: c6i
- Memory: r6i
- GPU: p4d, g5
- Storage: i3, d3

Use when:

• Need GPU/FPGA
• Windows applications
• Specific instance configurations
• Reserved capacity makes sense

Database Services

Decision Matrix

DynamoDB

Best for: Key-value and document data, serverless applications

Limits:
- Item size: 400 KB max
- Partition key: 2048 bytes
- Sort key: 1024 bytes
- GSI: 20 per table

Pricing:
- On-demand: $1.25 per million writes, $0.25 per million reads
- Provisioned: Per RCU/WCU

Data Modeling Example:

# Single-table design for e-commerce
PK                  SK                  Attributes
USER#123            PROFILE             {name, email, ...}
USER#123            ORDER#456           {total, status, ...}
USER#123            ORDER#456#ITEM#1    {product, qty, ...}
PRODUCT#789         METADATA            {name, price, ...}

Aurora

Best for: Relational data with complex queries

Limits:
- Storage: 128 TB max
- Replicas: 15 read replicas
- Connections: Instance-dependent

Pricing:
- Serverless: $0.12 per ACU-hour
- Standard: Instance + storage + I/O

Comparison: DynamoDB vs Aurora

Storage Services

S3 Storage Classes

Lifecycle Policy Example

{
  "Rules": [
    {
      "ID": "Archive old data",
      "Status": "Enabled",
      "Transitions": [
        {
          "Days": 30,
          "StorageClass": "STANDARD_IA"
        },
        {
          "Days": 90,
          "StorageClass": "GLACIER"
        },
        {
          "Days": 365,
          "StorageClass": "DEEP_ARCHIVE"
        }
      ],
      "Expiration": {
        "Days": 2555
      }
    }
  ]
}

Block and File Storage

Messaging and Events

Decision Matrix

EventBridge

Best for: Event-driven architectures, SaaS integration

# EventBridge rule pattern
{
    "source": ["orders.service"],
    "detail-type": ["OrderCreated"],
    "detail": {
        "total": [{"numeric": [">=", 100]}]
    }
}

SQS

Best for: Decoupling services, handling load spikes

# SQS with dead letter queue
import boto3

sqs = boto3.client('sqs')

def process_with_dlq(queue_url, dlq_url, max_retries=3):
    response = sqs.receive_message(
        QueueUrl=queue_url,
        MaxNumberOfMessages=10,
        WaitTimeSeconds=20,
        AttributeNames=['ApproximateReceiveCount']
    )

    for message in response.get('Messages', []):
        receive_count = int(message['Attributes']['ApproximateReceiveCount'])

        try:
            process(message)
            sqs.delete_message(QueueUrl=queue_url, ReceiptHandle=message['ReceiptHandle'])
        except Exception as e:
            if receive_count >= max_retries:
                sqs.send_message(QueueUrl=dlq_url, MessageBody=message['Body'])
                sqs.delete_message(QueueUrl=queue_url, ReceiptHandle=message['ReceiptHandle'])

Kinesis

Best for: Real-time streaming data, analytics

API and Integration

API Gateway vs AppSync

API Gateway Configuration

# Throttling and caching
Resources:
  ApiGateway:
    Type: AWS::ApiGateway::RestApi
    Properties:
      Name: my-api

  ApiStage:
    Type: AWS::ApiGateway::Stage
    Properties:
      StageName: prod
      MethodSettings:
        - HttpMethod: "*"
          ResourcePath: "/*"
          ThrottlingBurstLimit: 500
          ThrottlingRateLimit: 1000
          CachingEnabled: true
          CacheTtlInSeconds: 300

Step Functions

Best for: Workflow orchestration, long-running processes

{
  "StartAt": "ProcessOrder",
  "States": {
    "ProcessOrder": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:...:processOrder",
      "Next": "CheckInventory"
    },
    "CheckInventory": {
      "Type": "Choice",
      "Choices": [
        {
          "Variable": "$.inStock",
          "BooleanEquals": true,
          "Next": "ShipOrder"
        }
      ],
      "Default": "BackOrder"
    },
    "ShipOrder": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:...:shipOrder",
      "End": true
    },
    "BackOrder": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:...:backOrder",
      "End": true
    }
  }
}

Networking

VPC Components

VPC Design Pattern

VPC: 10.0.0.0/16

Public Subnets (AZ a, b, c):
  10.0.1.0/24, 10.0.2.0/24, 10.0.3.0/24
  - ALB, NAT Gateway, Bastion

Private Subnets (AZ a, b, c):
  10.0.11.0/24, 10.0.12.0/24, 10.0.13.0/24
  - Application servers, Lambda

Database Subnets (AZ a, b, c):
  10.0.21.0/24, 10.0.22.0/24, 10.0.23.0/24
  - RDS, ElastiCache

VPC Endpoints (Cost Savings)

# Interface endpoint for Secrets Manager
SecretsManagerEndpoint:
  Type: AWS::EC2::VPCEndpoint
  Properties:
    VpcId: !Ref VPC
    ServiceName: !Sub com.amazonaws.${AWS::Region}.secretsmanager
    VpcEndpointType: Interface
    SubnetIds: !Ref PrivateSubnets
    SecurityGroupIds:
      - !Ref EndpointSecurityGroup

Security and Identity

IAM Best Practices

// Least privilege policy example
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "dynamodb:GetItem",
        "dynamodb:PutItem",
        "dynamodb:Query"
      ],
      "Resource": "arn:aws:dynamodb:us-east-1:123456789:table/users",
      "Condition": {
        "ForAllValues:StringEquals": {
          "dynamodb:LeadingKeys": ["${aws:userid}"]
        }
      }
    }
  ]
}

Secrets Manager vs Parameter Store

Cognito Configuration

UserPool:
  Type: AWS::Cognito::UserPool
  Properties:
    UserPoolName: my-app-users
    AutoVerifiedAttributes:
      - email
    MfaConfiguration: OPTIONAL
    EnabledMfas:
      - SOFTWARE_TOKEN_MFA
    Policies:
      PasswordPolicy:
        MinimumLength: 12
        RequireLowercase: true
        RequireUppercase: true
        RequireNumbers: true
        RequireSymbols: true
    AccountRecoverySetting:
      RecoveryMechanisms:
        - Name: verified_email
          Priority: 1