New Event Source

Adding a New Event Source to Lambda Annotations

This skill guides you through adding a complete new event source attribute to the AWS Lambda .NET Annotations framework. Use this when a user asks to add support for a new AWS event source like Kinesis, Kafka, MQ, etc.

Prerequisites

Before starting, gather from the user:

1. Service name (e.g., "Kinesis", "Kafka", "MQ")
2. Primary resource identifier (e.g., stream ARN, topic ARN, broker ARN)
3. CloudFormation event type string (e.g., "Kinesis", "MSK", "MQ")
4. Event class name from the corresponding Amazon.Lambda.*Events NuGet package (e.g., KinesisEvent)
5. Optional properties the attribute should support (e.g., BatchSize, StartingPosition, Filters)
6. Whether @ references use Fn::GetAtt or Ref — event source mappings use Fn::GetAtt, subscriptions use Ref

Reference Examples

Read these files to understand existing patterns before creating new ones:

• SNS (simplest, subscription-based): Libraries/src/Amazon.Lambda.Annotations/SNS/SNSEventAttribute.cs
• SQS (event source mapping with batching): Libraries/src/Amazon.Lambda.Annotations/SQS/SQSEventAttribute.cs
• DynamoDB (stream-based): Libraries/src/Amazon.Lambda.Annotations/DynamoDB/DynamoDBEventAttribute.cs
• S3 (notification-based): Libraries/src/Amazon.Lambda.Annotations/S3/S3EventAttribute.cs

Steps

Step 1: Create the Event Attribute Class

Create: Libraries/src/Amazon.Lambda.Annotations/{ServiceName}/{ServiceName}EventAttribute.cs

Key patterns:

• Add copyright header: // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved. + // SPDX-License-Identifier: Apache-2.0
• Inherit from Attribute with [AttributeUsage(AttributeTargets.Method, AllowMultiple = true)] (use AllowMultiple = false for event types where multiple triggers on the same function don't make sense, e.g., Schedule events)
• Constructor takes the primary resource identifier as a required string parameter
• All optional properties use nullable backing fields with Is<PropertyName>Set internal properties
• Include auto-derived ResourceName property (strips @ prefix or extracts name from ARN)
• Include internal List<string> Validate() method with all validation rules
• Use Regex("^[a-zA-Z0-9]+$") for ResourceName validation

Step 2: Register Type Full Names

Modify: Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/TypeFullNames.cs

Add constants:

public const string {ServiceName}EventAttribute = "Amazon.Lambda.Annotations.{ServiceName}.{ServiceName}EventAttribute";
public const string {ServiceName}Event = "Amazon.Lambda.{ServiceName}Events.{ServiceName}Event";

Also add to EventType enum if needed in Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/EventType.cs.

Step 3: Create the Attribute Builder

Create: Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/Attributes/{ServiceName}EventAttributeBuilder.cs

Extracts attribute data from Roslyn AttributeData. Use consistent else if chaining. Reference: SNSEventAttributeBuilder.cs.

Step 4: Register in AttributeModelBuilder

Modify: Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/Attributes/AttributeModelBuilder.cs

Add else if block for the new attribute type after the existing event attribute blocks.

Step 5: Register in EventTypeBuilder

Modify: Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/EventTypeBuilder.cs

Add else if block mapping the attribute to the EventType enum value.

Step 6: Add DiagnosticDescriptor

Modify: Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Diagnostics/DiagnosticDescriptors.cs

Add descriptor with the next available AWSLambda0XXX ID for invalid attribute validation errors.

Modify: Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Diagnostics/AnalyzerReleases.Unshipped.md — add the new diagnostic ID.

Step 7: Add Validation in LambdaFunctionValidator

Modify: Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Validation/LambdaFunctionValidator.cs

1. Add Validate{ServiceName}Events() call in ValidateFunction method
2. Create private Validate{ServiceName}Events() method that validates:
   • Attribute properties via Validate() method
   • Method parameters (first must be event type, optional second is ILambdaContext)
   • Return type (usually void or Task)

Step 8: Add Dependency Check

Modify: Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Validation/LambdaFunctionValidator.cs

In ValidateDependencies, add check for Amazon.Lambda.{ServiceName}Events NuGet package.

Step 9: Check SyntaxReceiver

Check: Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/SyntaxReceiver.cs

Add the new attribute name if the SyntaxReceiver filters by attribute name strings.

Step 10: Add CloudFormation Writer Logic

Modify: Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Writers/CloudFormationWriter.cs

1. Add case AttributeModel<{ServiceName}EventAttribute> in the event processing switch
2. Create Process{ServiceName}Attribute() method that writes CF template properties
   • Event source mappings (SQS, DynamoDB, Kinesis): use Fn::GetAtt for @ references
   • Subscription events (SNS): use Ref for @ references
   • Track synced properties in metadata

Step 11: Create Attribute Unit Tests

Create: Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/{ServiceName}EventAttributeTests.cs

Cover: constructor, defaults, property tracking, ResourceName derivation, all validation paths. Reference: SQSEventAttributeTests.cs, DynamoDBEventAttributeTests.cs, SNSEventAttributeTests.cs.

Step 12: Create CloudFormation Writer Tests

Create: Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/WriterTests/{ServiceName}EventsTests.cs

This is a partial class CloudFormationWriterTests. Include tests for:

1. Verify{ServiceName}EventAttributes_AreCorrectlyApplied — Theory with JSON/YAML and property combinations
2. Verify{ServiceName}EventProperties_AreSyncedCorrectly — Synced properties update when attributes change
3. SwitchBetweenArnAndRef_For{Resource} — ARN to @ reference switching
4. Verify{Resource}CanBeSet_FromCloudFormationParameter — CF Parameters handling
5. VerifyManuallySet{ServiceName}EventProperties_ArePreserved — Hand-edited template preservation

Reference: SQSEventsTests.cs, DynamoDBEventsTests.cs, SNSEventsTests.cs.

Step 13: Create Valid Event Examples + Source Generator Test

Create: Libraries/test/TestServerlessApp/{ServiceName}EventExamples/Valid{ServiceName}Events.cs.txt Create: Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/{ServiceName}/ (generated handler snapshots) Create: Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/ServerlessTemplates/{serviceName}Events.template Modify: Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/SourceGeneratorTests.cs — add VerifyValid{ServiceName}Events() test

Step 14: Create Invalid Event Examples + Source Generator Test

Create: Libraries/test/TestServerlessApp/{ServiceName}EventExamples/Invalid{ServiceName}Events.cs.error

Cover: invalid property values, invalid params, invalid return type, multiple events, invalid ARN, invalid resource name.

Modify: Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/SourceGeneratorTests.cs — add VerifyInvalid{ServiceName}Events_ThrowsCompilationErrors() test with diagnostic assertions including line spans.

Step 15: Create Generated Code Snapshots

Create: Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/{ServiceName}/

Tip: Run the source generator once to get actual output, then use as snapshot.

Step 16: Create Integration Test

Create: Libraries/test/TestServerlessApp.IntegrationTests/{ServiceName}EventSourceMapping.cs Modify: Libraries/test/TestServerlessApp.IntegrationTests/IntegrationTestContextFixture.cs — resource lookup Modify: Libraries/test/TestServerlessApp.IntegrationTests/DeploymentScript.ps1 — if needed

Step 17: Update AnalyzerReleases.Unshipped.md

Modify: Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Diagnostics/AnalyzerReleases.Unshipped.md

File Map Summary

Important Conventions

• Copyright header on every new .cs file
• Consistent else if chaining in attribute builders (never if then if for the same loop)
• Both JSON and YAML template formats must be tested in writer tests
• Invalid event test spans must reference exact line numbers in the .cs.error file
• .cs.txt extension for valid test files (prevents deployment)
• .cs.error extension for invalid test files (prevents compilation)
• Use enums instead of strings when you need to represent a fixed, known set of constants that do not change frequently (e.g., StartingPosition, AuthType, HttpApiVersion). Enums provide compile-time type safety, eliminate the need for manual string validation in the Validate() method, and prevent invalid values from being set. In attribute builders, enum values come through Roslyn's AttributeData as their underlying int value and must be cast accordingly (e.g., (MyEnum)(int)pair.Value.Value). When writing enum values to CloudFormation templates, use .ToString() to convert back to the string representation.

Source: aws/aws-lambda-dotnet — distributed by TomeVault.