JSON serialization (Lambda function)

The rule

Lambda hands the function a JSON payload and expects JSON back. The [assembly: LambdaSerializer(...)] attribute at the top of LambdaFunction.cs sets the serializer for every function: LambdaFunctionJsonSerializer in src/Presentation/LambdaFunction/Serialization/. It's a SourceGeneratorLambdaJsonSerializer<LambdaFunctionJsonSerializerContext> from Amazon.Lambda.Serialization.SystemTextJson that writes camelCase, like the other templates. It uses the source-generated metadata in src/Presentation/LambdaFunction/Serialization/LambdaFunctionJsonSerializerContext.cs, so nothing is serialised with reflection, which keeps cold starts short.

Responses are trees: a list returns flat records, and a review holds a flat Movie and a flat Author. There's no cycle handling.

Steps

1. Add every new request and response type to LambdaFunctionJsonSerializerContext with [JsonSerializable(typeof(...))], including each Page<T> you return. The HTTP API functions also need Problem, APIGatewayHttpApiV2ProxyRequest and APIGatewayHttpApiV2ProxyResponse, which are already there.
2. Change how JSON looks in the LambdaFunctionJsonSerializer constructor, not on single properties, and keep the request and response tests in tests/Presentation.Tests.Unit/Serialization/LambdaFunctionJsonSerializerTests.cs in step. The serializer's own options replace the context's, so [JsonSourceGenerationOptions] on the context has no effect.
3. Add a serializer test for a new request type (it reads the JSON a caller sends) and for any response whose shape matters to callers.

Example

// src/Presentation/LambdaFunction/Serialization/LambdaFunctionJsonSerializerContext.cs
[JsonSerializable(typeof(ListRequest))]
[JsonSerializable(typeof(ListReviewsRequest))]
[JsonSerializable(typeof(Page<Movie>))]
[JsonSerializable(typeof(Page<Author>))]
[JsonSerializable(typeof(Page<Review>))]
public partial class LambdaFunctionJsonSerializerContext : JsonSerializerContext
{
}

Don't

• Switch to DefaultLambdaJsonSerializer or CamelCaseLambdaJsonSerializer. They use reflection and slow every cold start.
• Use [JsonPropertyName] to rename one property.
• Use Newtonsoft.Json or Amazon.Lambda.Serialization.Json.
• Let a response type refer back to its parent.

Check

dotnet test tests/Presentation.Tests.Unit --filter "FullyQualifiedName~Serializer"