Backends

Every backend is a separate module. The following backends are available:

• Servers (Node or Edge runtime)
• Middleware (If manually set to external)
• Image Optimization
• Revalidation
• Warmer
• Initializer

All these backends can be overrided to fit your needs. They share some common ground for the configuration and the way they are used.

Common Overrides

Every custom overrides (not string) share a BaseOverride types and needs to be wrapped this way:

type BaseOverride = {
  name: string;
};
 
// This is the type of the override
type LazyLoadedOverride<T extends BaseOverride> = () => Promise<T>;

Wrapper

The wrapper is the main entrypoint for the backend. This is the type of the wrapper:

type WrapperHandler<
  E extends BaseEventOrResult = InternalEvent,
  R extends BaseEventOrResult = InternalResult,
> = (
  handler: OpenNextHandler<E, R>,
  converter: Converter<E, R>,
) => Promise<(...args: any[]) => any>;
 
export type Wrapper<
  E extends BaseEventOrResult = InternalEvent,
  R extends BaseEventOrResult = InternalResult,
> = BaseOverride & {
  wrapper: WrapperHandler<E, R>;
  supportStreaming: boolean;
};

The handler is the original handler that is being wrapped. The converter is the converter that is being used to convert the event and the result.

Here is a list of the provided wrappers:

• aws-lambda - The default AWS Lambda wrapper - It is the default wrapper
• aws-lambda-streaming - The AWS Lambda wrapper with streaming support
• node - The Node wrapper - Create a node server, not suitable for serverless
• cloudflare - The Cloudflare wrapper - For Cloudflare Worker

Aws Lambda streaming

If you want to enable streaming in lambda, you need to use this wrapper. It is not enabled by default.

Be aware that you might encounter some unexpected behaviors when using streaming. Aws Lambda seems to apply some buffering on the response. In some rare cases it might mean that streaming won't properly start. This is an issue with the lambda runtime itself, but this should only impact TTFB (Time To First Byte) in those cases. See this github issue (opens in a new tab)

Converter

The converter is used to convert the event and the result. This is the type of the converter:

export type Converter<
  E extends BaseEventOrResult = InternalEvent,
  R extends BaseEventOrResult = InternalResult,
> = BaseOverride & {
  convertFrom: (event: any) => Promise<E>;
  convertTo: (result: R, originalRequest?: any) => any;
};

The convertFrom method is used to convert the event to the internal event used by the backend internal handler. The convertTo method is used to convert the result to the expected result. convertTo can usually be ignored when using streaming as the result is being streamed directly.

Here is a list of the provided converters:

• aws-apigw-v2 - The default AWS API Gateway v2 converter - It is the default converter
• aws-apigw-v1 - The AWS API Gateway v1 converter
• aws-cloudfront - The AWS CloudFront converter - Used for lambda@edge
• edge - The Edge converter - Converts from Request and to Response
• node - The Node converter - Converts from IncomingMessage and to ServerResponse
• sqs-revalidate - The SQS Revalidate converter - Used by default for the revalidation backend
• dummy - The Dummy converter - Does nothing