Feature request: manipulating the idempotent response

[am29d]

Use case

I wan to have an option to call a function when idempotency is triggered. In some cases I want to either change payload to signal to the caller that this request was idempotent (x-idempotency: true) or to have side effects, i.e. add custom metric.

While we have this option in python with idempotency_hook there is no option available int TypeScript.

Solution/User Experience

import type { IdempotencyRecord } from '@aws-lambda-powertools/idempotency/persistence';
import {
  IdempotencyConfig,
  makeIdempotent,
} from '@aws-lambda-powertools/idempotency';
import type { Context } from 'aws-lambda';
import { DynamoDBPersistenceLayer } from '@aws-lambda-powertools/idempotency/dynamodb';

interface HandlerReponse {
  message: string;
  statusCode: number;
  headers?: Record<string, string>;
}

const myCustomHook = async (
  response: HandlerReponse,
  record: IdempotencyRecord
) => {
  response.headers['x-idempotency-key'] = record.idempotencyKey;
  return response;
};

const config = new IdempotencyConfig({
  reponseHook: myCustomHook,
});

const hanlder = async (event: unknown, context: Context) => {
  // ... process your event
  return {
    message: 'success',
    statusCode: 200,
  };
};

const persistenceStore = new DynamoDBPersistenceLayer({
  tableName: 'idempotencyTableName',
});
const idempotentHandler = makeIdempotent(hanlder, {
  config: config,
  persistenceStore: persistenceStore,
});

Following the Python implementation, we would add resposneHook option to the config, that takes a function as a parameters. The response hook function has signature with response and idempotencyRecord and returns a modified version of the response. The response type should match the return type of the handler function.

Alternative solutions

No response

Acknowledgment

• This feature request meets Powertools for AWS Lambda (TypeScript) Tenets
• Should this be considered in other Powertools for AWS Lambda languages? i.e. Python, Java, and .NET

Future readers

Please react with 👍 and your use case to help us understand customer demand.

[dreamorosi]

Good point - we are missing this feature and it'd be good to have it for feature parity.

I'm going to mark it as contributions welcome.

Note

For those interested in contributing, please leave a comment below so that we can assign the issue to you and make sure we don't duplicate efforts. Also, if you have any further questions please don't hesitate to ask here or on our Discord channel.

[arnabrahman]

I would like to contribute to this but I need some pointers.

I believe if the custom hook function is provided, it should be called from here. Something like this?

const resultFromIdempotency = IdempotencyHandler.determineResultFromIdempotencyRecord(
    idempotencyRecord
);
returnValue.result = hasHook? customHook(resultFromIdempotency, idempotencyRecord) : resultFromIdempotency;
        

Once again, I need your expert opinion on this, @dreamorosi. Thanks.

[dreamorosi]

Hi @arnabrahman, thanks for giving some traction to this feature.

I've looked at the implementation in Powertools for AWS Lambda (Python) and it roughly matches the suggestion you shared above. In their implementation they call the hook in the equivalent of our IdempotencyHandler.determineResultFromIdempotencyRecord method just before returning, which is pretty much the same as you are suggesting¹.

Normally I would advocate for consistency across projects for this type of feature, mainly to reduce the cognitive load for recurrent maintainers by having similar implementation.

In this case we might not be able to the same unless we change the determineResultFromIdempotencyRecord() method from static to normal, so that it can access the config object. Assuming we make the method non-static, I would be inclined to add the logic within the determineResultFromIdempotencyRecord() method instead.

Regarding the actual implementation, I think the logic you're suggesting makes sense, so from my side we're good to move forward if you're still interested.

Footnotes

1. In their implementation they also do the equivalent of an undefined check - which is a bug that I have reported (Bug: response hook is not called when idempotent function returns None powertools-lambda-python#5150). ↩

[arnabrahman]

Yes, I am interested. Please assign me on this, @dreamorosi