How to use idempotency with path parameters

[tknisch]

Hi,

I'm building a REST API with usage of API Gateway and want to use idempotency for some routes.

Lets consider I have the following route:

PUT /notifications/{notificationId}

where the body might be

{
  "status": "READ"
}

When I now use

const config = new IdempotencyConfig({
  eventKeyJmesPath: "powertools_json(body)",
});

it will only use the body for the Idempotency Key, which is the same on different notification ids.
But if I use the whole event it will not cache because there are timestamps in the event too.

How can I build an idempotency key that is composed of several values like body and path?

[dreamorosi]

Hi @tknisch, thanks for opening this discussion.

To account for multiple fields, you can use square brackets and list all the fields you want to extract from the payload, in this case it would be [requestContext.http.path, powertools_json(body)] or [rawPath, powertools_json(body)] depending on which field you want to use to get the path.

For future reference, if you want to test different types of expressions, you can import directly the JMESPath function used by the Idempotency utility and try expressions directly, for example:

import { search } from '@aws-lambda-powertools/jmespath';
import { PowertoolsFunctions } from '@aws-lambda-powertools/jmespath/functions';

const extracted = search('[requestContext.http.path, powertools_json(body)]', event, {
  customFunctions: new PowertoolsFunctions(),
})

Alternatively, you can also use the online playground in the JMESPath website, they don't have custom functions like powertools_json() but you can still test most expressions like this one.

Hope this helps!

[tknisch]

Hi @dreamorosi ,
thank you very much for your fast response!

The solution with the square brackets is working fine!

Maybe this should be added to the documentation as well?

[dreamorosi]

Glad it helped!

We have a similar example with square brackets here (see L27).

It's slightly different but it shows how to get keys within an object. I'll keep it in mind and consider adding it to some other examples during the next update.

[tknisch]

Yes, I saw this examples. But it wasn't really clear for me that I can also use this to get different fields from the event, too.