Correct way to use uuid generation

[davidtimmsinshur]

I have a postman collection in which one prerequest script generates a uuid using the uuid module to a variable and then uses this in further requests. For example:

const { v4: uuidv4 } = require('uuid');
pm.variables.set("uniqueId", uuidv4() );

When using postman-to-k6, this breaks and those requests are not sent by k6 because it does not find the uuid module. Following these instructions https://k6.io/docs/examples/generating-uuids/ and browserifying uuid then importing into the flows.js does not work as k6 throws the following error:

level=error msg="Error: crypto.getRandomValues() not supported. See https://github.com/uuidjs/uuid#getrandomvalues-not-supported\n\tat

Based on the discussion here grafana/k6#725 the Crypto module is not supported by k6. After some further googling, this https://k6.io/docs/javascript-api/jslib/utils/uuidv4/ does work in my use case.

I therefore manually add the following at the top of the generated flows.js

import { uuidv4 } from 'https://jslib.k6.io/k6-utils/1.2.0/index.js';

My first question is whether I am using the correct approach and am I missing something? I'm also wondering whether this is potentially a bug with postman-to-k6 where uuid.js is not being added to the shims. There is a list of exclusions but I see no mention in the readme that the modules supported by Postman are not supported by postman-to-k6. I notice that uuid is a requirement of postman-to-k6 and so that module is available in the node_modules.

I realise there is some complexity because a module supported by Postman might not be directly supported by k6 however it would nevertheless be nice to not have to manually edit generated files (or write a script to do so) before execution if there is an overlap between the two tools. Ideally a compatible uuid.js would be available in the shims rather than having to download the k6-utils.

So I am not sure whether this is a bug or a feature request and whether my current approach is recommended in the mean time.

Thanks in advance.

[davidtimmsinshur]

We have changed tack slightly. Now we are not using the uuid library in postman and instead generating a uuid-like string by including this in the postman pre-request script instead of importing the third party library

// const { v4: uuidv4 } = require('uuid');
function guid() {
    function s4() {
        return Math.floor((1 + Math.random()) * 0x10000)
            .toString(16)
            .substring(1);
    }
    return s4() + s4() + '-' + s4() + '-' + s4() + '-' + s4() + '-' + s4() + s4() + s4();
}

pm.variables.set("myUuid", guid() );

This gets round the problem of then manually needing to import uuid to the flows.js after generation.

[thim81]

Hi David,

It is a best practice to keep the usage of third party libraries to a minimum.

Postman offers support for "dynamic" variables, which will allow you to generate all kind of variables, like an UUID by using {{$guid}} which will generate a uuid-v4 style guid.

postman-to-k6 fully supports the available Postman dynamic variables, see https://github.com/apideck-libraries/postman-to-k6/blob/main/changes-since-1.5.0-release.md#full-support-of-the-postman-random-functionsdynamic-variables

By using the build "dynamic" variables, there is no need to struggle with the usage of remote libraries.

[davidtimmsinshur]

Thanks, all good to know.

That being the case, I think it would be useful to update the readme to clarify that the external libraries supported by postman https://learning.postman.com/docs/writing-scripts/script-references/postman-sandbox-api-reference/#using-external-libraries are not supported by postman-to-k6.

For what it's worth, our use case required me to use the same uuid across multiple requests so we needed code to generate the uuid this way and save it to a variable. Due to the fact that pm.request is not supported either, it was not possible to use {{$guid}} in the request as it is then not possible to retrieve the generated value.

[thim81]

Hi David,

postman-to-k6 does support a large range of the external libraries: https://github.com/apideck-libraries/postman-to-k6/tree/main/lib/shim although we might need to review them.

To come back on your use-case, you could still use a dynamic variable and have it generated upfront, using the collection "Pre-request scripts" or reqest "Pre-request script"

const uniqueRef = pm.variables.replaceIn("{{$guid}}") // generates a GUID 
pm.environment.set('uniqueRef', uniqueRef) // set the generated GUID as Postman env variable

What would be the reason of using the pm.request function?

[davidtimmsinshur]

Oh interesting, replaceIn is not a syntax I've seen before. When you mentioned using {{$guid}} I was imagining setting the guid on the request json body as per...

{ "someGuid": "{{$guid}}" }

...Then having to read the pm.request.body.someGuid value (I probably got that wrong but hopefully you get the idea) to save as a variable in future requests.

So in fact the replaceIn method is something new for me and will help with some other solutions I've butchered previously!

Thanks

[thim81]

If you want to use a variable in the response body, you can use indeed

{ "someGuid": "{{$guid}}" }

Or any of the other dynamic variables ...
Example: { "reference": "{{$randomInt}}{{$randomPassword}}{{$randomInt}}{{$randomPassword}}"}

If you want pre-define or re-use a dynamic variable, the replaceIn function is the best way.

[thim81]

If you want to assign variables based on the response body, you can add the following in the "test" of the Postman request.

example:

// Set response object as internal variable
let jsonData = {};
try {jsonData = pm.response.json();}catch(e){}

// pm.collectionVariables - Set get-email-messages.message_id as variable for jsonData.message_id
if (jsonData?.message_id) {
   pm.collectionVariables.set("get-email-messages.message_id", jsonData.message_id);
   // console.log("- use {{get-email-messages.message_id}} as collection variable for value", jsonData.message_id);
} else {
   console.log('INFO - Unable to assign variable {{get-email-messages.message_id}}, as jsonData.message_id is undefined.');
};