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if you're registering outputs from your component it's needed. If you're not it's best practice, and we may enforce it in future

Do you provide it with the values? In this example it doesn't: https://www.pulumi.com/docs/tutorials/aws/s3-folder-component/#create-an-s3-folder-component but this one does: https://www.pulumi.com/docs/intro/concepts/resources/#registering-component-outputs

So it is definitely a little confusing. But

registerOutputs

is how pulumi keeps track of pending asynchronous tasks. In your

create-an-s3-folder-component

example they are not providing the output to the

registerOutputs

call because functionally it is not necessary - functionally all that example is doing is forwarding the

bucketName

output from the

aws.s3.Bucket

to the parent

ComponentResource

. So this works because the

aws.s3.Bucket

has already registered that output and no additional tracking is necessary. The

registering-component-outputs

example also technically doesn't need to provide the output to the function because functionally it is just forwarding an output that was already registered by the component that owns it. But consider the following:

var newOutput = bucket.Name.apply(bucketName => {
    // do some asynchronous work, make an API call
    return value;
});

this.output = newOutput;
this.registerOutputs();

In this scenario, since I did not provide

newOutput

to the

registerOutputs

call pulumi has no way of tracking the asynchronous work that I added. So you have introduced a race-condition, it is now possible for your

pulumi up

to finish before that asynchronous work has finished, and you might have leaky promises. Another thing to keep in mind is that

Output<T>

doesn't just convey asynchronous work, it also conveys dependency. So if you want your dependency tree to accurately reflect that consuming resources depend on the

ComponentResource

rather than the inner

aws.s3.Bucket

than you want to register it as a new output. Otherwise your dependency path will be

consumer -> aws.s3.Bucket

instead of

consumer -> componentResource -> aws.s3.Bucket

which conceptually breaks the logical grouping of your component resource. But this is less important because I can't think of scenario where the former dependency path would break anything, it just may not be a strictly accurate representation of your resources

Thanks a lot for the great explanation, makes way more sense now :)