Add implementation-check on startup

[JPSchellenberg]

In order to improve the development we have to make sure, that reported errors are not in fact a wrong implementation.
So we should have something like try/catch blocks around calls to the implementations and check if they fail.

Or is there a better way?

[sr258]

I'm not sure about this...

What would the catch blocks do? Just report that the error was raised in the implementation? You can already see this in the stack trace generated by a thrown error.

[JPSchellenberg]

Yes, I can see that. But when I get a question from someone else who is trying to implement it I often don't have the stack trace. It is mostly something like Cannot read property xzy of undefined at File and Line. And then I have to look up all the source-code files and generated code to know where the error comes from. I hope this would make the implementation more 'user-friendly' as they can see that the error is within the implementation and not our source-code.

[sr258]

Could you provide an example of how the catch block might look like? I'm a bit concerned that this might make debugging even harder as there is a risk that it swallows the actual error or obscures debug information (in particular the stack trace) that could be useful.

[JPSchellenberg]

The example is the get-method of the ContentTypeCache.

If this.storage.load('...') fails it returns undefined. When using the .filter() in line 143 it throws the Error: TypeError: Cannot read property 'filter' of undefined.

With the try/catch block we could throw something like: Missing load-method on content-storage. Please make sure your implementation of the ContentStorage is correct. For more information see..

    public async get(...machineNames: string[]): Promise<ContentType[]> {
        try {
            if (!machineNames || machineNames.length === 0) {
                return this.storage.load('contentTypeCache');
            }
            return (await this.storage.load('contentTypeCache')).filter(
                (contentType: ContentType) =>
                    machineNames.some(
                        (machineName: string) =>
                            machineName === contentType.machineName
                    )
            );
        } catch (error) {
            log(error);
            throw new Error(
                'Missing load-method on content-storage. Please make sure your implementation of the ContentStorage is correct. For more information see..'
            );
        }
    }

[JPSchellenberg]

Another idea would be to check the implementations on startup and throw errors there. Something like an implementation-validator that checks if everything works. But it is just an idea.

[sr258]

I would prefer the implementation check on startup (or some test cases that you can run?). In the example above the catch block would throw the error message regardless of what goes wrong, which might be something different than just a missing load method. It could be an error in the database of the implementation, an error in our code etc. All of this is obscured by the catch statement, and I think giving a proper error message is impossible.