Loose event validation

[lance]

Is your feature request related to a problem? Please describe.
Currently, when creating a CloudEvent instance, one of the final things that happens in the constructor before freezing the object is this.validate() is called. The validate() function will throw an exception if the event is invalid. This effectively means that there is no way to create an event that's invalid. In most cases, this is probably for the best. But what happens if you have a system with a known bug that's emitting invalid events, and some JavaScript service using this module is receiving them. There's no way to actually do that without forking this repo.

Describe the solution you would like to see

• Optionally, don't call this.validate() in the constructor based on an added boolean parameter (strict: boolean = true by default).
• Change Detector interface from feat: introduce Message, Serializer, Deserializer and Binding interfaces #324 to use an optional parameter for loose validation.
• Change the Deserializer interface which converts from a Message to a CloudEvent to use an optional parameter for loose validation.

Additional context
This is all in the context of creating events manually, or deserializing an incoming Message into a CloudEvent. I haven't thought much about the serialization step. If a user manually creates an invalid event, and then serializes it to a Message, should that throw an exception? I think maybe it should, but interested in other's opinions.

[lholmquist]

When i was first adding the validation to the CloudEvent constructor, i contemplated adding what you are talking about in. My main reason was because of how the test were written.

I can see this being a feature, but my only concern is if a user has a service where they send cloud events, and turn off the event validation. then another user uses that service to receive events, how will they know that the service has the validation turned off. They will probably get errors when trying to "accept" the event and think that maybe there is a bug in this library.

[grant]

Thanks for creating the issue!

According to the SDK spec, there are various methods described. One of them is creation and one is validation.
https://github.com/cloudevents/spec/blob/master/SDK.md#technical-requirements

I would assume the composition method and validation method are separate and folks can use them separately as they wish for whatever reason they want.

As such, I think the validation method should not be called by default within the composition method. (Maybe we can have it for convenience.) It should be default false. This is important because consumers of CloudEvent from major CE producers can expect to have a valid event and don't need this.

i.e. we don't need new CloudEvent({...}, false) everywhere.