Add init handler and invoke it after config get deserialized

[rucciva]

Sometimes, it is necesary to do some kine of initialization based on config of the plugin. Maybe invoke instanceConfig.Init() here?

[holowinski]

It would be great to have that capability.

[StarlightIbuki]

The initialization could be achieved with a function called only once when the access handler is invoked for the first time. Do we have some benefits to introduce a new API for this? Could this workaround work for you? Also, I feel that we need to find another name for this as it would be confused with the init phase (and it's different both in actual behavior and implementation)

[rucciva]

The initialization could be achieved with a function called only once when the access handler is invoked for the first time. Do we have some benefits to introduce a new API for this? Could this workaround work for you?

Actually, this is what i'm doing right now as a workaround. But i think its not simple, e.g. you need to guard with mutex to prevent more than once initialization.

Another thing regarding the current structure of the api to be considered is that it is quite easy to be trapped to do initialization inside the constructor, meanwhile the configuration is not yet loaded when the constructor is invoked so the initialization would be invalid. well at least thats what happened to me at first.

Also, I feel that we need to find another name for this as it would be confused with the init phase (and it's different both in actual behavior and implementation)

PreInit() or PostConfig ?

[fffonion]

Hi @rucciva, due to the design of PDK and because it derives from the Lua plugin design, each plugin instance (the instanceData instance you mentioned in go-pdk) must be implemented stateless. As Kong handle many connections in a non-blocking way, a (go) plugin instance could be handling lots of requests at the same time. So we are not providing this init API to prevent user to program a stateful plugin handler. That means either the init happens in global level or "per-instance" level, you will need a mutex to access it anyway.

[rucciva]

Hi @fffonion , thanks for responding, but forgive me, i'm not sure i understand your reasoning.

a (go) plugin instance could be handling lots of requests at the same time

having an init won't make the plugin unable to handle a lots of requests, it would be executed only once before the handling of any request.

So we are not providing this init API to prevent user to program a stateful plugin handler

I'm not sure what you means with "stateful". The init that i propose was to derive or extend the state that was loaded from the config passed by kong one time only. For example there is no easy way to specify dynamic object as kong's config, so one of the possible way is to specify it as json/yaml string and decode that string to get back the dynamic object.

That means either the init happens in global level or "per-instance" level

If the init is implemented and invoked here only after the config is loaded, won't that means it is "per-instance" level?

[StarlightIbuki]

@fffonion I think what they want is pretty much like a configure handler (https://github.com/Kong/kong/pull/11703). We could introduce this to Go PDK. What do you think?