CRON Functionality

[djsuperchief]

A CRON is required as a pre-requisite for things like FROM http.
It needs to be high performing and use standard CRON syntax and not some specialised version of it.

CRON needs to be part of the core library so that it can be the core From trigger.
We need to consider how this will affect onward components and how we need to structure FROM's going forward. This may be a breaking change.

Thoughts

Internal pub/sub might be the way to go on this one as we will need to think about how we trigger a FROM component from the CRON. FROM components are the base hosted service and as such will run in their own scope. The CRON cannot be the root from as this will mean that we will need to break current FROM component structure to allow this to execute.
Choices are:

1. FROM component gets split from single host component into a host and execution. The advantage of this is that it allows us to have a single CRON FROM and then every component can then be executed as part of the chain. The disadvantage of this is that it changes, fundamentally, how the chain is built.
2. CRON is a second hosted services and it publishes an event into an internal Pub/Sub which gets picked up by a listening FROM component. The advantage here is that from a user perspective, there is little change except saying the chain happens on a CRON. The disadvantage, again, is that each component will need to listen to the event in case it is specified as a CRON.

One way or another, the structure of the FROM has to change to allow a CRON. CRON does make sense to be part of the Core.

The other advantage of this is that a CRON will allow some non from components to now have a FROM that only gets triggered from a CRON.

Further Thoughts
Actually, we do not need to change the behaviour at all. CRON replaces FROM. The addressing all the same but you specify the cron as part of the construction.
You can have a "NONE" CRON which is a generic "run this load of Processing and TO" or you can have a Cron("componenturi", "cron"). The component has to have something implementing ICronable which is then up to the component how the inner implementation works. The basic gist is that FROM continues to work as is.

CRON is essentially a built in FROM component. Start / Stop all the same and the actual execution is done on...well...cron.

I think it could work and will mean we can do components that haven't got a FROM to have a FROM (SQL, S3 etc).

One thing worth considering is do we implement an overall Kyameru CRON with jobs or just implement per chain?