upstream-corteza/server/pkg/eventbus

# pkg/eventbus

Package handles event dispatching and handler registration

## Event

Event (in context of eventbus package) contains of resource type, event type and a matcher.

### Resource type

Resource types are identifiers of internal resources, like user, role, request, ...

### Event type

Type describes direct or indirect user action or some other event in the system.

#### Deferred events

Deferred events (onTimestamp and onInterval) are executed by scheduler package service.

#### Manual events

Manual events (onManual) are a special case.
These events are not (nor should be) dispatched directly or through event-bus.

We use it to route requests and pack attributes from the API to the Corredor service

### Event matcher

Matcher is a function on event that helps filtering fired events.
It decides (returns true or false) if fired event is a match for any of a registered handlers

Bus has basic internal filtering for resource & event type.
Other constraints are passed to event's matcher function.

.Matcher will receive 2+ string parameters:
 - name
 - operator
 - zero or more values

It's matcher responsibility to handle contents of name, operator and values parameters.

Constraint checker procedure will call matcher for each constraint.
All constraints must match.
First non-match will break constraint checking procedure.


## Handler

Handler is combination of (event matching) rules/constraints and handler callback function.
Function is called when a event compatible with registered rules & constraints is fired.

Handler can respond to multiple resource/event combinations.

### Constraints

Matcher function is called multiple times, once per each handler constraint.
If any of the calls result in a non-match, check

Handler without any constraints is considered a match.

### Weight

Weight controls order of execution.