Document how to achieve singleton event handler

[mostlyobvious]

There are situations when firing more instances of event handlers would be problematic. Show how one could achieve this (and there could be multiple ways to achieve it).

[paneq]

I am not sure I understand. Could you elaborate?

[mostlyobvious]

Once I did following stunt:

• there were domain events describing some fact in the system that carried geolocation in data
• I had a separate context that was interested in plotting that location on a map, more-or-less live
• for a reason I don't recall fully now it was beneficial to limit or even serialize event processing by that particular handler

What I meant was showing how one could achieve it, either via:

• sidekiq feature when you have sidekiq configured as an async event handler
• pessimistic lock with help of underlying database engine

Btw. this serialize event processing might be probably better suited to process managers documentation (we don't seem to have it?).