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# Recipe DSL method for event handler hooks
Allow cookbook authors to easily add custom logic on Chef events.
## Motivation
Chef has an extensive [event dispatch mechanism](https://github.com/chef/chef/blob/main/lib/chef/event_dispatch/base.rb).
But incorporating some custom logic against any of the events is an onerous process, which involves
subclassing the based event handler and adding it via the config. This RFC
proposes a recipe DSL method to ease this. For new Chef users, this will reduce
the entry barrier.
## Specification
Currently, `chef-client` sets up couple of default handlers -- e.g. doc, base -- during
initialization. An additional empty event handler -- a subclass
of the base handler without any custom logic -- can be added alongside the
existing handlers, which will used as a placeholder for user-specific hooks.
A top level (::Chef) method will be introduced (`event_handler`) to wrap the
main event handler DSL (`on`). Users can tap into one of the event types
-- as specified in base dispatcher -- using this DSL to execute their custom logic.
The additional top level method(`Chef.event_handler`) will allow the handler
DSL usage in and outside of recipes, and also ease writing backward-compatible
changes for the `on` method if need be.
The following is an example of sending hipchat notification on chef run failure.
```ruby
Chef.event_handler do
on :run_failed do |exception|
hipchat_notify exception.message
end
end
```
The following is another example of taking a distributed lock via `etcd`, to
prevent concurrent chef runs in different nodes.
```ruby
lock_key = "#{node.chef_environment}/#{node.name}"
Chef.event_handler do
on :converge_start do |run_context|
Etcd.lock_acquire(lock_key)
end
end
Chef.event_handler do
on :converge_complete do
Etcd.lock_release(lock_key)
end
end
```
The following is another example of sending a hipchat alert on a key config change.
```ruby
Chef.event_handler do
on :resource_updated do |resource, action|
if resource.to_s == 'template[/etc/nginx/nginx.conf]'
Helper.hipchat_message("#{resource} was updated by chef")
end
end
end
```
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