3.18. Checking System Events

In order to ensure that no events are lost, it is recommended to write
the index of the last processed event, in persistent storage.
Here, it is stored in a file called index.txt. In a production environment,
it will likely be stored in a database.
This is the function that is called to process the events. It prints
the identifier and description of each event.
Find the root of the tree of services:
Find the service that manages the collection of events:
If no index is stored yet, retrieve the last event and start with it.
Events are ordered by index, in ascending order. max=1 retrieves only one event,
the last event.
This loop retrieves the events, always starting from the last index. It waits
before repeating. The from parameter specifies that you want to retrieve
events that are newer than the last index that was processed. Note: the max
parameter is not used, so that all pending events will be retrieved.

# In order to ensure that no events are lost, it is recommended to write
# the index of the last processed event, in persistent storage.
# Here, it is stored in a file called index.txt. In a production environment,
# it will likely be stored in a database.
INDEX_TXT = 'index.txt'.freeze

def write_index(index)
  File.open(INDEX_TXT, 'w') { |f| f.write(index.to_s) }
end

def read_index
  return File.read(INDEX_TXT).to_i if File.exist?(INDEX_TXT)
  nil
end

# This is the function that is called to process the events. It prints
# the identifier and description of each event.
def process_event(event)
  puts("#{event.id} - #{event.description}")
end

# Find the root of the tree of services:
system_service = connection.system_service

# Find the service that manages the collection of events:
events_service = system_service.events_service

# If no index is stored yet, retrieve the last event and start with it.
# Events are ordered by index, in ascending order. max=1 retrieves only one event,
# the last event.
unless read_index
  events = events_service.list(max: 1)
  unless events.empty?
    first = events.first
    process_event(first)
    write_index(first.id.to_i)
  end
end

# This loop retrieves the events, always starting from the last index. It waits
# before repeating. The from parameter specifies that you want to retrieve
# events that are newer than the last index that was processed. Note: the max
# parameter is not used, so that all pending events will be retrieved.
loop do
  sleep(5)
  events = events_service.list(from: read_index)
  events.each do |event|
    process_event(event)
    write_index(event.id.to_i)
  end
end