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Watcher Classes

Watchers are the piece of Synapse that watch an external service registry and reflect those changes in the local configuration state. Watchers should conform to the interface specified by BaseWatcher and when your watcher has received an update from the service registry you should call set_backends(new_backends) to trigger a sync of your watcher state with local configuration (HAProxy, files, etc ...) state. See the Backend Interface section for what fields in registrations Synapse understands.

require "synapse/service_watcher/base"

class Synapse::ServiceWatcher
  class MyWatcher < BaseWatcher
    def start
      # write code which begins running service discovery

    def stop
      # write code which tears down the service discovery

    def ping?
      # write code to check in on the health of the watcher

    def validate_discovery_opts
      # here, validate any required options in @discovery

    ... setup watches, poll, etc ... and call set_backends when you have new
    ... backends to set


Watcher Plugin Inteface

Synapse deduces both the class path and class name from the method key within the watcher configuration. Every watcher is passed configuration with the method key, e.g. zookeeper or ec2tag.

Class Location

Synapse expects to find your class at synapse/service_watcher/#{method}. You must make your watcher available at that path, and Synapse can "just work" and find it.

Class Name

These method strings are then transformed into class names via the following function:

method_class  = method.split('_').map{|x| x.capitalize}.join.concat('Watcher')

This has the effect of taking the method, splitting on '_', capitalizing each part and recombining with an added 'Watcher' on the end. So zookeeper_dns becomes ZookeeperDnsWatcher, and zookeeper becomes Zookeeper. Make sure your class name is correct.

### Backend interface Synapse understands the following fields in service backends (which are pulled from the service registries):

host (string): The hostname of the service instance

port (integer): The port running the service on host

name (string, optional): The human readable name to refer to this service instance by

weight (float, optional): The weight that this backend should get when load balancing to this service instance. Full support for updating HAProxy based on this is still a WIP.

haproxy_server_options (string, optional): Any haproxy server options specific to this particular server. They will be applied to the generated server line in the HAProxy configuration. If you want Synapse to react to changes in these lines you will need to enable the state_file_path option in the main synapse configuration. In general the HAProxy backend level haproxy.server_options setting is preferred to setting this per server in your backends.