[feature request] make the dfget to get the list of supernodes more flexible

[lowzj]

Why you need it?

Now dfget gets the list of supernodes from configuration file or command line parameters.
It's inconvenient to update the list or

• inconvenient to update client's configuration when scale up or scale down the supernodes cluster.
• cannot customize the selection logic that better fits the user's scenario

How it could be?

Dragonfly defines the interface how to get supernodes and provides a default implementation that get supernodes from config/CLI. Then users can write a new implementation to select supernodes.

• SupernodeLocator is defined interface which dfget's core flow invokes
• StaticLocator is the default implementation Dragonfly provided
• DynamicLocator is a user customized implementation for example:
  • it get supernode from StaticLocator firstly, and return it to caller if exists
  • otherwise, it get the list of supernodes from remote manager and store the result into local cache for next time

Other related information