Module tower::balance::pool

This module defines a load-balanced pool of services that adds new services when load is high.

The pool uses poll_ready as a signal indicating whether additional services should be spawned to handle the current level of load. Specifically, every time poll_ready on the inner service returns Ready, Pool consider that a 0, and every time it returns Pending, Pool considers it a 1. Pool then maintains an exponential moving average over those samples, which gives an estimate of how often the underlying service has been ready when it was needed “recently” (see Builder::urgency). If the service is loaded (see Builder::loaded_above), a new service is created and added to the underlying Balance. If the service is underutilized (see Builder::underutilized_below) and there are two or more services, then the latest added service is removed. In either case, the load estimate is reset to its initial value (see Builder::initial to prevent services from being rapidly added or removed.

Structs

Builder

A builder that lets you configure how a Pool determines whether the underlying service is loaded or not. See the module-level documentation and the builder’s methods for details.

Pool

A dynamically sized, load-balanced pool of Service instances.

PoolDiscoverer

A wrapper around MakeService that discovers a new service when load is high, and removes a service when load is low. See Pool.