Module tower::balance::p2c [−][src]
Expand description
This module implements the “Power of Two Random Choices” load balancing algorithm.
It is a simple but robust technique for spreading load across services with only inexact load
measurements. As its name implies, whenever a request comes in, it samples two ready services
at random, and issues the request to whichever service is less loaded. How loaded a service is
is determined by the return value of Load.
As described in the Finagle Guide:
The algorithm randomly picks two services from the set of ready endpoints and selects the least loaded of the two. By repeatedly using this strategy, we can expect a manageable upper bound on the maximum load of any server.
The maximum load variance between any two servers is bound by
ln(ln(n))wherenis the number of servers in the cluster.
The balance service and layer implementations rely on service discovery to provide the
underlying set of services to balance requests across. This happens through the
Discover trait, which is essentially a Stream that indicates
when services become available or go away. If you have a fixed set of services, consider using
ServiceList.
Since the load balancer needs to perform random choices, the constructors in this module usually come in two forms: one that uses randomness provided by the operating system, and one that lets you specify the random seed to use. Usually the former is what you’ll want, though the latter may come in handy for reproducability or to reduce reliance on the operating system.
Structs
Efficiently distributes requests across an arbitrary number of services.
Constructs load balancers over dynamic service sets produced by a wrapped “inner” service.
A Balance in the making.