mdeuser commented on a change in pull request #3778: Add documentation to the
loadbalancer.
URL:
https://github.com/apache/incubator-openwhisk/pull/3778#discussion_r197458158
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File path:
core/controller/src/main/scala/whisk/core/loadBalancer/ShardingContainerPoolBalancer.scala
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@@ -45,10 +45,77 @@ import scala.concurrent.{ExecutionContext, Future, Promise}
import scala.util.{Failure, Success}
/**
- * A loadbalancer that uses "horizontal" sharding to not collide with fellow
loadbalancers.
+ * A loadbalancer that schedules workload based on a hashing-algorithm.
+ *
+ * ## Algorithm
+ *
+ * The loadbalancer schedules workload based on a hashing-algorithm. At first,
for every namespace + action pair a hash
+ * is calculated and then bounded to the number of invokers available. The
determined index is the so called "home
+ * invoker". This is the invoker where the following progression will
**always** start. If this invoker is healthy and
+ * if there is capacity on that invoker, the request is scheduled to it. If
one of these prerequisites is not true,
+ * the index is incremented by a step-size. The step-sizes available are the
all coprime numbers smaller than the amount
+ * of invokers available (coprime, to minimize collisions while progressing
through the invokers). The step-size is
+ * picked by the same hash calculated above, bounded to the number of
step-sizes available. The home-invoker-index is
+ * now incremented by the step-size and the checks (healthy + capacity) are
done on the invoker we land on now. This
+ * procedure is repeated until all invokers have been checked at which point
the "overload" strategy will be employed,
+ * which is to choose a healthy invoker randomly.
+ *
+ * An example:
+ * - availableInvokers: 10 (all healthy)
+ * - stepSizes: 1, 3, 7 (note how 2 and 5 is not part of this because it's not
coprime to 10)
+ * - hash: 13
+ * - homeInvoker: 13 % 10 = 3
+ * - stepSizeIndex: 13 % 3 = 1 => stepSize = 3
+ *
+ * Progression to check the invokers: 3, 6, 9, 2, 5, 8, 1, 4, 7, 0 --> done
+ *
+ * This heuristic is based on the assumption, that the chance to get a warm
container is the best on the home invoker
+ * and degrades the more hops you make. The hashing makes sure that all
loadbalancers in a cluster will always pick the
Review comment:
maybe use `steps` instead of `hops` for consistency with the prior language
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