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Holistic shard allocation strategy, #29543 (#29555)

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* The rebalance in the LeastShardAllocationStrategy is only comparing the region
  with most shards with the one with least shards. Makes the rebalance rather
  slow. By default it's only rebalancing 1 shard at a time.
* This new strategy looks at all current allocations to find the optimal
  number of shards per region and tries to adjust towards that value.
  Picking from all regions with more shards than the optimal.
* Absolute and relative limit on how many shards that can be rebalanced
  in one round.
* It's also not starting a new rebalance round until the previous has
  completed.
* unit tests
* second phase for fine grained rebalance, due to rounding it will not be perfect in the first phase
* randomized unit test
* configuration settings
* docs
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Patrik Nordwall 2020-09-11 08:49:45 +02:00 • committed by GitHub
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35
akka-docs/src/main/paradox/typed/cluster-sharding.md
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@ -164,9 +164,33 @@ The `number-of-shards` configuration value must be the same for all nodes in the
configuration check when joining. Changing the value requires stopping all nodes in the cluster.
The shards are allocated to the nodes in the cluster. The decision of where to allocate a shard is done
by a shard allocation strategy. The default implementation @apidoc[ShardCoordinator.LeastShardAllocationStrategy]
allocates new shards to the `ShardRegion` (node) with least number of previously allocated shards.
This strategy can be replaced by an application specific implementation.
by a shard allocation strategy.
The default implementation `LeastShardAllocationStrategy` allocates new shards to the `ShardRegion` (node) with least
number of previously allocated shards. This strategy can be replaced by an application specific implementation.
When a node is added to the cluster the shards on the existing nodes will be rebalanced to the new node.
The `LeastShardAllocationStrategy` picks shards for rebalancing from the `ShardRegion`s with most number
of previously allocated shards. They will then be allocated to the `ShardRegion` with least number of
previously allocated shards, i.e. new members in the cluster. The amount of shards to rebalance in each
round can be limited to make it progress slower since rebalancing too many shards at the same time could
result in additional load on the system. For example, causing many Event Sourced entites to be started
at the same time.
A new rebalance algorithm was included in Akka 2.6.10. It can reach optimal balance in a few rebalance rounds
(typically 1 or 2 rounds). For backwards compatibility the new algorithm is not enabled by default.
The new algorithm is recommended and will become the default in future versions of Akka.
You enable the new algorithm by setting `rebalance-absolute-limit` > 0, for example:
```
akka.cluster.sharding.least-shard-allocation-strategy.rebalance-absolute-limit = 20
```
The `rebalance-absolute-limit` is the maximum number of shards that will be rebalanced in one rebalance round.
You may also want to tune the `akka.cluster.sharding.least-shard-allocation-strategy.rebalance-relative-limit`.
The `rebalance-relative-limit` is a fraction (< 1.0) of total number of (known) shards that will be rebalanced
in one rebalance round. The lower result of `rebalance-relative-limit` and `rebalance-absolute-limit` will be used.
### External shard allocation
@ -538,7 +562,10 @@ properties are read by the `ClusterShardingSettings` when created with an ActorS
It is also possible to amend the `ClusterShardingSettings` or create it from another config section
with the same layout as below.
One important configuration property is `number-of-shards` as described in @ref:[Shard allocation](#shard-allocation)
One important configuration property is `number-of-shards` as described in @ref:[Shard allocation](#shard-allocation).
You may also need to tune the configuration properties is `rebalance-absolute-limit` and `rebalance-relative-limit`
as described in @ref:[Shard allocation](#shard-allocation).
@@snip [reference.conf](/akka-cluster-sharding/src/main/resources/reference.conf) { #sharding-ext-config }