Start the allocation architecture guide section

Author: DiannaHohensee

docs/internal/DistributedArchitectureGuide.md

@@ -229,19 +229,43 @@ works in parallel with the storage engine.)
 
 # Allocation
 
-(AllocationService runs on the master node)
-
-(Discuss different deciders that limit allocation. Sketch / list the different deciders that we have.)
-
-### APIs for Balancing Operations
-
-(Significant internal APIs for balancing a cluster)
-
-### Heuristics for Allocation
-
-### Cluster Reroute Command
-
-(How does this command behave with the desired auto balancer.)
+### Core Components
+
+The `DesiredBalanceShardsAllocator` is what runs shard allocation decisions. It leverages the `DesiredBalanceComputer` to produce
+`DesiredBalance` instances for the cluster based on the latest cluster changes (add/remove nodes, create/remove indices, load, etc). Then
+the `DesiredBalanceReconciler` is invoked to choose the next steps to take to move the cluster from the current shard allocation to the
+latest computed `DesiredBalance` shard allocation. The `Reconciler` will apply changes to a copy of the `RoutingNodes`, which is then
+published in a cluster state update that will reach the data nodes to start the individual shard creation/recovery/move work.
+
+The `Reconciler` is throttled by cluster settings, like the max number of concurrent shard moves and recoveries per cluster and node: this
+is why the `Reconciler` will make, and publish via cluster state updates, incremental changes to the cluster shard allocation. The
+`DesiredBalanceShardsAllocator` is the endpoint for reroute requests, which may trigger immediate requests to the `Reconciler`, but
+asynchronous requests to the `DesiredBalanceComputer` via the `ContinuousComputation` component. Cluster state changes that affect shard
+balancing (for example index deletion) all call some reroute method interface that reaches the `DesiredBalanceShardsAllocator` to run
+reconciliation and queue a request for the `DesiredBalancerComputer`, leading to desired balance computation and reconciliation actions.
+Asynchronous completion of a new `DesiredBalance` will also invoke a reconciliation action, as will cluster state updates completing shard
+moves/recoveries (unthrottling the next shard move/recovery).
+
+The `ContinuousComputation` maintains a queue of desired balance computation requests, each of which holds the latest cluster information at
+the time of the request, and a thread that runs the `DesiredBalanceComputer`. The ContinuousComputation thread grabs the latest request,
+with the freshest cluster information, feeds it into the `DesiredBalanceComputer` and publishes a `DesiredBalance` back to the
+`DesiredBalanceShardsAllocator` to use for reconciliation actions. Sometimes the `ContinuousComputation` thread's desired balance
+computation will be signalled to end early and get a `DesiredBalance` published more quickly, if newer cluster state changes are significant
+or to get unassigned shards started as soon as possible.
+
+### Rebalancing Process
+
+There are different priorities in shard allocation, reflected in which moves the `DesiredBalancerReconciler` selects to do first given that
+it can only move, recover, or remove a limited number of shards at once. The first priority is assigning unassigned shards, primaries being
+more important than replicas. The second is to move shards that violate node resource limits or hard limits. The `AllocationDeciders` holds
+a group of `AllocationDecider` types that place hard constraints on shard allocation. There is a decider that manages disk memory usage
+thresholds that does not allow further shard assignment or requires shards to be moved off; or another that excludes a configurable list of
+indices from certain nodes; or one that will not attempt to recover a shard on a certain node after so many failed retries. The third
+priority is to rebalance shards to even out the relative weight of shards on each node: the intention is to avoid, or ease, future
+hot-spotting on data nodes due to too many shards being placed on the same data node. Node shard weight is based on a sum of factors: disk
+memory usage, projected shard write load, total number of shards, and an incentive to spread out shards within the same index. See the
+`WeightFunction` and `NodeAllocationStatsAndWeightsCalculator` classes for more details on the weight calculations that support the
+`DesiredBalanceComputer` decisions.
 
 # Autoscaling