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After this change, the load balancer can make progress with active migrations. If the algorithm is called with active tablet migrations in tablet metadata, those are treated by load balancer as if they were already completed. This allows the algorithm to incrementally make decision which when executed with active migrations will produce the desired result. Overload of shards is limited by the fact that the algorithm tracks streaming concurrency on both source and target shards of active migrations and takes concurrency limit into account when producing new migrations. The coordinator executes the load balancer on edges of tablet state machine stransitions. This allows new migrations to be started as soon as tablets finish streaming. The load balancer is also continuously invoked as long as it produces a non-empty plan. This is in order to saturate the cluster with streaming. A single make_plan() call is still not saturating, due to the way algorithm is implemented.
Scylla developer documentation
This folder contains developer-oriented documentation concerning the ScyllaDB codebase. We also have a wiki, which contains additional developer-oriented documentation. There is currently no clear definition of what goes where, so when looking for something be sure to check both.
Seastar documentation can be found here.
User documentation can be found on docs.scylladb.com
For information on how to build Scylla and how to contribute visit HACKING.md and CONTRIBUTING.md.