RandomBalancerStrategy does not assign segments if there is only one historical

[pjain1]

Found while investigating #10067. RandomBalancerStrategy does not assign segments if there is only one historical.

Affected Version

All

Description

Setup - I start with one empty historical with server size enough to load one segment of size 4,821,713. Replication factor does not matter. This does not load any segments because of the logic here- https://github.com/apache/druid/blob/master/server/src/main/java/org/apache/druid/server/coordinator/RandomBalancerStrategy.java#L37. Here are the relevant coordinator logs -

2020-06-24T17:24:47,691 INFO [DatabaseRuleManager-Exec--0] org.apache.druid.metadata.SQLMetadataRuleManager - Polled and found 1 rule(s) for 2 datasource(s)
2020-06-24T17:24:47,786 INFO [org.apache.druid.metadata.SqlSegmentsMetadataManager-Exec--0] org.apache.druid.metadata.SqlSegmentsMetadataManager - Polled and found 1 segments in the database
2020-06-24T17:24:57,814 INFO [Coordinator-Exec--0] org.apache.druid.server.coordinator.duty.LogUsedSegments - Found [1] used segments.
2020-06-24T17:24:57,814 INFO [Coordinator-Exec--0] org.apache.druid.server.coordinator.ReplicationThrottler - [_default_tier]: Replicant create queue is empty.
2020-06-24T17:24:57,814 WARN [Coordinator-Exec--0] org.apache.druid.server.coordinator.rules.LoadRule - No available [_default_tier] servers or node capacity to assign primary segment[wikipedia_null_2015-09-12T00:00:00.000Z_2015-09-13T00:00:00.000Z_2019-07-10T06:10:13.432Z]! Expected Replicants[3]
2020-06-24T17:24:57,815 INFO [Coordinator-Exec--0] org.apache.druid.server.coordinator.rules.LoadRule - Loading in progress, skipping drop until loading is complete
2020-06-24T17:24:57,815 INFO [Coordinator-Exec--0] org.apache.druid.server.coordinator.duty.BalanceSegments - Found 1 active servers, 0 decommissioning servers
2020-06-24T17:24:57,815 WARN [Coordinator-Exec--0] org.apache.druid.server.coordinator.duty.BalanceSegments - [_default_tier]: insufficient active servers. Cannot balance.
2020-06-24T17:24:57,815 INFO [Coordinator-Exec--0] org.apache.druid.server.coordinator.duty.EmitClusterStatsAndMetrics - Load Queues:
2020-06-24T17:24:57,815 INFO [Coordinator-Exec--0] org.apache.druid.server.coordinator.duty.EmitClusterStatsAndMetrics - Server[localhost:8083, historical, _default_tier] has 0 left to load, 0 left to drop, 0 bytes queued, 0 bytes served.

[pjain1]

Fixed in #10070