Segments can be automatically loaded and dropped from the cluster based on a set of rules. For more information on rules, see [Rule Configuration](../operations/rule-configuration.md).
If a Historical process restarts or becomes unavailable for any reason, the Druid Coordinator will notice a process has gone missing and treat all segments served by that process as being dropped. Given a sufficient period of time, the segments may be reassigned to other Historical processes in the cluster. However, each segment that is dropped is not immediately forgotten. Instead, there is a transitional data structure that stores all dropped segments with an associated lifetime. The lifetime represents a period of time in which the Coordinator will not reassign a dropped segment. Hence, if a historical process becomes unavailable and available again within a short period of time, the historical process will start up and serve segments from its cache without any those segments being reassigned across the cluster.
To ensure an even distribution of segments across Historical processes in the cluster, the Coordinator process will find the total size of all segments being served by every Historical process each time the Coordinator runs. For every Historical process tier in the cluster, the Coordinator process will determine the Historical process with the highest utilization and the Historical process with the lowest utilization. The percent difference in utilization between the two processes is computed, and if the result exceeds a certain threshold, a number of segments will be moved from the highest utilized process to the lowest utilized process. There is a configurable limit on the number of segments that can be moved from one process to another each time the Coordinator runs. Segments to be moved are selected at random and only moved if the resulting utilization calculation indicates the percentage difference between the highest and lowest servers has decreased.
Each run, the Coordinator compacts segments by merging small segments or splitting a large one. This is useful when the size of your segments is not optimized which may degrade query performance.
See [Segment size optimization](../operations/segment-optimization.md) for details.
See [Automatic compaction configuration API](../operations/api-reference.md#automatic-compaction-configuration) and [Automatic compaction configuration](../configuration/index.md#automatic-compaction-dynamic-configuration) to enable and configure automatic compaction.
- If a task of a higher priority acquires a [time chunk lock](../ingestion/tasks.md#locking) for an interval overlapping with the interval of a compaction task, the compaction task fails.
Once a compaction task fails, the Coordinator simply checks the segments in the interval of the failed task again, and issues another compaction task in the next run.
Note that Compacting Segments Coordinator Duty is automatically enabled and run as part of the Indexing Service Duties group. However, Compacting Segments Coordinator Duty can be configured to run in isolation as a separate Coordinator duty group. This allows changing the period of Compacting Segments Coordinator Duty without impacting the period of other Indexing Service Duties. This can be done by setting the following properties. For more details, see [custom pluggable Coordinator Duty](../development/modules.md#adding-your-own-custom-pluggable-coordinator-duty).
Assuming that each segment is 10 MB and haven't been compacted yet, this policy first returns two segments of
`foo_2017-11-01T00:00:00.000Z_2017-12-01T00:00:00.000Z_VERSION` and `foo_2017-11-01T00:00:00.000Z_2017-12-01T00:00:00.000Z_VERSION_1` to compact together because
`2017-11-01T00:00:00.000Z/2017-12-01T00:00:00.000Z` is the most recent time chunk.
`bar_2017-10-01T00:00:00.000Z_2017-11-01T00:00:00.000Z_VERSION` and `bar_2017-10-01T00:00:00.000Z_2017-11-01T00:00:00.000Z_VERSION_1`.
Finally, `foo_2017-09-01T00:00:00.000Z_2017-10-01T00:00:00.000Z_VERSION` will be picked up even though there is only one segment in the time chunk of `2017-09-01T00:00:00.000Z/2017-10-01T00:00:00.000Z`.
If this is set, this policy will ignore the segments falling into the time chunk of (the end time of the most recent segment - `skipOffsetFromLatest`).
This is to avoid conflicts between compaction tasks and realtime tasks.
Note that realtime tasks have a higher priority than compaction tasks by default. Realtime tasks will revoke the locks of compaction tasks if their intervals overlap, resulting in the termination of the compaction task.
Historical processes never directly contact the Coordinator process. The Druid Coordinator tells the Historical processes to load/drop data via Zookeeper, but the Historical processes are completely unaware of the Coordinator.
Brokers also never contact the Coordinator. Brokers base their understanding of the data topology on metadata exposed by the Historical processes via ZK and are completely unaware of the Coordinator.
No. If the Druid Coordinator is not started up, no new segments will be loaded in the cluster and outdated segments will not be dropped. However, the Coordinator process can be started up at any time, and after a configurable delay, will start running Coordinator tasks.
This also means that if you have a working cluster and all of your Coordinators die, the cluster will continue to function, it just won’t experience any changes to its data topology.
