Docs: Add details of cost balancer strategy #17595
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Akshat-Jain
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Dec 21, 2024
docs/design/coordinator.md
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| @@ -79,11 +79,19 @@ On each run, the Coordinator determines and cleans up unneeded eternity tombston | |||
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| ## Segment availability | |||
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| If a Historical service restarts or becomes unavailable for any reason, the Coordinator will notice a service has gone missing and treat all segments served by that service as being dropped. Given a sufficient period of time, the segments may be reassigned to other Historical services 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 service becomes unavailable and available again within a short period of time, the Historical service will start up and serve segments from its cache without any those segments being reassigned across the cluster. | |||
| If a Historical service restarts or becomes unavailable for any reason, the Coordinator notices that a service has gone missing and treats all segments served by that service as being dropped. The segments are then reassigned to other Historical services 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 service becomes unavailable and available again within a short period of time, the Historical service will start up and serve segments from its cache without any those segments being reassigned across the cluster. | |||
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without any those segments being reassigned across the cluster -> without any of those segments being reassigned across the cluster
docs/design/coordinator.md
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| But in a tier with several Historicals (or a low replication factor), segment replication is not sufficient to attain balance. | ||
| Thus, the Coordinator constantly monitors the set of segments present on each Historical in a tier and employs one of the following strategies to identify segments that may be moved from one Historical to another to retain balance. | ||
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| - `cost` (default): For a given segment in a tier, this strategy picks the server with the minimum "cost" of placing that segment. The cost is a function of the data interval of the segment and the data intervals of all the segments already present on the candidate server. In essence, this strategy tries to avoid placing segments with adjacent or overlapping data intervals on the same server. This is based on the premise that adjacent-interval segments are more likely to be used together in a query and placing them on the same server may lead to skewed cpu usages of Historicals. |
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Nit: cpu usages -> CPU usages
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Thanks for the feedback, @Akshat-Jain ! |
Akshat-Jain
approved these changes
Dec 22, 2024
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Fixes #17530