capistrant opened a new pull request #11960: URL: https://github.com/apache/druid/pull/11960
<!-- Thanks for trying to help us make Apache Druid be the best it can be! Please fill out as much of the following information as is possible (where relevant, and remove it when irrelevant) to help make the intention and scope of this PR clear in order to ease review. --> <!-- Please read the doc for contribution (https://github.com/apache/druid/blob/master/CONTRIBUTING.md) before making this PR. Also, once you open a PR, please _avoid using force pushes and rebasing_ since these make it difficult for reviewers to see what you've changed in response to their reviews. See [the 'If your pull request shows conflicts with master' section](https://github.com/apache/druid/blob/master/CONTRIBUTING.md#if-your-pull-request-shows-conflicts-with-master) for more details. --> <!-- Replace XXXX with the id of the issue fixed in this PR. Remove this section if there is no corresponding issue. Don't reference the issue in the title of this pull-request. --> <!-- If you are a committer, follow the PR action item checklist for committers: https://github.com/apache/druid/blob/master/dev/committer-instructions.md#pr-and-issue-action-item-checklist-for-committers. --> ### Description <!-- Describe the goal of this PR, what problem are you fixing. If there is a corresponding issue (referenced above), it's not necessary to repeat the description here, however, you may choose to keep one summary sentence. --> <!-- Describe your patch: what did you change in code? How did you fix the problem? --> <!-- If there are several relatively logically separate changes in this PR, create a mini-section for each of them. For example: --> #### Fixed unintended code path execution in BalancerStrategy#pickSegmentsToMove https://github.com/apache/druid/pull/11257 Introduced a new way of sampling segments for balancing that is much more performant than the legacy way that exists today. There is a case in the code where the deprecated code can be executed despite, the dynamic config for batched segment sampling being enabled. In [this](https://github.com/yuanlihan/druid/blob/b02a9e659846a88c7be375ffb805340c38d1dbf5/server/src/main/java/org/apache/druid/server/coordinator/duty/BalanceSegments.java#L198) line of code, if `useBatchedSegmentSampler() == true` and `maxSegmentsToMove == 1`, you would expect the batched segment sampler code to execute. However, you can see [here](https://github.com/yuanlihan/druid/blob/b02a9e659846a88c7be375ffb805340c38d1dbf5/server/src/main/java/org/apache/druid/server/coordinator/BalancerStrategy.java#L82) that the new batched sampler is not used in this case because reservoir size is 1. This PR fixes this behavior by splitting `BalancerStrategy#pickSegmentsToMove(List<S erverHolder>, Set<String>, int, double)` into two methods using overloading - one for the new batched sampler, and one for the deprecated way of sampling one segment at a time. This change will also simplify cleanup of the deprecated code in what I assume will be the 0.24.X cycle. #### Cleaned up the code for the competing segment pick strategies (deprecated vs batched segment sampler) I did a few things to make the code more understandable and to make it abundantly clear that the old way of sampling segments to move with the help of the dynamic config `percentOfSegmentsToConsiderPerMove` is deprecated: * Added `@Deprecated` annotations to the `CoordinatorDynamicConfig` code related to `percentOfSegmentsToConsiderPerMove` * Updated the Coordinator Dynamic Config docs to explicitly state that `percentOfSegmentsToConsiderPerMove` is deprecated and will have no effect if `useBatchedSegmentSampler` dynamic config is `true` * Split `BalancerStrategy#pickSegmentsToMove` into two methods with the same name and overloaded arguments. One implementation is deprecated and takes a double for `percentOfSegmentsToConsiderPerMove` while the other is the new batched sampler which takes an `int` for the reservoir size. * This change also makes `BalanceSegments#balanceServers` more understandable to those with limited familiarity with this work to migrate to the batched segment sampler. Before the call to `BalancerStrategy#pickSegmentsToMove` was opaque because it included both the reservoir size and the percent of segments to consider per move despite only one of those arguments being used depending on the value of the reservoir size (this also relates to the unintended code execution I talk about above). The new way makes it clear that if batched segment sampling is enabled, the non-deprecated `pickSegmentsToMove` is called. And if it is not, the deprecated `pickSegmentsToMove` is called. <!-- In each section, please describe design decisions made, including: - Choice of algorithms - Behavioral aspects. What configuration values are acceptable? How are corner cases and error conditions handled, such as when there are insufficient resources? - Class organization and design (how the logic is split between classes, inheritance, composition, design patterns) - Method organization and design (how the logic is split between methods, parameters and return types) - Naming (class, method, API, configuration, HTTP endpoint, names of emitted metrics) --> <!-- It's good to describe an alternative design (or mention an alternative name) for every design (or naming) decision point and compare the alternatives with the designs that you've implemented (or the names you've chosen) to highlight the advantages of the chosen designs and names. --> <!-- If there was a discussion of the design of the feature implemented in this PR elsewhere (e. g. a "Proposal" issue, any other issue, or a thread in the development mailing list), link to that discussion from this PR description and explain what have changed in your final design compared to your original proposal or the consensus version in the end of the discussion. If something hasn't changed since the original discussion, you can omit a detailed discussion of those aspects of the design here, perhaps apart from brief mentioning for the sake of readability of this PR description. --> <!-- Some of the aspects mentioned above may be omitted for simple and small changes. --> <hr> ##### Key changed/added classes in this PR * `BalanceSegments` * `CoordinatorDynamicConfig` * `BalancerStrategy` <hr> <!-- Check the items by putting "x" in the brackets for the done things. Not all of these items apply to every PR. Remove the items which are not done or not relevant to the PR. None of the items from the checklist below are strictly necessary, but it would be very helpful if you at least self-review the PR. --> This PR has: - [ ] been self-reviewed. - [X] added documentation for new or modified features or behaviors. - [X] added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links. - [X] added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader. - [X] added unit tests or modified existing tests to cover new code paths, ensuring the threshold for [code coverage](https://github.com/apache/druid/blob/master/dev/code-review/code-coverage.md) is met. - [ ] been tested in a test Druid cluster. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
