Update follower read and learner read descriptions #21470
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Signed-off-by: zyguan <[email protected]>
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Summary of Changes
Hello @zyguan, I'm Gemini Code Assist1! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!
This pull request updates the documentation for TiDB's tidb_replica_read system variable, specifically clarifying the behavior of follower and learner read modes. The changes reflect recent adjustments in TiDB's replica reading logic and provide important version-specific details for older releases to ensure the documentation is accurate and up-to-date.
Highlights
followerread mode behavior: The description fortidb_replica_readset tofollowerhas been updated. It now clarifies that TiDB also considers reading from learner replicas with equal priority, and if no follower or learner replicas are available, it will fall back to reading from the leader replica.learnerread mode behavior: The description fortidb_replica_readset tolearnerhas been revised. Previously, it stated that an error would be returned if no learner replica was available; now, it clarifies that TiDB will attempt to read from available leader or follower replicas in such cases.- Version-specific notes: A new note has been added to specify the behavior for TiDB versions up to and including v7.5, detailing that
followerandlearnerread modes would return an error if no appropriate replicas were available, contrasting with the newer fallback behavior.
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Code Review
This pull request updates the documentation for follower and learner reads to reflect new behaviors in recent TiDB versions. The changes clarify that when tidb_replica_read is set to follower or learner, TiDB will now fall back to other replica types if the preferred type is unavailable, instead of returning an error. My review includes suggestions to improve clarity and conciseness, and to align the text with the documentation style guide. The style guide elements referenced in this review include clarity, simplicity, readability, present tense, and second person.
| @@ -33,7 +33,7 @@ Default: leader | |||
| This variable is used to set the expected data read mode. | |||
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| - When the value of `tidb_replica_read` is set to `leader` or an empty string, TiDB maintains its default behavior and sends all read operations to the leader replica to perform. | |||
| - When the value of `tidb_replica_read` is set to `follower`, TiDB selects a follower replica of the Region to perform all read operations. | |||
| - When the value of `tidb_replica_read` is set to `follower`, TiDB selects a follower replica of the Region to perform all read operations. When a Region has learner replicas, TiDB also considers reading data from learner replicas. In this case, follower replicas and learner replicas have the same priority. If there are no available follower or learner replicas in the current Region, TiDB will read from the leader replica. | |||
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For better readability and conciseness, combine the sentences describing the behavior for follower reads. This makes the logic easier to follow.12
Style Guide References
| - When the value of `tidb_replica_read` is set to `follower`, TiDB selects a follower replica of the Region to perform all read operations. When a Region has learner replicas, TiDB also considers reading data from learner replicas. In this case, follower replicas and learner replicas have the same priority. If there are no available follower or learner replicas in the current Region, TiDB will read from the leader replica. | |
| - When the value of `tidb_replica_read` is set to `follower`, TiDB selects a follower replica of the Region to perform read operations. If the Region has learner replicas, TiDB also considers them for reads with the same priority. If no available follower or learner replicas exist in the current Region, TiDB reads from the leader replica. |
Footnotes
| @@ -42,13 +42,18 @@ This variable is used to set the expected data read mode. | |||
| - If the estimated result of a read request is greater than or equal to the value of [`tidb_adaptive_closest_read_threshold`](/system-variables.md#tidb_adaptive_closest_read_threshold-new-in-v630), TiDB prefers to select a replica in the same availability zone for read operations. To avoid unbalanced distribution of read traffic across availability zones, TiDB dynamically detects the distribution of availability zones for all online TiDB and TiKV nodes. In each availability zone, the number of TiDB nodes whose `closest-adaptive` configuration takes effect is limited, which is always the same as the number of TiDB nodes in the availability zone with the fewest TiDB nodes, and the other TiDB nodes automatically read from the leader replica. For example, if TiDB nodes are distributed across 3 availability zones (A, B, and C), where A and B each contains 3 TiDB nodes and C contains only 2 TiDB nodes, the number of TiDB nodes whose `closest-adaptive` configuration takes effect in each availability zone is 2, and the other TiDB node in each of the A and B availability zones automatically selects the leader replica for read operations. | |||
| - If the estimated result of a read request is less than the value of [`tidb_adaptive_closest_read_threshold`](/system-variables.md#tidb_adaptive_closest_read_threshold-new-in-v630), TiDB can only select the leader replica for read operations. | |||
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| - When the value of `tidb_replica_read` is set to `learner`, TiDB reads data from the learner replica. If there is no learner replica in the Region, TiDB returns an error. | |||
| - When the value of `tidb_replica_read` is set to `learner`, TiDB reads data from the learner replica. If there is no available learner replica in the Region, TiDB will read data from available leader or follower replicas. | |||
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To improve clarity and adhere to the style guide's preference for present tense, rephrase this sentence to be more direct and concise.12
Style Guide References
| - When the value of `tidb_replica_read` is set to `learner`, TiDB reads data from the learner replica. If there is no available learner replica in the Region, TiDB will read data from available leader or follower replicas. | |
| - When the value of `tidb_replica_read` is set to `learner`, TiDB reads data from the learner replica. If no learner replica is available in the Region, TiDB reads from an available leader or follower replica. |
Footnotes
| > - When the value of `tidb_replica_read` is set to `follower` and no follower or learner replicas are available, TiDB returns an error. | ||
| > - When the value of `tidb_replica_read` is set to `learner` and no learner replicas are available, TiDB returns an error. |
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To align with the style guide's recommendation of using the second person ("you"), rephrase these points. This makes the documentation more user-centric.1
Style Guide References
| > - When the value of `tidb_replica_read` is set to `follower` and no follower or learner replicas are available, TiDB returns an error. | |
| > - When the value of `tidb_replica_read` is set to `learner` and no learner replicas are available, TiDB returns an error. | |
| - If you set `tidb_replica_read` to `follower` and no follower or learner replicas are available, TiDB returns an error. | |
| - If you set `tidb_replica_read` to `learner` and no learner replicas are available, TiDB returns an error. |
Footnotes
First-time contributors' checklist
What is changed, added or deleted? (Required)
The description of follower-read and learner-read is inaccurate, and related behaviors have been adjusted in the newer version of tidb, so the corresponding documentation has been revised accordingly.
Which TiDB version(s) do your changes apply to? (Required)
Tips for choosing the affected version(s):
By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.
For details, see tips for choosing the affected versions.
What is the related PR or file link(s)?
Do your changes match any of the following descriptions?