reference: update doc for follower read #20703
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Signed-off-by: you06 <[email protected]>
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| Follower Read 功能适用于以下场景： | ||
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| - 希望通过 follower 打散读热点。 | ||
| - 希望通过读取本地副本节省流量。 |
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这个“本地”是否展开解释成 “相同 zone tidb 访问 tikv“？
follower-read.md
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| ### Follower 副本选择策略 | ||
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| 由于 TiKV 的 Follower Read 不会破坏 TiDB 的 Snapshot Isolation 事务隔离级别，因此 TiDB 选择 follower 的策略可以采用 round robin 的方式。目前，对于 Coprocessor 请求，Follower Read 负载均衡策略粒度是连接级别的，对于一个 TiDB 的客户端连接在某个具体的 Region 上会固定使用同一个 follower，只有在选中的 follower 发生故障或者因调度策略发生调整的情况下才会进行切换。而对于非 Coprocessor 请求（点查等），Follower Read 负载均衡策略粒度是事务级别的，对于一个 TiDB 的事务在某个具体的 Region 上会固定使用同一个 follower，同样在 follower 发生故障或者因调度策略发生调整的情况下才会进行切换。如果同一事务内既有点查请求又有 Coprocessor 请求，两种请求都将按照上述调度策略分别进行读取，即使 Coprocessor 和点查出现在同一个 Region 上，TiDB 也会当作独立事件来处理。 | ||
| 由于 TiKV 的 Follower Read 不会破坏 TiDB 的 Snapshot Isolation 事务隔离级别，因此 TiDB 会根据 `tidb_replica_read` 选择满足要求的副本进行读取。当选中的 follower 节点出现无法访问的故障或其他错误时，会切换到 leader 提供服务。 |
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是否提一下这里的策略是第一轮请求处理，不包含后续内部 region error 处理和重试？
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| - 当一个读请求的预估返回结果小于变量 [`tidb_adaptive_closest_read_threshold`](/system-variables.md#tidb_adaptive_closest_read_threshold-从-v630-版本开始引入) 的值时，TiDB 会选择 leader 副本执行读取操作。 | ||
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| - 当设置为 `learner` 时，TiDB 会选择 learner 副本执行读取操作。在读取时，如果当前 Region 没有 learner 副本，TiDB 会报错。 | ||
| - 当设置为 `learner` 时，TiDB 会选择 learner 副本执行读取操作。在读取时，如果当前 Region 没有 learner 副本，TiDB 会从 leader 读取数据。 |
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Co-authored-by: you06 <[email protected]> Co-authored-by: Grace Cai <[email protected]>
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