Update description of tidb_replica_read variable values for followe…

…r reads

follower-read.md

@@ -32,17 +32,17 @@ Default: leader
 
 This variable is used to set the expected data read mode.
 
-- 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 `leader-and-follower`, TiDB can select any replicas to perform read operations. In this mode, read requests are load balanced between the leader and follower.
-- When the value of `tidb_replica_read` is set to `prefer-leader`, TiDB prefers to select the leader replica to perform read operations. If the leader replica is obviously slow in processing read operations (such as caused by disk or network performance jitter), TiDB will select other available follower replicas to perform read operations.
-- When the value of `tidb_replica_read` is set to `closest-replicas`, TiDB prefers to select a replica in the same availability zone to perform read operations, which can be a leader or a follower. If there is no replica in the same availability zone, TiDB reads from the leader replica.
+- 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 if the estimated waiting duration in the leader replica's kv store does not exceed [`tidb_load_based_replica_read_threshold`](/system-variables.md#tidb_load_based_replica_read_threshold-new-in-v700).
+- When the value of `tidb_replica_read` is set to `follower`, TiDB selects a follower or learner replica of the Region to perform all read operations.
+- When the value of `tidb_replica_read` is set to `leader-and-follower`, TiDB can select any replicas to perform read operations. In this mode, read requests are load balanced between the leader, follower and learner replicas.
+- When the value of `tidb_replica_read` is set to `prefer-leader`, TiDB prefers to select the leader replica to perform read operations. If the leader replica is obviously slow in processing read operations (such as caused by disk or network performance jitter), TiDB will select other available follower or learner replicas to perform read operations.
+- When the value of `tidb_replica_read` is set to `closest-replicas`, TiDB prefers to select a replica in the same availability zone to perform read operations, which can be a leader, follower or learner. If there is no replica in the same availability zone, TiDB reads from the leader replica.
 - When the value of `tidb_replica_read` is set to `closest-adaptive`:
 
     - 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.
 
-- 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 learner replica in the Region, TiDB will read either the from leader or follower replicas randomly.
 
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