BP-66: support throttling for zookeeper read during rereplication (#4258)
Signed-off-by: ZhangJian He <shoothzj@gmail.com>
Co-authored-by: ZhangJian He <shoothzj@gmail.com>
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+# BP-66: support throttling for zookeeper read during rereplication
+
+### Motivation
+
+Each time the cluster triggers the re-replication, all replicators will read data from zookeeper. This can cause a great
+pressure on Zookeeper. We need to support throttling for zookeeper read of re-replication.
+
+For example, in a Pulsar cluster, we enable auto-recovery for every bookie. There are 400 bookies in a cluster, which
+means there are 400 replicators in the cluster.
+And there are about 3000 ledgers in each bookie, 1/3 of them are small ledgers, whose size is less than 0.1MB, that is
+1000 small ledgers in each bookie.
+If one bookie is decommissioned, the read latency of zookeeper will increase to minutes.
+
+### Configuration
+
+add the following configuration:
+
+```
+zkReplicationTaskRateLimit
+```
+
+default value is 0, which means no limit.
+Value greater than 0 will enable the rate limit, and the value is the number of tasks that can be acquired per second.
+Decimals are allowed too, for example, 0.5 means 1 task every 2 seconds, 1 means 1 task per second, 2 means 2 tasks per
+second, and so on.
+
+### Proposed Changes
+
+Add a new configuration `zkReplicationTaskRateLimit` to control the rate of zookeeper read of re-replication.
+
+### Compatibility, Deprecation, and Migration Plan
+
+Full compatibility with the existing behavior. No deprecation needed.
