li4wang commented on code in PR #1961:
URL: https://github.com/apache/zookeeper/pull/1961#discussion_r1070184620
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zookeeper-server/src/main/java/org/apache/zookeeper/server/ZooKeeperServer.java:
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@@ -1151,6 +1213,9 @@ protected void setLocalSessionFlag(Request si) {
}
public void submitRequest(Request si) {
+ if (state == State.MAINTENANCE) {
+ throw new IllegalStateException("Zookeeper server is in
maintenance state");
+ }
Review Comment:
> Add MAINTENANCE as a running state to ZooKeeperServer.isRunning() method.
Adding MAINTENANCE to ZooKeeperServer.isRunning() will change the semantics
of "running" state and has bigger impact, we would want to avoid it if it's
possible.
> Do not alter the state of ZooKeeperServer, but use a separate volatile
variable in this class to block requests temporarly.
Good idea. Yes, we don't have to alter the state of ZooKeeper if we don't
think we need a generic MAINTENANCE state.
Instead of using volatile variable, how about adding a `CountDownLatch` to
more efficiently coordinate/sync between threads and utilize system resources.
The `restoreFromSnapshot()` API will instantiate a `CountDownLatch` with 1
count when restore starts and count it down when restore is completed. The
`submitRequest()` API will check and wait on the `CountDownLatch`, so requests
will be blocked if a restore is in progress.
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