keith-turner commented on a change in pull request #1088: Use LeaderLatch to
determine if oracle exists
URL: https://github.com/apache/fluo/pull/1088#discussion_r388578931
##########
File path:
modules/core/src/main/java/org/apache/fluo/core/client/FluoAdminImpl.java
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@@ -505,30 +505,28 @@ private String getJarsFromClasspath() {
}
public static boolean oracleExists(CuratorFramework curator) {
- try {
- return curator.checkExists().forPath(ZookeeperPath.ORACLE_SERVER) != null
- &&
!curator.getChildren().forPath(ZookeeperPath.ORACLE_SERVER).isEmpty();
- } catch (Exception e) {
- throw new RuntimeException(e);
- }
+ return numOracles(curator) > 0;
}
public boolean oracleExists() {
return oracleExists(getAppCurator());
}
- public int numOracles() {
- CuratorFramework curator = getAppCurator();
- if (oracleExists(curator)) {
- try {
+ public static int numOracles(CuratorFramework curator) {
+ int numOracles = 0;
+ try {
+ if (curator.checkExists().forPath(ZookeeperPath.ORACLE_SERVER) != null) {
Review comment:
The pattern of checking for existence before the operation has a race
condition. Its possible that the node is deleted after the existence check but
before the operation.
I took a look at the code and its possible this existence check may not be
needed. The following code for `getParticipants()` calls `getSortedChildren()`
https://github.com/apache/curator/blob/959c1ca34f6ebcb11370180dfafbd8e85320dcd2/curator-recipes/src/main/java/org/apache/curator/framework/recipes/leader/LeaderLatch.java#L448
and `getSortedChildren()` seems like it may return empty list when the node
does not exists.
https://github.com/apache/curator/blob/959c1ca34f6ebcb11370180dfafbd8e85320dcd2/curator-recipes/src/main/java/org/apache/curator/framework/recipes/locks/LockInternals.java#L172
Not sure if the version of Curator used by Fluo does this.
When dealing with ZK the pattern of trying the operation and catching
NoNodeException is better than checking for existence before the operation
because it avoids race conditions.
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