amahussein commented on a change in pull request #161:
URL: https://github.com/apache/tez/pull/161#discussion_r749556005
##########
File path:
tez-api/src/main/java/org/apache/tez/dag/api/client/DAGClientImpl.java
##########
@@ -221,13 +245,14 @@ protected DAGStatus getDAGStatusInternal(@Nullable
Set<StatusGetOpts> statusOpti
final DAGStatus dagStatus = getDAGStatusViaAM(statusOptions, timeout);
if (!dagCompleted) {
- if (dagStatus != null) {
- cachedDagStatus = dagStatus;
+ if (dagStatus != null) { // update the cached DAGStatus
+ cachedDAGStatusRef.setValue(dagStatus);
return dagStatus;
}
- if (cachedDagStatus != null) {
+ DAGStatus cachedDAG = cachedDAGStatusRef.getValue();
+ if (cachedDAG != null) {
// could not get from AM (not reachable/ was killed). return cached
status.
- return cachedDagStatus;
+ return cachedDAG;
Review comment:
Thanks for the feedback.
Yes, considering the implementation of
[TezJob.run()-Line222](https://github.com/apache/pig/blob/trunk/src/org/apache/pig/backend/hadoop/executionengine/tez/TezJob.java#L222)
in Pig:
Pig polls on the DAGStatus inside an infinite loop:
```java
while (true) {
try {
dagStatus = dagClient.getDAGStatus(null);
} catch (Exception e) {
log.info("Cannot retrieve DAG status", e);
break;
}
if (dagStatus.isCompleted()) {
// do something
// break;
}
sleep(1000);
}
```
Let's assume the following scenario on Tez Side:
- Pig first iteration calls `getDAGStatusViaAM()` which successfully pulls
the DAGStatus and updates the cachedDAGStatus to running.
- Pig sleeps 1000
- second call from Pig calls `getDAGStatusViaAM()` which encounters
`TezException` or `IOException`. The call would return the last cachedDAGStatus
(which is running), instead of null.
- Since the status is `running`, the Pig-thread sleeps
- This will keep going as long as the `getDAGStatusViaAM()` fails, and the
last valid DAGStatus is still cached.
The problem in this corner case is that the Pig client will keep looping
indefinitely as long as it does not receive a null or
dagClient.getDAGStatus(null) does not throw an exception.
From a client perspective, it is better to fail early in order to recover
faster.
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