DruidLeaderClient.goAsync() is broken

[jihoonson]

Affected Version

0.18.0

Description

This bug seems to be introduced after #9481. Before this PR, DruidLeaderClient used to use ServerDiscoverySelector to find the leader. Since only the leader Overlord and Coordinator announce themselves, ServerDiscoverySelector could return the current leader to DruidLeaderClient. But now, DruidLeaderClient picks up the first node returned from DruidNodeDiscovery which knows all announced nodes. Since DruidLeaderClient.goAsync() doesn't follow redirection (#8716), some methods of the system table which use goAsync() API returns an error with the Temporary Redirect status.

[jihoonson]

This is a release blocker for 0.18.0 since Druid doesn't work with multi-masters.

[jihoonson]

I think we can revert #9481 for both master and 0.18.0 branches since it doesn't have any changes other than removing ServerDiscoverySelector from DruidLeaderClient. @himanshug does it make sense?

[himanshug]

@jihoonson DruidLeaderClient is designed to follow redirects so as to land requests to leader. can you please describe, what functionality is broken by this change ?

[jihoonson]

When it tries to follow redirects, it checks the current known leader which calls pickOneHost(). Before #9481, this function used to use ServerDiscoverySelector which returns the known leader and fall back to DruidNodeDiscovery when there is no known leader, but now it always returns the first node from DruidNodeDiscovery.

druid/server/src/main/java/org/apache/druid/discovery/DruidLeaderClient.java

Lines 297 to 307 in cda9f41

	private String pickOneHost()
	{
	Iterator<DiscoveryDruidNode> iter = druidNodeDiscovery.getAllNodes().iterator();
	if (iter.hasNext()) {
	DiscoveryDruidNode node = iter.next();
	return StringUtils.format(
	"%s://%s",
	node.getDruidNode().getServiceScheme(),
	node.getDruidNode().getHostAndPortToUse()
	);
	}

[himanshug]

ignoring name of that method, when we make request to wrong leader, we should get a HTTP 302 back and that redirect is then followed to get the request to right node (see

druid/server/src/main/java/org/apache/druid/discovery/DruidLeaderClient.java

Line 210 in cda9f41

String redirectUrlStr = fullResponseHolder.getResponse().headers().get("Location");

) and newly discovered leader is then cached ( see

druid/server/src/main/java/org/apache/druid/discovery/DruidLeaderClient.java

Line 231 in cda9f41

currentKnownLeader.set(StringUtils.format(

)

[gianm]

The problem happens with DruidLeaderClient.goAsync, which lets callers provide their own HttpResponseHandler, but doesn't contain the leader-following logic. It is used by the Druid SQL system tables so they can avoid materializing the entire response for the APIs they are hitting (which could be large). Perhaps the best fix is that it should learn how to follow leaders.

[jihoonson]

Yep, sorry for confusing. @himanshug you're right about DruidLeaderClient.g(), but it happens with goAsync() as @gianm mentioned which is reported in #8716.

I agree that the best approach to fix this would fix #8716, but we are running out of time for 0.18.0, I'm still inclined to reverting #9481 for now.

[gianm]

Reverting the patch makes sense to me, then for the next release, adding it back along with implementing redirects for goAsync.

[himanshug]

ah, I see , wasn't aware of this change. I agree, right fix is to make it follow leader.

for now, I think right way to fix it to not revert #9481 but rather use DruidLeaderSelector interface to get the leader.. so as to not add hardcoded dependency back to zk. Similar to what router is doing DruidLeaderSelector.getCurrentLeader() for example https://github.com/apache/druid/blob/master/server/src/main/java/org/apache/druid/server/AsyncManagementForwardingServlet.java#L95

that will revert the behavior equivalent to using zk specific ServerDiscoverySelector

let me do a PR unless someone sees problems with that .. @gianm @jihoonson ?

[himanshug]

or rather, to expedite things, let us revert #9481 for now.

I will revive that along with bringing in DruidLeaderSelector.getCurrentLeader() or follow redirect in goAsync later.