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https://issues.apache.org/jira/browse/AMBARI-18456?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15848330#comment-15848330
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Jonathan Hurley commented on AMBARI-18456:
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Sure can! I was keeping it around in the event that we wanted to do more work
with removing state from the code, but that's silly since the branch is tied to
this Jira anyway.
> Refactor Unnecessary In-Memory Locks Around Business Objects
> ------------------------------------------------------------
>
> Key: AMBARI-18456
> URL: https://issues.apache.org/jira/browse/AMBARI-18456
> Project: Ambari
> Issue Type: Epic
> Components: ambari-server
> Affects Versions: 2.5.0
> Reporter: Jonathan Hurley
> Assignee: Jonathan Hurley
> Labels: branch-feature-AMBARI-18456
> Fix For: 2.5.0
>
>
> The top 4 business objects in Ambari:
> - ClusterImpl
> - ServiceImpl
> - ServiceComponentImpl
> - ServiceComponentHostImpl
> All use {{ReadWriteLock}} implementations to prevent dirty reads and
> concurrent writes. However, {{ClusterImpl}} exposes a "global"
> {{ReadWriteLock}} which the other business objects share. This causes
> tremendous problems with deadlocks, especially on slow databases.
> Consider the case where you have 3 threads:
> # thread-1 acquires {{ClusterReadLock}}
> # thread-2 acquires {{ServiceComponenWriteLock}}
> # thread-3 tries to get {{ClusterWriteLock}} and is blocked by {{thread-1}}
> # thread-2 tries to get {{ClusterReadLock}} and is blocked by {{thread-3}}
> # thread-1 tries to get {{ServiceComponentReadLock}} and is blocked by
> {{thread-2}}
> Essentially, the exposure of the "cluster global lock" causes problems since
> multiple threads can acquire other internal locks and be blocked waiting on
> the global lock.
> In general, I don't believe that the read locks help at all. Ambari usually
> encounters these locks while try to display web page information. Once
> displayed, the locks are removed and the information is already stale if
> there were write threads waiting.
> These locks should be investigated and, for the most part, except in some
> cases involving concurrent writes, removed.
> Part of the problem revolves around our assumption about how the
> ReadWriteLock works. The issue in the above scenario is that the
> clusterWriteLock request is pending. This actually blocks all subsequent
> readers even though the lock is not fair.
> FYI, this code shows that a reader, in unfair mode, will wait when there is a
> waiting writer:
> {noformat:title=Output}
> Waiting for a read lock...
> Read lock acquired!
> Waiting for a write lock...
> Trying to acquire a second read lock...
> {noformat}
> {code}
> import java.util.concurrent.locks.ReentrantReadWriteLock;
> public class Test {
> private static ReentrantReadWriteLock lock = new
> ReentrantReadWriteLock(false);
> public static void main(String[] args) throws InterruptedException {
> // A reader which takes too long to finish
> new Thread() {
> @Override
> public void run() {
> System.out.println("Waiting for a read lock...");
> lock.readLock().lock();
> System.out.println("Read lock acquired!");
> try {
> try {
> Thread.sleep(1000 * 60 * 60);
> } catch (InterruptedException e) {
> }
> } finally {
> lock.readLock().unlock();
> }
> }
> }.start();
> Thread.sleep(3000);
> // A writer which will be waiting
> new Thread() {
> @Override
> public void run() {
> System.out.println("Waiting for a write lock...");
> lock.writeLock().lock();
> System.out.println("Write lock acquired!");
> lock.writeLock().unlock();
> }
> }.start();
> Thread.sleep(3000);
> // Another reader
> new Thread() {
> @Override
> public void run() {
> System.out.println("Trying to acquire a second read lock...");
> lock.readLock().lock();
> try {
> System.out.println("Second read lock acquired successfully!!");
> } finally {
> lock.readLock().unlock();
> }
> }
> }.start();
> }
> }
> {code}
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