Jonathan Hurley created AMBARI-16131:
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Summary: Prevent Views From Causing a Loss of Service For Ambari
Key: AMBARI-16131
URL: https://issues.apache.org/jira/browse/AMBARI-16131
Project: Ambari
Issue Type: Bug
Affects Versions: 2.0.0
Reporter: Jonathan Hurley
Assignee: Jonathan Hurley
Priority: Critical
Fix For: 2.4.0
The underlying problem is that views are accessed off of the REST endpoint
({{/api/v1/views}}). This means that the Ambari REST API connector is going to
handle the request from its own threadpool. There is no way to configure Jetty
to use a different threadpool for the same connector. As a result, if a request
to load a view holds the Jetty thread hostage, eventually we will see thread
starvation and loss of service.
An example of this situation is a view which makes an innocent request to a
remote resource. If the view's request has a timeout of 60 seconds, then the
Jetty thread is going to be held for that amount of time. With concurrent users
and multiple instances of that view deployed, the Jetty threadpool can becomes
exhausted quickly.
Although there are more graceful ways of handling this situation, they mostly
involve substantial re-architecture and design:
- The use of a new connector and threadpool would require binding to another
port for view requests. This will cause problems with "local" views and their
assumption that if they run on the Ambari server they can share the same
session.
- The use of a
[Continuation|https://wiki.eclipse.org/Jetty/Feature/Continuations] in Jetty
which can suspend the incoming request. We would need the ability for views to
signal that they have completed their work in order to proceed with the
suspended request.
A quicker and far less invasive fix would be to create a filter which
intercepts requests for views. It will determine how many executing view
requests exist and decide if it will allow the new request through. For
example, if configured to allow a maximum of 10 concurrent view requests, then
the 11th request would be denied with an {{HTTP 503 - Service Unavailable}}.
Although the thread is temporarily used while the filter is processing, it's
quickly returned to the Jetty pool when it's determined there are too many
other running view requests.
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