Details
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Bug
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Status: Resolved
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Critical
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Resolution: Fixed
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2.0.0
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None
Description
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 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.