Add a Distinct Exception for Exceeding Max Retries

Add a Distinct Exception for Exceeding Max Retries

https://blueprints.launchpad.net/nova/+spec/no-valid-host-reporting

There are two separate and very different situations which raise a NoValidHost exception when attempting to spawn a new VM, which is confusing for operators who need to determine why a VM failed to spawn. This proposes to add a new exception class that will make the two failure modes more obvious to the operators.

Problem description

Currently, a NoValidHost exception is raised in two separate cases:

  • when the scheduler filters cannot find a host that meets the requirements of the request

  • when the maximum number of retries is exceeded when attempting to create the VM

Operators can have a difficult time distinguishing between the two situations when attempting to learn why a VM failed to spawn.

Use Cases

As an operator I want a less confusing way of determining why a VM failed to spawn. The current practice of having a NoValidHost exception raised for very different failure reasons is not as helpful as separate exception types raised for the different modes.

Project Priority

This is part of the scheduler improvement effort, which has been identified as a priority for Liberty.

Proposed change

A new exception class named MaxRetriesExceeded will be defined, and all places where NoValidHost is currently raised that are not due to all hosts being filtered out will be changed to raise this new exception. This new exception class will be a subclass of NoValidHost, so that any existing code that is catching NoValidHost exceptions will not break.

Alternatives

None.

Data model impact

None.

REST API impact

None.

Security impact

None.

Notifications impact

None.

Other end user impact

None.

Performance Impact

None.

Other deployer impact

None.

Developer impact

None.

Implementation

Assignee(s)

Primary assignee:

edleafe

Work Items

  • Define the new exception class

  • Search through the code for all cases where NoValidHost is raised, and change the exception class raised by those that are the result of exceeding the maximum number of retries to raise this new exception class instead.

Dependencies

None.

Testing

Existing tests will be updated to test for the new exception class instead of the old class wherever appropriate.

Documentation Impact

None.

References

None.

Creative Commons Attribution 3.0 License

Except where otherwise noted, this document is licensed under Creative Commons Attribution 3.0 License. See all OpenStack Legal Documents.

nova-specs