Add Support for Retry on NodeLocked Exceptions¶
Let’s reduce the pain of clients being presented errors due to conflicts with locking a particular node (NodeLocked exceptions) by adding multiple attempts to lock the node on the client’s behalf.
As an added benefit, this would also help with tempest testing where this error is seen on occasion .
Ironic clients may sometimes experience a NodeLocked exception/error when making certain REST API calls. This is because the conductor(s) will grab a lock on a node before performing any action that may change that node’s characteristics. Examples of where node locking occurs are:
Almost all of the RPC API calls (called to satisfy some REST API requests) lock the node in question except for: change_node_maintenance_mode, driver_vendor_passthru, get_console_information and validate_driver_interfaces.
The conductor periodic task to synchronize power states.
The conductor periodic task to check deploy timeouts.
The amount of time the node locks are held are typically not long. We could eliminate many of these errors by simply retrying the lock attempt. Admittedly, this would not totally eliminate the problem, but it would make it much less likely to occur for clients, and thus make for a much better experience.
The TaskManager class is used to control when nodes are locked. The lock itself is implemented by the database backend.
I propose we change the TaskManager.__init__() method to incorporate the retry logic, leaving the implementation of the lock itself (the database API layer) untouched. This lets us change the lock implementation later, if we choose, without having to migrate the retry logic.
A more permanent solution to this problem is being discussed in a spec that is defining an asynchronous REST API :
It is unlikely that spec will be approved in Juno because of the work that it entails to change the APIs. This proposal is simple and can be implemented quickly.
A second alternative would be to implement the retry logic at the RPC level. This has the disadvantage of increasing traffic on the RPC bus, and even lengthier waits for lock attempts.
A third alternative is to change the locking layer itself and provide it a value for timing out the attempt. This could potentially be a more complex change, and would need to be duplicated if we changed the mechanism used for locking.
Data model impact¶
REST API impact¶
Driver API impact¶
Nova driver impact¶
This could create a DoS opportunity based on configuration values for retry attempts and time between retries. This is eliminated by using configuration values that will force only a single lock attempt.
Other end user impact¶
The only impact this will have on the user is the reduced amount of node lock errors from API requests and potential delays in HTTP responses. See the Performance Impact section for more information on delays.
This does have the possibility to negatively impact scalability. A spawned worker thread within the conductor could potentially take longer to process the work if a NodeLocked expection is thrown. This impact can be mitigated by increasing the number of workers in the pool (the workers_pool_size option).
This will add additional processing time to REST API calls that happen to encounter a node lock error. This is due to repeated calls to the database API layer to attempt to successfully lock the node.
If the node is locked successfully on the first attempt, then performance is not impacted at all.
Other deployer impact¶
We should control the retry logic with configuration variables for maximum retry attempts and time in between attempts. Using sane defaults (setting the values such that no retry attempts are performed) should help alleviate much of this impact.
The following new configuration variables are proposed (and their default values) to be added to the conductor variable group:
node_locked_retry_attempts = 3 (default to 3 retry attempts)
node_locked_retry_interval = 1 (default to 1 second between attempts)
The default for node_locked_retry_attempts will be 3, which could potentially affect existing installations (see Performance Impact section) when upgraded, but should reduce NodeLocked errors encountered.
- Primary assignee:
The retrying  Python library would be of great use here, as it encasulates all of the logic we would want. Version 1.2.2 (the latest release as of this writing) would be the minimum version we would want since it contains an important bug fix related to retrying on certain exceptions.
NOTE: The global-requirements.txt value  for the retrying module will need to be modified to meet this minimum version requirement.
I don’t see how to test this in tempest successfully (other than eliminating the current errors from tempest due to this problem), but I imagine we can add unit tests to verify it’s working as we expect.
 https://bugs.launchpad.net/ironic/+bug/1321494  https://review.openstack.org/94923  https://pypi.python.org/pypi/retrying  https://github.com/openstack/requirements