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Mistral Issues

This section contains information on how to troubleshoot Mistral related issues.

Troubleshooting Mistral Database Upgrade

The mistral and mistral-api services must be stopped at time of upgrading the st2mistral package and the mistral database schema. If the st2mistral package has been upgraded and the services are started before the mistral-db-manage upgrade head command is run, then the mistral-db-manage upgrade head command may fail. When the mistral and mistral-api services run, SQLAlchemy automatically creates tables, relationships, and indices that do not exist. In the case where there are new database objects in the upgraded database schema, the mistral-db-manage upgrade head command will fail because the actual schema in the database is now different than its spec and it no longer can create the new database objects. When that happens, the new database tables, relationships, and indices must be deleted before the mistral-db-manage upgrade head command can be re-run. For more details, review the specific section for the version being upgraded in Upgrades.

Troubleshooting Mistral Workflow Completion Latency

BWC interacts with Mistral via HTTP APIs. This is true for kicking off a workflow execution or collecting the results for a running workflow. BWC queries Mistral to check on workflow execution status and the status of individual tasks. The process in BWC that does the querying is called st2resultstracker. It saves the state of outstanding workflow executions in the database, and once a workflow is complete, deletes the state from the database. The process uses eventlets to simultaneously query multiple workflow results. This can consume significant CPU cycles. There are two configurable values (as of BWC version 2.3 onwards). These are thread_pool_size (number of eventlets) and query_interval (interval to space out the subsequent queries to Mistral for a single execution). You can configure these values by editing the results_tracker section in /etc/st2/st2.conf:

[resultstracker]
query_interval = 5 # in seconds
thread_pool_size = 10

These numbers are subject to load conditions in your infrastructure and the number of workflows you run. The default value for query_interval is set to 20 (seconds). With BWC 2.2 and earlier, this value was 0.1. We’ve set the value to 20 seconds to be conservative. This also means the time to detect a completed workflow in Mistral by BWC could take as long as 20 seconds. If this is unacceptable for you, you can reduce the query_interval and also simultaneously check CPU usage for the st2resultstracker process. We are reworking the design to use HTTP callbacks from Mistral to BWC. Until then, these tunable knobs should help.