Stack Breakpoint

https://blueprints.launchpad.net/heat/+spec/stack-breakpoint

Orchestration template is a powerful automation tool when it works; however when it fails, troubleshooting can be quite difficult. During development, debugging failed template is simply part of the process, but in production, a previously working template can also fail for many reasons. Providing support for troubleshooting template will not only increase productivity but will also help the adoption of Heat template by allowing users to “look under the hood” and have a better handle on the automation.

Typically, the user would start by checking the logs to get some bearing on the error. If possible, the user may try to enhance the logs by adding more log message in the script. This initial approach should resolve many errors, but difficult error may require more active debugging. The user would need to stop at or before the point of template failure, inspect variables, check the environment, run command or script manually, etc. Since the template is declarative, the user would need to be able to recreate the error consistently.

Support for troubleshooting is broad and will require many blueprints to implement the different features to control the template flow, recreate the error, and inspect the elements. Related blueprints include troubleshooting-low-level-control, resolve-failed-stack-attributes, user-visible-logs, user-friendly-template-errors. This blueprint covers the particular scenario of how to better control the stack deployment while troubleshooting.

Problem description

With a failing stack, currently we can stop on the point of failure by disabling rollback: the stack will stop when a resource fails, leaving in place the resources that have been created successfully. There may be some false failures because some resources may be aborted, but they can be easily identified by displaying the state of the resource. This technique works well for troubleshooting stack-create; stack-update can be handled similarly once the blueprint update-failure-recovery is implemented.

In many cases however, the point of failure may be too late or too hard to debug because the original cause of the failure may not be obvious or the environment may have been changed. If we can pause the stack at a point before the failure, then we are in a better position to troubleshoot. For instance, we can check whether the state of the environment and the stack is what we expect, we can manually run the next step to see how it fails, etc.

While developing new template or resource type, it is also useful to bring up a stack to a point before the new code is to be executed. Then the developer can manually execute and debug the new code.

Proposed change

The usage would be as follows:

  • Run stack-create or stack-update with one or more resource name specified as breakpoint, for example:

    heat stack-create my_stack –template-file my_template.yaml –breakpoint failing_resource_name

    heat stack-update my_stack –template-file my_template.yaml –breakpoint failing_resource_name

  • The breakpoint can also be coded in the environment file pointing to a particular resource, for example:

    breakpoints:

    resource: failing_resource_name

  • As the engine traverses down the dependency graph, it would stop at the breakpoint resource and all dependent resources. Other resources with no dependency will proceed to completion before stopping. Multiple breakpoints can be set to control parallel paths in the graph.

  • Running resource-list or resource-show will show the resource at the breakpoint as “CREATE.INPROGRESS” or “UPDATE.INPROGRESS” and the resource is not created or updated yet. Running event-list will show that the breakpoint has occurred, and event-show will give more details on the breakpoint.

  • The breakpoint can be deleted on the command line by:

    heat stack-update my_stack –template-file my_template.yaml –nobreakpoint failing_resource_name

  • In the environment file, the breakpoint can be deleted simply by deleting the resource name in the breakpoint property. This would take effect the next time the environment file is specified on stack-update. The user is probably more likely to use the command line option.

  • After debugging, continue the stack by (done manually, but can also be automated by a high level debugger):

    • Stepping: remove current breakpoint, set breakpoint for next resource(s) in dependency graph, resume stack-create (or stack-update).
    • Running to completion: remove current breakpoint, resume stack-create or stack-update by running stack-update with the same template and parameters.

For nested stack, the breakpoint would be prefixed with the name of the nested template.

The change will include the heat client, api and environment to add the breakpoint option. For the Heat engine to stop at a resource, we will leverage the blueprint lifecycle-callbacks. Some code to set up and interface with the callback will be needed and the details will be determined when this blueprint is implemented.

Alternatives

The manual approach is simply to edit the template and delete any failing resources until the remaining resources can be created successfully. Then stepping each resource can be done by adding it back to the template and running stack-update. The full stack will need to be deleted and recreated for each iteration. This manual technique cannot be incorporated into high level tool.

Implementation

Assignee(s)

Primary assignee:
Ton Ngo

Milestones

Target Milestone for completion:
Juno-3 or further

Work Items

  • Heat client: add option to specify breakpoint
  • Heat API: add option to specify breakpoint
  • Environment: add option to specify breakpoint
  • Interface with lifecycle-callbacks

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