Multiple Environment Support

https://blueprints.launchpad.net/heat/+spec/multi-environments

We allow the user to specify multiple environment files in the client, but these get combined in the client so any redundant information, precendence, &c. is no longer available to heat-engine. This causes problems with PATCH updates of the environment, particularly with TripleO.

Problem description

On a PATCH update for a stack whose environment is a composite from multiple files, a user cannot (in the general case) safely update any of the environment files without explicitly passing all of the files previously included to the client again (since order matters, and since the engine has no knowledge of any given file’s position in the hierarchy. This leaves them in a similar position to where they would be without PATCH updates to the environment: having to remember all of the constituent parts of the stack’s environment.

This is a particular problem for TripleO, which may use many environment files even in a fairly typical deployment. If the user customises one of the default environment files, the change is not picked up because the TripleO client does not send environment files with its PATCH updates unless they are explicitly specified; conversely sending the default environment file(s) again risks overwriting user-specified environment (such as the one for network isolation) unless the user is required to always pass all of the environment files again.

Proposed change

Add an optional “environment_files” key to the body of a stack create or update request. Valid content for this key is a list of filenames. The filenames themselves may be arbitrary. The file content must be included in the “files” section of the request, keyed by the filename, in the same way as other extra files (scripts, nested templates) are.

The “environment_files” key is outside the existing environment section to ensure that it is only inserted by the client; this change does not modify the environment file format itself. This prevents any issues with circular inclusions and the like.

Multiple files will be combined by the engine in the same manner as they currently are by the client. Where environment files contain conflicting information, the last one specified wins.

Parameter values that are specified explicitly (i.e. outside the environment) will be applied after all environment files are merged, to maintain consistency with the current approach (where the file combination is done in the client and the parameters merged in heat-api). This means that the code to do this must be moved from heat-api to heat-engine.

On a PATCH update, any additional_environment files specified are appended to the list. Heat will recalculate the combined environment on each stack update, so that any changes to the “files” part of the environment are picked up.

Alternatives

We could allow includes in the environment file format itself. However, this would require us to deal with problems like circular includes, or to have a different format for the included environment files vs. the ‘main’ environment. While it would be nice to be able to record all of the environments for a given stack in a single place that can be stored e.g. in Git, it’s probably better to implement this purely on the client side as a CLI option to read the list of environments from a file instead of/as well as from multiple –environment options.

Implementation

Assignee(s)

Primary assignee:
jdob

Milestones

Target Milestone for completion:
Mitaka-1

Work Items

  • Add support for combining environments + parameters in heat-engine
  • Add support for passing the environment_files list + params over the RPC API
  • Add support for environment_files in heat-api
  • Modify the client to pass environments as a list
  • Add a CLI option to read the environments list from a file

Dependencies

None.