Immutable Parameters

Template authors should be able to mark template parameters as updatable or non-updatable to restrict updating parameters which have destructive effects on the application.

Problem description

Sometimes a template parameter is only intended to be passed during stack creation. For example, updating an SSH public key name parameter, which is passed to the keypair property of an OS::Nova::Server, would result in the server being replaced and risk downtime.

In some cases, an application’s architecture would not be tolerant of certain servers being rebuilt. The template author has the freedom to mark parameters which cause those servers to be rebuilt as immutable.

This feature is targeted to Heat service providers and operators of other services that offer a curated set of templates to end-users. These are users who may not have a lot of expertise with the application architecture. Expert users have the option of editing the template to remove the update restriction.

Proposed change

Add a new “updatable” boolean field to the parameters section in a HOT template. A value of False would result in the engine rejecting stack-updates that include changes to that parameter. When not specified in the template, “updatable” would default to True to ensure backwards compatibility with old templates.


  • Loosen the “each parameter can be in one parameter group” restriction and use parameter groups to mark parameters as immutable. Adding a new updatable field is a more user-friendly option.


This would include changes to heat/engine/hot/, where a new updatable field would be added, and heat/engine/, where we would restrict updates to parameters. Whenever a user attempts to update a restricted parameter, they will see a ImmutableParameterModified exception returned from the API before the actual stack-update begins.


Primary assignee:



Target Milestone for completion:


Work Items

  • Add updatable field to template (heat/engine/hot/

  • Restrict updates to parameters (heat/engine/

  • Add a ParameterUpdateNotAllowed exception (heat/common/

  • Add unit tests