[openstack-dev] [heat] Sofware Config progress

Steve Baker sbaker at redhat.com
Fri Dec 13 23:46:48 UTC 2013

I've been working on a POC in heat for resources which perform software
configuration, with the aim of implementing this spec

The code to date is here:

What would be helpful now is reviews which give the architectural
approach enough of a blessing to justify fleshing this POC out into a
ready to merge changeset.

Currently it is possible to:
- create templates containing OS::Heat::SoftwareConfig and
OS::Heat::SoftwareDeployment resources
- deploy configs to OS::Nova::Server, where the deployment resource
remains in an IN_PROGRESS state until it is signalled with the output values
- write configs which execute shell scripts and report back with output
values that other resources can have access to.

What follows is an overview of the architecture and implementation to
help with your reviews.

Like many heat resources, OS::Heat::SoftwareConfig and
OS::Heat::SoftwareDeployment are backed by "real" resources that are
invoked via a REST API. However in this case, the API that is called is
heat itself.

The REST API for these resources really just act as structured storage
for config and deployments, and the entities are managed via the REST
paths /{tenant_id}/software_configs and /{tenant_id}/software_deployments:
RPC layer of REST API:
DB layer of REST API:
heatclient lib access to REST API:

This data could be stored in a less structured datastore like swift, but
this API has a couple of important implementation details which I think
justify it existing:
- SoftwareConfig resources are immutable once created. There is no
update API to modify an existing config. This gives confidence that a
config can have a long lifecycle without changing, and a certainty of
what exactly is deployed on a server with a given config.
- Fetching all the deployments and configs for a given server is an
operation done repeatedly throughout the lifecycle of the stack, so is
optimized to be able to do in a single operation. This is called by
using the deployments index API call,
/{tenant_id}/software_deployments?server_id=<server_id>. The resulting
list of deployments include the their associated config data[1].

OS::Heat::SoftwareConfig resource
OS::Heat::SoftwareConfig can be used directly in a template, but it may
end be more frequently used in a resource provider template which
provides a resource aimed at a particular configuration management tool.
The contents of the config property will depend on the CM tool being
used, but at least one value in the config map will be the actual script
that the CM tool invokes.  An inputs and outputs schema is also defined
here. The group property is used when the deployments data is actually
delivered to the server (more on that later).

Since a config is immutable, any changes to a OS::Heat::SoftwareConfig
on stack update result in replacement.

OS::Heat::SoftwareDeployment resource
OS::Heat::SoftwareDeployment joins a OS::Heat::SoftwareConfig resource
with a OS::Nova::Server resource. It allows server-specific input values
to be specified that map to the OS::Heat::SoftwareConfig inputs schema.
Output values that are signaled to the deployment resource are exposed
as resource attributes, using the names specified in the outputs schema.
The OS::Heat::SoftwareDeployment resource remains in an IN_PROGRESS
state until it receives a signal (containing any outputs) from the server.

A deployment has its own actions and statuses that are specific to what
a deployment does, and OS::Heat::SoftwareDeployment maps this to heat
resource statuses and actions:

status (these could use some bikeshedding):

In the config outputs schema there is a special flag for error_output.
If the signal response contains any value for any of these error_output
outputs then the deployment resource is put into the FAILED state.

The SoftwareDeployment class subclasses SignalResponder which means that
a SoftwareDeployment creates an associated user and ec2 keypair. Since
the SoftwareDeployment needs to use the resource_id for the deployment
resource uuid, the user_id needs to be stored in resource-date instead.
This non-wip change enables that:

During create, the deployment REST API is polled until status goes from
WAITING to RECEIVED. When handle_signal is called, the deployment is
updated via the REST API to set the status to RECEIVED (or FAILED),
along with any output values that were received.

One alarming consequence of having a deployments API is that any tenant
user can create a deployment for any heat-created nova server and that
software will be deployed to that server, which is, um, powerful.

There will need to be a deployment policy (probably an OS::Nova::Server
property) which limits to scope of what deployments are allowed on that
server. This could default to deployments in the same stack, but could
still allow deployments from anywhere.

OS::Nova::Server support
A new user_data_format=SOFTWARE_CONFIG is currently used to denote that
this server is configured via software config deployments. Like
user_data_format=HEAT_CFNTOOLS, nova_utils.build_userdata is used to
build the cloud-init parts required to support software config. However
like user_data_format=RAW anything specified in user_data will be parsed
as cloud-init data. If user_data is multi-part data then the parts will
be appended to the parts created in nova_utils.build_userdata.

The agent used currently is os-collect-config. This is typically
configured to poll for metadata from a particular heat resource via the
CFN API using the configured ec2 keypair. In the current implementation
the resource which is polled is the OS::Nova::Server itself, since this
is the only resource known to exist at server boot time (deployment
resources depend on server resources, so have not been created yet). The
ec2 keypair comes from a user created implicitly with the server
(similar to SignalResponder resources). This means the template author
doesn't need to include User/AccessKey/AccessPolicy resources in their
templates just to enable os-collect-config metadata polling.

Until now, polling the metadata for a resource just returns the metadata
which has been stored in the stack resource database. This
implementation changes metadata polling to actually query the
deployments API to return the latest deployments data. This means
deployment state can be stored in one place, and there is no need to
keep various metadata stores updated with any changed state.

An actual template
This template contains:
- a config resource
- 2 deployments which deploy that config with 2 different sets of inputs
- stack outputs which output the results of the deployments
- a server resource
- an os-refresh-config script delivered via cloud-config[2] which
executes config scripts with deployment inputs and signals outputs to
the provided webhook.

/opt/stack/os-config-refresh/configure.d/55-heat-config-bash is a hook
specific for performing configuration via shell scripts, and only acts
on software config which has group=Heat::Shell. Each configuration
management tool will have its own hook, and will act on its own group
namespace. Each configuration management tool will also have its own way
of passing inputs and outputs. The hooks job is to invoke the CM tool
with the given inputs and script, then extract the outputs and signal heat.

The server needs to have the CM tool and the hook already installed,
either by building a golden image or by using cloud-config during boot.

Next steps
There is a lot left to do and I'd like to spread the development load.
What happens next entirely depends on feedback to this POC, but here is
my ideal scenario:
- any feedback which causes churn on many of the current changes I will
- a volunteer is found to take the REST API/RPC/DB/heatclient changes
and make them ready to merge
- we continue to discuss and refine the resources, the changes to
OS::Nova::Server, and the example shell hook
- volunteers write hooks for different CM tools, Chef and Puppet hooks
will need to be attempted soon to validate this approach.

Vaguely related changes include:
- Some solution for specifying cloud-init config, either the intrinsic
functions or cloud-init heat resources
- Some heatclient file inclusion mechanism - writing that python hook in
a heat yaml template was a bit painful ;)

Trying for yourself
- Using diskimage-builder, create an ubuntu image with
tripleo-image-elements os-apply-config, os-refresh-config and
- Create a local heat branch containing
https://review.openstack.org/#/q/topic:bp/cloud-init-resource,n,z and
- launch the above template with your created image


[1] https://review.openstack.org/#/c/58877/7/heat/engine/api.py
[2] This relies on these not-merged intrinsic functions
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