On 08/08/14 11:07, Tomas Sedovic wrote:
On 08/08/14 00:53, Zane Bitter wrote:
On 07/08/14 13:22, Tomas Sedovic wrote:
Hi all,

I have a ResourceGroup which wraps a custom resource defined in another

        type: OS::Heat::ResourceGroup
          count: 10
            type: my_custom_server
              prop_1: "..."
              prop_2: "..."

And a corresponding provider template and environment file.

Now I can get say the list of IP addresses or any custom value of each
server from the ResourceGroup by using `{get_attr: [servers,
ip_address]}` and outputs defined in the provider template.

But I can't figure out how to pass that list back to each server in the

This is something we use in TripleO for things like building a MySQL
cluster, where each node in the cluster (the ResourceGroup) needs the
addresses of all the other nodes.

Yeah, this is kind of the perpetual problem with clusters. I've been
hoping that DNSaaS will show up in OpenStack soon and that that will be
a way to fix this issue.

The other option is to have the cluster members discover each other
somehow (mDNS?), but people seem loath to do that.

Right now, we have the servers ungrouped in the top-level template so we
can build this list manually. But if we move to ResourceGroups (or any
other scaling mechanism, I think), this is no longer possible.

So I believe the current solution is to abuse a Launch Config resource
as a store for the data, and then later retrieve it somehow? Possibly
you could do something along similar lines, but it's unclear how the
'later retrieval' part would work... presumably it would have to involve
something outside of Heat closing the loop :(

Do you mean AWS::AutoScaling::LaunchConfiguration? I'm having trouble
figuring out how would that work. LaunchConfig represents an instance,

We can't pass the list to ResourceGroup's `resource_def` section because
that causes a circular dependency.

And I'm not aware of a way to attach a SoftwareConfig to a
ResourceGroup. SoftwareDeployment only allows attaching a config to a
single server.

Yeah, and that would be a tricky thing to implement well, because a
resource group may not be a group of servers (but in many cases it may
be a group of nested stacks that each contain one or more servers, and
you'd want to be able to handle that too).

Yeah, I worried about that, too :-(.

Here's a proposal that might actually work, though:

The provider resource exposes the reference to its inner instance by
declaring it as one of its outputs. A SoftwareDeployment would learn to
accept a list of Nova servers, too.

Provider template:

         type: OS::Nova::Server

       ... (some other resource hidden in the provider template)

         value: {get_resource: my_server}
         value: {get_attr: [controller_server, networks, private, 0]}

Based on my limited testing, this already makes it possible to use the
inner server with a SoftwareDeployment from another template that uses
"my_server" as a provider resource.


       type: OS::Heat::ResourceGroup
         count: 10
           type: custom::my_server

       type: OS::Heat::StructuredDeployment
         server: {get_attr: [a_cluster_of_my_servers,
         config: {get_resource: some_config}

So what if we allowed SoftwareDeployment to accept a list of servers in
addition to accepting just one server? Or add another resource that does

I approve of that in principle. Only Steve Baker can tell us for sure if there are any technical roadblocks in the way of that, but I don't see any.

Maybe if we had a new resource type that was internally implemented as a nested stack... that might give us a way of tracking the individual deployment statuses for free.


Then we could do:

       type: OS::Heat::StructuredDeployment
         server_list: {get_attr: [a_cluster_of_my_servers, inner_server]}
         config: {get_resource: mysql_cluster_config}
           cluster_ip_addresses: {get_attr: [a_cluster_of_my_servers,

This isn't that different from having a SoftwareDeployment accepting a
single server and doesn't have any of the problems of allowing a
ResourceGroup as a SoftwareDeployment target.

What do you think?

Is there a way to do this that I'm missing? And if there isn't, is this
something we could add to Heat? E.g. extending a SoftwareDeployment to
accept ResourceGroups or adding another resource for that purpose.


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