[chef] Re: Re: Concurrency node creation issue

["John E. Vincent (lusis)" < >]

I suppose I should record that screencast based on the demo I did in Mt. View eh?

Since Josh and Andrew mentioned it, this is one of the primary use cases for something like Noah (or even Zookeeper proper).

The way it works in the Noah LWRP is that you define blocking in the recipes where appropriate.

So in the demo I used the django quickstart and defined dependencies as 'noah_block' resources. Haproxy nodes blocked for django servers. Django servers blocked waiting on MySQL nodes to come up. The secondary django node blocked on the primary until it ran the initial database setup.

At the end of each node's run, it registered itself with Noah which is what each dependency was waiting for.

I need to move that dependency map into a databag but it does work.

I'll hopefully have the screencast recorded this week since I need it for something else as well.

There's also no reason that you couldn't simply use a custom LWRP that blocked until something came back from search results.

On Sep 6, 2011 11:24 PM, "Joshua Timberman" < "> > wrote:
> Hello,
>
> As Andrew points out, this is a race condition.
>
> What we typically do is the specificity route, where a single system who is to be master is assigned such as a role. For example the "database master" for an application will have a role ... "appname_database_master." Any other nodes that are slaves would have "appname_database_slave." Our "database" cookbook and its companion "application" follow this pattern and behave accordingly.
>
> However I'd love to see a solution that utilizes Noah or Zookeeper to solve the problem more dynamically.
>
> On Sep 4, 2011, at 3:57 PM, Daniel Cukier wrote:
>
>> Hi,
>>
>> I have in my infrastructure a topology where there's one master node and
>> many slave nodes.
>> What I want to do is to automatically detect if a node is a master or a
>> slave. The rule is:
>> 1) If there is no master node yet, the next node will be the master
>> 2) If there is a master, the next node will be slave and its master will be
>> the already existing master.
>>
>> The problem occurs when I have an empty infrastructure (zero nodes) and I
>> try to create the first 2 nodes simultaneously.
>> When both nodes get provisioned, both check that there is no master and both
>> are set to be master, which is a wrong configuration.
>> This is a very common synchronization problem, but I don't know how to deal
>> with it in the Chef environment.
>>
>> Here's the recipe to configure a node:
>>
>> @@master = node
>>
>> search(:node, 'role:myrole') do |n|
>> if n['myrole']['container_type'] == "master"
>> @@master = n
>> if n.name != node.name
>> node.set['myrole']['container_type'] = "slave"
>> end
>> end
>> end
>>
>> if @@master == node
>> node.set['myrole']['container_type'] = "master"
>> end
>>
>> template "#{node['myrole']['install_dir']}/#{ZIP_FILE.gsub('.zip',
>> '')}/conf/topology.xml" do
>> source "topology.xml.erb"
>> owner "root"
>> group "root"
>> mode "0644"
>> variables({:master => @@master})
>> end
>>
>> How can I avoid this problem?
>>
>> Thanks a lot
>>
>> Daniel Cukier
>
> --
> Opscode, Inc
> Joshua Timberman, Director of Training and Services
> IRC, Skype, Twitter, Github: jtimberman
>