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Ohai Doc Changes:

Shard Seed Plugin

The new shard_seed attribute provides a uniformly distributed integer ID value to all nodes. This value will be between 0 and 4294967295 (0xFFFFFFFF), based on a consistent hash of one or more pieces of node data. A consistent hash means that a node with the same source inputs will always result in the same shard seed, but collisions are possible.

The sources used to compute the seed value can be configured either in your Chef config file:

Ohai::Config[:plugin][:shard_seed][:sources] = [:fqdn]

or your Ohai config file if you run Ohai outside of Chef:

ohai.plugin[:shard_seed][:sources] = [:fqdn]

The default sources are [:machinename, :serial, :uuid] but this value should be considered experimental for the moment as it may be expanded to support additional platforms. Configuring a value that works for your infrastructure is highly recommended.

Shard Seed Sources

There are multiple sources of data that can be used as inputs to the hash:

  • :machinename – Everything before the first . in the machine's hostname.
  • :fqdn – The machine's fully-qualified name. Depending on your platform and configuration, it is possible that this value could be unstable during DNS failures or other network outages.
  • :hostname – The machine's raw hostname. As with :fqdn, this may be unstable during DNS failures or network outages.
  • :serial – The machine's serial number as reported by DMI, SystemProfiler, or WMI.
  • :uuid – The machine's UUID as reported by DMI, SystemProfiler, or WMI.
  • :machine_id – The systemd machine ID. Only available on platforms using systemd.

You can configure either one source or an array of sources which will be concatenated together.

Using the Shard Seed

Two main use cases for the seed value are slow rollouts of new features and computing service IDs.

To do slow rollouts, you can wrap new features or options in your recipe code with a conditional if statement like:

# Enable feature for 20% of nodes.
if (node['shard_seed'] % 100) < 20
  # Code goes here ...
end

For creating a service ID for something like ZooKeeper or MySQL replication:

# Write the ID to a file for safety, in case something changes.
file '/etc/zookeeper/myid' do
  action :create_if_missing
  # ZooKeeper ID is 1-255.
  content "#{(node['shard_seed'] % 255) + 1}"
end