lock_rdlm cookbook

Contains some functions that work with an external lock server.

Also supports managing the lock server using RDLM

Supported Platforms

For clients: Nothing native, so anything goes I guess.

For lock server: Tested on CentOS 6.7.

Attributes

Recipes

lock_rdlm::server

Installs and configures the simple RDLM server.

It uses the ['lock_rdlm']['port'] attribute to determine the port the daemon will be listening on.

I'm using an init file tested on CentOS 6.7.

Library resources

Common parameters

Used to locate the lock server, appear under additional_config in method descriptions:

• lock_url: How to reach the lock server. Defaults to building the url from attributes (if possible)
• lock_wait: Number of seconds to wait to acquire the lock before raising an error. Defaults to node['lock_rdlm']['wait']
• lock_lifetime: Number of seconds before the lock will expire on its own. Defaults to node['lock_rdlm']['lifetime']

LockRDLM::with_lock

Yields (runs a code block) while locking the resource specified on the lock.

Parameters:

• node: Calling node's object
• lock_resource: name for lock
• "additional_config" detailed above

Can be thought of like the synchronized keyword in java.

Example:

# Add apple to databag if it has none
LockRDLM::with_lock(node,'dbvendingjuice') do
  db=data_bag_item('vending','juice')
  db['kinds']<<'apple' unless db['kinds'].member? 'apple'
  db.save
end

This will ensure the databag has only one apple item, even if several clients run this recipe at the same time.

LockRDLM::assign_identity

Used to assign a unique identity.

Parameters:

• node: Calling node's object
• assignment_path: Path for attribute to store unique value in
• range: Pool of possibe values
• additional_config: Detailed above

Resulting effects

Upon successful completion of the function, this node will be assigned a unique identity in its node object.

The identity will be stored as a node attribute specified by assignment_path, where [:a,0,'bla'] is used to address the node attribute node[:a][0]['bla'].

The value is selected from range and is guranteed to be different from any value currently present on other nodes in the Chef server.

Selection is done without concurrency, thanks to the lock.

The resulting value is saved on both the current node object (for use in other parts of the recipe) and in the server's version of the node object immediately, to stop other nodes from choosing the same value. This is important to mention because by default, Chef client only updates the server's version of the node object if the run is successful. In our case, this attribute is updated during resource compilation.

There are several ways this method can fail:

• Can't talk to lock server
  Fix the server, firewalls etc.
• Can't get lock lock (other client is busy with lock).
  • My library might be buggy and not release the lock for some reason. Create a PR/Issue :)
  • Enumerating all of the nodes takes longer than the lock time constraints. Allow more wait time.
• Range is exhausted
  • Increase the range
  • Delete dead nodes from the Chef server

Basic walk-through

Assuming a working and reachable lock server:

1. Function is called with the following:
   • assignment_path = [:slasher,:id]
   • range = (1..5).to_a
   • lock_url = 'http://lock:8080/'
   • lock_wait = 5
   • lock_lifetime = 300
2. Current value of attribute is checked (node[:slasher][:id]). If not-empty, function returns. Assuming empty
3. Lock is locked for a normalized version of the assignment_path (slasherid)
4. If lock can't be locked for some reason, raise error
5. All node objects on Chef server are enumerated for that value. Resulting collection is filtered out from range.
6. Take a single element from range (first). If empty, raise error (release lock before). Assuming got 3.
7. Load our node object from Chef server, modify the node attribute and save it.
8. Update the current node's attribute (node.normal[:slasher][:id]=3)
9. Release lock
10. Return 3

LockRDLM::find_duplicate_identity

Parameters:

• node: Calling node's object
• assignment_path: Path for attribute unique value is stored in
• only_me: Only check for conflicts involving my node
• additional_config: Detailed above

Used to enforce uniqueness of the identity attribute without modifying anything.

Is useful in monitoring.

Return value

Empty hash ({}) if there are no duplicates.

If there are duplicates, the hash is composed of a key for the duplicate identity and the value is an array of nodes holding said value, like this:

{
  3 => ['node1.backslasher.net', 'node2.backslasher.net']
}

License and Authors

Licensed GPL v2

Author:: Nitzan Raz (backslasher)