How does chef-solo --daemonize work, and what's the point?

Question

I understand the purpose of chef-client --daemonize, because it's a service that Chef Server can connect to and interact with.

But chef-solo is a command that simply brings the current system inline with specifications and then is done.

So what is the point of chef-solo --daemonize, and what specifically does it do? For example, does it autodetect when the system falls out of line with spec? Does it do so via polling or tapping into filesystem events? How does it behave if you update the cookbooks and node files it depends on when it's already running?

Answer 1

You might also ask why chef-solo supports the --splay and --interval arguments.

Don't forget that chef-server is not the only source of data. Configuration values can rely on a bunch of other sources (APIs, OHAI, DNS...). The most classic one is OHAI - think of a cookbook that configures memcached. You would probably want to keep X amount of RAM for the operating system and the rest goes to memcached.

Available RAM can be changed when running inside a VM, even without rebooting it. That might be a good reason to run chef-solo as a daemon with frequent chef-runs, like you're used to when using chef-client with a chef-server.

As for your other questions:

Q: Does it autodetect when the system falls out of line with spec? Does it do so via polling or tapping into filesystem events?

A: Chef doesn't respond to changes. Instead, it runs frequently and makes sure the current state is in sync with the desired state - which can be based on chef-server inventory, API calls, OHAI attributes, etc. The desired state is constructed from scratch every time you run Chef, at the compile stage when all the resources are generated. Read about it here

---

Q: How does it behave if you update the cookbooks and node files it depends on when it's already running?

A: Usually when running chef-solo, one uses the --json flag to specify a JSON file with node attributes and a run-list. When running in --daemonize mode with chef-solo, the node attributes are read only for the first run. For the rest of the runs, it's as if you were running it without a --json flag. I couldn't figure out a way to make it work as if you were running it with --json all over again, however, you can use the --override-runlist option to at least make the runlist stick. Note that the attributes you're specifying in your JSON won't make it past the first run. This is possibly a bug.