Mac OS Launch Daemons and Agents

It is highly recommended to design daemons to be launchd compliant. Using launchd provides better performance and flexibility for daemons. It also improves the ability of administrators to manage the daemons running on a given system. Using launchd provides better performance and flexibility for daemons. It also improves the ability of administrators to manage the daemons running on a given system.

There are four ways to launch daemons using launchd. The preferred method is on-demand launching, but launchd can launch daemons that run continuously, and can replace inetdfor launching inetd-style daemons. In addition, launchd can start jobs at timed intervals.

 What launchd provides is a harness for launching your daemon as needed. To client programs, the port representing your daemon’s service is always available and ready to handle requests. In reality, the daemon may or may not be running. When a client sends a request to the port, launchd may have to launch the daemon so that it can handle the request. Once launched, the daemon can continue running or shut itself down to free up the memory and resources it holds. If a daemon shuts itself down, launchd once again relaunches it as needed to process requests.

In addition to the launch-on-demand feature, launchd provides the following benefits to daemon developers:

    1. Simplifies the process of making a daemon by handling many of the standard housekeeping chores normally associated with launching a daemon.

    2. Provides system administrators with a central place to manage daemons on the system.
    3. Supports inetd-style daemons.
    4. Eliminates the primary reason for running daemons as root. Because launchd runs as root, it can create low-numbered TCP/IP listen sockets and hand them off to the daemon
    5. Simplifies error handling and dependency management for inter-daemon communication. Because daemons launch on demand, communication requests do not fail if the daemon is not launched. They are simply delayed until the daemon can launch and process them.


Below is the launchd startup process for both user level and system level

After the system is booted and the kernel is running, launchd is run to finish the system initialization. As part of that initialization, it goes through the following steps:

    It loads the parameters for each launch-on-demand system-level daemon from the property list files found in /System/Library/LaunchDaemons/ and /Library/LaunchDaemons/.
    It registers the sockets and file descriptors requested by those daemons.
    It launches any daemons that requested to be running all the time.
    As requests for a particular service arrive, it launches the corresponding daemon and passes the request to it.
    When the system shuts down, it sends a SIGTERM signal to all of the daemons that it started.


The process for per-user agents is similar. When a user logs in, a per-user launchd is started. It does the following:

    It loads the parameters for each launch-on-demand user agent from the property list files found in /System/Library/LaunchAgents, /Library/LaunchAgents, and the user’s individual Library/LaunchAgents directory.
    It registers the sockets and file descriptors requested by those user agents.
    It launches any user agents that requested to be running all the time.
    As requests for a particular service arrive, it launches the corresponding user agent and passes the request to it.
    When the user logs out, it sends a SIGTERM signal to all of the user agents that it started.



References:
https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html#//apple_ref/doc/uid/10000172i-SW7-BCIEDDBJ