Speak, friend, and enter.

mellon-core is a Haskell package for controlling physical access devices designed for human factors, e.g., electric strikes. The access control protocol is quite simple: a device is either locked, or it is unlocked until a particular date and time (an expiration date). Once the expiration date passes, the device is automatically locked again. In the meantime, the device can be locked immediately, overriding the unlocked state; or the unlock period can be extended.

User programs incorporate mellon-core functionality via a controller, which is responsible for handling user lock and unlock commands, and for scheduling and canceling unlock expirations.

User programs must also adapt their physical access devices to the interface expected by the controller. For this purpose, mellon-core defines a device type with 2 simple IO actions for locking and unlocking the device. (mellon-core does not provide any useful device implementations; see the companion mellon-gpio package for a GPIO-driven implementation.)

Note that mellon-core does not provide authentication mechanisms or network services for interacting with controllers; that is the domain of higher-level packages which use the base mellon-core package (e.g., mellon-web).

On the use of UTC dates for timers

mellon-core uses UTC dates for unlock expiration, rather than a time delta or a monotonic clock. You might disagree with this decision based on the common wisdom that it's a bad idea to use "wall clock time" (of which UTC is one flavor) for timers. In general, the common wisdom is correct. Wall clocks have lots of problems: they may not be accurate, they may disagree from one system to the next, they may "jump around" if the system is running a time daemon such as NTP, and they occasionally do something unexpected like adding a leap second.

If your timers must be high-precision (i.e., this timer must run for exactly n microseconds, for some definition of "exactly"), then there's no argument: using a wall clock is a bad idea. However, as mellon-core is designed for use with physical access devices, which themselves are typically designed for human factors, accuracy to within a second or two is acceptable in most cases. (If you have higher-precision needs, especially for extreme safety- or security-related scenarios, you should probably be using a real-time system anyway, not a Haskell program.)

Once the need for high precision is eliminated, and assuming that the system(s) controlling your physical access devices use a synchronized time source such as that provided by NTP, the advantages of using UTC over most of the alternatives become apparent:

• Absolute time deltas without a common reference do not work well in networked environments, where network problems may appreciably delay the delivery of commands from client to server. If a user wants to unlock a device for 7 seconds, does that mean 7 seconds from the clock time T when the user presses "send," or does it mean 7 seconds from opening to close, regardless of when the server receives the command? Without a common reference, there is no way for the user to communicate her intent.

• Monotonic clocks never go backwards, which is a nice invariant and eliminates a problem that occurs in some NTP implementations. However, monotonic clocks are a) non-portable, and not even supported on all systems; b) usually system-dependent, which renders them useless when attempting to communicate time across two systems; c) sometimes even process-dependent, in which case they're not even useful for communicating time between two processes on the same system; and d) often idle while the system is suspending or sleeping, in which case the clock does not move forward while the system is suspended, rendering the clock useless for absolute timers if there's any possibility that the system will be suspended or otherwise go into a low-power mode.

Using the TAI coordinate system rather than UTC has the advantage of guaranteeing that every (TAI) day is exactly 86400 (TAI) seconds, unlike UTC and all of the time systems based on it, where very rarely a day may have 86401 seconds, i.e., one standard day plus 1 leap second. If TAI were well-supported and generally available, mellon-core would probably use it, but circa 2016 it is not. Anyway, at worst, a mellon-core unlock command which spans a time period in which a leap second is added will expire approximately 1 second too soon / too early, depending on whether the user accounted for the leap second when she issued the command. As this error is more or less within the expected accuracy of a mellon-core system under normal operation (due to the vagaries of thread scheduling, and not even accounting for clock drift and other real-world factors), it doesn't really seem worth the effort just to avoid the minor inconvenience of leap seconds.

In short, synchronizing time (and timers) across multiple systems is a very difficult problem, and one which the universally-supported Network Time Protocol attempts to address, mostly successfully. Given its intended application to controlling physical access for human beings, most likely in a networked environment, mellon-core makes the choice of relying on a working, accurate NTP (or other wall-clock synchronization) deployment for coordinating and synchronizing time across devices. If you cannot guarantee accurate wall clock time in your system, mellon-core will not work properly, and you should look for an alternative solution.

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