4.1 Slots of services

A service has the following slots, all of which can be initialized with a keyword (i.e. #:provides, used when creating the object) of the same name, except where stated otherwise. You should not access them directly with slot-ref or slot-set! usually, use the methods of the service class Methods of services instead.

• provides is a list of symbols that are provided by the service. A symbol can only be provided by one service running at a time, i.e. if two services provide the same symbol, only one of them can run, starting the other one will fail. Therefore, these symbols are mainly used to denote conflicting services. The first symbol in the list is the canonical name for the service, thus it must be unique. This slot has no default value and must therefore be initialized.
• requires is, like provides, a list of symbols that specify services. In this case, they name what this service depends on, i.e. before the service can be started, services that provide those symbols must be started. If a required symbol is provided by several services, one will be started. By default, this slot contains the empty list.
• running is a hook that can be used by each service in its own way. The default value is #f, which indicates that the service is not running. When an attempt is made to start the service, it will be set to the return value of the procedure in the start slot. It will also be passed as an argument to the procedure in the stop slot. This slot can not be initialized with a keyword.
• respawn? specifies whether the service should be respawned by dmd. If this slot has the value #t, then assume the running slot specifies a child process PID and restart the service if that process terminates. Otherwise this slot is #f, which is the default. See also the last-respawns slot.
• start contains the “constructor” for the service, which will be called to start the service. (Therefore, it is not a constructor in the sense that it initializes the slots of a <service> object.) This must be a procedure that accepts any amount of arguments, which will be the additional arguments supplied by the user. If the starting attempt failed, it must return #f. The value will be stored in the running slot. The default value is a procedure that returns #t and performs no further actions, therefore it is desirable to specify a different one usually.
• stop is, similar to start, a slot containing a procedure. But in this case, it gets the current value of the running slot as first argument and the user-supplied arguments as further arguments; it gets called to stop the service. Its return value will again be stored in the running slot, so that it should return #f if it is now possible again to start the service at a later point. The default value is a procedure that returns #f and performs no further actions.
• actions specifies the additional actions that can be performed on a service when it is running. A typical example for this is the restart action. The macro make-actions Service Convenience is provided to abstract the actual data representation format for this slot. (It actually is a hash currently.)
• enabled? cannot be initialized with a keyword, and contains #t by default. When the value becomes #f at some point, this will prevent the service from getting started. A service can be enabled and disabled with the methods enable and disable, respectively Methods of services.
• last-respawns cannot be initialized with a keyword and is only ever used when the respawn? slot contains #t; it is a circular list with (car respawn-limit) elements, where each element contains the time when it was restarted, initially all 0, later a time in seconds since the Epoch. The first element is the one that contains the oldest one, the last one the newest.
• stop-delay? being false causes the stop slot to be unused; instead, stopping the service will just cause the waiting-for-termination? slot be set to #t.
• waiting-for-termination? cannot be initialized with a keyword and should not be used by others, it is only used internally for respawnable services when the stop-delay? slot contains a true value. waiting-for-termination? contains #t if the service is still running, but the user requested that it be stopped, in which case if the service terminates the next time, the respawn handler will not start it again.

  otherwise #f.