A slight variant of the configurator/actor pair can be useful in
situations that require serialized access to a shared resource in a
batch mode; that is, when a well-defined job stream or sequence
of requests requires some shared resource, but no individual
job requires user interaction.
In this spooler/daemon pattern, the spooler or front end simply
drops job requests and data in a spool area. The job requests and data
are simply files; the spool area is typically just a directory. The
location of the directory and the format of the job requests are agreed
on by the spooler and daemon.
The daemon runs forever in background, polling the spool
directory, looking there for work to do. When it finds a job request,
it tries to process the associated data. If it succeeds, the job
request and data are deleted out of the spool area.
The classic example of this pattern is the Unix print spooler
front end is
it simply drops files to be printed in a spool area periodically
lpd. lpd's job
is simply to serialize access to the printer devices.
Another classic example is the pair
which schedules commands for execution at specified times. A
third example, historically important though no longer in wide use,
was UUCP — the Unix-to-Unix Copy Program commonly used as a mail
transport over dial-up lines before the Internet explosion of the
The spooler/daemon pattern remains important in mail-transport
programs (which are batchy by nature). The front ends of mail
transports such as
usually make one try at delivering mail immediately, through SMTP over an
outbound Internet connection. If that attempt fails, the mail will
fall into a spool area; a daemon version or mode of the mail transport
will retry the delivery later.
Typically, a spooler/daemon system has four parts: a job
launcher, a queue lister, a job-cancellation utility, and a
spooling daemon, In fact, the presence of the first three parts is
a sure clue that there is a spooler daemon behind them
The terms “spooler” and “daemon” are
well-established Unix jargon. (‘Spooler’ actually
dates back to early mainframe days.)