race condition

race condition

   Anomalous behavior due to unexpected critical dependence on
   the relative timing of events.

   For example, if one process writes to a file while another is
   reading from the same location then the data read may be the
   old contents, the new contents or some mixture of the two
   depending on the relative timing of the read and write
   operations.

   A common remedy in this kind of race condition is {file
   locking}; a more cumbersome remedy is to reorganize the system
   such that a certain processes (running a {daemon} or the like)
   is the only process that has access to the file, and all other
   processes that need to access the data in that file do so only
   via interprocess communication with that one process.

   As an example of a more subtle kind of race condition,
   consider a {distributed} {chat} {network} like {IRC}, where a
   {user} is granted channel-operator {privileges} in any channel
   he starts.  If two users on different {servers}, on different
   ends of the same network, try to start the same-named channel
   at the same time, each user's respective server will grant
   channel-operator privileges to each user, since neither will
   yet have received the other's signal that that channel has
   been started.

   In this case of a race condition, the "shared resource" is the
   conception of the {state} of the network (what channels exist,
   as well as what users started them and therefore have what
   privileges), which each server is free to change as long as it
   signals the other servers on the network about the changes so
   that they can update their conception of the state of the
   network.  However, the {latency} across the network makes
   possible the kind of race condition described.  In this case,
   heading off race conditions by imposing a form of control over
   access to the shared resource -- say, appointing one server to
   be in charge of who holds what privileges -- would mean
   turning the distributed network into a centralized one (at
   least for that one part of the network operation).  Where this
   is not acceptable, the more pragmatic solution is to have the
   system recognize when a race condition has occurred and to
   repair the ill effects.

   Race conditions also affect electronic circuits where the
   value output by a {logic gate} depends on the exact timing of
   two or more input signals.  For example, consider a two input
   AND gate fed with a logic signal X on input A and its
   negation, NOT X, on input B.  In theory, the output (X AND NOT
   X) should never be high.  However, if changes in the value of
   X take longer to propagate to input B than to input A then
   when X changes from false to true, there will be a brief
   period during which both inputs are true, and so the gate's
   output will also be true.  If this output is fed to an
   edge-sensitive component such as a counter or flip-flop then
   the temporary effect ("{glitch}") will become permanent.

   (2002-08-03)