Appendix: signals and race conditions

- -

Prior to Apache 1.2b9 there were several race - conditions involving the restart and die signals (a simply put, - a race condition is a time-sensitive problem - if something happens - at just the wrong time or things happen in the wrong order, - undesired behaviour will result. If the same thing happens at the right - time, all will be well). For those architectures that have the "right" - feature set we have eliminated as many as we can. But it should - be noted that race conditions do still exist on certain - architectures.

- -

Architectures that use an on-disk ScoreBoardFile can potentially have - their scoreboards corrupted. This can result in the "bind: - Address already in use" (after HUP) or "long lost - child came home!" (after USR1). The former is a fatal - error, while the latter just causes the server to lose a - scoreboard slot. So it may be advisable to use graceful - restarts, with an occasional hard restart. These problems are very - difficult to work around, but fortunately most architectures do - not require a scoreboard file. See the ScoreBoardFile documentation for - architecture which uses it.

- -

All architectures have a small race condition in each child - involving the second and subsequent requests on a persistent - HTTP connection (KeepAlive). It may exit after reading the - request line but before reading any of the request headers. - There is a fix that was discovered too late to make 1.2. In - theory this isn't an issue because the KeepAlive client has to - expect these events because of network latencies and server - timeouts. In practice it doesn't seem to affect anything either - -- in a test case the server was restarted twenty times per - second and clients successfully browsed the site without - getting broken images or empty documents.

Available Languages: de | Modified: httpd/httpd/branches/2.2.x/docs/manual/stopping.xml URL: http://svn.apache.org/viewvc/httpd/httpd/branches/2.2.x/docs/manual/stopping.xml?rev=413452&r1=413451&r2=413452&view=diff ============================================================================== --- httpd/httpd/branches/2.2.x/docs/manual/stopping.xml (original) +++ httpd/httpd/branches/2.2.x/docs/manual/stopping.xml Sun Jun 11 07:26:50 2006 @@ -226,43 +226,4 @@ logfiles at the same time may destroy each other's logfiles.

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Appendix: signals and race conditions - -

- -

Architectures that use an on-disk ScoreBoardFile can potentially have - their scoreboards corrupted. This can result in the "bind: - Address already in use" (after HUP) or "long lost - child came home!" (after USR1). The former is a fatal - error, while the latter just causes the server to lose a - scoreboard slot. So it may be advisable to use graceful - restarts, with an occasional hard restart. These problems are very - difficult to work around, but fortunately most architectures do - not require a scoreboard file. See the ScoreBoardFile documentation for - architecture which uses it.

- -

See also

Appendix: signals and race conditions