From 2e13a585301585b62b6386e42b1f575453e05713 Mon Sep 17 00:00:00 2001 From: "Eric S. Raymond" Date: Thu, 26 Mar 1998 21:51:29 +0000 Subject: More on the concurrency problem. svn path=/trunk/; revision=1731 --- design-notes.html | 22 +++++++++++++++++----- 1 file changed, 17 insertions(+), 5 deletions(-) (limited to 'design-notes.html') diff --git a/design-notes.html b/design-notes.html index b6ddec2d..3b3f1e88 100644 --- a/design-notes.html +++ b/design-notes.html @@ -10,7 +10,7 @@
Back to Fetchmail Home Page To Site Map -$Date: 1998/03/06 04:17:44 $ +$Date: 1998/03/26 21:51:29 $

Design Notes On Fetchmail

@@ -190,8 +190,20 @@ worth the hideous complexity increase it would require in the code.

Multiple concurrent instances of fetchmail

-What would be required for this is a per-host semaphore asserted -during each poll.

+Fetchmail locking is on a per-invoking-user because finer-grained +locks would be really hard to implement in a portable way. The +problem is that you don't want two fetchmails querying the same site +for the same remote user at the same time.

+ +To handle this optimally, multiple fetchmails would have to associate +a system-wide semaphore with each active pair of a remote user and +host canonical address. A fetchmail would have to block until getting +this semaphore at the start of a query, and release it at the end of a +query.

+ +This would be way too complicated to do just for an "it might be nice" +feature. Instead, you can run a single root fetchmail polling for +multiple users in either single-drop or multidrop mode.

The fundamental problem here is how an instance of fetchmail polling host foo can assert that it's doing so in a way visible to all other @@ -203,7 +215,7 @@ complicated and fragile, with a bunch of the standard problems (what happens if a fetchmail aborts before clearing its semaphore, and how do we recover reliably?)

. -I'm not satisfied that there's enough functional gain here to pay +I'm just not satisfied that there's enough functional gain here to pay for the large increase in complexity that adding these semaphores would entail.

@@ -463,7 +475,7 @@ all shaped the design in one way or another.

Back to Fetchmail Home Page To Site Map -$Date: 1998/03/06 04:17:44 $ +$Date: 1998/03/26 21:51:29 $

Eric S. Raymond <esr@snark.thyrsus.com>
-- cgit v1.2.3