[ViewVC] Diff of: cvs/AnyEvent/README

Comparing AnyEvent/README (file contents):
Revision 1.43 by root, Thu Jul 9 08:37:06 2009 UTC vs.
Revision 1.47 by root, Mon Jul 20 22:39:57 2009 UTC

 NAME
-    AnyEvent - provide framework for multiple event loops
+    AnyEvent - events independent of event loop implementation
     EV, Event, Glib, Tk, Perl, Event::Lib, Qt and POE are various supported
     event loops.
 SYNOPSIS
 INTRODUCTION/TUTORIAL
     This manpage is mainly a reference manual. If you are interested in a
     tutorial or some gentle introduction, have a look at the AnyEvent::Intro
     manpage.
+SUPPORT
+    There is a mailinglist for discussing all things AnyEvent, and an IRC
+    channel, too.
+    See the AnyEvent project page at the Schmorpforge Ta-Sa Software
+    Respository, at <http://anyevent.schmorp.de>, for more info.
 WHY YOU SHOULD USE THIS MODULE (OR NOT)
     Glib, POE, IO::Async, Event... CPAN offers event models by the dozen
     nowadays. So what is different about AnyEvent?
     invocation, and callback invocation will be synchronous. Synchronous
     means that it might take a while until the signal gets handled by the
     process, but it is guaranteed not to interrupt any other callbacks.
     The main advantage of using these watchers is that you can share a
-    signal between multiple watchers.
+    signal between multiple watchers, and AnyEvent will ensure that signals
+    will not interrupt your program at bad times.
-    This watcher might use %SIG, so programs overwriting those signals
+    This watcher might use %SIG (depending on the event loop used), so
-    directly will likely not work correctly.
+    programs overwriting those signals directly will likely not work
+    correctly.
     Example: exit on SIGINT
        my $w = AnyEvent->signal (signal => "INT", cb => sub { exit 1 });
+   Signal Races, Delays and Workarounds
+    Many event loops (e.g. Glib, Tk, Qt, IO::Async) do not support attaching
+    callbacks to signals in a generic way, which is a pity, as you cannot do
+    race-free signal handling in perl. AnyEvent will try to do it's best,
+    but in some cases, signals will be delayed. The maximum time a signal
+    might be delayed is specified in $AnyEvent::MAX_SIGNAL_LATENCY (default:
+    10 seconds). This variable can be changed only before the first signal
+    watcher is created, and should be left alone otherwise. Higher values
+    will cause fewer spurious wake-ups, which is better for power and CPU
+    saving. All these problems can be avoided by installing the optional
+    Async::Interrupt module. This will not work with inherently broken event
+    loops such as Event or Event::Lib (and not with POE currently, as POE
+    does it's own workaround with one-second latency). With those, you just
+    have to suffer the delays.
   CHILD PROCESS WATCHERS
     You can also watch on a child process exit and catch its exit status.
     The child process is specified by the "pid" argument (if set to 0, it
     of when you start the watcher.
     This means you cannot create a child watcher as the very first thing in
     an AnyEvent program, you *have* to create at least one watcher before
     you "fork" the child (alternatively, you can call "AnyEvent::detect").
+    As most event loops do not support waiting for child events, they will
+    be emulated by AnyEvent in most cases, in which the latency and race
+    problems mentioned in the description of signal watchers apply.
     Example: fork a process and wait for it
        my $done = AnyEvent->condvar;
   CONDITION VARIABLES
     If you are familiar with some event loops you will know that all of them
     require you to run some blocking "loop", "run" or similar function that
     will actively watch for new events and call your callbacks.
-    AnyEvent is different, it expects somebody else to run the event loop
+    AnyEvent is slightly different: it expects somebody else to run the
-    and will only block when necessary (usually when told by the user).
+    event loop and will only block when necessary (usually when told by the
+    user).
     The instrument to do that is called a "condition variable", so called
     because they represent a condition that must become true.
+    Now is probably a good time to look at the examples further below.
     Condition variables can be created by calling the "AnyEvent->condvar"
     method, usually without arguments. The only argument pair allowed is
     "cb", which specifies a callback to be called when the condition
     variable becomes true, with the condition variable as the first argument
     (but not the results).
     After creation, the condition variable is "false" until it becomes
     Condition variables are similar to callbacks, except that you can
     optionally wait for them. They can also be called merge points - points
     in time where multiple outstanding events have been processed. And yet
     another way to call them is transactions - each condition variable can
     be used to represent a transaction, which finishes at some point and
-    delivers a result.
+    delivers a result. And yet some people know them as "futures" - a
+    promise to compute/deliver something that you can wait for.
     Condition variables are very useful to signal that something has
     finished, for example, if you write a module that does asynchronous http
     requests, then a condition variable would be the ideal candidate to
     signal the availability of results. The user can either act when the
           after => 1,
           cb    => sub { $result_ready->send },
        );
        # this "blocks" (while handling events) till the callback
-       # calls send
+       # calls -<send
        $result_ready->recv;
     Example: wait for a timer, but take advantage of the fact that condition
-    variables are also code references.
+    variables are also callable directly.
        my $done = AnyEvent->condvar;
        my $delay = AnyEvent->timer (after => 5, cb => $done);
        $done->recv;
        ...
        my @info = $couchdb->info->recv;
-    And this is how you would just ste a callback to be called whenever the
+    And this is how you would just set a callback to be called whenever the
     results are available:
        $couchdb->info->cb (sub {
           my @info = $_[0]->recv;
        });
         Any arguments passed to the "send" call will be returned by all
         future "->recv" calls.
         Condition variables are overloaded so one can call them directly (as
-        a code reference). Calling them directly is the same as calling
+        if they were a code reference). Calling them directly is the same as
-        "send". Note, however, that many C-based event loops do not handle
+        calling "send".
-        overloading, so as tempting as it may be, passing a condition
-        variable instead of a callback does not work. Both the pure perl and
-        EV loops support overloading, however, as well as all functions that
-        use perl to invoke a callback (as in AnyEvent::Socket and
-        AnyEvent::DNS for example).
     $cv->croak ($error)
         Similar to send, but causes all call's to "->recv" to invoke
         "Carp::croak" with the given error message/object/scalar.
         This can be used to signal any errors to the condition variable
-        user/consumer.
+        user/consumer. Doing it this way instead of calling "croak" directly
+        delays the error detetcion, but has the overwhelmign advantage that
+        it diagnoses the error at the place where the result is expected,
+        and not deep in some event clalback without connection to the actual
+        code causing the problem.
     $cv->begin ([group callback])
     $cv->end
         These two methods can be used to combine many transactions/events
         into one. For example, a function that pings many hosts in parallel
         function will call "croak".
         In list context, all parameters passed to "send" will be returned,
         in scalar context only the first one will be returned.
+        Note that doing a blocking wait in a callback is not supported by
+        any event loop, that is, recursive invocation of a blocking "->recv"
+        is not allowed, and the "recv" call will "croak" if such a condition
+        is detected. This condition can be slightly loosened by using
+        Coro::AnyEvent, which allows you to do a blocking "->recv" from any
+        thread that doesn't run the event loop itself.
         Not all event models support a blocking wait - some die in that case
         (programs might want to do that to stay interactive), so *if you are
-        using this from a module, never require a blocking wait*, but let
+        using this from a module, never require a blocking wait*. Instead,
-        the caller decide whether the call will block or not (for example,
+        let the caller decide whether the call will block or not (for
-        by coupling condition variables with some kind of request results
+        example, by coupling condition variables with some kind of request
-        and supporting callbacks so the caller knows that getting the result
+        results and supporting callbacks so the caller knows that getting
-        will not block, while still supporting blocking waits if the caller
+        the result will not block, while still supporting blocking waits if
-        so desires).
+        the caller so desires).
-        Another reason *never* to "->recv" in a module is that you cannot
-        sensibly have two "->recv"'s in parallel, as that would require
-        multiple interpreters or coroutines/threads, none of which
-        "AnyEvent" can supply.
-        The Coro module, however, *can* and *does* supply coroutines and, in
-        fact, Coro::AnyEvent replaces AnyEvent's condvars by coroutine-safe
-        versions and also integrates coroutines into AnyEvent, making
-        blocking "->recv" calls perfectly safe as long as they are done from
-        another coroutine (one that doesn't run the event loop).
         You can ensure that "-recv" never blocks by setting a callback and
         only calling "->recv" from within that callback (or at a later
         time). This will work even when the event loop does not support
         blocking waits otherwise.
         by "PERL_ANYEVENT_MODEL".
         When set to 2 or higher, cause AnyEvent to report to STDERR which
         event model it chooses.
+        When set to 8 or higher, then AnyEvent will report extra information
+        on which optional modules it loads and how it implements certain
+        features.
     "PERL_ANYEVENT_STRICT"
         AnyEvent does not do much argument checking by default, as thorough
         argument checking is very costly. Setting this variable to a true
         value will cause AnyEvent to load "AnyEvent::Strict" and then to
         thoroughly check the arguments passed to most method calls. If it
         finds any problems, it will croak.
         In other words, enables "strict" mode.
-        Unlike "use strict", it is definitely recommended to keep it off in
+        Unlike "use strict" (or it's modern cousin, "use common::sense", it
-        production. Keeping "PERL_ANYEVENT_STRICT=1" in your environment
+        is definitely recommended to keep it off in production. Keeping
+        "PERL_ANYEVENT_STRICT=1" in your environment while developing
-        while developing programs can be very useful, however.
+        programs can be very useful, however.
     "PERL_ANYEVENT_MODEL"
         This can be used to specify the event model to be used by AnyEvent,
         before auto detection and -probing kicks in. It must be a string
         consisting entirely of ASCII letters. The string "AnyEvent::Impl::"
     "PERL_ANYEVENT_CA_FILE", "PERL_ANYEVENT_CA_PATH".
         When neither "ca_file" nor "ca_path" was specified during
         AnyEvent::TLS context creation, and either of these environment
         variables exist, they will be used to specify CA certificate
         locations instead of a system-dependent default.
+    "PERL_ANYEVENT_AVOID_GUARD" and "PERL_ANYEVENT_AVOID_ASYNC_INTERRUPT"
+        When these are set to 1, then the respective modules are not loaded.
+        Mostly good for testing AnyEvent itself.
 SUPPLYING YOUR OWN EVENT MODEL INTERFACE
     This is an advanced topic that you do not normally need to use AnyEvent
     in a module. This section is only of use to event loop authors who want
     to provide AnyEvent compatibility.
     SIGCHLD
         A handler for "SIGCHLD" is installed by AnyEvent's child watcher
         emulation for event loops that do not support them natively. Also,
         some event loops install a similar handler.
-        If, when AnyEvent is loaded, SIGCHLD is set to IGNORE, then AnyEvent
+        Additionally, when AnyEvent is loaded and SIGCHLD is set to IGNORE,
-        will reset it to default, to avoid losing child exit statuses.
+        then AnyEvent will reset it to default, to avoid losing child exit
+        statuses.
     SIGPIPE
         A no-op handler is installed for "SIGPIPE" when $SIG{PIPE} is
         "undef" when AnyEvent gets loaded.
         it is that this way, the handler will be restored to defaults on
         exec.
         Feel free to install your own handler, or reset it to defaults.
+RECOMMENDED/OPTIONAL MODULES
+    One of AnyEvent's main goals is to be 100% Pure-Perl(tm): only perl (and
+    it's built-in modules) are required to use it.
+    That does not mean that AnyEvent won't take advantage of some additional
+    modules if they are installed.
+    This section epxlains which additional modules will be used, and how
+    they affect AnyEvent's operetion.
+    Async::Interrupt
+        This slightly arcane module is used to implement fast signal
+        handling: To my knowledge, there is no way to do completely
+        race-free and quick signal handling in pure perl. To ensure that
+        signals still get delivered, AnyEvent will start an interval timer
+        to wake up perl (and catch the signals) with some delay (default is
+        10 seconds, look for $AnyEvent::MAX_SIGNAL_LATENCY).
+        If this module is available, then it will be used to implement
+        signal catching, which means that signals will not be delayed, and
+        the event loop will not be interrupted regularly, which is more
+        efficient (And good for battery life on laptops).
+        This affects not just the pure-perl event loop, but also other event
+        loops that have no signal handling on their own (e.g. Glib, Tk, Qt).
+        Some event loops (POE, Event, Event::Lib) offer signal watchers
+        natively, and either employ their own workarounds (POE) or use
+        AnyEvent's workaround (using $AnyEvent::MAX_SIGNAL_LATENCY).
+        Installing Async::Interrupt does nothing for those backends.
+    EV  This module isn't really "optional", as it is simply one of the
+        backend event loops that AnyEvent can use. However, it is simply the
+        best event loop available in terms of features, speed and stability:
+        It supports the AnyEvent API optimally, implements all the watcher
+        types in XS, does automatic timer adjustments even when no monotonic
+        clock is available, can take avdantage of advanced kernel interfaces
+        such as "epoll" and "kqueue", and is the fastest backend *by far*.
+        You can even embed Glib/Gtk2 in it (or vice versa, see EV::Glib and
+        Glib::EV).
+    Guard
+        The guard module, when used, will be used to implement
+        "AnyEvent::Util::guard". This speeds up guards considerably (and
+        uses a lot less memory), but otherwise doesn't affect guard
+        operation much. It is purely used for performance.
+    JSON and JSON::XS
+        This module is required when you want to read or write JSON data via
+        AnyEvent::Handle. It is also written in pure-perl, but can take
+        advantage of the ultra-high-speed JSON::XS module when it is
+        installed.
+        In fact, AnyEvent::Handle will use JSON::XS by default if it is
+        installed.
+    Net::SSLeay
+        Implementing TLS/SSL in Perl is certainly interesting, but not very
+        worthwhile: If this module is installed, then AnyEvent::Handle (with
+        the help of AnyEvent::TLS), gains the ability to do TLS/SSL.
+    Time::HiRes
+        This module is part of perl since release 5.008. It will be used
+        when the chosen event library does not come with a timing source on
+        it's own. The pure-perl event loop (AnyEvent::Impl::Perl) will
+        additionally use it to try to use a monotonic clock for timing
+        stability.
 FORK
     Most event libraries are not fork-safe. The ones who are usually are
     because they rely on inefficient but fork-safe "select" or "poll" calls.
     Only EV is fully fork-aware.
     If you have to fork, you must either do so *before* creating your first
-    watcher OR you must not use AnyEvent at all in the child.
+    watcher OR you must not use AnyEvent at all in the child OR you must do
+    something completely out of the scope of AnyEvent.
 SECURITY CONSIDERATIONS
     AnyEvent can be forced to load any event model via
     $ENV{PERL_ANYEVENT_MODEL}. While this cannot (to my knowledge) be used
     to execute arbitrary code or directly gain access, it can easily be used

Diff Legend

-–
+Removed lines
-+
+Added lines
-<
+Changed lines
->
+Changed lines

Comparing AnyEvent/README (file contents): Revision 1.43 by root, Thu Jul 9 08:37:06 2009 UTC vs. Revision 1.47 by root, Mon Jul 20 22:39:57 2009 UTC

Diff Legend

Comparing AnyEvent/README (file contents):
Revision 1.43 by root, Thu Jul 9 08:37:06 2009 UTC vs.
Revision 1.47 by root, Mon Jul 20 22:39:57 2009 UTC