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Update the whatsnew-2.0.txt document
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Nick Mathewson
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whatsnew-2.0.txt
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whatsnew-2.0.txt
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What's New In Libevent 2.0 so far:
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1. About this document
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1. Meta-issues
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1.1. About this document
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This document describes the key differences between Libevent 1.4 and
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Libevent 2.0, from a user's point of view. It was most recently
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updated based on features in subversion trunk as of 18 November 2009.
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updated based on features in git master as of August 2010.
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NOTE 1: If any features or fixes get backported from trunk to 1.4,
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they should get moved from here into whatsnew-14.txt, since they
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will no longer be differences between 1.4 and this version.
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NOTE 2: We may have missed some things on this list. Caveat haxxor.
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NOTE 2: I am very sure that I missed some thing on this list. Caveat
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haxxor.
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1.2. Better documentation
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There is now a book-in-progress that explains how to use Libevent and its
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growing pile of APIs. As of this writing, it covers everything except the
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http and rpc code. Check out the latest draft at
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http://www.wangafu.net/~nickm/libevent-book/ .
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2. New and Improved Event APIs
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@ -28,7 +38,7 @@ What's New In Libevent 2.0 so far:
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preserving binary compatibility between releases. We'll try harder in the
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future, though: see 2.1 below.
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2.1. New header layout for improved compatibility
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2.1. New header layout for improved forward-compatibility
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Libevent 2.0 has a new header layout to make it easier for programmers to
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write good, well-supported libevent code. The new headers are divided
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@ -68,7 +78,7 @@ What's New In Libevent 2.0 so far:
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evutil.h) will continue to work by including the corresponding new
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headers. Old code should not be broken by this change.
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2.2. New thread-safe, binary-compatibile APIs
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2.2. New thread-safe, binary-compatibile, harder-to-mess-up APIs
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Some aspects of the historical Libevent API have encouraged
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non-threadsafe code, or forced code built against one version of Libevent
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@ -204,11 +214,13 @@ What's New In Libevent 2.0 so far:
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2.8. evthread_* functions for thread-safe structures.
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Libevent structures can now be built with locking support. You can
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enable this on a per-event-base level by writing functions to implement
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mutexes and thread IDs, and passing them to evthread_set_locking_callback
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and evthread_set_id_callback. This makes it safe to add, remove,
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and activate events on an event base from a different thread.
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Libevent structures can now be built with locking support. You can enable
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this on a per-event-base level by writing functions to implement mutexes
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and thread IDs, and passing them to evthread_set_lock_callbacks. This
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makes it safe to add, remove, and activate events on an event base from a
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different thread. (Previously, if you wanted to write multithreaded code
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with Libevent, you could only an event_base or its events in one thread at
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a time.)
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If you want threading support and you're using pthreads, you can just
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call evthread_use_pthreads(). (You'll need to link against the
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@ -243,11 +255,40 @@ What's New In Libevent 2.0 so far:
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The corresponding event_config feature is EV_FEATURE_ET; see 2.4 for more
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information.
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3. Backend-specific improvements.
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2.10. Better support for huge numbers of timeouts
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3.1. kqueue event ordering consistency
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The heap-based priority queue timer implementation for Libevent 1.4 is good
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for randomly distributed timeouts, but suboptimal if you have huge numbers
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of timeouts that all expire in the same amount of time after their
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creation. The new event_base_init_common_timeout() logic lets you signal
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that a given timeout interval will be very common, and should use a linked
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list implementation instead of a priority queue.
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TODO(niels)
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2.11. Improved debugging support
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It's been pretty easy to forget to delete all your events before you
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re-initialize them, or otherwise put Libevent in an internally inconsistent
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state. You can tell libevent to catch these and other common errors with
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the new event_enable_debug_mode() call. Just invoke it before you do
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any calls to other libevent functions, and it'll catch many common
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event-level errors in your code.
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2.12. Functions to access all event fields
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So that you don't have to access the struct event fields directly, Libevent
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now provides accessor functions to retrieve everything from an event that
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you set during event_new() or event_assign().
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3. Backend-specific and performance improvements.
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3.1. Change-minimization on O(1) backends
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With previous versions of Libevent, if you called event_del() and
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event_add() repeatedly on a single event between trips to the backend's
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dispatch function, the backend might wind up making unnecessary calls or
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passing unnecessary data to the kernel. The new backend logic batches up
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redundant adds and deletes, and performs no more operations than necessary
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at the kernel level.
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3.2. Improved notification on Linux
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@ -255,6 +296,29 @@ What's New In Libevent 2.0 so far:
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an epollfd to do so, instead of a socketpair. This is supposed to be
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faster.
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3.3. Windows: better support for everything
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Bufferevents on windows can use a new mechanism (off-by-default; see below)
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to send their data via windows overlapped IO and get their notifications
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via the IOCP API. This should be much faster than using event-based
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notification.
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Other functions throughout the code have been fixed to work more
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consistently with Windows. Libevent now builds on Windows using either
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mingw, or using MSVC (with nmake). Libevent works fine with UNICODE
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defined, or not.
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Data structures are a little smarter: our lookups from socket to pending
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event are now done with O(1) hash tables rather than O(lg n) red-black
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trees.
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Unfortunately, the main windows backend is still select()-based: from
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testing the IOCP backends on the mailing list, it seems that there isn't
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actually a way to tell for certain whether a socket is writable with IOCP
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without . Libevent 2.1 may add a multithreaded WaitForMultipleEvents-based
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backend for better performance with many inactive sockets and better
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integration with Windows events.
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4. Improvements to evbuffers
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Libevent has long had an "evbuffer" implementation to wrap access to an
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There are probably some bugs remaining in this code. On some platforms
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(like Windows), it just reads the relevant parts of the file into RAM.
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4.4. Support for zero-copy writes in evbuffers.
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4.4. Support for zero-copy ("scatter/gather") writes in evbuffers.
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You can add a piece of memory to an evbuffer without copying it.
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Instead, Libevent adds a new element to the evbuffer's linked list of
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part of its public API, but wants users to treat it as a pure source or
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sink.
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There's an evbuffer_copyout() that looks at the data at the start of an
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evbuffer without doing a drain.
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You can have an evbuffer defer all of its callbacks, so that rather than
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being invoked immediately when the evbuffer's length changes, they are
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invoked from within the event_loop. This is useful when you have a
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allocated bufferevents with bufferevent_new().
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Current implementations of the bufferevent interface are described below.
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See also section TODO(nickm).
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5.2. bufferevent_socket_new() replaces bufferevent_new()
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bufferevent_openssl_filter_new() function. If you want to do SSL
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on a socket directly, call bufferevent_openssl_socket_new().
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This is tricky code; there are probably some bugs hiding here.
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5.5. IOCP support for bufferevents on Windows
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There is now a bufferevents backend that supports IOCP on Windows.
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The functions to do this are bufferevent_socket_connect and
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bufferevent_socket_connect_hostname.
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6. Extras improvements
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5.7. Rate-limiting for bufferevents
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If you need to limit the number of bytes read/written by a single
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bufferevent, or by a group of them, you can do this with a new set of
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bufferevent rate-limiting calls.
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6. Other improvements
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6.1. DNS
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The evdns code now lets you have nameservers whose addresses are IPv6.
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6.1.2: Support for the 0x20 hack
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6.1.2: Better security.
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6.1.3: Better security.
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Libevent 2.0 tries harder to resist DNS answer-sniping attacks than
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earlier versions of evdns. See comments in the code for full details.
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TODO(nickm) writeme
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Notably, evdns now supports the "0x20 hack" to make it harder to
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impersonate a DNS server. Additionally, Libevent now uses a strong
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internal RNG to generate DNS transaction IDs, so you don't need to supply
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your own.
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6.1.4. Getaddrinfo support
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6.1.3. Getaddrinfo support
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There's now an asynchronous getaddrinfo clone, evdns_getaddrinfo(),
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to make the results of the evdns functions more usable. It doesn't
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platforms that don't have one, and smooth over the differences in
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various platforms implementations of RFC3493.
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Bufferevents provide bufferevent_connect_hostname(), which combines
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the name lookup and connect operations.
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6.1.4. No more evdns globals
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Like an event base, evdns operations are now supposed to use an evdns_base
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argument. This makes them easier to wrap for other (more OO) languages,
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and easier to control the lifetime of. The old evdns functions will
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still, of course, continue working.
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6.2. Listener support
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You can now more easily automate setting up a bound socket to listen for
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TCP connections. Just use the evconnlistener_*() functions in the
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event2/listener.h header.
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The listener code supports IOCP on windows if available.
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6.3. Secure RNG support
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Network code very frequently needs a secure, hard-to-predict random number
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generator. Some operating systems provide a good C implementation of one;
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others do not. Libevent 2.0 now provides a consistent implementation
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based on the arc4random code originally from OpenBSD. Libevent (and you)
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can use the evutil_secure_rng_*() functions to access a fairly secure
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random stream of bytes.
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7. Infrastructure improvements
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7.1. Better unit test framework
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