Nov 29, 2014

cl-async - now using libuv

Cl-async has swapped its backend from libevent to libuv. Please see the v0.6.x upgrade guide if you have not already.

There are a number of reasons for swapping out libevent:

  • Libuv is more actively maintained.
  • Libuv provides more abstractions, including filesystem IO and UDP sockets, which cl-async did not have access to before (however filesystem and UDP are not currently implemented in cl-async).
  • Libuv fixes a number of bugs and warts that libevent has had for quite a while now.
  • Libuv uses IOCP on windows, where libevent uses select() (this means that cl-async will now run a lot faster on windows).

Overall, we had enough issues with libevent and got enough push from the cl-async community to rewrite the backend to use libuv. Although this upgrade will be a lot better for cl-async in the future, it may cause some pain for people in the meantime. Please feel free to open an issue on our github or email me directly at orthecreedence [at] gmail if you run into problems or have questions.


There are a number of great things this rewrite has given cl-async.

First and foremost, faster TCP IO. We swapped out the manual byte-by-byte copies between C <–> lisp, instead using static-vectors for transfering data between C and lisp. On top of this, all socket buffering is done using fast-io which is another speed boost on top of static-vectors.

cl-async now also has a simpler threading abstraction. Previously, you had to lock the entire event loop and prevent its execution to add an event to it from another thread. By using notifiers instead, you can trigger callbacks in the event loop thread without going through the dance of adding dummy events and triggering them. And because notifiers provide a simpler abstraction, it’s no longer necessary to manually turn on threading support in cl-async: it’s always on now. See the updated threading docs for more info.

SSL has also been updated. Previously, cl-async used libevent’s SSL wrapper. This worked very well for most cases, but fell over in others. cl-async’s SSL support has been completely re-written from scratch, allowing it to be changed and fixed as needed going forward. Libuv doesn’t provide a wrapper for SSL, so all the work is done by cl-async itself, using OpenSSL’s memory BIO feature to allow async communication over SSL/TLS.

Idlers are also a nice update, allowing you to run code on each pass of the event loop.


The v0.6.x upgrade goal was to switch libevent to libuv with minimal change to the cl-async API. Coming up, we’re planning to wrap the filesystem API for both polling and IO, as well as async UDP support.

Also, there are bound to be bugs and problems after the switch to libuv, so as we go forward, we’ll be tightening all the bolts.