I recently embarked on a project to rebuild the main functionality of Turtl in common lisp. This requires embedding lisp (using ECL) into node-webkit (or soon, Firefox, as node-webkit is probably getting dumped).

To allow lisp and javascript to communicate, I made a simple messaging layer in C that both sides could easily hook into. While this worked, I stumbled on nanomsg and figured it couldn't hurt to give it a shot.

So I wrote up some quick bindings for nanomsg in lisp and wired everything up. So far, it works really well. I can't tell if it's faster than my previous messaging layer, but one really nice thing about it is that it uses file descriptors which can be easily monitored by an event loop (such as cl-async), making polling and strange thread < --> thread event loop locking schemes a thing of the past (although cl-async handles all this fairly well).

This simplified a lot of the Turtl code, and although right now it's only using the nanomsg "pair" layout type, it could be easily expanded in the future to allows different pieces of the app to communicate. In other words, it's a lot more future-proof than the old messaging system and probably a lot more resilient (dedicated messaging library authored by 0MQ mastermind beats hand-rolled, hard-coded simple messaging built by non-C expert).

Lately I've been neck-deep in embedding. Currently, I'm building a portable (hopefully) version of Turtl's core features in ECL.

Problem is, when embedding turtl-core into Node-webkit or Firefox, any output that ECL writes to STDOUT triggers:

C operation (write) signaled an error. C library explanation: Bad file descriptor.

Well it turns out Windows doesn't let you write to STDOUT unless a console is available, and even if using msys, it doesn't create a console for GUI apps. So here's a tool (in lisp, of course) that will let you convert an executable between GUI and console.

Seems to work great. Special thanks to death.

A while ago, I released cl-async, a library for non-blocking programming in Common Lisp. I've been updating it a lot over the past month or two to add features and fix bugs, and it's really coming along.

My goal for this project is to create a general-purpose library for asynchronous programming in lisp. I think I have achieved this. With the finishing of the futures implementation, not only is the library stable, but there is now a platform to build drivers on top of. This will be my next focal point over the next few months.

There are a few reasons I decided to build something new. Here's an overview of the non-blocking libraries I could find:

• IOLib - An IO library for lisp that has a built-in event loop, only works on *nix.
• Hinge - General purpose, non-blocking library. Only works on *nix, requires libev and ZeroMQ.
• Conserv - A nice layer on top of IOLib (so once again, only runs on *nix). Includes TCP client/server and HTTP server implementations. Very nice.
• teepeedee2 - A non-blocking, performant HTTP server written on top of IOLib.

I created cl-async because of all the available libraries, they are either non-portable, not general enough, have too many dependencies, or a combination of all three. I wanted a library that worked on Linux and Windows. I wanted a portable base to start from, and I also wanted tools to help make drivers.

Keeping all this in mind, I created bindings for libevent2 and built cl-async on top of them. There were many good reasons for choosing libevent2 over other libraries, such as libev and libuv (the backend for Node.js). Libuv would have been my first choice because it supports IOCP in Windows (libevent does not), however wrapping it in CFFI was like getting a screaming toddler to see the logic behind your decision to put them to bed. It could have maybe happened if I'd written a compatibility layer in C, but I wanted to have a maximum of 1 (one) dependency. Libevent2 won. It's fast, portable, easy to wrap in CFFI, and on top of that, has a lot of really nice features like an HTTP client/server, TCP buffering, DNS, etc etc etc. The list goes on. That means less programming for me.

Like I mentioned, my next goal is to build drivers. I've already built a few, but I don't consider them stable enough to release yet. Drivers are the elephant in the room. Anybody can implement non-blocking IO for lisp, but the real challenge is converting everything that talks over TCP/HTTP to be async. If lisp supported coroutines, this would be trivial, but alas, we're stuck with futures and the lovely syntax they afford.

I'm going to start with drivers I use every day: beanstalk, redis, cl-mongo, drakma, zs3, and cl-smtp. These are the packages we use at work in our queue processing system (now threaded, soon to be evented + threaded). Once a few of these are done, I'll update the cl-async drivers page with best practices for building drivers (aka wrapping async into futures). Then I will take over the world.

Another goal I have is to build a real HTTP server on top of the bare http-server implementation provided by cl-async. This will include nice syntax around routing (allowing REST interfaces), static file serving, etc.

Cl-async is still a work in progress, but it's starting to become stabilized (both in lack of bugs and the API itself), so check out the docs or the github project and give it a shot. All you need is a lisp and libevent =].

We're building a queuing system for Musio written in common lisp. To be accurate, we already built a queuing system in common lisp, and I recently needed to add a worker to it that communicates with MongoDB via cl-mongo. Each worker spawns four worker threads, each thread grabbing jobs from beanstalkd via cl-beanstalk. During my testing, each worker was updating a Mongo collection with some values scraped from our site. However, after a few seconds of processing jobs, the worker threads begin to spit out USOCKET errors and eventually Clozure CL enters it's debugger of death (ie, lisp's version of a segfault). SBCL didn't fare too much better, either.

The way cl-mongo's connections work is that it has a global hash table that holds connections: cl-mongo::*mongo-registry*. When the threads are all running and communicating with MongoDB, they are using the same hash table without any inherent locking or synchronization. There are a few options to fix this. You can implement a connection pool that supports access from multiple threads (complicated), you can give each thread its own connection and force the each thread to use its connection when communicating, or you can take advantage of special variables in lisp (the easiest, simplest, and most elegant IMO). Let's check out the last option.

Although it's not in the CL spec, just about all implementations allow you to have global thread-local variables by using (defparameter) or (defvar), both of which create special variables (read: dynamic variables, as opposed to lexical). Luckily, cl-mongo uses defvar to create *mongo-registry*. This means in our worker, we can re-bind this variable above the top level loop using (let) and all subsequent calls to MongoDB will use our new thread-local version of *mongo-registry* instead of the global one that all the threads we're bumping into each other using:

;; Main worker loop, using global *mongo-registry* (broken)
(defun start-worker ()
  (loop
    (let ((job (get-job)))
      (let ((results (process-job job)))
        ;; this uses the global registry. not good if running multiple threads.
        (with-mongo-connection (:db "musio")
          (db.save "scraped" results))))))

New version:

;; Replace *mongo-registry* above worker loop, creating a local version of the
;; registry for this thread.
(defun start-worker ()
  ;; setting to any value via let will re-create the variable as a local thread
  ;; variable. nil will do just fine.
  (let ((cl-mongo::*mongo-registry* nil))
    (loop
      (let ((job (get-job)))
        (let ((results (process-job job)))
          ;; with-mongo-connection now uses the local registry, which stops the
          ;; threads from touching each other.
          (with-mongo-connection (:db "musio")
            (db.save "scraped" results)))))))

BOOM everything works great after this change, and it was only a one line change. It may not be as efficient as connection pooling, but that's a lot more prone to strange errors and synchronization issues than just segregating the connections from each other and calling it a day. One issue: *mongo-registry* is not exported by cl-mongo, which is why we access it via cl-mongo::*mongo-registry* (notice the double colon instead of single). This means in future versions, the variable name may change, breaking our above code. So, don't update cl-mongo without testing. Not hard.

Hopefully this helps a few people out, let me know if you have better solutions to this issue!