Facebook uses Erlang

Here is proof: http://www.joyent.com/?gclid=CLeIy4La-pQCFRMJewoddC7c3A

For Facebook Chat, we rolled our own subsystem for logging chat messages (in C++) as well as an epoll-driven web server (in Erlang) that holds online users’ conversations in-memory and serves the long-polled HTTP requests. Both subsystems are clustered and partitioned for reliability and efficient failover. Why Erlang? In short, because the problem domain fits Erlang like a glove. Erlang is a functional concurrency-oriented language with extremely low-weight user-space “processes”, share-nothing message-passing semantics, built-in distribution, and a “crash and recover” philosophy proven by two decades of deployment on large soft-realtime production systems.

Up and running with Emacs, Erlang, and Distel

Umm… did I mention that I yearn for hacking Erlang code, too? Following Bill Clementson’s lead, I set up  Emacs+Erlang on my Ubuntu hardy box.

Step 1: Install erlang and erlang-mode

I used the version of erlang and erlang-mode for emacs in Ubuntu’s repositories. I might come to regret it some day, but for now the following was just too simple to resist:

$ sudo aptitude install erlang
# The above will install erlang-mode too;
# if it does not just "apt-get install erlang-mode"

Step 2: Configure erlang-mode in Emacs

I just put the following in my .emacs:

;; Erlang-mode
(require 'erlang-start)
(add-to-hook 'erlang-mode-hook
             (lambda ()
            ;; when starting an Erlang shell in Emacs, the node name
            ;; by default should be "emacs"
            (setq inferior-erlang-machine-options '("-sname" "emacs"))
            ;; add Erlang functions to an imenu menu
            (imenu-add-to-menubar "imenu")))

Now I can launch Emacs and open an erlang file, which puts me in Erlang mode.  I can start and Erlang shell with “C-c C-z”, compile the Erlang code with “C-c C-k”, and view the compilation result in the Erlang buffer (if the buffer is hidden) with “C-c C-l”.  Or, I can just switch to the Erlang shell with “C-c C-z”.  The customizations above enable me to get a menu item in the Emacs menubar with a list of functions defined in the file I am visiting, which is a big help. I am used to “ecb-mode” when coding Python; ECB does not grok Erlang yet, so the imenu is a handy substitute when coding Erlang.

Step 3: Install Distel

Distel is to Erlang and erlang-mode what SLIME is to Lisp and lisp-mode.

To get distel:

$ cd ~/.emacs.d/
$ svn co http://distel.googlecode.com/svn/trunk/ distel
$ cd distel
$ make
$ cd doc
$ make postscript && make postscript # must run twice
$ make info && sudo make install # install the Info documentation
$ info distel # read the distel info documentation

Step 4: Configure Emacs to use Distel

I just need to put this in my .emacs:

(push "/home/parijat/.emacs.d/distel/elisp/" load-path)
(require 'distel)
(distel-setup)

Step 5: Configure Erlang

Distel is designed to work in a distributed Erlang system. It can connect to specified Erlang nodes. My strategy is to use the standard erlang-mode command “C-c C-z” to start an Erlang shell, and connect to it using Distel by “C-c C-d n”. The latter asks for a nodename, which is going to be “emacs@beowulf” (because beowulf is the short hostname of my laptop).

But before we start using remote Erlang nodes, we should create on each physical machine that we will use a ~/.erlang.cookie file with a password, for inter-Erlang node authentication:

$ echo "secret" > ~/.erlang.cookie
$ chmod 0400 ~/.erlang.cookie

Step 6: Play with Distel

So, now we can launch an Erlang node, either via Emacs using “C-c C-z”, or on the command line using “erl -sname mynode”. Then, from within Emacs, we can connect Distel to this node using “C-c C-d n” specifiying the nodename on the prompt. Now we can ask Distel to interrogate the Erlang node for various things.

C-c C-d l ; list erlang processes
PID/Name              Initial Call                              Reds     Msgs
init                  otp_ring0:start/2                         3821        0
erl_prim_loader       erlang:apply/2                           87239        0
error_logger          proc_lib:init_p/5                          229        0
application_controlle erlang:apply/2                            2501        0
<0.7.0>               proc_lib:init_p/5                           45        0
<0.8.0>               application_master:start_it/4               91        0
kernel_sup            proc_lib:init_p/5                         1498        0
rex                   proc_lib:init_p/5                          493        0
global_name_server    proc_lib:init_p/5                           69        0
<0.12.0>              erlang:apply/2                              25        0
<0.13.0>              erlang:apply/2                               4        0
<0.14.0>              erlang:apply/2                               3        0
inet_db               proc_lib:init_p/5                          129        0
net_sup               proc_lib:init_p/5                          312        0
erl_epmd              proc_lib:init_p/5                          147        0