On-the-fly recompiling and reloading in Erlang. Code without friction.
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Stay in Sync

What is Sync?

Sync is a developer utility. It recompiles and reloads your Erlang code on-the-fly. With Sync, you can code without friction.

Successful compilation image.

What does "code without friction" mean? It means that with Sync running, you no longer need to worry about running make, or c:l(Module) again. Just write code, save the file, and watch as Erlang automatically detects your changes, recompiles the code, and reloads the module.

How can I use Sync?

The recommended approach is to put sync in your $ERL_LIBS directory.

git clone git@github.com:rustyio/sync.git
(cd sync; make)

Then, go in the Erlang console of an application you are developing, run sync:go().. You can also start sync using application:start(sync).

Starting up:

(rustyio@> sync:go().

Starting Sync (Automatic Code Compiler / Reloader)
Scanning source files...
08:34:18.609 [info] Application sync started on node 'rustyio@'

Successfully recompiling a module:

08:34:43.255 [info] /Code/Webmachine/src/webmachine_dispatcher.erl:0: Recompiled.
08:34:43.265 [info] webmachine_dispatcher: Reloaded! (Beam changed.)


08:35:06.660 [info] /Code/Webmachine/src/webmachine_dispatcher.erl:33: Warning: function dispatch/3 is unused


08:35:16.881 [info] /Code/Webmachine/src/webmachine_dispatcher.erl:196: Error: function reconstitute/1 undefined
/Code/Webmachine/src/webmachine_dispatcher.erl:250: Error: syntax error before: reconstitute

Desktop Notifications

Sync can pop success / warning / failure notifications onto your desktop to keep you informed of compliation results. All major operating systems are supported: Mac via Growl, Linux via Libnotify, Windows via Notifu and Emacs via lwarn / message command. Below are Growl screenshots.


Successful compilation image.


Compilation warnings image.


Compilation errors image.

Disabling Desktop Notifications

If you find the desktop notifications annoying, you can disable them in one of two ways:

1. As an environment variable called from the erlang command line:

erl -sync growl false
erl -sync growl true   # Default

2. From within the Erlang shell:

sync:growl(true).    % Enable notifications
sync:growl(false).   % Disable notifications

Troubleshooting Growl Notifications

Sync attempts to auto-detect the notification package to use via the os:type() command.

If this isn't working for you, or you would like to override the default, use the executable environment variable:

erl -sync executable TYPE

Where TYPE is:

  • growlnotify for Mac / Growl.
  • notify-send for Linux / libnotify.
  • notifu for Windows / Notifu.
  • emacsclient for Emacs notifications.

Remote Server "Patching"

If you are developing an application that runs on an Erlang cluster, you may need to recompile a module on one node, and then broadcast the changed module to other nodes. Sync helps you do that with a feature called "patching."

To use the patching feature:

  1. Connect to any machine in your cluster via distributed erlang. A simple net_adm:ping(Node) should suffice.

  2. Run sync:patch(). This will start the Sync application if it's not already started, and enable "patch mode".

  3. Start editing code.

Sync will detect changes to code, recompile your modules, and then sent the updated modules to every Erlang node connected to your cluster. If the module already exists on the node, then it will be overwritten on disk with the new .beam file and reloaded. If the module doesn't exist on the new node, then it is simply updated in memory.

How does Sync work?

Upon startup, Sync gathers information about loaded modules, ebin directories, source files, compilation options, etc.

Sync then periodically checks the last modified date of source files. If a file has changed since the last scan, then Sync automatically recompiles the module using the previous set of compilation options. If compilation was successful, it loads the updated module. Otherwise, it prints compilation errors to the console.

Sync also periodically checks the last modified date of any beam files, and automatically reloads the file if it has changed.

The scanning process adds 1% to 2% CPU load on a running Erlang VM. Much care has been taken to keep this low. Shouldn't have to say this, but this is for development mode only, don't run it in production.

Using Sync with the Nitrogen Web Framework

If you are running sync with the Nitrogen Web Framework, be sure to add the following line to your etc/vm.args file:

-sync sync_mode nitrogen