Question

In my Rails project, I noticed that sometimes actions are not written into the log file development.log immediately after they are done, but rather after some other actions are done (or maybe just after some amount of time, I’m not sure.) Exactly when is the development.log file written?

Answer 1

Updated for Rails 3.2

The default logging for Rails is buffered, so log message are not written immediately, but happen in batches. You can force the logger to flush the buffer with:

That was the old way, as of 3.2 the filesystem handles buffering before writing the log:

ActiveSupport::BufferedLogger#auto_flushing is deprecated. Either set
the sync level on the underlying file handle like this. Or tune your
filesystem. The FS cache is now what controls flushing.

You can change your config/environments/development.rb to auto flush with every log message:

config.autoflush_log = true

This is used by bootstrap.rb to force the sync of the log (e.g. f.sync = true). This will noticeable impact performance.