Logging provides a way to record messages from a running application. The messages can be used for debugging or analysis at a later time. Rails produces very verbose, informative, colored logs with each request. Each component of Rails is configured with a logger during initialization.

All of the most commonly used classes in Rails provide access to a logger logger accessor for convenient access to the logger. This includes Active Record models, controllers, views, and Active Resource models. The logger accessor returns the currently configured logger for the class.

The logger has a method for each of the log levels. Adding a message to the log is as simple as calling the method for the log level you want the message to be logged at. The following code logs a message at the :info log level:

logger.info "Successful login for #{@user.name}"

Whether or not a particular message is logged by the logger depends on the log level. If the severity of the message being logged is less than the current log level then the message is ignored. The logging severities supported by Rails compatible loggers can be seen in Table 16.1, “Logging severities”.

By default, the Rails test and development environments log with a level of :debug. The Rails production environment uses a default log level of :info. The logger will not log messages logged at a level lower than its current log level. This means that the development environment will log all messages, whereas the production environment will ignore all log messages at a log level lower than :info. This allows you to place different logging statements throughout your code with different log levels.

The easiest way to debug these problems is to monitor the log files. An easy way to monitor your logs is to use the Unix or Linux tail command. Execute the following from your project's root directory to have tail monitor the log file.

$ tail -f log/development.log

The -f flag tells tail to output appended data as the file grows.

Memcached is a distributed memory object caching system.

$ sudo port install memcached

$ sudo launchctl load -w /Library/LaunchDaemons/org.macports.memcached.plist

$ telnet localhost 11211
Trying ::1...
Connected to localhost.
Escape character is '^]'.
stats
STAT pid 527
STAT uptime 441
STAT time 1230761183
STAT version 1.2.6
STAT pointer_size 32
STAT rusage_user 0.006001
STAT rusage_system 0.012105
STAT curr_items 0
STAT total_items 0
STAT bytes 0
STAT curr_connections 6
STAT total_connections 9
STAT connection_structures 7
STAT cmd_get 0
STAT cmd_set 0
STAT get_hits 0
STAT get_misses 0
STAT evictions 0
STAT bytes_read 57
STAT bytes_written 942
STAT limit_maxbytes 67108864
STAT threads 4
END
quit
Connection closed by foreign host.