Linux Signals

For Linux, Unix, and Mac, Semantic Logger can register callbacks for specific signals.

On startup Semantic Logger does not register any signals so that it does not interfere with any existing signal handlers. The signal handler must be explicitly registered by calling SemanticLogger.add_signal_handler

Add a Rails initializer:

# config/initializers/semantic_logger.rb
SemanticLogger.add_signal_handler

Or, enabling Log Level Signal handler for a stand-alone application:

# Enable signal handling for this process
SemanticLogger.add_signal_handler

The following capabilities are available by using the corresponding signal:

Changing the log_level (USR2)

The global default logging level can be changed by sending the process a SIGUSR2 signal without having to restart the process.

When the signal is sent, the global default log level rotates through the following log levels in the following order, starting from the current global default level:

  :warn, :info, :debug, :trace

If the current level is :trace it wraps around back to :warn

Example (where the target ruby process id is 1234) :

kill -SIGUSR2 1234

Note: The changes to the logging level will not change for any classes where the level was set explicity within the application itself. The above signal only changes the global default level, which is used by loggers when their log level has not been overridden.

Logging a Ruby thread dump (TTIN)

When the TTIN signal is sent to this process, Semantic Logger will write the list of threads to the log file, along with their back-traces when available

For JRuby users this thread dump differs form the standard QUIT triggered Java thread dump which includes system threads and Java stack traces.

It is recommended to name any threads you create in the application, by calling the following from within the thread itself:

Thread.current.name = 'My Worker'

Example (where the target ruby process id is 1234) :

kill -TTIN 1234

development.log

JRuby Garbage collection event logging

For JRuby, any Garbage Collection events that take longer than 10ms are logged as a warning to the regular log file and other appenders.

This information is critical in high load scenarios to give visibility on any Garbage Collection that could affect active web requests or other processing.

development.log

Overriding signals

To only register one of the signal handlers, set the other to nil Set gc_log_microseconds to nil to not enable JRuby Garbage collections

# Set the log level change signal to USR1
# Set the thread dump signal to 'USR2'
# Set the Garbage collection minimum threshold to 100,000 micro-seconds
SemanticLogger.add_signal_handler('USR1', 'USR2', 100000)

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