Custom Appender

To write your own log appender it should meet the following requirements:

The #log method takes the Log Struct as a parameter. For the format of the Log Struct, see Log Struct

Basic outline for an Appender:

require "semantic_logger"

class SimpleAppender < SemanticLogger::Subscriber
  def initialize(level=nil, &block)
    # Set the log level and formatter if supplied
    super(level, &block)

  # Display the log struct and the text formatted output
  def log(log)
    # Ensure minimum log level is met, and check filter
    return false if (level_index > (log.level_index || 0)) || !include_message?(log)

    # Display the raw log structure
    p log

    # Display the formatted output

  # Optional
  def flush
    puts "Flush :)"

Sample program calling the above appender:

SemanticLogger.default_level = :trace
# Log to file dev.log
SemanticLogger.add_appender(file_name: "dev.log")
# Also log the above sample appender

logger = SemanticLogger["Hello"] "Hello World"

Look at the existing appenders for good examples


To have your custom appender included in the standard list of appenders, submit it along with complete working tests. See the Graylog Appender Test for an example.


This section introduces the internal design of Semantic Logger, which will be helpful for anyone that wants to contribute changes for others in the community to take advantage of.

Log message flow diagram

Shows how log messages events are emitted from the various log instances, placed in the in-memory queue, and then written to one or more appenders on a separate thread.

Log message flow diagram

Class Diagram

Class diagram

Next: Log Event ==>