Programming Guide

Logging levels

The following logging levels are available through Semantic Logger

:trace, :debug, :info, :warn, :error, :fatal

The log levels are listed above in the order of precedence with the most detail to the least. For example :debug would include :info, :warn, :error, :fatal levels but not :trace. :fatal only logs :fatal error messages and nothing else.

:trace is a new level that is often used for tracing low level calls such as the data sent or received to external web services. It is also commonly used in the development environment for low level trace logging of methods calls etc.

Changing the global default log level

By default Semantic Logger will only log :info and above. To log everything to the log file set the global default log level to :trace:

require "semantic_logger"

# Override the default log level of :info so that :debug and :trace are also logged
SemanticLogger.default_level = :trace

SemanticLogger.add_appender(file_name: "development.log", formatter: :color)

logger = SemanticLogger["MyClass"] "Hello World"
logger.trace "Low level trace information"

All loggers and appenders use the global SemanticLogger.default_level by default unless they have been explicity set to another level. In which case changing SemanticLogger.default_level will not affect that particular logger or appender.

Creating an instance of a logger

To create a stand-alone logger instance by supplying the name of the class/application:

logger = SemanticLogger["MyClass"]

Or, even better, pass the actual class itself:

logger = SemanticLogger[MyClass]

A logger instance should be created for every class so that its log entries can be uniquely identified from logging entries from other classes.

All logger instances forward their log messages to the global Appender Thread that does the actual logging to each appender, see Appenders for a list of available Appenders.

Using the SemanticLogger::Loggable Mixin

Rather than creating logger instances above inside classes it is recommended to use the SemanticLogger::Loggable Mixin by adding the following include:

  include SemanticLogger::Loggable

For Example:

class Supplier
  # Include class and instance logger variables
  include SemanticLogger::Loggable

  def self.some_class_method
    logger.debug("logger is accessible from class methods")

  def call_supplier
    logger.debug("logger is accessible from instance methods")

This will result in the log output identifying the log entry as from the Supplier class

2012-08-30 15:37:29.474 I [48308:main] Supplier -- logger is accessible from instance methods

Standard Logging methods

The Semantic Logger logging API supports the existing logging interface for the Rails and Ruby Loggers. For example:"Hello World")

Or to query whether a specific log level is set

The following traditional logging methods are available

logger.trace("Low level trace information such as data sent over a socket")
logger.debug("Debugging information to aid with problem determination")"Informational message such as request received")
logger.warn("Warn about something in the system")
logger.error("An error occurred during processing")
logger.fatal("Oh no something really bad happened")

Each of the above calls can take additional parameters, for example:, payload_or_exception = nil, exception = nil, &block)



logger.debug("Calling Supplier")

logger.debug("Calling Supplier", request: "update", user: "Jack")

logger.trace { "A total of #{result.inject(0) {|sum, i| i+sum }} were processed" }

An alternative API is to pass everything in a Hash, including the message.

logger.debug(message: "Calling Supplier")

logger.debug(message: "Calling Supplier", payload: {request: "update", user: "Jack"})

# Log a complete exception
logger.error(message: "Calling Supplier", exception: exception)

# Add a 100ms duration to the log entry
logger.error(message: "Calling Supplier", duration: 100)

# Add a count metric ( with a value of 1 )
logger.error(message: "Calling Supplier", metric: "Supplier/inquiry")

# Add a count metric with a value of 21
logger.error(message: "Calling Supplier", metric: "Supplier/inquiry", metric_amount: 21)

# Add a duration metric
logger.error(message: "Calling Supplier", metric: "Supplier/inquiry", duration: 100)


The Semantic Logger adds an optional parameter to the existing log methods so that a corresponding Exception can be logged in a standard way

  # ... Code that can raise an exception
rescue Exception => exception

  # Log the exception as an error
  logger.error("Oops external call failed", exception)

  # Re-raise or handle the exception
  raise exception


The Semantic Logger adds an extra parameter to the existing log methods so that additional payload can be logged, such as a Hash or a Ruby Exception object."Oops external call failed", result: :failed, reason_code: -10)

The additional payload is machine readable so that we don’t have to write complex regular expressions so that a program can analyze log output. With the MongoDB appender the payload is written directly to MongoDB as part of the document and is therefore fully searchable

Measure Everything

It is important to “measure everything” in a production application so that when things slow down it is obvious where the slow down is coming from.

logger.measure_info "Called external interface" do
  # Code to call external service ...

The following output will be written to file:

2012-08-30 15:37:29.474 I [48308:ScriptThreadProcess: script/rails] (5.2ms) Rails -- Called external interface

The log message is only written once the block completes and includes how long it took to complete.

If an exception is raised during the block, the exception message will be logged at the same log level as the measure along with the duration and message. After logging the exception is re-raised unchanged.

The following measuring methods are available:

logger.measure_trace("Low level trace information such as data sent over a socket") do ... end
logger.measure_debug("Debugging information to aid with problem determination") do ... end
logger.measure_info("Informational message such as request received") do ... end
logger.measure_warn("Warn about something in the system") do ... end
logger.measure_error("An error occurred during processing") do ... end
logger.measure_fatal("Oh no something really bad happened") do ... end

Elastic Logging

With elastic logging the log message should only be logged when the minimum duration has been exceeded.

For example, Memcache calls are usually very fast, log when it takes longer than 3 ms to complete:

logger.measure_warn "Called memcache", min_duration: 3 do
  # Code to call memcache ...

Metrics / Dashboards

By measuring the time taken to execute a block of code and then assigning a metric to it, the duration can be aggregated into a dashboard.

logger.measure_info "Called external interface", metric: "Supplier/inquiry" do
  # Code to call external service ...

The log level can be supplied dynamically as the first parameter to:

logger.measure(:info, "Informational message such as request received")

Each of the above calls take additional optional parameters:

log.measure_info(message, params=nil) do
  # Measure how long it takes to run this block of code

Measuring calls take two parameters, the first is a mandatory text message, the second is a Hash of settings:


logger.measure_info "Called external interface",
    log_exception: :full,
    min_duration:  100,
    metric:        "Custom/Supplier/process" do
  # Code to call external service ...

If the duration is already available, it is possible to use the same measure logging and manually supply the duration without a block. This ensures that the duration is logged in a semantic way rather than inserting the duration into the text message itself.

duration = - start_time
logger.measure_info "Called external interface", duration: duration

Note: Either a code block or :duration must be supplied on all measure calls

Tagged Logging

Tagged logging adds the specified tags to every log message within the supplied block. If a new thread is created within the block the logging tags are not automatically copied to that thread. See Parallel Minion for a library that creates threads and automatically copies across any logging tags to the new thread.

Using Tagged logging is critical in any highly concurrent environment so that one can quickly find all related log entries across all levels of code, and threads.

tracking_number = "15354128"

SemanticLogger.tagged(tracking_number) do
  # All log entries in this block will include the "tracking_number" logging tag
  logger.debug("Hello World")

It is recommended to name tags since using just the tag values can be confusing, especially when a system grows over time. Named tags are easier to identify and alert off of when a centralized logging system is being written to.

SemanticLogger.tagged(user: "Jack", zip_code: 12345) do
  # All log entries in this block will include the above named tags
  logger.debug("Hello World")

Named threads

Semantic Logger logs the name or id of the thread in every log message.

On Ruby MRI the thread name is by default the thread’s object_id, For example: 70184354571980

2013-11-07 16:25:14.279627 I [35841:70184354571980] (0.0ms) ExternalSupplier -- Calling external interface

To set a custom name for any thread so that it shows up in the logger: = "User calculation thread 32"

Sample output:

2013-11-07 16:26:02.744139 I [35841:User calculation thread 32] (0.0ms) ExternalSupplier -- Calling external interface

When running JRuby, will also set the underlying thread name in the JVM which is very useful when monitoring the JVM via JMX using tools such as jconsole.


Make sure that the assigned thread name is unique otherwise it will be difficult to distinguish between concurrently running threads if they have the same name.

For example, use the current thread object_id to ensure uniqueness: = "Worker Thread:#{Thread.current.object_id}"

Silencing noisy logs

Silence noisy log levels by changing the default_level within the block.

This setting is thread-safe and only applies to the current thread.

Any threads spawned from within the block will not be affected by silence.

# Silence all logging below :error level
logger.silence do "this will _not_ be logged"
 logger.warn "this neither"
 logger.error "but errors will be logged"

silence can also lower the log level within the supplied block. For example to increase log information in diagnosing a specific issue.

# Perform trace level logging within the block, even when the default is higher
SemanticLogger.default_level = :info

logger.debug "this will _not_ be logged"

logger.silence(:trace) do
 logger.debug "this will be logged"


silence does not affect any loggers which have had their log level set explicitly. I.e. That do not rely on the global default level.

Debug logging as Trace

Some third party gems log a large amount of information at debug level since they do not use Semantic Logger and do not have access to the :trace level for logging.

To map the :debug logging calls for these existing libraries to :trace, replace its logger with an instance of DebugAsTraceLogger::SemanticLogger

# Example, log debug level messages as trace:
logger ="NoisyLibrary")

# This will be logged as :trace
logger.debug "Some very low level noisy message"

Changing the log level for a single class at runtime

Since the logger is class specific, its log level can be changed dynamically at runtime. For example, to temporarily set the log level to :trace to diagnose an issue:

require "semantic_logger"

SemanticLogger.default_level = :info
SemanticLogger.add_appender(file_name: "example.log", formatter: :color)

class ExternalSupplier
  # Lazy load logger class variable on first use
  include SemanticLogger::Loggable

  def call_supplier(amount, name)
    logger.trace "Calculating with amount", { :amount => amount, :name => name }

    # Measure and log on completion how long the call took to the external supplier
    logger.measure_info "Calling external interface" do
      # Code to call the external supplier ...

# Create and use the class
supplier =
supplier.call_supplier(100, "Jack")

# Now change the log level to :trace
ExternalSupplier.logger.level = :trace

# Call the supplier, this time including trace level messages
supplier.call_supplier(100, "Jack")

# Change the log level back to the global default level
ExternalSupplier.logger.level = nil

Below is the output from the above example showing the :trace log level message that was written during the second call to the ExternalSupplier:

2013-11-07 16:19:26.496 I [35674:main] (0.0ms) ExternalSupplier -- Calling external interface

2013-11-07 16:19:26.683 T [35674:main] ExternalSupplier -- Calculating with amount -- {:amount=>100, :name=>"Jack"}
2013-11-07 16:19:26.683 I [35674:main] (0.0ms) ExternalSupplier -- Calling external interface

Change the log level without using signals

If the application has another means of communicating without needing signals, the global default log level can be modified using SemanticLogger.default_level=

# Change the global default logging level for active loggers
SemanticLogger.default_level = :debug

Flushing the logs

Semantic Logger automatically flushes all appenders ( log files, etc. ) when a process exits.

The flush method is not defined for Semantic Logger instances since there could be many logging instances, sometimes one per class.

To perform a global flush of all appenders and to wait for any outstanding queued messages to be written:

# Flush all appenders and wait for them to complete flushing

Replacing loggers in other Gems

Rails Semantic Logger already replaces the loggers for the following Gems, but if Semantic Logger is being used stand-alone, then these need to be called from within your code as needed:

# Replace the Resque Logger
Resque.logger = SemanticLogger[Resque] if defined?(Resque) && Resque.respond_to?(:logger)

# Replace the Sidekiq logger
Sidekiq::Logging.logger = SemanticLogger[Sidekiq] if defined?(Sidekiq)

# Replace the Mongoid Logger
Mongoid.logger = SemanticLogger[Mongoid] if defined?(Mongoid)
Moped.logger   = SemanticLogger[Moped] if defined?(Moped)

Causal Exceptions

When an exception is caught and then a new one is raised, Ruby automatically adds the original caught exception to the new exception as its cause.

Example function that raises a new exception:

def oh_no
  f ="filename", "w")
  # Will raise: IOError: not opened for reading
rescue IOError
  raise"Failed to write to file")

Calling the above function and then logging the exception:

require "semantic_logger"
SemanticLogger.add_appender(io: $stdout, formatter: :color)
logger = SemanticLogger["Demo"]

rescue StandardError => exception
  # Semantic Logger will log both the exception and the causing exception
  logger.error("Failed calling oh_no", exception)

Both the RuntimeError and the IOError will be logged:

2017-05-03 09:45:38.948029 E [17641:70311685126260 demo.rb:17] Demo -- Failed calling oh_no -- Exception: RuntimeError: Failed to write to file
demo.rb:6:in `rescue in oh_no'
demo.rb:2:in `oh_no'
demo.rb:14:in `<main>'
Cause: IOError: not opened for reading
demo.rb:4:in `read'
demo.rb:4:in `oh_no'
demo.rb:14:in `<main>'

The output above contains 2 stack traces, with the second stack trace starting at Cause: IOError: not opened for reading.

Synchronous Operation

Some users have requested the ability to bypass the separate logging thread. It forces logging to be performed in the current thread, instead of being performed asynchronously. Of course this means that all logging is performed in the current thread slowing it down, depending on how many and which log appenders are being used.

It is not recommended to use this feature since it disables a core design principle in Semantic Logger itself. If your application is single threaded and is not time critical, performing logging in the current may be useful for your application though.

Some scenarious where it could be useful.

Run Semantic Logger in Synchronous mode:


Note: The above config option must be called before any appenders have been added.

To guarantee that the above sync config option is enabled, replace the regular require statement with one that forces synchronous logging:

require "semantic_logger/sync"

Or, when using a Gemfile:

gem "semantic_logger", require: "semantic_logger/sync"

Next: Testing ==>