Component Architecture

Slic provides a simple and easy to use logging interface for applications.

Basic Component Architecture of Slic.

The basic component architecture of Slic, depicted in :numref:`figs/slic_architecture`, consists of three main components:

A static logger API. This serves as the primary interface to the application.
One or more Logger object instances. In addition to the root logger, which is created when Slic is initialized, an application may create additional loggers. However, at any given instance, there is only a single active logger to which all messages are logged.
- A Logger consists of four log message levels: ERROR, WARNING, INFO and DEBUG.
- Each :ref:`logMessageLevel` can have one or more :ref:`LogStream` instances, which, specify the output destination, format and filtering of messages.
One or more :ref:`LogStream` instances, bound to a particular logger, that can be shared between log message levels.

The application logs messages at an appropriate :ref:`logMessageLevel`, i.e., ERROR, WARNING, INFO, or, DEBUG using the static API. Internally, the Logger routes the message to the corresponding :ref:`LogStream` instances that are bound to the associated :ref:`logMessageLevel`.

The following sections discuss some of these concepts in more detail.

Log Message Level

The :ref:`logMessageLevel` indicates the severity of a message. Slic provides four levels of messages ranked from highest to lowest as follows:

Message Level	Usage Description
`ERROR`	Indicates that the application encountered a critical error or a faulty state. Includes also stacktrace.
`WARNING`	Indicates that the application encountered an error, but, the application should proceed.
`INFO`	General information reported by an application.
`DEBUG`	Information useful to application developers.

Note

ERROR messages by default will cause the application to abort. This behavior may be toggled by calling slic::enableAbortOnError() and slic::disableAbortOnError(). See the `Slic Doxygen API Documentation`_ for more details.

An application may adjust at runtime the severity level of messages to capture by calling slic::setLoggingMsgLevel(). For example, the following code snippet, sets the severity level to WARNING

slic::setLoggingMsgLevel( slic::message::Warning );

This indicates that all messages with a level of severity of WARNING and higher will be captured, namely WARNING and ERROR messages. Thereby, enable the application to filter out messages with lower severity.

Warning

All messages will be ignored until the first call to slic::setLoggingMsgLevel().

Log Stream

The :ref:`LogStream` class, is an abstract base class that facilitates the following:

Specifying the :ref:`logMessageFormat` and output destination of log messages.
Implementing logic for handling and filtering messages.
Defines a pure abstract interface for all :ref:`LogStream` instances.

Since :ref:`LogStream` is an abstract base class, it cannot be instantiated and used directly. Slic provides a set of :ref:`BuiltInLogStreams`, which provide concrete implementations of the :ref:`LogStream` base class that support common use cases for logging, e.g., logging to a file or output to the console.

Applications requiring custom functionality, may extend the :ref:`LogStream` class and provide a concrete :ref:`LogStream` instance implementation that implements the abstract interface defined by the :ref:`LogStream` base class. See the :ref:`addACustomLogStream` section for details.

A concrete :ref:`LogStream` instance can be attached to one or more :ref:`logMessageLevel` by calling slic::addStreamToMsgLevel() and slic::addStreamToAllMsgLevels(). See the `Slic Doxygen API Documentation`_ for more details.

Log Message Format

The :ref:`logMessageFormat` is specified as a string consisting of keywords that are encapsulated in <...>, which, Slic knows to interpret when assembling the log message.

The list of keywords is summarized in the table below.

keyword	Replaced With
<TIMESTAMP>	A textual representation of the time a message is logged, as returned by `std::asctime()`.
<LEVEL>	The :ref:`logMessageLevel`, i.e., `ERROR`, `WARNING`, `INFO`, or `DEBUG`.
<MESSAGE>	The supplied message that is being logged.
<FILE>	The file from where the message was emitted.
<LINE>	The line location where the message was emitted.
<TAG>	A string tag associated with a given message, e.g., for filtering during post-processing, etc.
<RANK>	The MPI rank that emitted the message. Only applicable when the `Axom Toolkit`_ is compiled with MPI enabled and with MPI-aware :ref:`LogStream` instances, such as, the :ref:`SynchronizedStream` and :ref:`LumberjackStream`.

These keywords can be combined in a string to specify a template for a log message.

For example, the following code snippet, specifies that all reported log messages consist of the level, enclosed in brackets followed by the user-supplied log message.

std::string format = "[<LEVEL>]: <MESSAGE>";

To get the file and line location within the file where the message was emitted, the format string above could be amended with the following:

std::string format = "[<LEVEL>]: <MESSAGE> \n\t<FILE>:<LINE>";

This indicates that the in addition to the level and user-supplied, the resulting log messages will have an additional line consisting of the file and line where the message was emitted.

Default Message Format

If the :ref:`logMessageFormat` is not specified, the :ref:`LogStream` base class defines a default format that is set to the following:

std::string DEFAULT_FORMAT = "*****\n[<LEVEL>]\n\n <MESSAGE> \n\n <FILE>\n<LINE>\n****\n"

Built-In Log Streams

The :ref:`BuiltInLogStreams` provided by Slic are summarized in the following table, followed by a brief description for each.

Log Stream	Use & Availability
:ref:`GenericOutputStream`	Always available. Used in serial applications, or, for logging on rank zero.
:ref:`SynchronizedStream`	Requires MPI. Used with MPI applications.
:ref:`LumberjackStream`	Requires MPI. Used with MPI applications.

Generic Output Stream

The :ref:`GenericOutputStream`, is a concrete implementation of the :ref:`LogStream` base class, that can be constructed by specifying:

A C++ std::ostream object instance, e.g., std::cout`, ``std::cerr for console output, or to a file by passing a C++ std::ofstream object, and,
Optionally, a string that specifies the :ref:`logMessageFormat`.

For example, the following code snippet registers a :ref:`GenericOutputStream` object that is bound to the the std::cout.

slic::addStreamToAllMsgLevels(
  new slic::GenericOutputStream( &std::cout, format ) );

Similarly, the following code snippet, registers a :ref:`GenericOutputStream` object that is bound to a file.

std::ofstream log_file;
log_file.open( "logfile.dat" );

slic::addStreamToAllMsgLevels(
  new slic::GenericOutputStream( &log_file, format ) );

Synchronized Stream

The :ref:`SynchronizedStream` is intended to be used with parallel MPI applications, primarily for debugging. The :ref:`SynchronizedStream` provides similar functionality to the :ref:`GenericOutputStream`, however, the log messages are synchronized across the MPI ranks of the specified communicator.

Similar to the :ref:`GenericOutputStream` the :ref:`SynchronizedStream` is constructed by specifying:

A C++ std::ostream object instance, e.g., std::cout`, ``std::cerr for console output, or to a file by passing a C++ std::ofstream object.
The MPI communicator, and,
Optionally, a string that specifies the :ref:`logMessageFormat`.

The following code snippet illustrates how to register a :ref:`SynchronizedStream` object with Slic to log messages to std::cout.

slic::addStreamToAllMsgLevels(
   new slic::SynchronizedStream( &std::cout, mpi_comm, format ) );

Note

Since, the :ref:`SynchronizedStream` works across MPI ranks, logging messages using the :ref:`SlicMacros` or the static API directly only logs the messages locally. To send the messages to the output destination the application must call slic::flushStreams() explicitly, which, in this context is a collective call.

Warning

In the event of an abort in :ref:`SynchronizedStream`, SLIC calls slic::outputLocalMessages(), which will output the locally stored messages to the output destination for log messages (file or console) and then SLIC calls MPI_Abort(). The call to slic::outputLocalMessages() is non-collective, and does not guarantee all locally stored messages will be outputted by all ranks.

Lumberjack Stream

The :ref:`LumberjackStream` is intended to be used with parallel MPI applications. In contrast to the :ref:`SynchronizedStream`, which logs messages from all ranks, the :ref:`LumberjackStream` uses `Lumberjack`_ internally to filter out duplicate messages that are emitted from multiple ranks.

The :ref:`LumberjackStream` is constructed by specifying:

A C++ std::ostream object instance, e.g., std::cout`, ``std::cerr for console output, or to a file by passing a C++ std::ofstream object.
The MPI communicator,
An integer that sets a limit on the number of duplicate messsages reported per rank, and,
Optionally, a string that specifies the :ref:`logMessageFormat`.

The following code snippet illustrates how to register a :ref:`LumberjackStream` object with Slic to log messages to std::cout.

slic::addStreamToAllMsgLevels(
   new slic::LumberjackStream( &std::cout, mpi_comm, 5, format ) );

Note

Since, the :ref:`LumberjackStream` works across MPI ranks, logging messages using the :ref:`SlicMacros` or the static API directly only logs the messages locally. To send the messages to the output destination the application must call slic::flushStreams() explicitly, which, in this context is a collective call.

Warning

In the event of an abort in :ref:`LumberjackStream`, SLIC calls slic::outputLocalMessages(), which will output the locally stored messages to the output destination for log messages (file or console) and then SLIC calls MPI_Abort(). The call to slic::outputLocalMessages() is non-collective, and does not guarantee all locally stored messages will be outputted by all ranks.

Add a Custom Log Stream

Slic can be customized by implementing a new subclass of the :ref:`LogStream`. This section demonstrates the basic steps required to :ref:`addACustomLogStream` by walking through the implementation of a new :ref:`LogStream` instance, which we will call MyStream.

Note

MyStream provides the same functionality as the :ref:`GenericOutputStream`. The implementation presented herein is primarily intended for demonstrating the basic process for extending Slic by providing a custom :ref:`LogStream`.

Create a LogStream Subclass

First, we create a new class, MyStream, that is a subclass of the :ref:`LogStream` class, as illustrated in the code snippet below.

class MyStream : public LogStream
{
 public:

   MyStream( ) = delete;
   MyStream( std::ostream* os, const std::string& format );

   virtual ~MyStream();

   /// \see LogStream::append
   virtual void append( message::Level msgLevel,
                       const std::string& message,
                       const std::string& tagName,
                       const std::string& fileName,
                       int line,
                       bool filter_duplicates );
 private:
   std::ostream* m_stream;

   // disable copy & assignment
   MyStream( const MyStream & ) = delete;
   MyStream& operator=(const MyStream&) = delete;

   // disable move & assignment
   MyStream( const MyStream&& ) = delete;
   MyStream& operator=(const MyStream&&) = delete;
};

The class has a pointer to a C++ std::ostream object as a private class member. The std::ostream object holds a reference to the output destination for log messages, which can be any std::ostream instance, e.g., std::cout, std::cerr, or a file std::ofstream, etc.

The reference to the std::ostream is specified in the class constructor and is supplied by the application when a MyStream object is instantiated.

Since MyStream is a concrete instance of the :ref:`LogStream` base class, it must implement the append() method, which is a pure virtual method.

Implement LogStream::append()

The MyStream class implements the LogStream::append() method of the :ref:`LogStream` base class, as demonstrated in the code snippet below.

 void MyStream::append( message::Level msgLevel,
                       const std::string& message,
                       const std::string& tagName,
                       const std::string& fileName,
                       int line,
                       bool AXOM_UNUSED_PARAM(filtered_duplicates) )
{
   assert( m_stream != nillptr );

   (*m_stream) << this->getFormatedMessage( message::getLevelAsString(msgLevel),
                                            message,
                                            tagName,
                                            "",
                                            fileName,
                                            line );
}

The append() method takes all the metadata associated with a message through its argument list:

The :ref:`logMessageLevel`
The user-specified message
A tag associated with the message, may be set MSG_IGNORE_TAG
The file where the message was emitted
The line location within the file where the message was emitted

The append() method calls LogStream::getFormatedMessage(), a method implemented in the :ref:`LogStream` base class, which, applies the :ref:`logMessageFormat` according to the specified format string supplied to the MyStream class constructor, when the class is instantiated.

Register the new class with Slic

The new :ref:`LogStream` class may be used with Slic in a similar manner to any of the :ref:`BuiltInLogStreams`, as demonstrated in the code snippet below:

slic::addStreamToAllMsgLevels( new MyStream( &std::cout, format ) );

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architecture.rst

architecture.rst

Component Architecture

Log Message Level

Log Stream

Log Message Format

Default Message Format

Built-In Log Streams

Generic Output Stream

Synchronized Stream

Lumberjack Stream

Add a Custom Log Stream

Create a LogStream Subclass

Implement LogStream::append()

Register the new class with Slic

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Component Architecture

Log Message Level

Log Stream

Log Message Format

Default Message Format

Built-In Log Streams

Generic Output Stream

Synchronized Stream

Lumberjack Stream

Add a Custom Log Stream

Create a LogStream Subclass

Implement LogStream::append()

Register the new class with Slic