architecture.rst

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 figs/slic_architecture, consists of three main components:

1. A static logger API. This serves as the primary interface to the application.
2. 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 logMessageLevel can have one or more logStream instances, which, specify the output destination, format and filtering of messages.
3. One or more logStream instances, bound to a particular logger, that can be shared between log message levels.

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

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

Log Message Level

The 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.

Log Stream

The logStream class, is an abstract base class that facilitates the following:

• Specifying the logMessageFormat and output destination of log messages.
• Implementing logic for handling and filtering messages.
• Defines a pure abstract interface for all logStream instances.

Since logStream is an abstract base class, it cannot be instantiated and used directly. Slic provides a set of BuiltInLogStreams, which provide concrete implementations of the 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 logStream class and provide a concrete logStream instance implementation that implements the abstract interface defined by the logStream base class. See the addACustomLogStream section for details.

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

Log Message Format

The 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 logMessageLevel, i.e., ERROR, WARNING, INFO, or DEBUG.
<MESSAGE>	The supplied message that is being logged.
<FILE>	The file from where the message was emmitted.
<LINE>	The line location where the message was emmitted.
<TAG>	A string tag associated with a given message, e.g., for filtering during post-processing, etc.
<RANK>	The MPI rank that emmitted the message. Only applicable when the Axom Toolkit is compiled with MPI enabled and with MPI-aware logStream instances, such as, the SynchronizedOutputStream and 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 logMessageFormat is not specified, the 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 BuiltInLogStreams provided by Slic are summarized in the following table, followed by a brief description for each.

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

Generic Output Stream

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

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

For example, the following code snippet registers a 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 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 Output Stream

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

Similar to the GenericOutputStream the SynchronizedOutputStream is constructed by specifying:

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

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

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

Note

Since, the SynchronizedOutputStream works across MPI ranks, logging messages using the 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.

Lumberjack Stream

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

The LumberjackStream is constructed by specifying:

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

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

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

Note

Since, the LumberjackStream works across MPI ranks, logging messages using the 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.

Add a Custom Log Stream

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

Note

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

Create a LogStream Subclass

First, we create a new class, MyStream, that is a subclass of the 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 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 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_NOT_USED(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 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 logStream base class, which, applies the 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 logStream class may be used with Slic in a similar manner to any of the BuiltInLogStreams, as demonstrated in the code snippet below:

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