Runtime Logging

Mapnik >= 2.1.x supports a variety of logging options to make it possible for the developer or user to see what Mapnik is doing under the hood.

The amount of possible logging functionality is determined at compile time, but the logging verbosity can also be controlled at runtime.

Compile time options

When you configure Mapnik you have several options that can be set during the configure stage of building which impact how logging behaves

• DEFAULT_LOG_SEVERITY - Default error. Other named severity levels are: ['debug', warn, none`.
• ENABLE_LOG - Default False. If set to True then logging of all severity types will be enabled. This is True by default if DEBUG=True (see option below).
• ENABLE_STATS -
• DEBUG

The runtime logger

TODO

Logging where it counts when developing in C++

When you develop Mapnik, and need to output log strings that needs to print common info, debug or warning and error strings, then you need to use the newer logger interface. Be sure you have set this option in config.py (or you are building in DEBUG):

ENABLE_LOG = True

Then if we have a file called cairo_renderer.cpp and need to debug a string:

#include <mapnik/debug.hpp>
...
// here we need to output a string that will be output in DEBUG severity level:
MAPNIK_LOG_DEBUG(cairo_renderer) << "Log my data, visible at DEBUG severity level";

All the logging facilities in C++

// Output a string in WARN severity level
MAPNIK_LOG_WARN(object_name) << "This is INFO";

// Output a string in DEBUG severity level
MAPNIK_LOG_DEBUG(object_name) << "This is DEBUG";

// Output a string in ERROR severity level
MAPNIK_LOG_ERROR(object_name) << "This is ERROR";

Best practices

This has the advantages that it will be optimized automatically by the compiler when ENABLE_LOG is set to False. But if you need to perform complex code before outputting a string to debug, then it's better to disable those expensive calls completely when log is not enabled:

#include <mapnik/debug.hpp>
...
#ifdef MAPNIK_LOG
  // here we need to output a string that will be output in DEBUG severity level:
  const double a = 1.0 / sin(x);
  const double z = a * connection->get_zoom_from_sql_call();
  MAPNIK_LOG_DEBUG(cairo_renderer) << "Log the variable " << z << ", visible at DEBUG severity level";
#endif