Comparing `folly::logging` to other log libraries

This document attempts to briefly describe the differences between the folly logging library and other C++ logging libraries. This is not necessarily comprehensive, and may become out of date as libraries change.

Google Logging (glog)

The folly logging library is similar to glog in many ways.

Like folly logging, glog also provides very cheap debug log messages with its VLOG macro. (However, the glog LOG macro is not lazy, and always evaluates its arguments, even if the log message is disabled.)

The primary difference between folly logging and glog is that folly offers more flexibility in turning debug messages on or off. The VLOG() macros can be enabled or disabled on per-file basis through a --vmodule command line flag on non-Windows platforms. This flag does support regular expressions to match groups of files, but the expression only applies to the last component of the file name. This makes it more difficult to control logging for specific libraries and subcomponents of a project.

Other advantages of folly logging over glog:

• Logging I/O can be performed in a separate thread. glog performs all logging I/O in the thread that generated the log message. This can block processing while waiting for logging I/O to complete.
• Unprintable characters in multi-line log messages are escaped by default. This helps avoid problematic or dangerous terminal escape sequences from appearing in log messages.
• Better support for multi-line log messages. Folly logging adds a log message header after each internal new line in log messages.
• Full functionality on Windows. The VLOG() macro from glog has somewhat diminished functionality on Windows, since it cannot be controlled on a per-module basis.

Advantages of glog over folly logging:

• Smaller generated code size. Due to automatically picking a log category name, folly logging's XLOG() macros currently result in slightly larger generated code compared to VLOG().

Log4j Clones

There are a number of Log4j-like libraries for C++ (log4cxx, log4cpp, log4cplus).

Conceptually folly logging behaves similarly to most of these libraries. Much of folly logging's hierarchical log category behavior was modeled after log4j functionality, like these libraries.

The main difference between folly logging and most of these libraries is low overhead for disabled log messages. The folly logging code ensures that disabled log messages boil down to a single conditional level check. Most of the other C++ log4j clones always evaluate log message arguments, and some also perform more complex hierarchical log level checks at log time.