Boost study: boost log

The log system is used to record the key information during software operation , It's big CAx An important component of a software system .

This paper intends to Boost.Log Module for analysis , To record its technical points .

notes 1： Limited to the author's research level , It is hard to avoid misunderstanding , Welcome criticism and correction .

notes 2： The content of the article will be updated from time to time , Welcome to exchange and discussion .

One 、 working principle

Boost.Log In general, the Logging Sources、Logging Core、Logging Sinks Other components ：Logging Sources establish log records;Logging Core Yes log records To deal with ;Logging Sinks Complete the final formatting and output .

 Ref. from Boost.Log Design Overview

Logging sources

Getting back to the figure, in the left side your application emits log records with help of loggers - special objects that provide streams to format messages that will eventually be put to log. The library provides a number of different logger types and you can craft many more yourself, extending the existing ones. Loggers are designed as a mixture of distinct features that can be combined with each other in any combination. You can simply develop your own feature and add it to the soup. You will be able to use the constructed logger just like the others - embed it into your application classes or create and use a global instance of the logger. Either approach provides its benefits. Embedding a logger into some class provides a way to differentiate logs from different instances of the class. On the other hand, in functional-style programming it is usually more convenient to have a single global logger somewhere and have a simple access to it.

Generally speaking, the library does not require the use of loggers to write logs. The more generic term "log source" designates the entity that initiates logging by constructing a log record. Other log sources might include captured console output of a child application or data received from network. However, loggers are the most common kind of log sources.

Logging core and filtering

When the set of attribute values is composed, the logging core decides if this log record is going to be processed in sinks. This is called filtering. There are two layers of filtering available: the global filtering is applied first within the logging core itself and allows quickly wiping away unneeded log records; the sink-specific filtering is applied second, for each sink separately. The sink-specific filtering allows directing log records to particular sinks. Note that at this point it is not significant which logging source emitted the record, the filtering relies solely on the set of attribute values attached to the record.

Sinks and formatting

If a log record passes filtering for at least one sink the record is considered to be consumable. If the sink supports formatted output, this is the point when log message formatting takes place. The formatted message along with the composed set of attribute values is passed to the sink that accepted the record. Note that formatting is performed on the per-sink basis so that each sink can output log records in its own specific format.

Ref. from Boost.Log

Reference material

Boost.Loghttps://www.boost.org/doc/libs/1_79_0/libs/log/doc/html/index.html

Log4Cpphttp://log4cpp.sourceforge.net/