Crate log4rs [] [src]

log4rs is a highly configurable logging framework modeled after Java's Logback and log4j libraries.


The basic units of configuration are appenders, encoders, filters, and loggers.


An appender takes a log record and logs it somewhere, for example, to a file, the console, or the syslog.


An encoder is responsible for taking a log record, transforming it into the appropriate output format, and writing it out. An appender will normally use an encoder internally.


Filters are associated with appenders and, like the name would suggest, filter log events coming into that appender.


A log event is targeted at a specific logger, which are identified by string names. The logging macros built in to the log crate set the logger of a log event to the one identified by the module containing the invocation location.

Loggers form a hierarchy: logger names are divided into components by "::". One logger is the ancestor of another if the first logger's component list is a prefix of the second logger's component list.

Loggers are associated with a maximum log level. Log events for that logger with a level above the maximum will be ignored. The maximum log level for any logger can be configured manually; if it is not, the level will be inherited from the logger's parent.

Loggers are also associated with a set of appenders. Appenders can be associated directly with a logger. In addition, the appenders of the logger's parent will be associated with the logger unless the logger has its additivity set to false. Log events sent to the logger that are not filtered out by the logger's maximum log level will be sent to all associated appenders.

The "root" logger is the ancestor of all other loggers. Since it has no ancestors, its additivity cannot be configured.


log4rs can be configured programmatically by using the builders in the config module to construct a log4rs Config object, which can be passed to the init_config function.

The more common configuration method, however, is via a separate config file. The init_file function takes the path to a config file as well as a Deserializers object which is responsible for instantiating the various objects specified by the config file. The file module documentation covers the exact configuration syntax, but an example in the YAML format is provided below.

log4rs makes heavy use of Cargo features to enable consumers to pick the functionality they wish to use. File-based configuration requires the file feature, and each file format requires its own feature as well. In addition, each component has its own feature. For example, YAML support requires the yaml_format feature and the console appender requires the console_appender feature.

By default, the all_components, gzip, file, and yaml_format features are enabled.

As a convenience, the all_components feature activates all logger components.


Configuration via a YAML file:

# Scan this file for changes every 30 seconds
refresh_rate: 30 seconds

  # An appender named "stdout" that writes to stdout
    kind: console

  # An appender named "requests" that writes to a file with a custom pattern encoder
    kind: file
    path: "log/requests.log"
      pattern: "{d} - {m}{n}"

# Set the default logging level to "warn" and attach the "stdout" appender to the root
  level: warn
    - stdout

  # Raise the maximum log level for events sent to the "app::backend::db" logger to "info"
    level: info

  # Route log events sent to the "app::requests" logger to the "requests" appender,
  # and *not* the normal appenders installed at the root
    level: info
      - requests
    additive: false
log4rs::init_file("log4rs.yml", Default::default()).unwrap();

Programmatically constructing a configuration:

extern crate log;
extern crate log4rs;

use log::LogLevelFilter;
use log4rs::append::console::ConsoleAppender;
use log4rs::append::file::FileAppender;
use log4rs::encode::pattern::PatternEncoder;
use log4rs::config::{Appender, Config, Logger, Root};

fn main() {
    let stdout = ConsoleAppender::builder().build();

    let requests = FileAppender::builder()
        .encoder(Box::new(PatternEncoder::new("{d} - {m}{n}")))

    let config = Config::builder()
        .appender(Appender::builder().build("stdout", Box::new(stdout)))
        .appender(Appender::builder().build("requests", Box::new(requests)))
        .logger(Logger::builder().build("app::backend::db", LogLevelFilter::Info))
            .build("app::requests", LogLevelFilter::Info))

    let handle = log4rs::init_config(config).unwrap();

    // use handle to change logger configuration at runtime





log4rs configuration




Support for log4rs configuration from files.





A handle to the active logger.



An error initializing the logging framework from a file.



Initializes the global logger as a log4rs logger with the provided config.


Initializes the global logger as a log4rs logger configured via a file.