Create an example of logging with file rotation (estk#295)

* Create an example of logging with file rotation

* Fix filename extension

* Add example to Cargo.toml

* Move configuration values out of main to top

* Correct Merge Conflict Correctly

Co-authored-by: Bryan Conn <[email protected]>

* Use rustfmt on nightly

---------

Co-authored-by: Bryan Conn <[email protected]>

Cargo.toml

@@ -97,6 +97,10 @@ required-features = ["console_appender", "file_appender", "rolling_file_appender
 name = "compile_time_config"
 required-features = ["yaml_format", "config_parsing"]
 
+[[example]]
+name = "log_to_file_with_rolling"
+required-features = ["file_appender", "rolling_file_appender", "size_trigger"]
+
 [[example]]
 name = "multi_logger_config"
 required-features = ["yaml_format", "config_parsing"]

examples/log_to_file_with_rolling.rs

@@ -0,0 +1,108 @@
+//! Example showing how to use logging with a rolling trigger based on size
+//!
+//! NB: The size used in the example is intentionally small so multiple file
+//! will be created in the 2 seconds that the example is set to run and is not
+//! intended for practical for usage
+
+/// This is the size at which a new file should be created. For the demo it is
+/// set to 2KB which is very small and only for demo purposes
+const TRIGGER_FILE_SIZE: u64 = 2 * 1024;
+
+/// Delay between log messages for demo purposes
+const TIME_BETWEEN_LOG_MESSAGES: Duration = Duration::from_millis(10);
+
+/// Number of archive log files to keep
+const LOG_FILE_COUNT: u32 = 3;
+
+/// Time demo is set to run for (Set to be long enough for multiple files to be created)
+const RUN_TIME: Duration = Duration::from_secs(2);
+
+/// Location where logs will be written to
+const FILE_PATH: &str = "/tmp/foo.log";
+
+/// Location where log archives will be moved to
+/// For Pattern info See:
+///     https://docs.rs/log4rs/*/log4rs/append/rolling_file/policy/compound/roll/fixed_window/struct.FixedWindowRollerBuilder.html#method.build
+const ARCHIVE_PATTERN: &str = "/tmp/archive/foo.{}.log";
+
+use std::{
+    thread::sleep,
+    time::{Duration, Instant},
+};
+
+use log::{debug, error, info, trace, warn, LevelFilter, SetLoggerError};
+use log4rs::{
+    append::{
+        console::{ConsoleAppender, Target},
+        rolling_file::policy::compound::{
+            roll::fixed_window::FixedWindowRoller, trigger::size::SizeTrigger, CompoundPolicy,
+        },
+    },
+    config::{Appender, Config, Root},
+    encode::pattern::PatternEncoder,
+    filter::threshold::ThresholdFilter,
+};
+
+fn main() -> Result<(), SetLoggerError> {
+    let level = log::LevelFilter::Info;
+
+    // Build a stderr logger.
+    let stderr = ConsoleAppender::builder().target(Target::Stderr).build();
+
+    // Create a policy to use with the file logging
+    let trigger = SizeTrigger::new(TRIGGER_FILE_SIZE);
+    let roller = FixedWindowRoller::builder()
+        .base(0) // Default Value (line not needed unless you want to change from 0 (only here for demo purposes)
+        .build(ARCHIVE_PATTERN, LOG_FILE_COUNT) // Roll based on pattern and max 3 archive files
+        .unwrap();
+    let policy = CompoundPolicy::new(Box::new(trigger), Box::new(roller));
+
+    // Logging to log file. (with rolling)
+    let logfile = log4rs::append::rolling_file::RollingFileAppender::builder()
+        // Pattern: https://docs.rs/log4rs/*/log4rs/encode/pattern/index.html
+        .encoder(Box::new(PatternEncoder::new("{l} - {m}\n")))
+        .build(FILE_PATH, Box::new(policy))
+        .unwrap();
+
+    // Log Trace level output to file where trace is the default level
+    // and the programmatically specified level to stderr.
+    let config = Config::builder()
+        .appender(Appender::builder().build("logfile", Box::new(logfile)))
+        .appender(
+            Appender::builder()
+                .filter(Box::new(ThresholdFilter::new(level)))
+                .build("stderr", Box::new(stderr)),
+        )
+        .build(
+            Root::builder()
+                .appender("logfile")
+                .appender("stderr")
+                .build(LevelFilter::Trace),
+        )
+        .unwrap();
+
+    // Use this to change log levels at runtime.
+    // This means you can change the default log level to trace
+    // if you are trying to debug an issue and need more logs on then turn it off
+    // once you are done.
+    let _handle = log4rs::init_config(config)?;
+
+    error!("Goes to stderr and file");
+    warn!("Goes to stderr and file");
+    info!("Goes to stderr and file");
+    debug!("Goes to file only");
+    trace!("Goes to file only");
+
+    // Generate some log messages to trigger rolling
+    let instant = Instant::now();
+    while instant.elapsed() < RUN_TIME {
+        info!("Running for {:?}", instant.elapsed());
+        sleep(TIME_BETWEEN_LOG_MESSAGES);
+    }
+    info!(
+        "See '{}' for log and '{}' for archived logs",
+        FILE_PATH, ARCHIVE_PATTERN
+    );
+
+    Ok(())
+}