Describes the level of verbosity of a span or event.

Comparing Levels

Level implements the PartialOrd and Ord traits, allowing two Levels to be compared to determine which is considered more or less verbose. Levels which are more verbose are considered "greater than" levels which are less verbose, with Level::ERROR considered the lowest, and Level::TRACE considered the highest.

For example:

use tracing_core::Level;

assert!(Level::TRACE > Level::DEBUG);
assert!(Level::ERROR < Level::WARN);
assert!(Level::INFO <= Level::DEBUG);
assert_eq!(Level::TRACE, Level::TRACE);

Filtering

Levels are typically used to implement filtering that determines which spans and events are enabled. Depending on the use case, more or less verbose diagnostics may be desired. For example, when running in development, DEBUG-level traces may be enabled by default. When running in production, only INFO-level and lower traces might be enabled. Libraries may include very verbose diagnostics at the DEBUG and/or TRACE levels. Applications using those libraries typically chose to ignore those traces. However, when debugging an issue involving said libraries, it may be useful to temporarily enable the more verbose traces.

The LevelFilter type is provided to enable filtering traces by verbosity. Levels can be compared against LevelFilters, and LevelFilter has a variant for each Level, which compares analogously to that level. In addition, LevelFilter adds a LevelFilter::OFF variant, which is considered "less verbose" than every other Level. This is intended to allow filters to completely disable tracing in a particular context.

For example:

use tracing_core::{Level, LevelFilter};

assert!(LevelFilter::OFF < Level::TRACE);
assert!(LevelFilter::TRACE > Level::DEBUG);
assert!(LevelFilter::ERROR < Level::WARN);
assert!(LevelFilter::INFO <= Level::DEBUG);
assert!(LevelFilter::INFO >= Level::INFO);

Examples

Below is a simple example of how a Subscriber could implement filtering through a LevelFilter. When a span or event is recorded, the Subscriber::enabled method compares the span or event's Level against the configured LevelFilter. The optional Subscriber::max_level_hint method can also be implemented to allow spans and events above a maximum verbosity level to be skipped more efficiently, often improving performance in short-lived programs.

use tracing_core::{span, Event, Level, LevelFilter, Subscriber, Metadata};
# use tracing_core::span::{Id, Record, Current};

#[derive(Debug)]
pub struct MySubscriber {
    /// The most verbose level that this subscriber will enable.
    max_level: LevelFilter,

    // ...
}

impl MySubscriber {
    /// Returns a new `MySubscriber` which will record spans and events up to
    /// `max_level`.
    pub fn with_max_level(max_level: LevelFilter) -> Self {
        Self {
            max_level,
            // ...
        }
    }
}
impl Subscriber for MySubscriber {
    fn enabled(&self, meta: &Metadata<'_>) -> bool {
        // A span or event is enabled if it is at or below the configured
        // maximum level.
        meta.level() <= &self.max_level
    }

    // This optional method returns the most verbose level that this
    // subscriber will enable. Although implementing this method is not
    // *required*, it permits additional optimizations when it is provided,
    // allowing spans and events above the max level to be skipped
    // more efficiently.
    fn max_level_hint(&self) -> Option<LevelFilter> {
        Some(self.max_level)
    }

    // Implement the rest of the subscriber...
    fn new_span(&self, span: &span::Attributes<'_>) -> span::Id {
        // ...
        # drop(span); Id::from_u64(1)
    }

    fn event(&self, event: &Event<'_>) {
        // ...
        # drop(event);
    }

    // ...
    # fn enter(&self, _: &Id) {}
    # fn exit(&self, _: &Id) {}
    # fn record(&self, _: &Id, _: &Record<'_>) {}
    # fn record_follows_from(&self, _: &Id, _: &Id) {}
}

It is worth noting that the tracing-subscriber crate provides additional APIs for performing more sophisticated filtering, such as enabling different levels based on which module or crate a span or event is recorded in.