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Level in tracing - Rust

pub struct Level();

Describes the level of verbosity of a span or event.

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);

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);

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};

#[derive(Debug)]
pub struct MySubscriber {
    max_level: LevelFilter,

    }

impl MySubscriber {
    pub fn with_max_level(max_level: LevelFilter) -> Self {
        Self {
            max_level,
            }
    }
}
impl Subscriber for MySubscriber {
    fn enabled(&self, meta: &Metadata<'_>) -> bool {
        meta.level() <= &self.max_level
    }

    fn max_level_hint(&self) -> Option<LevelFilter> {
        Some(self.max_level)
    }

    fn new_span(&self, span: &span::Attributes<'_>) -> span::Id {
        }

    fn event(&self, event: &Event<'_>) {
        }

    }

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.

The “error” level.

Designates very serious errors.

The “warn” level.

Designates hazardous situations.

The “info” level.

Designates useful information.

The “debug” level.

Designates lower priority information.

The “trace” level.

Designates very low priority, often extremely verbose, information.

Returns the string representation of the Level.

This returns the same string as the fmt::Display implementation.

Converts to this type from the input type.

The associated error which can be returned from parsing.

Parses a string

to return a value of this type.

Tests for self and other values to be equal, and is used by ==.

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

Tests for self and other values to be equal, and is used by ==.

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

Tests for self and other values to be equal, and is used by ==.

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

This method returns an ordering between

and

values if one exists.

Tests less than (for

and

) and is used by the

operator.

Tests less than or equal to (for

and

) and is used by the

operator.

Tests greater than (for

and

) and is used by the

operator.

Tests greater than or equal to (for

and

) and is used by the

operator.

This method returns an ordering between

and

values if one exists.

Tests less than (for

and

) and is used by the

operator.

Tests less than or equal to (for

and

) and is used by the

operator.

Tests greater than (for

and

) and is used by the

operator.

Tests greater than or equal to (for

and

) and is used by the

operator.

This method returns an ordering between

and

values if one exists.

Tests less than (for

and

) and is used by the

operator.

Tests less than or equal to (for

and

) and is used by the

operator.

Tests greater than (for

and

) and is used by the

operator.

Tests greater than or equal to (for

and

) and is used by the

operator.

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