Struct Context

pub struct Context { /* private fields */ }

An execution-scoped collection of values.

A Context is a propagation mechanism which carries execution-scoped values across API boundaries and between logically associated execution units. Cross-cutting concerns access their data in-process using the same shared context object.

Contexts are immutable, and their write operations result in the creation of a new context containing the original values and the new specified values.

Context state

Concerns can create and retrieve their local state in the current execution state represented by a context through the get and with_value methods. It is recommended to use application-specific types when storing new context values to avoid unintentionally overwriting existing state.

Managing the current context

Contexts can be associated with the caller’s current execution unit on a given thread via the attach method, and previous contexts can be restored by dropping the returned ContextGuard. Context can be nested, and will restore their parent outer context when detached on drop. To access the values of the context, a snapshot can be created via the Context::current method.

Examples

use opentelemetry::Context;

// Application-specific `a` and `b` values
#[derive(Debug, PartialEq)]
struct ValueA(&'static str);
#[derive(Debug, PartialEq)]
struct ValueB(u64);

let _outer_guard = Context::new().with_value(ValueA("a")).attach();

// Only value a has been set
let current = Context::current();
assert_eq!(current.get::<ValueA>(), Some(&ValueA("a")));
assert_eq!(current.get::<ValueB>(), None);

{
    let _inner_guard = Context::current_with_value(ValueB(42)).attach();
    // Both values are set in inner context
    let current = Context::current();
    assert_eq!(current.get::<ValueA>(), Some(&ValueA("a")));
    assert_eq!(current.get::<ValueB>(), Some(&ValueB(42)));
}

// Resets to only the `a` value when inner guard is dropped
let current = Context::current();
assert_eq!(current.get::<ValueA>(), Some(&ValueA("a")));
assert_eq!(current.get::<ValueB>(), None);

Implementations

impl Context

pub fn new() -> Self

Creates an empty Context.

The context is initially created with a capacity of 0, so it will not allocate. Use with_value to create a new context that has entries.

pub fn current() -> Self

Returns an immutable snapshot of the current thread’s context.

Examples

use opentelemetry::Context;

#[derive(Debug, PartialEq)]
struct ValueA(&'static str);

fn do_work() {
    assert_eq!(Context::current().get(), Some(&ValueA("a")));
}

let _guard = Context::new().with_value(ValueA("a")).attach();
do_work()

pub fn map_current<T>(f: impl FnOnce(&Context) -> T) -> T

Applies a function to the current context returning its value.

This can be used to build higher performing algebraic expressions for optionally creating a new context without the overhead of cloning the current one and dropping it.

Note: This function will panic if you attempt to attach another context while the current one is still borrowed.

pub fn current_with_value<T: 'static + Send + Sync>(value: T) -> Self

Returns a clone of the current thread’s context with the given value.

This is a more efficient form of Context::current().with_value(value) as it avoids the intermediate context clone.

Examples

use opentelemetry::Context;

// Given some value types defined in your application
#[derive(Debug, PartialEq)]
struct ValueA(&'static str);
#[derive(Debug, PartialEq)]
struct ValueB(u64);

// You can create and attach context with the first value set to "a"
let _guard = Context::new().with_value(ValueA("a")).attach();

// And create another context based on the fist with a new value
let all_current_and_b = Context::current_with_value(ValueB(42));

// The second context now contains all the current values and the addition
assert_eq!(all_current_and_b.get::<ValueA>(), Some(&ValueA("a")));
assert_eq!(all_current_and_b.get::<ValueB>(), Some(&ValueB(42)));

pub fn get<T: 'static>(&self) -> Option<&T>

Returns a reference to the entry for the corresponding value type.

Examples

use opentelemetry::Context;

// Given some value types defined in your application
#[derive(Debug, PartialEq)]
struct ValueA(&'static str);
#[derive(Debug, PartialEq)]
struct MyUser();

let cx = Context::new().with_value(ValueA("a"));

// Values can be queried by type
assert_eq!(cx.get::<ValueA>(), Some(&ValueA("a")));

// And return none if not yet set
assert_eq!(cx.get::<MyUser>(), None);

pub fn with_value<T: 'static + Send + Sync>(&self, value: T) -> Self

Returns a copy of the context with the new value included.

Examples

use opentelemetry::Context;

// Given some value types defined in your application
#[derive(Debug, PartialEq)]
struct ValueA(&'static str);
#[derive(Debug, PartialEq)]
struct ValueB(u64);

// You can create a context with the first value set to "a"
let cx_with_a = Context::new().with_value(ValueA("a"));

// And create another context based on the fist with a new value
let cx_with_a_and_b = cx_with_a.with_value(ValueB(42));

// The first context is still available and unmodified
assert_eq!(cx_with_a.get::<ValueA>(), Some(&ValueA("a")));
assert_eq!(cx_with_a.get::<ValueB>(), None);

// The second context now contains both values
assert_eq!(cx_with_a_and_b.get::<ValueA>(), Some(&ValueA("a")));
assert_eq!(cx_with_a_and_b.get::<ValueB>(), Some(&ValueB(42)));

pub fn attach(self) -> ContextGuard

Replaces the current context on this thread with this context.

Dropping the returned ContextGuard will reset the current context to the previous value.

Examples

use opentelemetry::Context;

#[derive(Debug, PartialEq)]
struct ValueA(&'static str);

let my_cx = Context::new().with_value(ValueA("a"));

// Set the current thread context
let cx_guard = my_cx.attach();
assert_eq!(Context::current().get::<ValueA>(), Some(&ValueA("a")));

// Drop the guard to restore the previous context
drop(cx_guard);
assert_eq!(Context::current().get::<ValueA>(), None);

Guards do not need to be explicitly dropped:

use opentelemetry::Context;

#[derive(Debug, PartialEq)]
struct ValueA(&'static str);

fn my_function() -> String {
    // attach a context the duration of this function.
    let my_cx = Context::new().with_value(ValueA("a"));
    // NOTE: a variable name after the underscore is **required** or rust
    // will drop the guard, restoring the previous context _immediately_.
    let _guard = my_cx.attach();

    // anything happening in functions we call can still access my_cx...
    my_other_function();

    // returning from the function drops the guard, exiting the span.
    return "Hello world".to_owned();
}

fn my_other_function() {
    // ...
}

Sub-scopes may be created to limit the duration for which the span is entered:

use opentelemetry::Context;

#[derive(Debug, PartialEq)]
struct ValueA(&'static str);

let my_cx = Context::new().with_value(ValueA("a"));

{
    let _guard = my_cx.attach();

    // the current context can access variables in
    assert_eq!(Context::current().get::<ValueA>(), Some(&ValueA("a")));

    // exiting the scope drops the guard, detaching the context.
}

// this is back in the default empty context
assert_eq!(Context::current().get::<ValueA>(), None);

Trait Implementations

impl BaggageExt for Context

fn with_baggage<T: IntoIterator<Item = I>, I: Into<KeyValueMetadata>>( &self, baggage: T, ) -> Self

Returns a clone of the given context with the included name/value pairs.

fn current_with_baggage<T: IntoIterator<Item = I>, I: Into<KeyValueMetadata>>( kvs: T, ) -> Self

Returns a clone of the current context with the included name/value pairs.

fn with_cleared_baggage(&self) -> Self

Returns a clone of the given context with no baggage.

fn baggage(&self) -> &Baggage

Returns a reference to this context’s baggage, or the default empty baggage if none has been set.

impl Clone for Context

fn clone(&self) -> Context

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Context

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for Context

fn default() -> Context

Returns the “default value” for a type.

impl TraceContextExt for Context

fn current_with_span<T: Span + Send + Sync + 'static>(span: T) -> Self

Returns a clone of the current context with the included Span.

fn with_span<T: Span + Send + Sync + 'static>(&self, span: T) -> Self

Returns a clone of this context with the included span.

fn span(&self) -> SpanRef<'_>

Returns a reference to this context’s span, or the default no-op span if none has been set.

fn has_active_span(&self) -> bool

Returns whether or not an active span has been set.

fn with_remote_span_context(&self, span_context: SpanContext) -> Self

Returns a copy of this context with the span context included.