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pub struct Context { /* private fields */ }
Expand description

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_api::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

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.

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

Examples
use opentelemetry_api::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()

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_api::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)));

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

Examples
use opentelemetry_api::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);

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

Examples
use opentelemetry_api::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)));

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_api::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_api::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_api::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

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

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

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

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

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

Returns the “default value” for a type. Read more

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

Returns a clone of this context with the included span. Read more

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

Returns whether or not an active span has been set. Read more

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

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Attaches the provided Context to this type, returning a WithContext wrapper. Read more

Attaches the current Context to this type, returning a WithContext wrapper. Read more

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The resulting type after obtaining ownership.

Creates owned data from borrowed data, usually by cloning. Read more

Uses borrowed data to replace owned data, usually by cloning. Read more

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.