Crate retry

Crate retry provides utilities for retrying operations that can fail.

Usage

Retry an operation using the retry function. retry accepts an iterator over Durations and a closure that returns a Result (or OperationResult; see below). The iterator is used to determine how long to wait after each unsuccessful try and how many times to try before giving up and returning Result::Err. The closure determines either the final successful value, or an error value, which can either be returned immediately or used to indicate that the operation should be retried.

Any type that implements Iterator<Item = Duration> can be used to determine retry behavior, though a few useful implementations are provided in the delay module, including a fixed delay and exponential back-off.

let mut collection = vec![1, 2, 3].into_iter();

let result = retry(Fixed::from_millis(100), || {
    match collection.next() {
        Some(n) if n == 3 => Ok("n is 3!"),
        Some(_) => Err("n must be 3!"),
        None => Err("n was never 3!"),
    }
});

assert!(result.is_ok());

The Iterator API can be used to limit or modify the delay strategy. For example, to limit the number of retries to 1:

let mut collection = vec![1, 2, 3].into_iter();

let result = retry(Fixed::from_millis(100).take(1), || {
    match collection.next() {
        Some(n) if n == 3 => Ok("n is 3!"),
        Some(_) => Err("n must be 3!"),
        None => Err("n was never 3!"),
    }
});

assert!(result.is_err());

To apply random jitter to any delay strategy, the jitter function can be used in combination with the Iterator API:

let mut collection = vec![1, 2, 3].into_iter();

let result = retry(Exponential::from_millis(10).map(jitter).take(3), || {
    match collection.next() {
        Some(n) if n == 3 => Ok("n is 3!"),
        Some(_) => Err("n must be 3!"),
        None => Err("n was never 3!"),
    }
});

assert!(result.is_ok());

To deal with fatal errors, return retry::OperationResult, which is like std's Result, but with a third case to distinguish between errors that should cause a retry and errors that should immediately return, halting retry behavior. (Internally, OperationResult is always used, and closures passed to retry that return plain Result are converted into OperationResult.)

use retry::OperationResult;
let mut collection = vec![1, 2].into_iter();
let value = retry(Fixed::from_millis(1), || {
    match collection.next() {
        Some(n) if n == 2 => OperationResult::Ok(n),
        Some(_) => OperationResult::Retry("not 2"),
        None => OperationResult::Err("not found"),
    }
}).unwrap();

assert_eq!(value, 2);

If your operation needs to know how many times it's been tried, use the retry_with_index function. This works the same as retry, but passes the number of the current try to the closure as an argument.

let mut collection = vec![1, 2, 3, 4, 5].into_iter();

let result = retry_with_index(Fixed::from_millis(100), |current_try| {
    if current_try > 3 {
        return OperationResult::Err("did not succeed within 3 tries");
    }

    match collection.next() {
        Some(n) if n == 5 => OperationResult::Ok("n is 5!"),
        Some(_) => OperationResult::Retry("n must be 5!"),
        None => OperationResult::Retry("n was never 5!"),
    }
});

assert!(result.is_err());

Features

• random: offer some random delay utilities (on by default)

Modules

delay

Different types of delay for retryable operations.

Enums

Error	An error with a retryable operation.
OperationResult	A result that represents either success, retryable failure, or immediately-returning failure.

Functions

retry	Retry the given operation synchronously until it succeeds, or until the given Duration iterator ends.
retry_with_index	Retry the given operation synchronously until it succeeds, or until the given Duration iterator ends, with each iteration of the operation receiving the number of the attempt as an argument.