Retry Strategies

Message retry strategies allow you to specify how much of a delay should occur before retrying a message.
Delays between retries can be useful when messages fail handling due to race conditions, service unavailability, or concurrency reasons.
By default, @node-ts/bus uses a DefaultRetryStrategy that exponentially increases the delay between each retry attempt. Additionally, it will introduce a random variance of 10% for each delay to help unblock messages that are failing when processed at the same time. 
Additional strategies can be implemented to suit your application.
Choosing a Retry Strategy
A retry strategy can be provided to the bus configuration on initialization by using .withRetryStrategy()
For example:
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const bus = await Bus.configure()
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  .withRetryStrategy(DefaultRetryStrategy)
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  .initialize()
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Custom retry strategies
A custom retry strategy can be provided by implementing the RetryStrategy from @node-ts/bus-core.` 
retry-strategy.ts
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export type Milliseconds = number
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​
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/**
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 * Defines how a message retry strategy is to be implemented that calculates the delay between subsequent
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 * retries of a message.
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 */
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export interface RetryStrategy {
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  /**
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   * Calculate the delay between retrying a failed message
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   * @param currentAttempt How many attempts at handling the message have failed
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   * @returns The number of milliseconds to delay retrying a failed message attempt
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   */
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  calculateRetryDelay (currentAttempt: number): Milliseconds
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}
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​
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An example of a retry strategy is as follows
default-retry-strategy.ts
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import { Milliseconds, RetryStrategy } from './retry-strategy'
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​
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const MAX_DELAY_MS = 2.5 * 60 * 60 * 1000 // 2.5 hours
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const JITTER_PERCENT = 0.1
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​
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/**
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 * A default message retry strategy that exponentially increases the delay between retries
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 * from 5ms to 2.5 hrs for the first 10 attempts. Each retry delay includes a jitter of
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 * up to 10% to avoid deadlock-related errors from continually blocking.
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 */
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export class DefaultRetryStrategy implements RetryStrategy {
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  calculateRetryDelay (currentAttempt: number): Milliseconds {
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    const numberOfFailures = currentAttempt + 1
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    const constantDelay: Milliseconds = Math.pow(5, numberOfFailures)
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    const jitterAmount = Math.random() * JITTER_PERCENT * constantDelay
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    const jitterDirection = Math.random() > 0.5 ? 1 : -1
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    const jitter = jitterAmount * jitterDirection
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    const delay = Math.round(constantDelay + jitter)
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    return Math.min(delay, MAX_DELAY_MS)
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  }
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}
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​
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​

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Choosing a Retry Strategy

Custom retry strategies