How to implement a waiter pattern for an API?

The implementation of the Waiter pattern for APIs involves cyclically checking the resource's state or the operation's completion until the desired condition is met or a specified timeout expires.

Main steps:

1. Define the condition to check: Clearly understand what state of the API or resource is considered "ready" (e.g., request status changed, new record appeared, expected data returned).
2. Choose a checking mechanism: Determine how the status will be checked. This could be a repeated request to the same endpoint, a request to a special status endpoint, or checking the response content.
3. Set wait parameters: Specify the maximum wait time (timeout) and the interval between checks.
4. Implement the loop: Create a loop that performs the status check at the specified interval.
5. Handle exit conditions:
   • Exit when the expected condition is met.
   • Exit when the timeout occurs, accompanied by an exception or error.
6. Handle errors during checking: Provide for handling possible errors during the check inside the loop (e.g., temporary API unavailability).

Example pseudocode:

# wait_for_resource_ready(api_endpoint, expected_condition, timeout_seconds, interval_seconds)

start_time = current_time
while current_time - start_time < timeout_seconds:
    response = make_api_request(api_endpoint) # Make a request to the API

    if check_condition(response, expected_condition): # Check readiness condition
        return True # Condition met

    sleep(interval_seconds) # Wait before the next check
    current_time = current_time # Update current time

raise TimeoutError("Resource not ready within timeout") # Timeout

# check_condition(response, expected_condition):
# Logic to check readiness condition based on API response
# For example, return response['status'] == 'ready'

Python example using the requests library:

import requests
import time

def wait_for_status(url, expected_status, timeout=60, interval=5):
    """
    Waits until the endpoint returns a specific status.
    """
    start_time = time.time()

    while time.time() - start_time < timeout:
        try:
            response = requests.get(url)
            if response.status_code == expected_status:
                print(f"[{time.strftime('%H:%M:%S')}] Status {expected_status} received.")
                return True
        except requests.exceptions.RequestException as e:
            print(f"[{time.strftime('%H:%M:%S')}] Request error: {e}. Retry in {interval} seconds.")

        time.sleep(interval)

    raise TimeoutError(f"Failed to get status {expected_status} within {timeout} seconds.")

# Usage example
# try:
#     wait_for_status("http://api.example.com/status", 200)
# except TimeoutError as e:
#     print(e)

The specific implementation choice depends on the API's specifics and testing requirements. It is important to ensure flexibility in setting timeouts and intervals, as well as informative timeout messages.