Feature: Parameters: add support for manual value refresh (WIP)

[nmoutschen]

Is your feature request related to a problem? Please describe.

Describe the solution you'd like

This would add a method to the parameter provider to force-refresh a parameter value. This could be done through a flag.

Use cases: username/password that have expired and need to be refreshed.

Describe alternatives you've considered

Additional context

[michaelbrewer]

@nmoutschen @heitorlessa would this be via some kind of exception handler that picks up when the credentials failed?

[michaelbrewer]

@nmoutschen a version of this is in this PR : #341

UX might be better for a decorator.

[heitorlessa]

@nmoutschen could you expand on what you meant with this feature request so we can get it added in Wednesday's release?

Do you mean a parameter like what @michaelbrewer implemented on a per get_parameter call, or an explicit method to refresh what's already in the cache e.g. .refresh_parameter(name='/dev/service/parameter')?

If it's the former, @michaelbrewer I'd suggest a more explicit name like ignore_cache as that's what it seems to be happening in the implementation.

[nmoutschen]

Hey @michaelbrewer & @heitorlessa!

I didn't have a specific implementation in mind and thought of 2 possibilities: pass a flag to the get_parameter call, or manually flush a specific cache value. Overall, I think @michaelbrewer's implementation is simpler (same function, with just an additional parameter).

From a name point of view, ignore_cache would imply that the parameter wouldn't be stored in local cache, so I personally prefer force_update. force_fetch is good too.

[heitorlessa]

Thanks @nmoutschen that makes sense. force_fetch in this case makes more sense -- @michaelbrewer could you do a quick refactor on that PR with that and we can merge ahead of Wed's release?

[michaelbrewer]

@heitorlessa i have made the changes just thinking of a good example for the docs, here it was i have so far:

import os
from aws_lambda_powertools.utilities import parameters


class InvalidCredentials(Exception):
    """Raised when client get an invalid credentials error"""


class Client:
    password = None

    def configure(self, password):
        self.password = password

    def read_record(self):
        """Might raise an InvalidCredentials exception """


DB_PASS_PARAM = os.environ["DB_PASS_PARAM"]
db_client = Client()
db_client.configure(parameters.get_parameter(DB_PASS_PARAM))


def refresh_db_password():
    db_client.configure(parameters.get_parameter(DB_PASS_PARAM, force_fetch=True))


def read_record(is_retry=False):
    try:
        return db_client.read_record()
    except InvalidCredentials:
        if not is_retry:  # avoid infinite recursion
            refresh_db_password()  # force parameter refresh
            return read_record(is_retry=True)


def handler(event, context):
    return {"record": read_record()}

[heitorlessa]

This is now available in 1.12.0 :) Thanks @michaelbrewer !

https://github.com/awslabs/aws-lambda-powertools-python/releases/tag/v1.12.0