Motivation

Disclaimer: targeting master for now to validate all tests, and we will set the STRICT_SERVICE_LOADING variable to default to True in the release branch afterwards. This variable won't be advertised and is used to decouple eager service loading from "strict" service loading, and might be removed after the release.

When some services are problematic, or we want to use LocalStack for only one or more services, we should be able to use the SERVICES variable coupled with STRICT_SERVICE_LOADING=1 in order to be able to only enable the listed services.

The EAGER_SERVICE_LOADING functionality is untouched for now, and is applied on top: meaning if you don't have STRICT_SERVICE_LOADING set then you'll have all available services, and will only eager load the services in SERVICES.

If STRICT_SERVICE_LOADING=1 is set, then the services in SERVICES will be the only ones available. You can additionally eager load them by specifying EAGER_SERVICE_LOADING =1.

Example: you only want S3 and SQS loaded:
SERVICES=sqs,s3 + STRICT_SERVICE_LOADING=1

You want kinesis eager loaded at startup, but still lazy load other services:
SERVICES=kinesis + EAGER_SERVICE_LOADING =1

You want to only use kinesis, dynamodb, s3 and sqs, but with eager loading:
SERVICES=kinesis,dynamodb,sqs,s3 + STRICT_SERVICE_LOADING=1 + EAGER_SERVICE_LOADING =1

We should also do a new pass on the API_DEPENDENCIES constant to validate the inter-service dependencies, and be sure we will load the necessary services.

\cc @steffyP

Changes

Change the logic for get_enabled_apis, which is now linked to STRICT_SERVICE_LOADING.
Create new logic for eager loading, linked to the get_preloaded_services method and EAGER_SERVICE_LOADING .
Add a test to validate the new behaviour of disabling services.