Cache describe_regions using lru_cache from stdlib (#803)

Caches EC2:DescribeRegion API calls response.

On high-volume deployments, ESF can hit the EC2:DescribeRegions API requests limit, causing throttling errors like the following:

```text
An error occurred (RequestLimitExceeded) when calling the DescribeRegions operation (reached max retries: 4): Request limit exceeded.
```

ESF uses the list of existing regions to parse incoming events from the `cloudwatch-logs` input. Since new AWS region additions do not happen frequently, picking up and caching the list of existing regions at function startup seems adequate.

CHANGELOG.md

@@ -1,3 +1,7 @@
+### v1.17.1 - 2024/09/23
+##### Bug fixes
+* Cache EC2:DescribeRegion API response to avoid throttling and improve performance [803](https://github.com/elastic/elastic-serverless-forwarder/pull/803).
+
 ### v1.17.0 - 2024/07/10
 ##### Features
 * Add dead letter index for ES outputs [733](https://github.com/elastic/elastic-serverless-forwarder/pull/733).

handlers/aws/utils.py

@@ -2,6 +2,7 @@
 # or more contributor license agreements. Licensed under the Elastic License 2.0;
 # you may not use this file except in compliance with the Elastic License 2.0.
 import os
+from functools import lru_cache
 from typing import Any, Callable, Optional
 
 import boto3
@@ -385,6 +386,16 @@ def get_account_id_from_arn(lambda_arn: str) -> str:
     return arn_components[4]
 
 
+@lru_cache()
+def describe_regions(all_regions: bool = True) -> Any:
+    """
+    Fetches all regions from AWS and returns the response.
+
+    :return: The response from the describe_regions method
+    """
+    return get_ec2_client().describe_regions(AllRegions=all_regions)
+
+
 def get_input_from_log_group_subscription_data(
     config: Config, account_id: str, log_group_name: str, log_stream_name: str
 ) -> tuple[str, Optional[Input]]:
@@ -395,12 +406,9 @@ def get_input_from_log_group_subscription_data(
     In order to not hardcode the list of regions we rely on ec2 DescribeRegions - as much weird as it is - that I found
     no information about having any kind of throttling. We add IAM permissions for it in deployment.
     """
-    all_regions = get_ec2_client().describe_regions(AllRegions=True)
+    all_regions = describe_regions(all_regions=True)
     assert "Regions" in all_regions
     for region_data in all_regions["Regions"]:
-
-        # arn:aws:logs:region:account-id:log-group:log_group_name:*
-
         region = region_data["RegionName"]
 
         aws_or_gov = "aws"

share/version.py

@@ -2,4 +2,4 @@
 # or more contributor license agreements. Licensed under the Elastic License 2.0;
 # you may not use this file except in compliance with the Elastic License 2.0.
 
-version = "1.17.0"
+version = "1.17.1"

tests/handlers/aws/test_utils.py

@@ -9,6 +9,7 @@
 from typing import Any
 from unittest import TestCase
 
+import mock
 import pytest
 
 from handlers.aws.utils import (
@@ -66,6 +67,20 @@ def _get_random_digit_string_of_size(size: int) -> str:
     return "".join(random.choices(string.digits, k=size))
 
 
+def _describe_regions(AllRegions: bool) -> dict[str, Any]:
+    return {
+        "Regions": [
+            {
+                "RegionName": "us-west-2",
+            },
+        ]
+    }
+
+
+_ec2_client_mock = mock.MagicMock()
+_ec2_client_mock.describe_regions = _describe_regions
+
+
 @pytest.mark.unit
 class TestGetTriggerTypeAndConfigSource(TestCase):
     def test_get_trigger_type_and_config_source(self) -> None:
@@ -408,3 +423,48 @@ def test_cloudwatch_id_less_than_512bytes(self) -> None:
 
         generated_id = cloudwatch_logs_object_id(relevant_fields_for_id)
         assert _utf8len(generated_id) <= MAX_ES_ID_SIZ_BYTES
+
+
+@pytest.mark.unit
+class TestDescribeRegions(TestCase):
+
+    @mock.patch("handlers.aws.utils.get_ec2_client", lambda: _ec2_client_mock)
+    def test_cache_miss(self) -> None:
+        from handlers.aws.utils import describe_regions
+
+        # Reset the cache info before running the test
+        describe_regions.cache_clear()
+
+        # First call should be a cache miss
+        response = describe_regions(all_regions=False)
+        assert response["Regions"] is not None
+        assert len(response["Regions"]) == 1
+        assert response["Regions"][0]["RegionName"] == "us-west-2"
+
+        cache_info = describe_regions.cache_info()
+
+        assert cache_info.hits == 0
+        assert cache_info.misses == 1
+        assert cache_info.currsize == 1
+
+    @mock.patch("handlers.aws.utils.get_ec2_client", lambda: _ec2_client_mock)
+    def test_cache_hits(self) -> None:
+        from handlers.aws.utils import describe_regions
+
+        # Reset the cache info before running the test
+        describe_regions.cache_clear()
+
+        # First call should be a cache miss and populate the cache
+        # Second and third calls should be cache hits.
+        response = describe_regions(all_regions=False)
+        response = describe_regions(all_regions=False)
+        response = describe_regions(all_regions=False)
+        assert response["Regions"] is not None
+        assert len(response["Regions"]) == 1
+        assert response["Regions"][0]["RegionName"] == "us-west-2"
+
+        cache_info = describe_regions.cache_info()
+
+        assert cache_info.hits == 2
+        assert cache_info.misses == 1
+        assert cache_info.currsize == 1