Pre-import cdk libraries at the beginning of cluster create and update

Import of cdk libraries at lambda cold start takes 12 to 16 seconds, sometimes causing lambda timeout.
Importing them in a separate thread, started at the beginning of create and update operations
and executed concurrently with stack validation allows to reduce the time to complete the operation.

cli/src/pcluster/models/cluster.py

@@ -66,6 +66,7 @@
 from pcluster.models.s3_bucket import S3Bucket, S3BucketFactory, S3FileFormat, create_s3_presigned_url, parse_bucket_url
 from pcluster.schemas.cluster_schema import ClusterSchema
 from pcluster.templates.cdk_builder import CDKTemplateBuilder
+from pcluster.templates.import_cdk import start as start_cdk_import
 from pcluster.utils import (
     datetime_to_epoch,
     generate_random_name_with_prefix,
@@ -352,6 +353,7 @@ def create(
         raises ClusterActionError: in case of generic error
         raises ConfigValidationError: if configuration is invalid
         """
+        start_cdk_import()
         creation_result = None
         artifact_dir_generated = False
         try:
@@ -887,6 +889,7 @@ def update(
         raises ConfigValidationError: if configuration is invalid
         raises ClusterUpdateError: if update is not allowed
         """
+        start_cdk_import()
         try:
             target_config, changes, ignored_validation_failures = self.validate_update_request(
                 target_source_config, validator_suppressors, validation_failure_level, force

cli/src/pcluster/templates/import_cdk.py

@@ -0,0 +1,27 @@
+import logging
+import time
+from threading import Thread
+
+LOGGER = logging.getLogger(__name__)
+
+
+def import_cdk():
+    """Import cdk libraries."""
+    LOGGER.info("Start importing")
+    begin = time.time()
+    from aws_cdk.core import App  # noqa: F401 pylint: disable=import-outside-toplevel
+
+    from pcluster.templates.cluster_stack import ClusterCdkStack  # noqa: F401 pylint: disable=import-outside-toplevel
+
+    LOGGER.info("Import complete in %i seconds", time.time() - begin)
+
+
+def start():
+    """
+    Import cdk libraries in a separate thread.
+
+    :return: thread importing cdk libraries
+    """
+    thread = Thread(target=import_cdk)
+    thread.start()
+    return thread