feat: fetch RO Crates from MinIO store

- Addresses #31, #3
- RO-Crate fetched from MinIO based on the provided crate ID

app/utils/minio_utils.py

@@ -0,0 +1,70 @@
+"""Utility methods for interacting with MinIO."""
+
+# Author: Alexander Hambley
+# License: MIT
+# Copyright (c) 2025 eScience Lab, The University of Manchester
+
+from minio import Minio, S3Error
+import os
+import tempfile
+import logging
+
+from dotenv import load_dotenv
+
+logger = logging.getLogger(__name__)
+
+
+def fetch_ro_crate_from_minio(crate_id: str) -> str:
+    """
+    Fetches an RO-Crate from MinIO based on the provided crate ID.
+    Downloads the crate as a file and returns the local file path.
+
+    :param crate_id: The ID of the RO-Crate to fetch from MinIO.
+    :return: The local file path where the RO-Crate is saved.
+    :raises S3Error: If an error occurs during the MinIO operation.
+    :raises ValueError: If the required environment variables are not set.
+    :raises Exception: If an unexpected error occurs during the operation.
+    """
+    load_dotenv()
+
+    try:
+        minio_client = Minio(
+            endpoint=os.environ.get("MINIO_ENDPOINT"),
+            access_key=os.environ.get("MINIO_ROOT_USER"),
+            secret_key=os.environ.get("MINIO_ROOT_PASSWORD"),
+            secure=False,
+        )
+        logging.info("MinIO client initialised successfully.")
+
+        bucket_name = os.environ.get("MINIO_BUCKET_NAME")
+
+        if not bucket_name:
+            raise ValueError(
+                "RO Crate MINIO_BUCKET_NAME is not set in the environment variables."
+            )
+
+        object_name = f"{crate_id}.zip"
+
+        # Prepare temporary file path to store RO Crate for validation:
+        temp_dir = tempfile.mkdtemp()
+        file_path = os.path.join(temp_dir, object_name)
+
+        logging.info(
+            f"Fetching RO-Crate {object_name} from MinIO bucket {bucket_name}. File path: {file_path}"
+        )
+        minio_client.fget_object(bucket_name, object_name, file_path)
+        logging.info(
+            f"RO-Crate {object_name} fetched successfully and saved to {file_path}."
+        )
+
+        return file_path
+
+    except S3Error as s3_error:
+        logging.error(f"MinIO S3 Error: {s3_error}")
+        raise
+    except ValueError as value_error:
+        logging.error(f"Configuration Error: {value_error}")
+        raise
+    except Exception as e:
+        logging.error(f"Unexpected error fetching RO-Crate from MinIO: {e}")
+        raise