Refreshing External Tables Automatically for Google Cloud Storage

This topic provides instructions for triggering external table metadata refreshes using Google Cloud Pub/Sub messages for Google Cloud Storage (GCS) events.

Note

To complete the steps described in this topic, you must use a role that has the CREATE STAGE and CREATE EXTERNAL TABLE privileges on a schema.

In addition, you must have administrative access to Google Cloud Platform (GCP). If you are not a GCP administrator, ask your GCP administrator to complete the Prerequisites steps.

In this Topic:

Cloud Platform Support

Triggering automated external metadata refreshes using GCS Pub/Sub event messages is supported by Snowflake accounts hosted on the following cloud platforms:

  • Amazon Web Services (AWS)

  • Google Cloud Platform

The instructions for configuring this support are identical for accounts on either cloud hosting platform.

Configuring Secure Access to Cloud Storage

Note

If you have already configured secure access to the GCS bucket that stores your data files, you can skip this section.

This section describes how to configure a Snowflake storage integration object to delegate authentication responsibility for cloud storage to a Snowflake identity and access management (IAM) entity.

This section describes how to use storage integrations to allow Snowflake to read data from and write to a Google Cloud Storage bucket referenced in an external (i.e. Cloud Storage) stage. Integrations are named, first-class Snowflake objects that avoid the need for passing explicit cloud provider credentials such as secret keys or access tokens; instead, integration objects reference a Cloud Storage service account. An administrator in your organization grants the service account permissions in the Cloud Storage account.

Administrators can also restrict users to a specific set of Cloud Storage buckets (and optional paths) accessed by external stages that use the integration.

Note

Completing the instructions in this section requires access to your Cloud Storage project as a project editor. If you are not a project editor, ask your Cloud Storage administrator to perform these tasks.

The following diagram shows the integration flow for a Cloud Storage stage:

Google Cloud Storage Stage Integration Flow
  1. An external (i.e. Cloud Storage) stage references a storage integration object in its definition.

  2. Snowflake automatically associates the storage integration with a Cloud Storage service account created for your account. Snowflake creates a single service account that is referenced by all GCS storage integrations in your Snowflake account.

  3. A project editor for your Cloud Storage project grants permissions to the service account to access the bucket referenced in the stage definition. Note that many external stage objects can reference different buckets and paths and use the same integration for authentication.

When a user loads or unloads data from or to a stage, Snowflake verifies the permissions granted to the service account on the bucket before allowing or denying access.

In this Section:

Step 1: Create a Cloud Storage Integration in Snowflake

Create an integration using the CREATE STORAGE INTEGRATION command. An integration is a Snowflake object that delegates authentication responsibility for external cloud storage to a Snowflake-generated entity (i.e. a Cloud Storage service account). For accessing Cloud Storage buckets, Snowflake creates a service account that can be granted permissions to access the bucket(s) that store your data files.

A single storage integration can support multiple external (i.e. GCS) stages. The URL in the stage definition must align with the GCS buckets (and optional paths) specified for the STORAGE_ALLOWED_LOCATIONS parameter.

Note

Only account administrators (users with the ACCOUNTADMIN role) or a role with the global CREATE INTEGRATION privilege can execute this SQL command.

CREATE STORAGE INTEGRATION <integration_name>
  TYPE = EXTERNAL_STAGE
  STORAGE_PROVIDER = GCS
  ENABLED = TRUE
  STORAGE_ALLOWED_LOCATIONS = ('gcs://<bucket>/<path>/', 'gcs://<bucket>/<path>/')
  [ STORAGE_BLOCKED_LOCATIONS = ('gcs://<bucket>/<path>/', 'gcs://<bucket>/<path>/') ]

Where:

  • integration_name is the name of the new integration.

  • bucket is the name of a Cloud Storage bucket that stores your data files (e.g. mybucket). The required STORAGE_ALLOWED_LOCATIONS parameter and optional STORAGE_BLOCKED_LOCATIONS parameter restrict or block access to these buckets, respectively, when stages that reference this integration are created or modified.

  • path is an optional path that can be used to provide granular control over objects in the bucket.

The following example creates an integration that explicitly limits external stages that use the integration to reference either of two buckets and paths. In a later step, we will create an external stage that references one of these buckets and paths.

Additional external stages that also use this integration can reference the allowed buckets and paths:

CREATE STORAGE INTEGRATION gcs_int
  TYPE = EXTERNAL_STAGE
  STORAGE_PROVIDER = GCS
  ENABLED = TRUE
  STORAGE_ALLOWED_LOCATIONS = ('gcs://mybucket1/path1/', 'gcs://mybucket2/path2/')
  STORAGE_BLOCKED_LOCATIONS = ('gcs://mybucket1/path1/sensitivedata/', 'gcs://mybucket2/path2/sensitivedata/');

Step 2: Retrieve the Cloud Storage Service Account for your Snowflake Account

Execute the DESCRIBE INTEGRATION command to retrieve the ID for the Cloud Storage service account that was created automatically for your Snowflake account:

DESC STORAGE INTEGRATION <integration_name>;

Where:

For example:

DESC STORAGE INTEGRATION gcs_int;

+-----------------------------+---------------+-----------------------------------------------------------------------------+------------------+
| property                    | property_type | property_value                                                              | property_default |
+-----------------------------+---------------+-----------------------------------------------------------------------------+------------------|
| ENABLED                     | Boolean       | true                                                                        | false            |
| STORAGE_ALLOWED_LOCATIONS   | List          | gcs://mybucket1/path1/,gcs://mybucket2/path2/                               | []               |
| STORAGE_BLOCKED_LOCATIONS   | List          | gcs://mybucket1/path1/sensitivedata/,gcs://mybucket2/path2/sensitivedata/   | []               |
| STORAGE_GCP_SERVICE_ACCOUNT | String        | service-account-id@project1-123456.iam.gserviceaccount.com                  |                  |
+-----------------------------+---------------+-----------------------------------------------------------------------------+------------------+

The STORAGE_GCP_SERVICE_ACCOUNT property in the output shows the Cloud Storage service account created for your Snowflake account (e.g. service-account-id@project1-123456.iam.gserviceaccount.com). We provision a single Cloud Storage service account for your entire Snowflake account. All Cloud Storage integrations use that service account.

Step 3: Grant the Service Account Permissions to Access Bucket Objects

The following step-by-step instructions describe how to configure IAM access permissions for Snowflake in your Google Cloud Platform Console so that you can use a Cloud Storage bucket to load and unload data:

Creating a Custom IAM Role

Create a custom role that has the permissions required to access the bucket and get objects.

  1. Log into the Google Cloud Platform Console as a project editor.

  2. From the home dashboard, choose IAM & admin » Roles.

  3. Click Create Role.

  4. Enter a name, and description for the custom role.

  5. Click Add Permissions.

  6. Filter the list of permissions, and add the following from the list:

    Data loading only
    • storage.buckets.get

    • storage.objects.get

    • storage.objects.list

    Data loading with purge option

    Note

    This permission also allows executing the REMOVE command on the stage:

    • storage.buckets.get

    • storage.objects.delete

    • storage.objects.get

    • storage.objects.list

    Data loading and unloading
    • storage.buckets.get

    • storage.objects.create

    • storage.objects.delete

    • storage.objects.get

    • storage.objects.list

    Data unloading only
    • storage.buckets.get

    • storage.objects.create

    • storage.objects.delete

    • storage.objects.list

  7. Click Create.

Assigning the Custom Role to the Cloud Storage Service Account

  1. Log into the Google Cloud Platform Console as a project editor.

  2. From the home dashboard, choose Cloud Storage » Browser:

    Bucket List in Google Cloud Platform Console
  3. Select a bucket to configure for access.

  4. Click SHOW INFO PANEL in the upper-right corner. The information panel for the bucket slides out.

  5. Click the ADD PRINCIPAL button.

  6. In the New principals field, search for the service account name from the DESCRIBE INTEGRATION output in Step 2: Retrieve the Cloud Storage Service Account for your Snowflake Account (in this topic).

    Bucket Information Panel in Google Cloud Platform Console
  7. From the Select a role dropdown, select Custom » <role>, where <role> is the custom Cloud Storage role you created in Creating a Custom IAM Role (in this topic).

  8. Click the Save button. The service account name is added to the Storage Object Viewer role dropdown in the information panel.

    Storage Object Viewer role list in Google Cloud Platform Console

Granting the Cloud Storage Service Account Permissions on the Cloud Key Management Service Cryptographic Keys

Note

This step is required only if your GCS bucket is encrypted using a key stored in the Google Cloud Key Management Service (Cloud KMS).

  1. Log into the Google Cloud Platform Console as a project editor.

  2. From the home dashboard, choose Security » Cryptographic keys.

  3. Select the key ring that is assigned to your GCS bucket.

  4. Click SHOW INFO PANEL in the upper-right corner. The information panel for the key ring slides out.

  5. Click the ADD PRINCIPAL button.

  6. In the New principals field, search for the service account name from the DESCRIBE INTEGRATION output in Step 2: Retrieve the Cloud Storage Service Account for your Snowflake Account (in this topic).

  7. From the Select a role dropdown, select the Cloud KMS CrytoKey Encryptor/Decryptor role.

  8. Click the Save button. The service account name is added to the Cloud KMS CrytoKey Encryptor/Decryptor role dropdown in the information panel.

Configuring Automation Using GCS Pub/Sub

Prerequisites

The instructions in this topic assume the following items have been created and configured:

GCP account

For instructions, see the Pub/Sub documentation.

Snowflake
  • Target table in the Snowflake database where your data will be loaded.

Creating the Pub/Sub Topic

Create a Pub/Sub topic using Cloud Shell or Cloud SDK.

Execute the following command to create the topic and enable it to listen for activity in the specified GCS bucket:

$ gsutil notification create -t <topic> -f json gs://<bucket-name>

Where:

  • <topic> is the name for the topic.

  • <bucket-name> is the name of your GCS bucket.

If the topic already exists, the command uses it; otherwise, a new topic is created.

For more information, see Using Pub/Sub notifications for Cloud Storage in the Pub/Sub documentation.

Creating the Pub/Sub Subscription

Create a subscription with pull delivery to the Pub/Sub topic using the Cloud Console, gcloud command-line tool, or the Cloud Pub/Sub API. For instructions, see Managing topics and subscriptions in the Pub/Sub documentation.

Note

  • Only Pub/Sub subscriptions that use the default pull delivery are supported with Snowflake. Push delivery is not supported.

Retrieving the Pub/Sub Subscription ID

The Pub/Sub topic subscription ID is used in these instructions to allow Snowflake access to event messages.

  1. Log into the Google Cloud Platform Console as a project editor.

  2. From the home dashboard, choose Big Data » Pub/Sub » Subscriptions.

  3. Copy the ID in the Subscription ID column for the topic subscription

Step 1: Create a Notification Integration in Snowflake

Create a notification integration using the CREATE NOTIFICATION INTEGRATION command. The notification integration references your Pub/Sub subscription. Snowflake associates the notification integration with a GCS service account created for your account. Snowflake creates a single service account that is referenced by all GCS notification integrations in your Snowflake account.

Note

  • Only account administrators (users with the ACCOUNTADMIN role) or a role with the global CREATE INTEGRATION privilege can execute this SQL command.

  • The GCS service account for notification integrations is different from the service account created for storage integrations.

CREATE NOTIFICATION INTEGRATION <integration_name>
  TYPE = QUEUE
  NOTIFICATION_PROVIDER = GCP_PUBSUB
  ENABLED = true
  GCP_PUBSUB_SUBSCRIPTION_NAME = '<subscription_id>';

Where:

For example:

CREATE NOTIFICATION INTEGRATION my_notification_int
  TYPE = QUEUE
  NOTIFICATION_PROVIDER = GCP_PUBSUB
  ENABLED = true
  GCP_PUBSUB_SUBSCRIPTION_NAME = 'projects/project-1234/subscriptions/sub2';

Step 2: Grant Snowflake Access to the Pub/Sub Subscription

  1. Execute the DESCRIBE INTEGRATION command to retrieve the Snowflake service account ID:

    DESC NOTIFICATION INTEGRATION <integration_name>;
    

    Where:

    For example:

    DESC NOTIFICATION INTEGRATION my_notification_int;
    
  2. Record the service account name in the GCP_PUBSUB_SERVICE_ACCOUNT column, which has the following format:

    <service_account>@<project_id>.iam.gserviceaccount.com
    
  3. Log into the Google Cloud Platform Console as a project editor.

  4. From the home dashboard, choose Big Data » Pub/Sub » Subscriptions.

  5. Select the subscription to configure for access.

  6. Click SHOW INFO PANEL in the upper-right corner. The information panel for the subscription slides out.

  7. Click the ADD PRINCIPAL button.

  8. In the New principals field, search for the service account name you recorded.

  9. From the Select a role dropdown, select Pub/Sub Subscriber.

  10. Click the Save button. The service account name is added to the Pub/Sub Subscriber role dropdown in the information panel.

  11. Navigate to the Dashboard page in the Cloud Console, and select your project from the dropdown list.

  12. Click the ADD PEOPLE TO THIS PROJECT button.

  13. Add the service account name you recorded.

  14. From the Select a role dropdown, select Monitoring Viewer.

  15. Click the Save button. The service account name is added to the Monitoring Viewer role.

Step 3: Create a Stage (If Needed)

Create an external stage that references your GCS bucket using the CREATE STAGE command. Snowflake reads your staged data files into the external table metadata. Alternatively, you can use an existing external stage.

Note

  • To configure secure access to the cloud storage location, see Configuring Secure Access to Cloud Storage (in this topic).

  • To reference a storage integration in the CREATE STAGE statement, the role must have the USAGE privilege on the storage integration object.

The following example creates a stage named mystage in the active schema for the user session. The cloud storage URL includes the path files. The stage references a storage integration named my_storage_int.

USE SCHEMA mydb.public;

CREATE STAGE mystage
  URL='gcs://load/files/'
  STORAGE_INTEGRATION = my_storage_int;

Step 4: Create an External Table

Create an external table using the CREATE EXTERNAL TABLE command.

For example, create an external table in the mydb.public schema that reads JSON data from files staged in the mystage stage with the path1/ path.

The INTEGRATION parameter references the my_notification_int notification integration you created in Step 1: Create a Notification Integration in Snowflake. The integration name must be provided in all uppercase.

Note that the AUTO_REFRESH parameter is TRUE by default:

CREATE OR REPLACE EXTERNAL TABLE ext_table
 INTEGRATION = 'MY_NOTIFICATION_INT'
 WITH LOCATION = @mystage/path1/
 FILE_FORMAT = (TYPE = JSON);

The external stage with auto-refresh is now configured!

When new or updated data files are added to the GCS bucket, the event notification informs Snowflake to scan them into the external table metadata.

Step 5: Manually Refresh the External Table Metadata

Manually refresh the external table metadata once using ALTER EXTERNAL TABLE with the REFRESH parameter, e.g.:

ALTER EXTERNAL TABLE ext_table REFRESH;

+---------------------------------------------+----------------+-------------------------------+
| file                                        | status         | description                   |
|---------------------------------------------+----------------+-------------------------------|
| files/path1/file1.json                      | REGISTERED_NEW | File registered successfully. |
| files/path1/file2.json                      | REGISTERED_NEW | File registered successfully. |
| files/path1/file3.json                      | REGISTERED_NEW | File registered successfully. |
+---------------------------------------------+----------------+-------------------------------+

This step synchronizes the metadata with the list of files in the stage and path in the external table definition. Also, this step ensures the external table can read the data files in the specified stage and path, and that no files were missed in the external table definition.

If the list of files in the file column does not match your expectations, verify the paths in the external table definition and external stage definition. Any path in the external table definition is appended to any path specified in the stage definition. For more information, see CREATE EXTERNAL TABLE.

Important

If this step is not completed successfully at least once after the external table is created, querying the external table returns no results until a Pub/Sub notification refreshes the external table metadata automatically for the first time.

This step ensures the metadata is synchronized with any changes to the file list that occurred since Step 4. Thereafter, Pub/Sub notifications trigger the metadata refresh automatically.

Step 6: Configuring Security

For each additional role that will be used to query the external table, grant sufficient access control privileges on the various objects (i.e. the database(s), schema(s), stage, and table) using GRANT <privileges> … TO ROLE:

Object

Privilege

Notes

Database

USAGE

Schema

USAGE

Named stage

USAGE , READ

Named file format

USAGE

Optional; only needed if the stage you created in Step 3: Create a Stage (If Needed) references a named file format.

External table

SELECT

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