Refreshing external tables automatically for Google Cloud Storage¶

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.

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/');

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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:

• integration_name is the name of the integration you created in Step 1: Create a Cloud Storage Integration in Snowflake (in this topic).

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                  |                  |
+-----------------------------+---------------+-----------------------------------------------------------------------------+------------------+

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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 console so that you can use a Cloud Storage bucket to load and unload data:

Prerequisites¶

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

GCP account:

• Pub/Sub topic that receives event messages from the GCS bucket. For more information, see Creating the Pub/Sub Topic (in this topic).

• Subscription that receives event messages from the Pub/Sub topic. For more information, see Creating the Pub/Sub Subscription (in this topic).

For instructions, see the Pub/Sub documentation.

Snowflake:

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

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

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.

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

Where:

• integration_name is the name of the new integration.

• subscription_id is the subscription name you recorded in Retrieving the Pub/Sub Subscription ID.

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>;
   

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   Where:

   • integration_name is the name of the integration you created in Step 1: Create a Notification Integration in Snowflake.

   For example:

   DESC NOTIFICATION INTEGRATION my_notification_int;
   

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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
   

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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.

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;

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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. |
+---------------------------------------------+----------------+-------------------------------+

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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.

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>: