Create a Snowpark Container Services service that uses caller’s rights¶

Introduction¶

In this tutorial you explore building a service, presenting a web UI, that uses the caller’s rights feature when executing SQL queries on behalf of the users.

You create a service (named query_service) that executes a query provided in the request. By default, application containers connect to Snowflake as the service user using the service’s owner role. But this application uses the caller’s rights feature to connect to the service endpoint as the end user and using privileges granted to that user.

When testing, you use the service from a web browser because the caller’s rights feature is only supported when accessing a service using network ingress. The caller’s rights feature is not available when accessing a service using a service function.

The service does the following:

• Exposes one public endpoint.

• When a user logs in to the endpoint, the service provides a Web UI to provide a query. The service executes the query in Snowflake and returns the results. In this tutorial you execute the following SQL command:

  SELECT CURRENT_USER(), CURRENT_ROLE();
  

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  The command returns the name of the currently logged-in user and the currently active role, both of which depend on whether caller’s rights is used.

  • When caller’s rights is used, the service connects to Snowflake as the calling user and the user’s default role. The command returns your user name and default role.

  • When caller’s rights is not used, the default behavior kicks in where the service connects to Snowflake as the service user and the service’s owner role. Therefore, the command returns the service user name in the form: SF$SERVICE$unique-id, TEST_ROLE.

There are two parts to this tutorial:

Part 1: Create and test a service. You download code provided for this tutorial and follow step-by-step instructions:

1. Download the service code for this tutorial.

2. Build a Docker image for Snowpark Container Services, and upload the image to a repository in your account.

3. Create a service.

4. Communicate with the service using network ingress to connect with the public endpoint that the service exposes. Using a web browser, you login to the public endpoint and execute the SELECT CURRENT_USER(); command. Verify the command output to ensure that the container executed the command as the logged-in user.

Part 2: Understand the service. This section provides an overview of the service code and highlights how the application code uses the caller’s rights.

Prepare¶

Follow Common Setup to configure prerequisites and create snowflake resources that are required for all Snowpark Container Services tutorials provided in this documentation.

Download the service code¶

Code (a Python application) is provided to create the query service.

1. Download SnowparkContainerServices-Tutorials.zip.

2. Unzip the content, which includes one directory for each tutorial. The Tutorial-6-callers-rights directory has the following files:

   • Dockerfile

   • main.py

   • templates/basic_ui.html

Build an image and upload¶

Build an image for the linux/amd64 platform that Snowpark Container Services supports, and then upload the image to the image repository in your account (see Common Setup).

You will need information about the repository (the repository URL and the registry hostname) before you can build and upload the image. For more information, see Registry and Repositories.

Get information about the repository

1. To get the repository URL, execute the SHOW IMAGE REPOSITORIES SQL command.

   SHOW IMAGE REPOSITORIES;
   

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   • The repository_url column in the output provides the URL. An example is shown:

     <orgname>-<acctname>.registry.snowflakecomputing.cn/tutorial_db/data_schema/tutorial_repository
     

   • The host name in the repository URL is the registry host name. An example is shown:

     <orgname>-<acctname>.registry.snowflakecomputing.cn
     

Build image and upload it to the repository

1. Open a terminal window, and change to the directory containing the files you unzipped.

2. To build a Docker image, execute the following docker build command using the Docker CLI. Note that the command specifies the current working directory (.) as the PATH for files to use to build the image.

   docker build --rm --platform linux/amd64 -t <repository_url>/<image_name> .
   

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   • For image_name, use query_service:latest.

   Example

   docker build --rm --platform linux/amd64 -t myorg-myacct.registry.snowflakecomputing.cn/tutorial_db/data_schema/tutorial_repository/query_service:latest .
   

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3. Upload the image to the repository in your Snowflake account. For Docker to upload an image on your behalf to your repository, you must first authenticate Docker with Snowflake.

   1. To authenticate Docker with the Snowflake registry, execute the following command:

      docker login <registry_hostname> -u <username>
      

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      • For username, specify your Snowflake username. Docker will prompt you for your password.

   2. To upload the image, execute the following command:

      docker push <repository_url>/<image_name>
      

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      Example

      docker push myorg-myacct.registry.snowflakecomputing.cn/tutorial_db/data_schema/tutorial_repository/query_service:latest
      

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Create a service¶

In this section you create a service (query_service).

1. Verify that the compute pool is ready and that you are in the right context to create the service.

   1. Previously, you set the context in the Common Setup step. To ensure that you’re in the right context for the SQL statements in this step, execute the following:

   USE ROLE test_role;
   USE DATABASE tutorial_db;
   USE SCHEMA data_schema;
   USE WAREHOUSE tutorial_warehouse;
   

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   1. To ensure that the compute pool you created in the common setup is ready, execute DESCRIBE COMPUTE POOL, and verify that the state is ACTIVE or IDLE. If the state is STARTING, you need to wait until the state changes to either ACTIVE or IDLE.

   DESCRIBE COMPUTE POOL tutorial_compute_pool;
   

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2. To create the service, execute the following command using test_role:

   CREATE SERVICE query_service
     IN COMPUTE POOL tutorial_compute_pool
     FROM SPECIFICATION $$
       spec:
         containers:
         - name: main
           image: /tutorial_db/data_schema/tutorial_repository/query_service:latest
           env:
             SERVER_PORT: 8000
           readinessProbe:
             port: 8000
             path: /healthcheck
         endpoints:
         - name: execute
           port: 8000
           public: true
       capabilities:
         securityContext:
           executeAsCaller: true
       serviceRoles:
       - name: ui_usage
         endpoints:
         - execute
   $$;
   

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   Note

   If a service with that name already exists, use the DROP SERVICE command to delete the previously created service, and then create this service.

3. Execute the following SQL commands to get detailed information about the service you just created. For more information, see Snowpark Container Services: Working with services.

   • To list services in your account, execute the SHOW SERVICES command:

     SHOW SERVICES;
     

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   • To get the status of your service, execute the SHOW SERVICE CONTAINERS IN SERVICE command:

     SHOW SERVICE CONTAINERS IN SERVICE query_service;
     

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   • To get information about your service, execute the DESCRIBE SERVICE command:

     DESCRIBE SERVICE query_service;
     

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Use the service¶

In this section you verify that the caller’s rights configured for the service work. You log in to the public endpoint from a browser, execute a query, and verify that the Snowflake session that the service created operates as the calling user, instead of as the service user.

First, to set up the context for the SQL statements in this section, execute the following:

USE ROLE test_role;
USE DATABASE tutorial_db;
USE SCHEMA data_schema;
USE WAREHOUSE tutorial_warehouse;

The service exposes a public endpoint (see the inline specification provided in the CREATE SERVICE command); therefore, first log in to the endpoint using a web browser, then use the web UI that the service exposes to the internet to send query requests to the service endpoint.

1. Find the URL of the public endpoint that the service exposes:

   SHOW ENDPOINTS IN SERVICE query_service;
   

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   The ingress_url column in the response provides the URL.

   Example

   p6bye-myorg-myacct.snowflakecomputing.app
   

2. Append /ui to the endpoint URL, and paste it in the web browser. This causes the service to execute the ui() function (see main.py).

   Note that the first time you access the endpoint URL, you will be asked to log in to Snowflake.

3. Use the same user that you used to create the service. Upon successful login, the service shows the following Web UI.

   

   Enter the following command in the text box and press enter to see the results.

   SELECT CURRENT_USER(), CURRENT_ROLE()DONE;
   

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   Because you included the executeAsCaller capability in the service specification, when a request arrives, Snowflake inserts the Sf-Context-Current-User-Token header in the request and then forwards the request to your service endpoint.

   For illustration purposes, the service code in this tutorial executes the query both as the caller and the service user.

   

   • Executes the query on behalf of the caller (ingress user): In this case, the code uses the user token that Snowflake provides to construct a login token for connecting with Snowflake. Thus, the service uses the caller’s rights. Snowflake executes the query on behalf of the caller, displaying the caller’s name and active role name in the query result. For example:

     ['TESTUSER, PUBLIC']
     

   • Executes the query on behalf of the service user: In this case, the code doesn’t use the user token that Snowflake provides in the request when constructing the login token to connect with Snowflake. Thus, the service doesn’t utilize the caller’s rights, causing Snowflake to execute the query on behalf of the service user. The query result shows the service user’s name (which is the same as the service name) and the active role.

     ['QUERY_SERVICE, TEST_ROLE']
     

When the service executes the query (SELECT CURRENT_USER(), CURRENT_ROLE();) on behalf of the caller, Snowflake doesn’t need the user’s warehouse to execute this simple query. Therefore, the service didn’t need any caller grants. In the next section, the service executes a non-trivial query on behalf of the calling user that requires you to grant caller grants to the service.

Use the service with caller grants¶

In this section, the service executes the following query on behalf of the caller (the user who logs in the service’s ingress endpoint).

SELECT * FROM ingress_user_db.ingress_user_schema.ingress_user_table;

The service doesn’t have permissions to access the table and doesn’t have permission to run the query in the default warehouse. To enable the service to execute this query on behalf of the caller, you grant the required caller grants to the service.

To demonstrate the scenario, you create a new role (ingress_user_role) and a table (ingress_user_table) that’s accessible to the new role but not to the service’s owner role (test_role). Therefore, when the service attempts to execute the query using the service credentials, Snowflake returns an error. But when the service executes the query on behalf of the user, Snowflake executes the query and returns the result.

Cleanup¶

You should remove billable resources that you created. For more information, see Step 5 in Tutorial 3.

Reviewing the service code¶

This section covers the following topics:

• Examining the tutorial code: Review the code files that implement the query service.

Examining the tutorial code¶

The zip file you downloaded in Step 1 includes the following files:

• Dockerfile

• main.py

• templates/basic_ui.html

You also use service specification when creating the service. The following section explains how these code components work together to create the service.

main.py file¶

This Python file contains the code that implements a minimal HTTP server that executes a query in the request and returns query results. The code provides a web user interface (UI) for submitting echo requests.

from flask import Flask
from flask import request
from flask import render_template
import logging
import os
import sys

from snowflake.snowpark import Session
from snowflake.snowpark.exceptions import *

# Environment variables below will be automatically populated by Snowflake.
SNOWFLAKE_ACCOUNT = os.getenv("SNOWFLAKE_ACCOUNT")
SNOWFLAKE_HOST = os.getenv("SNOWFLAKE_HOST")
SNOWFLAKE_DATABASE = os.getenv("SNOWFLAKE_DATABASE")
SNOWFLAKE_SCHEMA = os.getenv("SNOWFLAKE_SCHEMA")

# Custom environment variables
SNOWFLAKE_USER = os.getenv("SNOWFLAKE_USER")
SNOWFLAKE_PASSWORD = os.getenv("SNOWFLAKE_PASSWORD")
SNOWFLAKE_ROLE = os.getenv("SNOWFLAKE_ROLE")
SNOWFLAKE_WAREHOUSE = os.getenv("SNOWFLAKE_WAREHOUSE")

SERVICE_HOST = os.getenv("SERVER_HOST", "0.0.0.0")
SERVER_PORT = os.getenv("SERVER_PORT", 8080)


def get_logger(logger_name):
    logger = logging.getLogger(logger_name)
    logger.setLevel(logging.DEBUG)
    handler = logging.StreamHandler(sys.stdout)
    handler.setLevel(logging.DEBUG)
    handler.setFormatter(
        logging.Formatter("%(name)s [%(asctime)s] [%(levelname)s] %(message)s")
    )
    logger.addHandler(handler)
    return logger


def get_login_token():
    """
    Read the login token supplied automatically by Snowflake. These tokens
    are short lived and should always be read right before creating any new connection.
    """
    with open("/snowflake/session/token", "r") as f:
        return f.read()


def get_connection_params(ingress_user_token=None):
    """
    Construct Snowflake connection params from environment variables.
    """
    if os.path.exists("/snowflake/session/token"):
        if ingress_user_token:
            logger.info("Creating a session on behalf of user.")
            token = get_login_token() + "." + ingress_user_token
        else:
            logger.info("Creating a session as service user.")
            token = get_login_token()

        return {
            "account": SNOWFLAKE_ACCOUNT,
            "host": SNOWFLAKE_HOST,
            "authenticator": "oauth",
            "token": token,
            "warehouse": SNOWFLAKE_WAREHOUSE,
            "database": SNOWFLAKE_DATABASE,
            "schema": SNOWFLAKE_SCHEMA,
        }
    else:
        return {
            "account": SNOWFLAKE_ACCOUNT,
            "host": SNOWFLAKE_HOST,
            "user": SNOWFLAKE_USER,
            "password": SNOWFLAKE_PASSWORD,
            "role": SNOWFLAKE_ROLE,
            "warehouse": SNOWFLAKE_WAREHOUSE,
            "database": SNOWFLAKE_DATABASE,
            "schema": SNOWFLAKE_SCHEMA,
        }


logger = get_logger("query-service")
app = Flask(__name__)


@app.get("/healthcheck")
def readiness_probe():
    return "I'm ready!"


@app.route("/ui", methods=["GET", "POST"])
def ui():
    """
    Main handler for providing a web UI.
    """
    if request.method == "POST":
        # get ingress user token
        ingress_user = request.headers.get("Sf-Context-Current-User")
        ingress_user_token = request.headers.get("Sf-Context-Current-User-Token")

        if ingress_user:
            logger.info(f"Received a request from user {ingress_user}")

        # getting input in HTML form
        query = request.form.get("query")
        if query:
            logger.info(f"Received a request for query: {query}.")
            query_result_ingress_user = (
                run_query(query, ingress_user_token)
                if ingress_user_token
                else "Token is missing. Can't execute as ingress user."
            )
            query_result_service_user = run_query(query)
            return render_template(
                "basic_ui.html",
                query_input=query,
                query_result_ingress_user=query_result_ingress_user,
                query_result_service_user=query_result_service_user,
            )
    return render_template("basic_ui.html")


@app.route("/query", methods=["GET"])
def query():
    """
    Main handler for providing programmatic access.
    """
    # get ingress user token
    query = request.args.get("query")
    logger.info(f"Received query request: {query}.")
    if query:
        ingress_user = request.headers.get("Sf-Context-Current-User")
        ingress_user_token = request.headers.get("Sf-Context-Current-User-Token")

        if ingress_user:
            logger.info(f"Received a request from user {ingress_user}")

        res = run_query(query, ingress_user_token)
        return str(res)
    return "DONE"


def run_query(query, ingress_user_token=None):
    # start a Snowflake session as the ingress user
    try:
        with Session.builder.configs(
            get_connection_params(ingress_user_token)
        ).create() as session:
            logger.info(
                f"Snowflake connection established (id={session.session_id}). Now executing query: {query}."
            )
            try:
                res = session.sql(query).collect()
                logger.info(f"Query execution done: {query}.")
                return (
                    "[Empty Result]"
                    if len(res) == 0
                    else [", ".join(row) for row in res]
                )
            except Exception as e:
                return "Encountered an error when executing query: " + str(e)
    except Exception as e:
        return "Encountered an error when connecting to Snowflake: " + str(e)

if __name__ == '__main__':
  app.run(host=SERVICE_HOST, port=SERVER_PORT)

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In the code:

• The ui function displays the following web form and handles query requests submitted from the web form.

  

  This function uses the @app.route() decorator to specify that requests for /ui are handled by this function:

  @app.route("/ui", methods=["GET", "POST"])
  def ui():
  

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  The query service exposes the execute endpoint publicly (see the inline service specification you provided when creating the service), enabling communication with the service over the web. When you load the URL of the public endpoint with /ui appended in your browser, the browser sends an HTTP GET request for this path, and the server routes the request to this function. The function executes and returns a simple HTML form for the user to enter a query.

  After the user enters a query and submits the form, the browser sends an HTTP POST request for this path. Because the service specification includes the executeAsCaller capability, Snowflake adds the Sf-Context-Current-User-Token header to the incoming request and forwards the request to this same function (see Connecting to Snowflake using caller’s rights).

  The code executes the run_query function twice:

  • As the ingress user. In this case, the login token is concatenation of both OAuth token and ingress user token.

    token = get_login_token() + "." + ingress_user_token
    

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  • As the service user. In this case the login token is only the OAuth token.

    token = get_login_token()
    

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• The readiness_probe function uses the @app.get() decorator to specify that requests for /healthcheck are handled by this function:

  @app.get("/healthcheck")
  def readiness_probe():
  

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  This function enables Snowflake to check the readiness of the service. When the container starts, Snowflake wants to confirm that the application is working and that the service is ready to serve the requests. Snowflake sends an HTTP GET request with this path (as a health probe, readiness probe) to ensure that only healthy containers serve traffic. The function can do whatever you want.

• The get_logger function helps set up logging.

Dockerfile¶

This file contains all the commands to build an image using Docker.

ARG BASE_IMAGE=python:3.10-slim-buster
FROM $BASE_IMAGE
COPY main.py ./
COPY templates/ ./templates/
RUN pip install --upgrade pip && pip install flask snowflake-snowpark-python
CMD ["python", "main.py"]

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The Dockerfile contains instructions to install the Flask library ind the Docker container. The code in main.py relies on the Flask library to handle HTTP requests.

/template/basic_ui.html¶

The query service exposes the echoendpoint endpoint publicly (see service specification), enabling communication with the service over the web. When you load the public endpoint URL with /ui appended in your browser, the query service displays this form.

You can enter a query in the form and submit the form, and the service returns the results in an HTTP response.

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Welcome to the query service!</title>
  </head>
  <body>
    <h1>Welcome to the query service!</h1>
    <form action="{{ url_for("ui") }}" method="post">
      <label for="query">query:<label><br>
      <input type="text" id="query" name="query" size="50"><br>
    </form>
    <h2>Query:</h2>
    {{ query_input }}
    <h2>Result (executed on behalf of ingress user):</h2>
    {{ query_result_ingress_user }}
    <h2>Result (executed as service user):</h2>
    {{ query_result_service_user }}
  </body>
</html>

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Service specification¶

Snowflake uses information you provide in this specification to configure and run your service.

spec:
  containers:
  - name: main
    image: /tutorial_db/data_schema/tutorial_repository/query_service:latest
    env:
      SERVER_PORT: 8000
    readinessProbe:
      port: 8000
      path: /healthcheck
  endpoints:
  - name: execute
    port: 8000
    public: true
capabilities:
  securityContext:
    executeAsCaller: true
serviceRoles:
- name: ui_usage
  endpoints:
  - execute

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In the service specification, the spec, capabilities, and serviceRoles are the top-level fields.

• spec provides specification details (see Service specification reference). Note that the service exposes one public endpoint (execute) that enables ingress access to the service from the public web.

• capabilities Specifies the executeAsCaller capability. This tells Snowflake that the application intends to use caller’s rights.

• serviceRoles specifies one service role (ui_usage) and endpoint name (execute) to grant the USAGE privilege on.

• The readinessProbe field identifies the port and path that Snowflake can use to send an HTTP GET request to the readiness probe to verify that the service is ready to handle traffic.

  The service code (echo_python.py) implements the readiness probe as follows:

  @app.get("/healthcheck")
  def readiness_probe():
  

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  Therefore, the specification file includes the container.readinessProbe field accordingly.

For more information about service specifications, see Service specification reference.

What’s next?¶

Now that you’ve completed this tutorial, you can return to Working with Services to explore other topics.