Python handler examples for stored procedures¶

Running concurrent tasks with worker processes¶

You can run concurrent tasks using Python worker processes. You might find this useful when you need to run parallel tasks that take advantage of multiple CPU cores on warehouse nodes.

Note

Snowflake recommends that you not use the built-in Python multiprocessing module.

To work around cases where the Python Global Interpreter Lock prevents a multi-tasking approach from scaling across all CPU cores, you can execute concurrent tasks using separate worker processes, rather than threads.

You can do this on Snowflake warehouses by using the joblib library’s Parallel class, as in the following example.

CREATE OR REPLACE PROCEDURE joblib_multiprocessing_proc(i INT)
  RETURNS STRING
  LANGUAGE PYTHON
  RUNTIME_VERSION = 3.9
  HANDLER = 'joblib_multiprocessing'
  PACKAGES = ('snowflake-snowpark-python', 'joblib')
AS $$
import joblib
from math import sqrt

def joblib_multiprocessing(session, i):
  result = joblib.Parallel(n_jobs=-1)(joblib.delayed(sqrt)(i ** 2) for i in range(10))
  return str(result)
$$;

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Note

The default backend used for joblib.Parallel differs between Snowflake standard and Snowpark-optimized warehouses.

Standard warehouse default: threading
Snowpark-optimized warehouse default: loky (multiprocessing)

You can override the default backend setting by calling the joblib.parallel_backend function, as in the following example.

import joblib
joblib.parallel_backend('loky')

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Using Snowpark APIs for asynchrononous processing¶

The following examples illustrate how you can use Snowpark APIs to begin asynchronous child jobs, as well as how those jobs behave under different conditions.

Checking the status of an asynchronous child job¶

In the following example, the checkStatus procedure executes an asynchronous child job that waits 60 seconds. The procedure then checks on the status of the job before it can have finished, so the check returns False.

CREATE OR REPLACE PROCEDURE checkStatus()
RETURNS VARCHAR
LANGUAGE PYTHON
RUNTIME_VERSION = 3.9
PACKAGES = ('snowflake-snowpark-python')
HANDLER='async_handler'
EXECUTE AS CALLER
AS $$
def async_handler(session):
    async_job = session.sql("select system$wait(60)").collect_nowait()
    return async_job.is_done()
$$;

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The following code calls the procedure.

CALL checkStatus();

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+-------------+
| checkStatus |
|-------------|
| False       |
+-------------+

Cancelling an asynchronous child job¶

In the following example, the cancelJob procedure uses SQL to insert data into the test_tb table with an asynchronous child job that would take 10 seconds to finish. It then cancels the child job before it finishes and the data has been inserted.

CREATE OR REPLACE TABLE test_tb(c1 STRING);

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CREATE OR REPLACE PROCEDURE cancelJob()
RETURNS VARCHAR
LANGUAGE PYTHON
RUNTIME_VERSION = 3.9
PACKAGES = ('snowflake-snowpark-python')
HANDLER = 'async_handler'
EXECUTE AS OWNER
AS $$
def async_handler(session):
    async_job = session.sql("insert into test_tb (select system$wait(10))").collect_nowait()
    return async_job.cancel()
$$;

CALL cancelJob();

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The following code queries the test_tb table, but returns no results because no data has been inserted.

SELECT * FROM test_tb;

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+----+
| C1 |
|----|
+----+

Waiting and blocking while an asynchronous child job runs¶

In the following example, the blockUntilDone procedure executes an asynchronous child job that takes 5 seconds to finish. Using the snowflake.snowpark.AsyncJob.result method, the procedure waits and returns when the job has finished.

CREATE OR REPLACE PROCEDURE blockUntilDone()
RETURNS VARCHAR
LANGUAGE PYTHON
RUNTIME_VERSION = 3.9
PACKAGES = ('snowflake-snowpark-python')
HANDLER='async_handler'
EXECUTE AS CALLER
AS $$
def async_handler(session):
    async_job = session.sql("select system$wait(5)").collect_nowait()
    return async_job.result()
$$;

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The following code calls the blockUntilDone procedure, which returns after waiting 5 seconds.

CALL blockUntilDone();

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+------------------------------------------+
| blockUntilDone                               |
|------------------------------------------|
| [Row(SYSTEM$WAIT(5)='waited 5 seconds')] |
+------------------------------------------+

Returning an error after requesting results from an unfinished asynchronous child job¶

In the following example, the earlyReturn procedure executes an asynchronous child job that takes 60 seconds to finish. The procedure then attempts to return a DataFrame from the job’s result before it can have finished. The result is an error.

CREATE OR REPLACE PROCEDURE earlyReturn()
RETURNS VARCHAR
LANGUAGE PYTHON
RUNTIME_VERSION = 3.9
PACKAGES = ('snowflake-snowpark-python')
HANDLER='async_handler'
EXECUTE AS CALLER
AS $$
def async_handler(session):
    async_job = session.sql("select system$wait(60)").collect_nowait()
    df = async_job.to_df()
    try:
        df.collect()
    except Exception as ex:
        return 'Error: (02000): Result for query <UUID> has expired'
$$;

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The following code calls the earlyReturn procedure, returning the error.

CALL earlyReturn();

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+------------------------------------------------------------+
| earlyReturn                                                 |
|------------------------------------------------------------|
| Error: (02000): Result for query <UUID> has expired        |
+------------------------------------------------------------+

Finishing a parent job before a child job finishes, canceling the child job¶

In the following example, the earlyCancelJob procedure executes an asynchronous child job to insert data into a table and takes 10 seconds to finish. However, the parent job — async_handler — returns before the child job finishes, which cancels the child job.

CREATE OR REPLACE PROCEDURE earlyCancelJob()
RETURNS VARCHAR
LANGUAGE PYTHON
RUNTIME_VERSION = 3.9
PACKAGES = ('snowflake-snowpark-python')
HANDLER='async_handler'
EXECUTE AS OWNER
AS $$
def async_handler(session):
    async_job = session.sql("insert into test_tb (select system$wait(10))").collect_nowait()
$$;

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The following code calls the earlyCancelJob procedure. It then queries the test_tb table, which returns no result because no data was inserted by the canceled child job.

CALL earlyCancelJob();
SELECT * FROM test_tb;

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+----+
| C1 |
|----|
+----+