Whenever new data are inserted, extract BigQuery tables as csv files and store them in Cloud Storage.

My plan: Set up an Eventarc trigger based on Event method google.cloud.bigquery.v2.JobService.InsertJob for Cloud Function to identify when new data are inserted into BigQuery.

from google.cloud import bigquery
import functions_framework

client = bigquery.Client()
bucket_name = "bucket"
project = "astute-coda-410816"
dataset_id = "dataset"

def move_data(table):
    #extract BigQuery table
    destination_uri = f"gs://{bucket_name}/{table}/{table}-*.csv"
    dataset_ref = bigquery.DatasetReference(project,dataset_id)
    table_ref = dataset_ref.table(f"{table}")
    job_config = bigquery.job.ExtractJobConfig(print_header=False)
    client.extract_table(table_ref, destination_uri, location="US",job_config=job_config)
    print(f"Exported {project}:{dataset_id}.{table} to {destination_uri}")

def transfer(cloudevent):
    payload = cloudevent.data.get("protoPayload")
    status = payload.get("status")
    if not status: #if status is empty, the insert job is successful and tables should be extracted to Cloud Storage

My questions are:

  • Anything else I should do to improve my code?
  • Is there a way to run in parallel the data transfer for all 3 tables?

1 Answer 1


Looks good.

optional type annotation

This would be a better signature:

def move_data(table: str) -> None:

It turns out we're really passing in a table name, rather than some fancy table object. I didn't learn that detail until I got down to the calling code. This line mislead me:

    ... = dataset_ref.table(f"{table}")

I have no idea why you're calling str(table), given that table should already be a str.

In the interest of naming consistency, consider calling it just bucket rather than bucket_name.

nit: Running $ black -S *.py on this wouldn't hurt, to tidy up the spacing a bit.

meaningful identifier

Thank you for this helpful comment, I appreciate it.

    if not status:  # if status is empty, the insert job is successful and tables should be extracted to Cloud Storage

I imagine Google's docs refer to the return value as a status.

Here, it might be more helpful to name it errors, and then there would be no need for that comment.

Let's talk about the missing else: clause. If there are errors, wouldn't you like for a logger to report the details?


a way to run in parallel the data transfer for all 3 tables?

import multiprocessing
    tables = [f"table_{i}" for i in range(1, 4)]

    with multiprocessing.Pool() as pool:
        pool.map(move_data, tables)

Consider using one of the variant mappers, such as imap_unordered(), which grant greater latitude to the scheduler by relaxing the ordering constraints.

This code achieves its design goals.

I would be willing to delegate or accept maintenance tasks on it.

  • \$\begingroup\$ You're correct, table is already a string. For the f"{table}" line, it was originally f"{table}_i" with i being part of for i in range(len(table_list)). I later changed this but forgot to remove the f string \$\endgroup\$
    – hashaf
    Feb 26 at 12:42

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