Build a Python App with CockroachDB and psycopg2

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As of November 18, 2022, CockroachDB v21.1 is no longer supported. For more details, refer to the Release Support Policy.

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This tutorial shows you how build a simple Python application with CockroachDB and the psycopg2 driver. For the CockroachDB back-end, you'll use a temporary local cluster.

Step 1. Install the psycopg2 driver

To install the Python psycopg2 driver, run the following command:

$ pip install psycopg2-binary

For other ways to install psycopg2, see the official documentation.

Step 2. Start CockroachDB

Choose whether to run a temporary local cluster or a free CockroachDB cluster on CockroachDB Serverless. The instructions below will adjust accordingly.

Create a free cluster


Organizations without billing information on file can only create one CockroachDB Serverless cluster.

  1. If you haven't already, sign up for a CockroachDB Cloud account.
  2. Log in to your CockroachDB Cloud account.
  3. On the Clusters page, click Create Cluster.
  4. On the Select a plan page, select Serverless.
  5. On the Cloud & Regions page, select a cloud provider (GCP or AWS) in the Cloud provider section.
  6. In the Regions section, select a region for the cluster. Refer to CockroachDB Cloud Regions for the regions where CockroachDB Serverless clusters can be deployed. To create a multi-region cluster, click Add region and select additional regions. A cluster can have at most six regions.
  7. Click Next: Capacity.
  8. On the Capacity page, select Start for free. Click Next: Finalize.
  9. On the Finalize page, click Create cluster.

    Your cluster will be created in a few seconds and the Create SQL user dialog will display.

Set up your cluster connection

Once your cluster is created, the Connect to cluster-name dialog displays. Use the information provided in the dialog to set up your cluster connection for the SQL user that was created by default:

  1. In your terminal, run the second command from the dialog to create a new certs directory on your local machine and download the CA certificate to that directory:

    curl --create-dirs -o ~/.postgresql/root.crt -O<cluster-id>/cert

    Your cert file will be downloaded to ~/.postgresql/root.crt.

    curl --create-dirs -o ~/.postgresql/root.crt -O<cluster-id>/cert

    Your cert file will be downloaded to ~/.postgresql/root.crt.

    mkdir -p $env:appdata\.postgresql\; Invoke-WebRequest -Uri<cluster-id>/cert -OutFile $env:appdata\.postgresql\root.crt

    Your cert file will be downloaded to %APPDATA%/.postgresql/root.crt.

  2. Copy the connection string provided, which will be used in the next steps (and to connect to your cluster in the future).


    This connection string contains your password, which will be provided only once. If you forget your password, you can reset it by going to the SQL Users page for the cluster, found at<CLUSTER ID>/users.

    cockroach sql --url 'postgresql://<username>:<password>@<serverless-host>:26257/defaultdb?sslmode=verify-full&sslrootcert='$HOME'/.postgresql/root.crt'

    cockroach sql --url 'postgresql://<username>:<password>@<serverless-host>:26257/defaultdb?sslmode=verify-full&sslrootcert='$HOME'/.postgresql/root.crt'
    cockroach sql --url "postgresql://<username>:<password>@<serverless-host>:26257/defaultdb?sslmode=verify-full&sslrootcert=$env:appdata/.postgresql/root.crt"


    • <username> is the SQL user. By default, this is your CockroachDB Cloud account username.
    • <password> is the password for the SQL user. The password will be shown only once in the Connection info dialog after creating the cluster.
    • <serverless-host> is the hostname of the CockroachDB Serverless cluster.
    • <cluster-id> is a unique string used to identify your cluster when downloading the CA certificate. For example, 12a3bcde-4fa5-6789-1234-56bc7890d123.

    You can find these settings in the Connection parameters tab of the Connection info dialog.

  1. If you haven't already, download the CockroachDB binary.
  2. Run the cockroach demo command:

    $ cockroach demo \

    This starts a temporary, in-memory cluster and opens an interactive SQL shell to the cluster. Any changes to the database will not persist after the cluster is stopped.


    If cockroach demo fails due to SSL authentication, make sure you have cleared any previously downloaded CA certificates from the directory ~/.postgresql.

  3. Take note of the (sql) connection string in the SQL shell welcome text:

    # Connection parameters:
    #   (webui)
    #   (sql)      postgres://demo:demo76950@
    #   (sql/unix) postgres://demo:demo76950@?host=%2Fvar%2Ffolders%2Fc8%2Fb_q93vjj0ybfz0fz0z8vy9zc0000gp%2FT%2Fdemo070856957&port=26257

Step 3. Create a database

  1. In the SQL shell, create the bank database that your application will use:

  2. Create a SQL user for your app:

    > CREATE USER <username> WITH PASSWORD <password>;

    Take note of the username and password. You will use it in your application code later.

  3. Give the user the necessary permissions:

    > GRANT ALL ON DATABASE bank TO <username>;
  1. If you haven't already, download the CockroachDB binary.
  2. Start the built-in SQL shell using the connection string you got from the CockroachDB Cloud Console earlier:

    $ cockroach sql \
    --url='postgres://<username>:<password>@<global host>:26257/<cluster_name>.defaultdb?sslmode=verify-full&sslrootcert=<certs_dir>/cc-ca.crt'

    In the connection string copied from the CockroachDB Cloud Console, your username, password and cluster name are pre-populated. Replace the <certs_dir> placeholder with the path to the certs directory that you created earlier.

  3. In the SQL shell, create the bank database that your application will use:

  4. Exit the SQL shell:

    > \q

Step 4. Run the Python code

Now that you have a database, you'll run the code shown below to:

  • Create an accounts table and insert some rows.
  • Transfer funds between two accounts inside a transaction.
  • Delete the accounts from the table before exiting so you can re-run the example code.

To handle transaction retry errors, the code uses an application-level retry loop that, in case of error, sleeps before trying the funds transfer again. If it encounters another retry error, it sleeps for a longer interval, implementing exponential backoff.

Get the code

Download the file, or create the file yourself and copy the code into it.

If you prefer, you can also clone a version of the code:

$ git clone
#!/usr/bin/env python3
Test psycopg with CockroachDB.

import logging
import os
import random
import time
import uuid
from argparse import ArgumentParser, RawTextHelpFormatter

import psycopg2
from psycopg2.errors import SerializationFailure
import psycopg2.extras

def create_accounts(conn):
    ids = []
    id1 = uuid.uuid4()
    id2 = uuid.uuid4()
    with conn.cursor() as cur:
            "CREATE TABLE IF NOT EXISTS accounts (id UUID PRIMARY KEY, balance INT)"
            "UPSERT INTO accounts (id, balance) VALUES (%s, 1000), (%s, 250)", (id1, id2))
        logging.debug("create_accounts(): status message: %s",
    return ids

def delete_accounts(conn):
    with conn.cursor() as cur:
        cur.execute("DELETE FROM accounts")
        logging.debug("delete_accounts(): status message: %s",

def print_balances(conn):
    with conn.cursor() as cur:
        cur.execute("SELECT id, balance FROM accounts")
        logging.debug("print_balances(): status message: %s",
        rows = cur.fetchall()
        print(f"Balances at {time.asctime()}:")
        for row in rows:
            print("account id: {0}  balance: ${1:2d}".format(row['id'], row['balance']))

def transfer_funds(conn, frm, to, amount):
    with conn.cursor() as cur:

        # Check the current balance.
        cur.execute("SELECT balance FROM accounts WHERE id = %s", (frm,))
        from_balance = cur.fetchone()['balance']
        if from_balance < amount:
            raise RuntimeError(
                f"insufficient funds in {frm}: have {from_balance}, need {amount}"

        # Perform the transfer.
            "UPDATE accounts SET balance = balance - %s WHERE id = %s", (
                amount, frm)
            "UPDATE accounts SET balance = balance + %s WHERE id = %s", (
                amount, to)

    logging.debug("transfer_funds(): status message: %s", cur.statusmessage)

def run_transaction(conn, op, max_retries=3):
    Execute the operation *op(conn)* retrying serialization failure.

    If the database returns an error asking to retry the transaction, retry it
    *max_retries* times before giving up (and propagate it).
    # leaving this block the transaction will commit or rollback
    # (if leaving with an exception)
    with conn:
        for retry in range(1, max_retries + 1):

                # If we reach this point, we were able to commit, so we break
                # from the retry loop.

            except SerializationFailure as e:
                # This is a retry error, so we roll back the current
                # transaction and sleep for a bit before retrying. The
                # sleep time increases for each failed transaction.
                logging.debug("got error: %s", e)
                logging.debug("EXECUTE SERIALIZATION_FAILURE BRANCH")
                sleep_ms = (2**retry) * 0.1 * (random.random() + 0.5)
                logging.debug("Sleeping %s seconds", sleep_ms)

            except psycopg2.Error as e:
                logging.debug("got error: %s", e)
                logging.debug("EXECUTE NON-SERIALIZATION_FAILURE BRANCH")
                raise e

        raise ValueError(
            f"transaction did not succeed after {max_retries} retries")

def main():
    opt = parse_cmdline()
    logging.basicConfig(level=logging.DEBUG if opt.verbose else logging.INFO)
        # Attempt to connect to cluster with connection string provided to
        # script. By default, this script uses the value saved to the
        # DATABASE_URL environment variable.
        # For information on supported connection string formats, see
        db_url = opt.dsn
        conn = psycopg2.connect(db_url, 
                                application_name="$ docs_simplecrud_psycopg2", 
    except Exception as e:
        logging.fatal("database connection failed")
    ids = create_accounts(conn)

    amount = 100
    toId = ids.pop()
    fromId = ids.pop()

        run_transaction(conn, lambda conn: transfer_funds(
            conn, fromId, toId, amount))

    except ValueError as ve:
        # Below, we print the error and continue on so this example is easy to
        # run (and run, and run...).  In real code you should handle this error
        # and any others thrown by the database interaction.
        logging.debug("run_transaction(conn, op) failed: %s", ve)



    # Close communication with the database.

def parse_cmdline():
    parser = ArgumentParser(description=__doc__,

    parser.add_argument("-v", "--verbose",
                        action="store_true", help="print debug info")

database connection string\
 (default: value of the DATABASE_URL environment variable)

    opt = parser.parse_args()
    if opt.dsn is None:
        parser.error("database connection string not set")
    return opt

if __name__ == "__main__":

Run the code

The Python code is a command-line utility that accepts the connection string to CockroachDB as a command-line argument:

$ python3 \

Before running the command, update the connection string as follows:

  • Replace <username> and <password> with the SQL username and password that you created earlier.
  • Replace <hostname> and <port> with the hostname and port in the (sql) connection string from SQL shell welcome text.
$ python3 \

Before running the command, update the connection string that you copied earlier from the Connection info dialog as follows:

  • Replace <username> and <password> with the SQL username and password that you created earlier.
  • Replace <globalhost> with the name of the CockroachDB Serverless host (e.g.,
  • Replace <cluster-name> with the name of your cluster.
  • Replace <certs_directory> with the path to the cc-ca.crt file that you downloaded from the CockroachDB Cloud Console.

If you are using the connection string that you copied from the Connection info dialog, your username, password, and cluster name will be pre-populated.

The output should show the account balances before and after the funds transfer:

Balances at Fri Oct 30 18:27:00 2020:
(1, 1000)
(2, 250)
Balances at Fri Oct 30 18:27:00 2020:
(1, 900)
(2, 350)

What's next?

Read more about using the Python psycopg2 driver.

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