Let me provide you with a practical example of how to run shell commands (like pg_dump) in a Prefect flow. There are two main approaches you can use: 1. Using Python's

subprocess

module (recommended) 2. Using the community

prefect-shell

package Here's a complete example using

subprocess

, which is the most straightforward approach:

python
from prefect import flow, task
import subprocess
from datetime import datetime

@task
def run_pg_dump(database_name: str, output_path: str) -> str:
    """Run pg_dump and return the output path"""
    try:
        # Construct the pg_dump command
        command = [
            "pg_dump",
            "-d", database_name,
            "-f", output_path,
            "--format=custom"
        ]
        
        # Run the command
        result = subprocess.run(
            command,
            check=True,  # This will raise an exception if the command fails
            capture_output=True,
            text=True
        )
        
        return output_path
    except subprocess.CalledProcessError as e:
        print(f"Error output: {e.stderr}")
        raise Exception(f"pg_dump failed with exit code {e.returncode}")

@flow
def backup_database(database_name: str):
    """Flow to backup a PostgreSQL database"""
    # Create a timestamp for the backup file
    timestamp = datetime.now().strftime("%Y%m%d_%H%M%S")
    output_path = f"backup_{database_name}_{timestamp}.dump"
    
    # Run the backup
    result_path = run_pg_dump(database_name, output_path)
    print(f"Database backup completed successfully: {result_path}")
    return result_path

# Run the flow
if __name__ == "__main__":
    backup_database("your_database_name")

Alternative approach using

prefect-shell

(requires installation):

python
# First install: pip install prefect-shell
from prefect import flow
from prefect_shell import ShellOperation

@flow
def backup_database(database_name: str):
    shell = ShellOperation()
    timestamp = datetime.now().strftime("%Y%m%d_%H%M%S")
    output_path = f"backup_{database_name}_{timestamp}.dump"
    
    command = f"pg_dump -d {database_name} -f {output_path} --format=custom"
    result = shell.run(command=command)
    
    if result.return_code == 0:
        print(f"Database backup completed successfully: {output_path}")
        return output_path
    else:
        raise Exception(f"Backup failed: {result.stderr}")

Key points about running shell commands in Prefect: 1. Error Handling: Always include proper error handling as shown in the examples. Shell commands can fail for various reasons. 2. Security Considerations: - Be careful with string formatting in commands to avoid shell injection - Use

subprocess.run()

with a list of arguments rather than shell=True when possible - Consider using environment variables for sensitive information 3. Logging: Prefect will automatically capture the logs from your tasks and flows, but you might want to explicitly log the command output for debugging purposes. 4. Environment: Make sure the shell command (like pg_dump) is available in the environment where your flow will run. This is especially important when deploying flows. You can extend these examples to run any shell command. Here's a more generic template:

python
@task
def run_shell_command(command: list[str]) -> str:
    """Generic task to run any shell command"""
    try:
        result = subprocess.run(
            command,
            check=True,
            capture_output=True,
            text=True
        )
        return result.stdout
    except subprocess.CalledProcessError as e:
        print(f"Error output: {e.stderr}")
        raise Exception(f"Command failed with exit code {e.returncode}")

@flow
def run_command_flow(command: list[str]):
    """Flow to run any shell command"""
    result = run_shell_command(command)
    print(f"Command output: {result}")
    return result

This can be used for any command: ```python # Example usage

command = ["ls", "-la"] # or ["pg_dump", "-d", "database", "-f", "output.dump"] run_command_flow(command) ``` Remember to: - Always validate and sanitize input commands - Handle errors appropriately - Consider using environment variables for sensitive information - Test the commands locally before deploying - Ensure all required command-line tools are available in your deployment environment

I'll help you create a robust implementation for pg_dump and pg_restore with proper waiting and error handling. Let me create a practical example that ensures the dump completes before restore begins. ```python from prefect import flow, task import subprocess from datetime import datetime from pathlib import Path import time from typing import Optional @task def run_pg_dump( database_name: str, output_path: str, host: str = "localhost", port: str = "5432", username: Optional[str] = None ) -> str: """Run pg_dump and ensure it completes""" try: # Build the command with basic authentication command = ["pg_dump"] if username: command.extend(["-U", username]) command.extend([ "-h", host, "-p", port, "-d", database_name, "-f", output_path, "--format=custom" # Using custom format for better restore control ]) print(f"Starting backup of database {database_name}") # Run pg_dump and wait for completion result = subprocess.run( command, check=True, capture_output=True, text=True ) # Verify the dump file exists and has size > 0 dump_file = Path(output_path) if not dump_file.exists(): raise Exception(f"Dump file {output_path} was not created") if dump_file.stat().st_size == 0: raise Exception(f"Dump file {output_path} is empty") print(f"Backup completed successfully: {output_path}") return output_path except subprocess.CalledProcessError as e: print(f"pg_dump error output: {e.stderr}") raise Exception(f"pg_dump failed with exit code {e.returncode}") @task def run_pg_restore( backup_file: str, target_database: str, host: str = "localhost", port: str = "5432", username: Optional[str] = None ) -> bool: """Run pg_restore and ensure it completes""" try: # Verify backup file exists before attempting restore if not Path(backup_file).exists(): raise Exception(f"Backup file {backup_file} does not exist") # Build the restore command command = ["pg_restore"] if username: command.extend(["-U", username]) command.extend([ "-h", host, "-p", port, "-d", target_database, "--clean", # Clean (drop) database objects before recreating "--if-exists", # Don't error if objects don't exist backup_file ]) print(f"Starting restore to database {target_database}") # Run pg_restore and wait for completion result = subprocess.run( command, check=True, capture_output=True, text=True ) print(f"Restore completed successfully to database {target_database}") return True except subprocess.CalledProcessError as e: print(f"pg_restore error output: {e.stderr}") raise Exception(f"pg_restore failed with exit code {e.returncode}") @task def verify_database_size( database_name: str, host: str = "localhost", port: str = "5432", username: Optional[str] = None ) -> bool: """Verify the database has data after restore""" try: # Build command to check database size command = ["psql"] if username: command.extend(["-U", username]) command.extend([ "-h", host, "-p", port, "-d", database_name, "-c", "SELECT pg_database_size(current_database());" ]) result = subprocess.run( command, check=True, capture_output=True, text=True

) # Parse the size from the output size = int(result.stdout.strip().split("\n")[2].strip()) if size <= 0: raise Exception(f"Database {database_name} appears to be empty") return True except subprocess.CalledProcessError as e: print(f"Database verification error: {e.stderr}") raise Exception("Failed to verify database size") @flow def backup_and_restore_database( source_database: str, target_database: str, host: str = "localhost", port: str = "5432", username: Optional[str] = None, backup_dir: str = "./backups" ): """Flow to backup a database and restore it to another database""" # Ensure backup directory exists Path(backup_dir).mkdir(parents=True, exist_ok=True) # Create timestamp for backup file timestamp = datetime.now().strftime("%Y%m%d_%H%M%S") backup_path = str(Path(backup_dir) / f"backup_{source_database}_{timestamp}.dump") try: # Step 1: Run the backup and wait for completion backup_file = run_pg_dump( database_name=source_database, output_path=backup_path, host=host, port=port, username=username ) # Step 2: Run the restore only if backup was successful restore_success = run_pg_restore( backup_file=backup_file, target_database=target_database, host=host, port=port, username=username ) # Step 3: Verify the restored database if restore_success: verify_success = verify_database_size( database_name=target_database, host=host, port=port, username=username ) if verify_success: print("Backup and restore completed successfully!") return True except Exception as e: print(f"Backup and restore failed: {str(e)}") raise return False # Example usage if name == "__main__": backup_and_restore_database( source_database="source_db", target_database="target_db", username="postgres", backup_dir="/path/to/backup/directory" )

This implementation includes several important features:

1. *Proper Sequencing*: 
   - The flow ensures that pg_dump completes before pg_restore starts
   - Each step is a separate task for better observability
   - Tasks wait for completion before proceeding

2. *Verification Steps*:
   - Verifies the dump file exists and isn't empty
   - Checks the restored database size
   - Includes error handling at each step

3. *Flexibility*:
   - Supports optional authentication parameters
   - Configurable host and port
   - Customizable backup directory

4. *Error Handling*:
   - Comprehensive error catching and reporting
   - Proper subprocess management
   - Verification of each step's success

To use this in production, you might want to add:

```python
from prefect.blocks.system import Secret

@flow
def secure_backup_and_restore(
    source_database: str,
    target_database: str
):
    # Load credentials securely
    postgres_password = Secret.load("postgres-password")
    
    # Set password in environment
    import os
    os.environ["PGPASSWORD"] = postgres_password.get()
    
    return backup_and_restore_database(
        source_database=source_database,
        target_database=target_database,
        username="postgres"
    )

Key considerations: 1. Environment Variables: PostgreSQL commands typically use environment variables like

PGPASSWORD

for authentication. In production, use Prefect Blocks for secure credential management. 2. Error Handling: The implementation includes comprehensive error handling, but you might want to add specific handling for your use case.

3. Logging: Prefect automatically handles task logging, but you might want to add additional logging for specific events. 4. Cleanup: Consider adding a cleanup task to remove old backup files. 5. Retries: You might want to add retries for network-related issues:

python
from prefect import task
from prefect.tasks import task_input_hash
from datetime import timedelta

@task(
    retries=3,
    retry_delay_seconds=30,
    cache_key_fn=task_input_hash,
    cache_expiration=timedelta(hours=1)
)
def run_pg_dump(...):
    # ... same implementation as above

This implementation ensures that your database operations are reliable, observable, and maintainable within your Prefect workflows.