@Marvin in prefect 2, how can i limit the number of concurrent tasks in a flow?

In Prefect 2.x, you can limit the number of concurrent tasks in a flow using task concurrency limits. This is achieved by tagging tasks and setting concurrency limits based on these tags. Here's how you can do it: 1. Tagging Tasks: You can specify task tags and set a concurrency limit for tasks with a given tag. If a task has multiple tags, it will run only if all tags have available concurrency. 2. Configuring Concurrency Limits: Concurrency limits are set at the work pool and/or work queue level. For tasks, the limits are configured via tags. 3. Using the Concurrency Context Manager: Prefect also provides a

concurrency

context manager which you can use to limit the number of certain types of tasks that can run concurrently. Here's an example of how to use the

concurrency

context manager in a flow:

python
from prefect import flow, task, concurrency
import psycopg2

@task
def database_query(query):
    with concurrency("database", occupy=1):
        connection = psycopg2.connect("<connection_string>")
        cursor = connection.cursor()
        cursor.execute(query)
        result = cursor.fetchall()
        connection.close()
        return result

@flow
def my_flow():
    queries = ["SELECT * FROM table1", "SELECT * FROM table2", "SELECT * FROM table3"]
    for query in queries:
        database_query.submit(query)

if __name__ == "__main__":
    my_flow()

In this example: - The

database_query

task uses the

concurrency

context manager to limit the number of concurrent database connections. - The flow

my_flow

submits multiple

database_query

tasks, but due to the concurrency limit, only one task will run at a time if the limit is set to 1. For more details, you can refer to the documentation on task concurrency limits and global concurrency limits.