How to properly exec a long-running database operation during the launching phase?

[tumluliu]

Hi all,

I am trying to make a long-running (~1min) function call which queries the database and constructs some data objects for upcoming usage. But it will give me a runtime panic if I create a postgres::Client within the launch function.

thread 'main' panicked at 'Cannot start a runtime from within a runtime. This happens because a function (like `block_on`) attempted to block the current thread while the thread is being used to drive asynchronous tasks.'

a minimal sample code snippet will be like this:

use postgres::{Client, NoTls};
use my_crate::my_libs::fill_data_model_from_db;

#[macro_use] extern crate rocket;

#[get("/world")]
fn world() -> &'static str {
    "Hello, world!"
}

#[launch]
fn rocket() -> _ {
    let mut client: Client = Client::connect("host=localhost dbname=testdb", NoTls).unwrap();
    // long running db queries for data preparation
    // let d = fill_data_model_from_db(&mut client);
    // will use State to encapsulate `d`
    rocket::build().mount("/hello", routes![world])
}

It's a bit strange to me that although the postgres::Client is actually a synchronous PostgreSQL client as its document says, why the creation of it will cause a nested tokio runtime issue? I already took a look at the rocket_db_pools, but could not figure out how the db pool would help here. I'm still pretty new to rust and rocket, and looking forward to your hints. Thanks a lot in advance!

[the10thWiz]

It's a bit strange to me that although the postgres::Client is actually a synchronous PostgreSQL client as its document says, why the creation of it will cause a nested tokio runtime issue?

Looking into the postgres crate, it looks like it starts it's own async runtime and runs an async client on the runtime it creates. It's technically a sync client (since it doesn't need an async runtime), but it can't actually be started within a tokio runtime due to some things tokio does. The simple solution here would actually be to use something like tokio-postgres, an async client (since Rocket already creates a tokio runtime, you might as well use it). The postgres crate actually uses tokio-postgres behind the scenes, so it shouldn't be too hard to translate your code to work with the async runtime.

[SergioBenitez]

To add on, instead of using tokio-Postgres, use rocket_db_pools instead.

[tumluliu]

Thank you both for your explanations and suggestions!