vweb: implement database pool

[Casper64]

At this moment it is only possible to have 1 ongoing datbase connection for an entire vweb application. Problems arise when multiple workers need to access that same database connection over multiple threads, see #18006.

This pullrequest adds the option to have a pool of database connections.

Usage

Example with postgres as database.

module main

import vweb
import db.pg

struct App {
	vweb.Context
	db_handle vweb.DatabasePool[pg.DB]
mut:
	db pg.DB
}

fn get_database_connection() pg.DB {
	return pg.connect(user: 'dev', password: 'password', dbname: 'database') or { panic(err) }
}

fn main() {
	pool := vweb.database_pool(handler: get_database_connection, nr_workers: 20)

	vweb.run_at(&App{
		db_handle: pool
	},
		port: 8080
		nr_workers: 20
	)!
}

In this example there are 20 workers and there must be 1 database connection for each worker (thread).

The default nr of workers is the same as for a default app:

[params]
pub struct PoolParams[T] {
	handler    fn () T [required]
	nr_workers int = runtime.nr_jobs()
}

DIfferent databases

The above example configured for mysql:

module main

import vweb
import db.mysql

struct App {
	vweb.Context
	vweb.Controller
	db_handle vweb.DatabasePool[mysql.Connection]
mut:
	db mysql.Connection
}

fn get_database_connection() mysql.Connection {
	mut connection := mysql.Connection{
		// host: ''
		// port: 3306
		username: 'root'
		// password: ''
		dbname: 'mysql'
	}
	connection.connect() or { panic(err) }
	return connection
}

fn main() {
	pool := vweb.database_pool(handler: get_database_connection, nr_workers: 20)

	vweb.run_at(&App{
		db_handle: pool
	},
		port: 8080
		nr_workers: 20
	)!
}

Only the types in the App struct need to be changed and the implementation of the pool handler.

Compatibility

All older vweb applications with a single db connection will still work so no breaking changes.

Performance

Test script run.sh:

#!/bin/bash

sleep 1
wrk -t 4 -c 20 -d10s http://127.0.0.1:8080/ &
wrk -t 4 -c 20 -d10s http://127.0.0.1:8080/other &

Output:

Running 10s test @ http://127.0.0.1:8080/
Running 10s test @ http://127.0.0.1:8080/other
  4 threads and 20 connections
  4 threads and 20 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     3.15ms  642.50us  16.56ms   80.83%
    Req/Sec     0.91k   413.37     1.64k    68.00%
  36058 requests in 10.01s, 3.61MB read
Requests/sec:   3603.88
Transfer/sec:    369.54KB
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     3.14ms  639.29us  12.51ms   81.35%
    Req/Sec     0.96k    36.87     1.08k    76.50%
  38082 requests in 10.01s, 3.81MB read
Requests/sec:   3804.78
Transfer/sec:    390.14KB

Tested on intel i9 12900K with 24 threads (23 workers).

The test was perfomed with this script.
The load is equally distributed in the app averaging at $3604+ 3805 = 7409$ request per second.

[medvednikov]

Looks good, but the get_connection field doesn't feel right.

db or db_handle?

[einar-hjortdal]

I would love for a database connection pool for MySQL too. Great work

[Casper64]

Done! Updated the field to db_handle and managed for it to work with all databases. Will add the documentation by tomorrow.

[medvednikov]

Great work!