Error in connector: Error creating a database connection. (Operation timed out (mobc timeout))

Bug description

Sometimes, due to DB connections limit, the client fails with this error:

Error in connector: Error creating a database connection. (Operation timed out (mobc timeout))
    at PrismaClientFetcher.request (/home/nicosampler/develop/loan-system/node_modules/@prisma/client/runtime/index.js:78786:15)
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async eval (webpack-internal:///./graphql/migration/resolver.ts:139:17)
(node:106069) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 75)

my DB configuration is like so: DATABASE_URL="mysql://someUser:somePass@host/db?connection_limit=10"

Note: connection_limit

How to reproduce

It happened to me when trying to create/update/delete several entities at the same time, for example:

const res = [largeArray]
res.forEach(async (client) => {
       const createdClient = await context.prisma.client.create({data: {client}})
})

Expected behavior

The client should be able to recover from a timeout connection.

Environment & setup

• OS: Ubuntu
• Database: Mysql
• Node.js version: 14
• Prisma version: 2.11.0

[pantharshit00]

I can reproduce this with a heroku database with connection limit of 10 and this simple script.

All timed out calls doesn’t recover. Connections in the heroku reaches max of 11 then it starts to give timeouts.

Reproduction: https://github.com/harshit-test-org/prisma-issue-4280

[JonathanCallewaert]

I am using the latest version of prisma (2.15.0). It’s throwing the new error (https://github.com/prisma/prisma-engines/pull/1515). But the error message doesn’t make sense. I have 150 connections as limit, 10 connections are used but it still fires an error:

Timed out fetching a new connection from the pool. Please consider reducing the number of requests or increasing the `connection_limit` parameter (https://www.prisma.io/docs/concepts/components/prisma-client/connection-management#connection-pool). Current limit: 150, amount of connections currently in use: 10.

The connection limit of my DB (postgres) is 180