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Connection pool

The query engine manages a connection pool of database connections. The pool is created when Prisma Client opens the first connection to the database, which can happen in one of two ways:

Relational database connectors use Prisma ORM's own connection pool, and the MongoDB connectors uses the MongoDB driver connection pool.

Relational databases​

The relational database connectors use Prisma ORM's connection pool. The connection pool has a connection limit and a pool timeout, which are controlled by connection URL parameters.

How the connection pool works​

The following steps describe how the query engine uses the connection pool:

  1. The query engine instantiates a connection pool with a configurable pool size and pool timeout.
  2. The query engine creates one connection and adds it to the connection pool.
  3. When a query comes in, the query engine reserves a connection from the pool to process query.
  4. If there are no idle connections available in the connection pool, the query engine opens additional database connections and adds them to the connection pool until the number of database connections reaches the limit defined by connection_limit.
  5. If the query engine cannot reserve a connection from the pool, queries are added to a FIFO (First In First Out) queue in memory. FIFO means that queries are processed in the order they enter the queue.
  6. If the query engine cannot process a query in the queue for before the time limit, it throws an exception with error code P2024 for that query and moves on to the next one in the queue.

If you consistently experience pool timeout errors, you need to optimize the connection pool .

Connection pool size​

Default connection pool size​

The default number of connections (pool size) is calculated with the following formula:

num_physical_cpus * 2 + 1

num_physical_cpus represents the number of physical CPUs on the machine your application is running on. If your machine has four physical CPUs, your connection pool will contain nine connections (4 * 2 + 1 = 9).

Although the formula represents a good starting point, the recommended connection limit also depends on your deployment paradigm - particularly if you are using serverless.

Setting the connection pool size​

You can specify the number of connections by explicitly setting the connection_limit parameter in your database connection URL. For example, with the following datasource configuration in your Prisma schema the connection pool will have exactly five connections:

datasource db {
  provider = "postgresql"
  url      = "postgresql://johndoe:mypassword@localhost:5432/mydb?connection_limit=5"
}

Viewing the connection pool size​

The number of connections Prisma Client uses can be viewed using logging and metrics.

Using the info logging level, you can log the number of connections in a connection pool that are opened when Prisma Client is instantiated.

For example, consider the following Prisma Client instance and invocation:

import { PrismaClient } from '@prisma/client'

const prisma = new PrismaClient({
  log: ['info'],
})

async function main() {
  await prisma.user.findMany()
}

main()
Show CLI results
prisma:info Starting a postgresql pool with 21 connections.

When the PrismaClient class was instantiated, the logging notified stdout that a connection pool with 21 connections was started.

warning

Note that the output generated by log: ['info'] can change in any release without notice. Be aware of this in case you are relying on the output in your application or a tool that you're building.

If you need even more insights into the size of your connection pool and the amount of in-use and idle connection, you can use the metrics feature (which is currently in Preview).

Consider the following example:

import { PrismaClient } from '@prisma/client'

const prisma = new PrismaClient()

async function main() {
  await Promise.all([prisma.user.findMany(), prisma.post.findMany()])

  const metrics = await prisma.$metrics.json()
  console.dir(metrics, { depth: Infinity })
}

main()
Show CLI results
{
  "counters": [
    // ...
    {
      "key": "prisma_pool_connections_open",
      "labels": {},
      "value": 2,
      "description": "Number of currently open Pool Connections"
    }
  ],
  "gauges": [
    // ...
    {
      "key": "prisma_pool_connections_busy",
      "labels": {},
      "value": 0,
      "description": "Number of currently busy Pool Connections (executing a datasource query)"
    },
    {
      "key": "prisma_pool_connections_idle",
      "labels": {},
      "value": 21,
      "description": "Number of currently unused Pool Connections (waiting for the next datasource query to run)"
    },
    {
      "key": "prisma_pool_connections_opened_total",
      "labels": {},
      "value": 2,
      "description": "Total number of Pool Connections opened"
    }
  ],
  "histograms": [
    /** ... **/
  ]
}
info

For more details on what is available in the metrics output, see the About metrics section.

Connection pool timeout​

Default pool timeout​

The default connection pool timeout is 10 seconds. If the Query Engine does not get a connection from the database connection pool within that time, it throws an exception and moves on to the next query in the queue.

Setting the connection pool timeout​

You can specify the pool timeout by explicitly setting the pool_timeout parameter in your database connection URL. In the following example, the pool times out after 2 seconds:

datasource db {
  provider = "postgresql"
  url      = "postgresql://johndoe:mypassword@localhost:5432/mydb?connection_limit=5&pool_timeout=2"
}

Disabling the connection pool timeout​

You disable the connection pool timeout by setting the pool_timeout parameter to 0:

datasource db {
  provider = "postgresql"
  url      = "postgresql://johndoe:mypassword@localhost:5432/mydb?connection_limit=5&pool_timeout=0"
}

You can choose to disable the connection pool timeout if queries must remain in the queue - for example, if you are importing a large number of records in parallel and are confident that the queue will not use up all available RAM before the job is complete.

MongoDB​

The MongoDB connector does not use the Prisma ORM connection pool. The connection pool is managed internally by the MongoDB driver and configured via connection string parameters.

External connection poolers​

You cannot increase the connection_limit beyond what the underlying database can support. This is a particular challenge in serverless environments, where each function manages an instance of PrismaClient - and its own connection pool.

Consider introducing an external connection pooler like PgBouncer to prevent your application or functions from exhausting the database connection limit.

Manual database connection handling​

When using Prisma ORM, the database connections are handled on an engine-level. This means they're not exposed to the developer and it's not possible to manually access them.