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Connection Pools

What is it?

A cache of database connections maintained so that the connections can be reused when future requests to the database are required

  • after a connection is created, it is placed in the pool and it is used again so that a new connection does not have to be established

The client pool allows you to have a reusable pool of clients you can check out, use, and return.

  • You generally want a limited number of pools in your application, and usually just 1. Creating an unbounded number of pools defeats the purpose of pooling at all.

Why do it?

Connecting a new client to the database server requires a handshake which can take 20-30 milliseconds. During this time passwords are negotiated, SSL may be established, and configuration information is shared with the client & server. Incurring this cost every time we want to execute a query would substantially slow down our application.

The client pool allows you to have a reusable pool of clients you can check out, use, and return

  • used to enhance the performance of executing commands on a database
  • If you're working on a web application or other software which makes frequent queries you'll want to use a connection pool.

How does it work?

  1. When the connection pool is created by the client (most often at the same time as when the client server starts), it pre-establishes a set number of database connections. These connections sit ready to be used (ie. with status available) by clients to the database server.
  2. When .connect() is called on the database client, one of the connections is borrowed from the pool. The connection that is assigned to the client gets marked as inUse
  3. After the client finishes the query, it calls '.close(), which returns the connection back to the pool (instead of actually closing it), and gets marked as available
  4. If a client request comes in and there are no available connections, the Pool Manager will either:
    • create a new connection (up to the configured maximum limit)
    • or wait until a connection becomes available.

We can configure the pool to have a minimum number of idle connections, and a maximum number of connections that can possibly be created

Typically, each instance of a database client would have its own pool

  • ex. If there are 4 Express servers running, then there will be 4 pools, and between those 4 servers and the users sending requests to them sits a load balancer

The pool sits between the database client (e.g. application server) and the database server:

front end client → load balancer → application server → pool → database

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