Why should I close or keep Redis connections open?

Question

I'm using Redis in a PHP project. I use phpredis as a client. Sometimes, during long CLI-scripts, I experience PHP segmentation faults.

I've experienced before that phpredis has problems when the connection times out. As my Redis config is configured to automatically close idle connections after 300 seconds, I guess that causes the segmentation fault.

In order to be able to choose whether to increase the connection timeout or default it to 0 (which means "never timeout"), I would like to know what the possible advantages and disadvantages are?

Why should I never close a connection?
Why should I make sure connections don't stay open?

Thanks

Consanguinity

Answer 1

Generally, opening a connection is an expensive operation so modern best practices are to keep them open. On the other hand, open connections requires resources (from the database) to manage so keeping a lot of idle connections open can also be problematic. This trade off is usually resolved via the use of connection pools.

That said, what's more interesting is why does PHP segfault. The timeout is, evidently, caused by a long running command (CLI script in your case) that blocks Redis (which is mostly single threaded) from attending to the PHP app's connections. While this is a well-known Redis behavior, I would expect PHP (event without featuring reconnect at the client library) not to s**t its pants so miserably.

Ectomorph

Answer 2

The answer to your question much depends on cases of redis usage in your application. So, should your never close a connection with idle connection timeout?

In general no, your should keep it default - 0. Why or when:

• Any types of long living application. Such as CLI-script ot background worker. Why - phpredis do not has builded in reconnection feature so your should take care about this by yourself or do not your idle timeout.
• Each time your request processed or CLI script die - all connections would be closed by php engine. Redis server close all connection for closed client sockets. You will have no problems like zombie connection or something like that. As extension, phpredis close connection in destructor - so your may be sure connections don't stay open.

p.s. Of course your can implement reconnection insome proxy class in php by yourself. We have redis in high load environment - ~4000 connections per second on instance. After 2.4 version we do not use idle connection timeout. And do not have any types of troubles with that.

Popple