Comments on: Checking for a live database connection considered harmful

By: Bottomless

Bottomless — Sat, 26 Jun 2010 05:36:10 +0000

Shouldn’t there be some sort of sleeping introduced in the code? If so for how long? (or better a back-off algorithm as suggested by “J”)

Otherwise it will just result in a storm of queries in a very short period of time, not allowing the system to recover from probably a temporary overload (and probably adding to the overload too) that caused the first query to fail in the first place.

Thanks

By: sys

sys — Sat, 15 May 2010 09:56:51 +0000

The Oracle database adapter shipped with the Sun (now Oracle) Java CAPS (Composite Application Platform System) actually used to get a sorted list of users in the database, for checking if a connection is alive.

After much frantic yelling at Support we got it down to _merely_ doing something similar to select 1 from dual.

By: Baron Schwartz

Baron Schwartz — Sat, 15 May 2010 08:09:56 +0000

Syd, that’s not true. See above about “common case.” If the uncommon case (a dead connection) is rare, which it should be, then a “heavy exception” that happens once in a few million queries is going to be a lot cheaper than a “cheap check” that happens once every query.

By: Syd

Syd — Sat, 15 May 2010 05:14:55 +0000

Depends on the actual implementation & overhead. It may or may not be faster to manually check rather than relying on an exception, which can be heavy, depending on your platform.

By: Haile

Haile — Sat, 15 May 2010 05:13:10 +0000

Yes, this is a general principle that applies outside databases. Checking whether a file exists before trying to open it is the classic example of LBYL and has the same problems as the database example.

By: Brigham

Brigham — Fri, 14 May 2010 23:08:02 +0000

Thanks for the interesting article, especially the info about what goes on in the server when you make a check. I think the check at any point is really useless for the most part. I would not perform a retry either because if the connection is down something is really wrong. I would report the error back to the consumer of the query and let it decide how to handle.

By: Waldo Jaquith - links for 2010-05-14

Waldo Jaquith - links for 2010-05-14 — Fri, 14 May 2010 23:05:58 +0000

Baron Schwartz: Checking for a live database connection considered harmful Baron explains why checking for a working DB connection at the head of your code in website development is actually a pretty bad idea. And I'll be damned if he's not right. I've been doing this for years, but I'll stop now. (tags: mysql php programming database)

By: Jay

Jay — Fri, 14 May 2010 20:52:45 +0000

Roland,

He does retry to connect as many times as specified in the parameter.
Because of the while loop it will try to reconnect and requery

By: Scott S. McCoy

Scott S. McCoy — Fri, 14 May 2010 20:50:25 +0000

I would consider this type of continual query attempt to be harmful in many instances. I prefer that applications simply error if the connection becomes stale or breaks â€” I would like not to have partial and corrupt data in most instances.

By: developingElegance();

developingElegance(); — Fri, 14 May 2010 20:47:04 +0000

Don’t check for a live database connection…

Baron Schwartz from the MySQL Performance Blog chimes in with a great point on reducing the load on your MySQL server – use a try / catch functionality instead of an if statement to figure out if you’re going to have a problem with the Â MyS…