How are "simultaneous client connections" quantified in mysql

Sorry for the newb factor, but I was reading about "Too many connections" to mysql.

http://dev.mysql.com/doc/refman/5.5/en/too-many-connections.html

How are "simultaneous client connections" quantified in mysql?

For example if 20 million people are on gmail (let's say they use mysql with only 1 table to store everything just for sake of example) and all those people simultaneously all click on an email to open up, doe开发者_JS百科s that mean there are 20 million simultaneous connections or just one connection since all the users are connecting to the same table?

EDIT: I'm trying to understand what the term 'client' means. Is a 'client' someone who is using the application, or is a 'client' the part of the application (ex. php script) that is connecting to the database?

When a visitor goes to your website and the server-side script connects to the database it is 1 connection - you can make as many queries as necessary during that connection to any number of tables/databases - and on termination of the script the connection ends. If 31 people request a page (and hence a db connection) and your limit is 30, then the 31st person will get an error.

You can upgrade server hardware so MySQL can efficiently handle loads of connections or spread the load across multiple database servers. It is possible to have your server-side scripting environment maintain a persistent connection to MySQL in which case all scripts make queries through that single connection. This will probably have adverse effects on the correct queuing of queries and their order to maintain usable speeds under high load, and ultimately doesn't solve the CPU/memory/disk bottlenecks with handling large numbers of queries.

In the case of a webmail application, the query to check for new messages runs so fast (in the milliseconds) that hitting server limits isn't likely unless it's on a large scale.

Google's applications scale on a level previously unheard of. Check out the docs on MapReduce, GoogleFS, etc. It's awesome.

In answer to your edit - anything that connects directly to MySQL is considered a client in this case. Each PHP script that connects to MySQL is a client, as is the MySQL console on the command line, or anything else.

Hope that helps

The connections mentioned are server connection. Every client has one or more. For example if your php script connects mysql, there may be more web requests at a time and thus more connections to db.

Sometimes you can ran out of them, because they are not closed properly after they become useless.

And I thing Gmail is stored different way than in one large mysql db :]