My site has always used persistent connections, based on my understanding of them there's no reason not to. Why close the connection when it can be reused? I have a site that in total accesses about 7 databases. It's not a huge traffic site, but it's big enough. What's your take on persistent, should I use them?
With persistent connections:
You cannot build transaction processing effectively
impossible user sessions on the same connection
app are not scalable. With time you may need to extend it and it will require management/tracking of persistent connections
if the script, for whatever reason, could not release the lock on the table, then any following scripts will block indefinitely and one should restart the db server. Using transactions, transaction block will also pass to the next script (using the same connection) if script execution ends before the transaction block completes, etc.
Persistent connections do not bring anything you can do with non-persistent connections.
Then, why to use them, at all?
The only possible reason is performance, to use them when overhead of creating a link to your SQL Server is high. And this depends on many factors like:
database type
whether MySQl server is on the same machine and, if not, how far? might be out of your local network /domain?
how much overloaded by other processes the machine on which MySQL sits
One always can replace persistent connections with non-persistent connections. It might change the performance of the script, but not its behavior!
Commercial RDMS might be licensed by the number of concurrent opened connections and here the persistent connections can misserve
My knowledge on the area is kinda limited so I can't give you many details on the subject but, as far as I know, the process of creating connections and handing them to a thread really costs resources, so I would avoid it if I were you. Anyhow I think that most of this decisions can't be generalized and depend on the business.
If, for instance, your application communicates continuously with the Database and will only stop when the application is closed, then perhaps persistent connections are the way to go, for you avoid the process mentioned before.
However, if your application only communicates with the Database sporadically to get minor information then closing the connection might be more sane, for you won't waste resources on opened connections that are not being used.
Also there is a technique called "Connection Pooling", in which you create a series of connections a priori and keep them there for other applications to consume. In this case connections are persistent to the database but non-persistent to the applications.
Note: Connections in MSSQL are always persistent to the database because connection pooling is the default behavior.
Related
Assuming I'll execute every 2 seconds a query, should I open the connection on every request or should I keep the connection alive until the application(server) stops?
In my experience, establishing connections is unlikely to be a bottleneck for a mysql server (connection overhead is fairly low in mysql). That having been said, reusing existing connections is often an appropriate approach, but it requires some careful considerations: if the database server is temporarily unavailable, the code must reconnect; if the server is replaced, it must reconnect (mysql implementations tend towards failover solutions rather than true high availability); if the application uses multiple connections to mysql, you must be sure not to cross your connections between users or sessions (active database, timezone, charset, and so on are sessions variables, essentially tied to a connection). If you're not up to the task of making your reusable connection reliable in these and other edge cases, creating a new connection every 2 seconds may provide this durability for free.
In short, there can be less-than-obvious benefits to short lived connections. I would not bother to add intelligence around maintaining a persistent connection unless you have reason to believe it will actually make a meaningful difference in your case (eg benchmarks).
I made a program that receives user input and stores it on a MySQL database. I want to implement this program on several computers so users can upload information to the same database simoultaneously. The database is very simple, it has just seven columns and the user will only enter four of them.
There would be around two-three hundred computers uploading information (not always at the same time but it can happen). How reliable is this? Is that even possible?
It's my first script ever so I appreciate if you could point me in the right direction. Thanks in advance.
Having simultaneous connections from the same script depends on how you're processing the requests. The typical choices are by forking a new Python process (usually handled by a webserver), or by handling all the requests with a single process.
If you're forking processes (new process each request):
A single MySQL connection should be perfectly fine (since the total number of active connections will be equal to the number of requests you're handling).
You typically shouldn't worry about multiple connections since a single MySQL connection (and the server), can handle loads much higher than that (completely dependent upon the hardware of course). In which case, as #GeorgeDaniel said, it's more important that you focus on controlling how many active processes you have and making sure they don't strain your computer.
If you're running a single process:
Yet again, a single MySQL connection should be fast enough for all of those requests. If you want, you can look into grouping the inserts together, as well as multiple connections.
MySQL is fast and should be able to easily handle 200+ simultaneous connections that are writing/reading, regardless of how many active connections you have open. And yet again, the performance you get from MySQL is completely dependent upon your hardware.
Yes, it is possible to have up to that many number of mySQL connectins. It depends on a few variables. The maximum number of connections MySQL can support depends on the quality of the thread library on a given platform, the amount of RAM available, how much RAM is used for each connection, the workload from each connection, and the desired response time.
The number of connections permitted is controlled by the max_connections system variable. The default value is 151 to improve performance when MySQL is used with the Apache Web server.
The important part is to properly handle the connections and closing them appropriately. You do not want redundant connections occurring, as it can cause slow-down issues in the long run. Make sure when coding that you properly close connections.
I'm currently developing plugins for bukkit and a lot of them need a database connection. Now I'm thinking about if could be better to have just one plugin that handles the connection for all plugins.
The question behind that is if it is good or not to keep a connection up even if there are no queries for some minutes (that may happen). Otherwise I would need to establish a new connection for each query?
It is a good idea to have one class/plugin for handling database, but the connection state should not be open all the time,make sure the connection is opened only for the time taken by the query.
Many applications use connection pools to have a number of connections readily available to run queries over. It reduces the number of protocol re-negotiations that the database driver has to do. This is especially useful for applications that need fast access times to the underlying data, yet have larger downtimes between requests. E-Commerce applications like webshops are a good example.
I'm doing my first foray with mysql and I have a doubt about how to handle the connection(s) my applications has.
What I am doing now is opening a connection and keeping it alive until I terminate my program. I do a mysql_ping() every now and then and the connection is started with MYSQL_OPT_RECONNECT.
The other option (I can think of), would be to start a new connection before doing anything that requires my connection to the database and closing it after I'm done with it.
What are the pros and cons of these two approaches?
what are the "side effects" of a long connection?
What is the most used method of handling this?
Cheers ;)
Some extra details
At this point I am keeping the connection alive and I ping it every now and again to now it's status and reconnect if needed.
In spite of this, when there is some consistent concurrency with queries happening in quick succession, I get a "Server has gone away" message and after a while the connection is re-established.
I'm left wondering if this is a side effect of a prolonged connection or if this is just a case of bad mysql server configuration.
Any ideas?
In general there is quite some amount of overhead incurred when opening a connection. Depending on how often you expect this to happen it might be ok, but if you are writing any kind of application that executes more than just a very few commands per program run, I would recommend a connection pool (for server type apps) or at least a single or very few connections from your standalone app to be kept open for some time and reused for multiple transactions.
That way you have better control over how many connections get opened at the application level, even before the database server gets involved. This is a service an application server offers you, but it can also be rolled up rather easily if you want to keep it smaller.
Apart from performance reasons a pool is also a good idea to be prepared for peaks in demand. When a lot of requests come in and each of them tries to open a separate connection to the database - or as you suggested even more (per transaction) - you are quickly going to run out of resources. Keep in mind that every connection consumes memory inside MySQL!
Also you want to make sure to use a non-root user to connect, because if you don't (I think it is tied to the MySQL SUPER privilege), you might find yourself locked out. MySQL reserves at least one connection for an administrator for problem fixing, but if your app connects with that privilege, all connections would already be used up when you try to put out the fire manually.
Unless you are worried about having too many connections open (i.e. over 1,000), you she leave the connection open. There is overhead in connecting/reconnecting that will only slow things down. If you know you are going to need the connection to stay open for a while, run this query instead of pinging periodically:
SET SESSION wait_timeout=#
Where # is the number of seconds to leave an idle connection open.
What kind of application are you writing? If it's a webscript: keep it open. If it's an executable, pool your connections (if necessary, most of the times a singleton will do).
I've been thinking, why does Apache start a new connection to the MySQL server for each page request? Why doesn't it just keep ONE connection open at all times and send all sql queries through that one connection (obviously with client id attached to each req)?
It cuts down on the handshake time overhead, and a couple of other advantages that I see.
It's like plugging in a computer every time you want to use it. Why go to the outlet each time when you can just leave it plugged in?
MySQL does not support multiple sessions over a single connection.
Oracle, for instance, allows this, and you can setup Apache to mutliplex several logical sessions over a single TCP connection.
This is limitation of MySQL, not Apache or script languages.
There are modules that can do session pooling:
Precreate a number of connections
Pick a free connection on demand
Create additional connections if not free connection is available.
the reason is: it's simpler.
to re-use connections, you have to invent and implement connection pooling. this adds another almost-layer of code that has to be developed, maintained, etc.
plus pooled connections invite a whole other class of bugs that you have to watch out for while developing your application. for example, if you define a user variable but the next user of that connection goes down a code path that branches based on the existence of that variable or not then that user runs the wrong code. other problems include: temporary tables, transaction deadlocks, session variables, etc. all of these become very hard to duplicate because it depends on the subsequent actions of two different users that appear to have no ties to each other.
besides, the connection overhead on a mysql connection is tiny. in my experience, connection pooling does increase the number of users a server can support by very much.
Because that's the purpose of the mod_dbd module.