I am using sql with Go, using the mysql driver and database/sql.
While reading about it I found that this driver handles the things related to connection pooling itself and db, err := db.Open("conn string") returns the connection pool instead of a single connection and when we use db.Query("some query"), it actually picks a free connection from connection pool (if no connection in pool is free it opens a new one) and uses it.
Please correct me, if I am wrong in my above understanding.
Now my question is, what happens when I call db.Close():
Will it close all connections in the connection pool?
Will it close then even if they they are passed to different goroutines and are being used there?
Related
How safe is connection pooling with regard to connection settings? When a connection is released back to the pool, does the pool issue some sort of reset command to restore the connection to a known state, or is the connection just naively returned to the pool?
This is something that always bothers me when writing code that uses SET FOREIGN_KEY_CHECKS=0. If I forget/fail to call SET FOREIGN_KEY_CHECKS=1 before releasing the connection back to the pool, does the next process end up with an unsafe connection with foreign key checks disabled?
Since this may be specific to the pooling software, I'm mainly asking about node-mysql and JDBC. I looked at the code for node-mysql and couldn't see anything that resets the connection. I have no idea about JDBC.
I am using MySQL Workbench. If I open a connection (in Visual Basic) to MySQL it will show that a connection is opened. If I close the connection, (in MySQL Workbench), it shows the connection is still open. If I try to reopen the same connection, it will open and it will close without any problem. However, MySQL Workbench shows that the first connection is still open. It stays open until I exit the application.
Try
Using myConnection As New MySqlConnection(_ServerString_vip)
myConnection.Open()
myConnection.Close()
End Using
End Using
Catch ex As Exception
MsgBox("Error getting Username Information.")
End Try
You should do some reading on connection pooling in ADO.NET.
A DbConnection object is actually quite light-weight and exists at a higher level than the actual database connection. When you open a DbConnection object, a low-level database connection is opened. When you close the DbConnection object, the low-level database connection remains open for some time. That's because opening and closing it is an expensive operation. If you open another DbConnection with the same connection string, the same low-level database connection will be reused. The low-level connection is only closed if it remains unused for some time.
This is not something you have to worry about. This is the way it is and has always been in ADO.NET and the system handles it automatically. You keep doing what you're doing, i.e. creating DbConnection objects with Using statements, and the system will take care of the rest.
By the way, there's no point opening the connection explicitly when calling Fill or Update on a data adapter. Both those methods will automatically open the connection and close it again afterwards. You should only open the connection explicitly if you want it to stay open after the Fill or Update call, i.e. if you have more work to do over that connection.
jmcilhinney is absolutely right. I was looking all over the Internet and I couldn't find an answer for that. Hopefully, this question will help a lot of people. And eventually the connection will die automatically.
Is it possible to cache database connections when using PHP like you would in a J2EE container? If so, how?
There is no connection pooling in php.
mysql_pconnect and connection pooling are two different things.
There are many problems connected with mysql_pconnect and first you should read the manual and carefully use it, but this is not connection pooling.
Connection pooling is a technique where the application server manages the connections. When the application needs a connection it asks the application server for it and the application server returns one of the pooled connections if there is one free.
We can do connection scaling in php for that please go through following link: http://www.oracle.com/technetwork/articles/dsl/white-php-part1-355135.html
So no connection pooling in php.
As Julio said apache releases all resources when the request ends for the current reques. You can use mysql_pconnect but you are limited with that function and you must be very careful. Other choice is to use singleton pattern, but none of this is pooling.
This is a good article: https://blogs.oracle.com/opal/highly-scalable-connection-pooling-in-php
Also read this one http://www.apache2.es/2.2.2/mod/mod_dbd.html
Persistent connections are nothing like connection pooling. A persistent connection in php will only be reused if you make multiple db connects within the same request/script execution context. In most typical web dev scenarios you'll max out your connections way faster if you use mysql_pconnect because your script will have no way to get a reference to any open connections on your next request. The best way to use db connections in php is to make a singleton instance of a db object so that the connection is reused within the context of your script execution. This still incurs at least 1 db connect per request, but it's better than making multiple db connects per reqeust.
There is no real db connection pooling in php due to the nature of php. Php is not an application server that can sit there in between requests and manage references to a pool of open connections, at least not without some kind of major hack. I think in theory you could write an app server in php and run it as a commandline script that would just sit there in the background and keep a bunch of db connections open and pass references to them to your other scripts, but I don't know if that would be possible in practice, how you'd pass the references from your commandline script to other scripts, and I sort of doubt it would perform well even if you could pull it off. Anyway that's mostly speculation. I did just notice the link someone else posted to an apache module to allow connection pooling for prefork servers such as php. Looks interesting:
https://github.com/junamai2000/mod_namy_pool#readme
I suppose you're using mod_php, right?
When a PHP file finishes executing all it's state is killed so there's no way (in PHP code) to do connection pooling. Instead you have to rely on extensions.
You can mysql_pconnect so that your connections won't get closed after the page finishes, that way they get reused in the next request.
This might be all that you need but this isn't the same as connection pooling as there's no way to specify the number of connections to maintain opened.
You can use MySQLi.
For more info, scroll down to Connection pooling section # http://www.php.net/manual/en/mysqli.quickstart.connections.php#example-1622
Note that Connection pooling is also dependent on your server (i.e. Apache httpd) and its configuration.
If an unused persistent connection for a given combination of "host, username, password, socket, port and default database can not be found" in the open connection pool, then only mysqli opens a new connection otherwise it would reuse already open available persistent connections, which is in a way similar to the concept of connection pooling. The use of persistent connections can be enabled and disabled using the PHP directive mysqli.allow_persistent. The total number of connections opened by a script can be limited with mysqli.max_links (this may be interesting to you to address max_user_connections issue hitting hosting server's limit). The maximum number of persistent connections per PHP process can be restricted with mysqli.max_persistent.
In wider programming context, it's a task of web/app server however in this context, it's being handled by mysqli directive of PHP itself in a way supporting connection re-usability. You may also implement a singleton class to get a static instance of connection to reuse just like in Java. Just want to remind that java also doesn't support connection pooling as part of its standard JDBC, they're being different module/layers on top of JDBC drivers.
Coming to PHP, the good thing is that for the common databases in the PHP echosystem it does support Persistent Database Connections which persists the connection for 500 requests (config of max_requests in php.ini) and this avoids creating a new connection in each request. So check it out in docs in detail, it solves most of your challenges. Please note that PHP is not so much sophisticated in terms of extensive multi-threading mechanism and concurrent processing together with powerful asynchronous event handling, when compared to strictly object oriented Java. So in a way it is very less effective for PHP to have such in-built mechanism like pooling.
You cannot instantiate connection pools manually.
But you can use the "built in" connection pooling with the mysql_pconnect function.
I would like to suggest PDO::ATTR_PERSISTENT
Persistent connections are links that do not close when the execution of your script ends. When a persistent connection is requested, PHP checks if there's already an identical persistent connection (that remained open from earlier) - and if it exists, it uses it. If it does not exist, it creates the link.
Connection pooling works at MySQL server side like this.
If persistence connection is enabled into MySQL server config then MySQL keep a connection open and in sleep state after requested client (php script) finises its work and die.
When a 2nd request comes with same credential data (Same User Name, Same Password, Same Connection Parameter, Same Database name, Maybe from same IP, I am not sure about the IP) Then MySQL pool the previous connection from sleep state to active state and let the client use the connection. This helps MySQL to save time for initial resource for connection and reduce the total number of connection.
So the connection pooling option is actually available at MySQL server side. At PHP code end there is no option. mysql_pconnect() is just a wrapper that inform PHP to not send connection close request signal at the end of script run.
For features such as connection pooling - you need to install swoole extension first: https://openswoole.com/
It adds async features to php.
After that its trivial to add mysql and redis connection pooling:
https://github.com/open-smf/connection-pool
Some PHP frameworks come with pooling built-in: https://hyperf.wiki/2.2/#/en/pool
With the node-mysql module, there are two connection options - a single connection and a connection pool. What is the best way to set up a connection to a MySQL database, using a single global connection for all requests, or creating a pool of connections and taking one from the pool for each request? Or is there a better way to do this? Will I run in to problems using just a single shared connection for all requests?
Maintaining a single connection for the whole app might be a little bit tricky.
Normally, You want to open a connection to your mysql instance, and wait for it to be established.
From this point you can start using the database (maybe start a HTTP(S) server, process the requests and query the database as needed.)
The problem is when the connection gets destroyed (ex. due to a network error).
Since you're using one connection for the whole application, you must reconnect to MySQL and somehow queue all queries while the connection is being established. It's relatively hard to implement such functionality properly.
node-mysql has a built-in pooler. A pooler, creates a few connections and keeps them in a pool. Whenever you want to close a connection obtained from the pool, the pooler returns it to the pool instead of actually closing it. Connections on the pool can be reused on next open calls.
IMO using a connection pool, is obviously simpler and shouldn't affect the performance much.
I found a very good module (node-mysql) to connect to Mysql database.
The module is very good, I Only have a question about "WHEN" to open the connection to Mysql.
I always have used php-mysql before starting with node, for each request i opened a connection...then query....then close.
Is the same with node? for each request do I have to open a connection and then close it? or can i use persistent connection?
Thank you
The open-query-close pattern generally relies on connection pooling to perform well. Node-mysql doesn't have any built in connection pooling, so if you use this pattern you'll be paying the cost of establishing a new connection each time you run a query (which may or may not be fine in your case).
Because node is single threaded, you can get away with a single persistent connection (especially since node-mysql will attempt to reconnect if the connection dies), but there are possible problems with that approach if you intend to use transactions (since all users of the node client are sharing the same connection and so same transaction state). Also, a single connection can be a limit in throughput since only one sql command can be executed at a time.
So, for transactional safety and for performance, the best case is really to use some sort of pooling. You could build a simple pool yourself in your app or investigate what other packages are out there to provide that capability. But either open-query-close, or persistent connection approaches may work in your case also.
felixge/node-mysql now has connection pooling (at the time of this writing.)
https://github.com/felixge/node-mysql#pooling-connections
Here's a sample code from the above link:
var mysql = require('mysql');
var pool = mysql.createPool(...);
pool.getConnection(function(err, connection) {
// Use the connection
connection.query( 'SELECT something FROM sometable', function(err, rows) {
// And done with the connection.
connection.end();
// Don't use the connection here, it has been returned to the pool.
});
});
So to answer your question (and same as #Geoff Chappell's answer): best case would be to utilize pooling to manage connections.