The Situation:
I am using a db class as a wrapper (dbwrapper) to open and close db connections via PHP PDO
I have 3 separate classes which extend this db wrapper
I instantiate 3 objects from said classes and in the course of their existence they each make db queries
MySQL used in this case.
When the methods of the 3 classes require db access they each create PDO objects internally, making use of the dbwrapper which they extended. Each object is storing its PDO object in a member/field for reference by itself.
The Question: My question is this... is each object creating a separate connection to the database, making 3 in total? Is each client of Apache creating only one connection to the database or is each object using the db in a php application creating a connection. (sounds inefficient!)
The Reason: I would like to understand how these connections are handled.
I am trying to decide if it would be better to have each class extend the dbwrapper or if it would be better to initialize the dbwrapper without making an automatic connection the db, handle that when its needed. Instead I would pass the dbwrapper to each objects constructor as they are initialized... letting them use that reference. If multiple db connections are happening then I think this would be the best way to minimize overhead while overcoming issues of object's scope and access.
Thanks in advance!
is each object creating a separate connection to the database, making 3 in total?
Maybe. I don't know, but here's how to find out. While your script is running, connect to MySQL via your client of choice (like the command line) and issue the command SHOW PROCESSLIST; You'll get a list of active connections.
You might need to insert a sleep in your script to keep it alive long enough for you to run the process list command when you've instantiated and are working on all three objects.
You'll see either one connection or three. Then you'll have your answer.
(The answer will vary depending on the underlying driver. Some drivers will reuse the connection if the DSN is identical.)
Instead I would pass the dbwrapper to each objects constructor as they are initialized... letting them use that reference
This is the common practice. Database handles are prime candidates for the Singleton pattern as well.
Related
I have written a data class for handling MySQL queries and then all other classes such as login, products extend this class to make use of the database. As a result, each page load creates 2 or more DB connections due to something similar to the following:
$login, parent::__construct(); // check login via db
$products, parent::__construct(); // fetch products from db
Is there a way around this such as adding some code in the constructor to verify whether an existing DB connection has already been established?
A fellow developer I work with writes in procedural style and simply uses a single global $_db object for all queries and, this seems a lot more efficient as it only creates 1 DB connection.
For many smaller applications, I make my database instance global to the whole application, along with configuration and other application-wide classes, such as logging. This is not necessarily the preferred method, as it couples code to expect things named certain ways, and could possibly cause conflicts down the road. However, for small utility applications it is convenient.
For anything larger, I usually utilize my DB from an ORM anyway, so it becomes a non issue.
Michael's example of creating a static class and how to use it helped resolve the problem.
https://stackoverflow.com/a/26981461/541091
I have the following setup for a game:
One database with all users, one database with users that didn't complete the tutorial. In the first database I have a flag that tells me if user "Gogu" completed the tutorial or not. If he didn't, I need to connect to the second database and get some data. After some research I found this: connecting to two different databases with Zend Framework.
The thing is that since only like 5% of the users will be in tutorial progress is no use to keep both connections so I only need to connect while in a controller, get what I need and close the connection.
Any idea how to do this?
You don't have to worry about 2 connections, because Zend_Db "lazy loads" the connection. From the ZF manual:
Creating an instance of an Adapter class does not immediately connect to the RDBMS server. The Adapter saves the connection parameters, and makes the actual connection on demand, the first time you need to execute a query. This ensures that creating an Adapter object is quick and inexpensive. You can create an instance of an Adapter even if you are not certain that you need to run any database queries during the current request your application is serving.
http://framework.zend.com/manual/en/zend.db.adapter.html#zend.db.adapter.connecting.getconnection
Just note that the method used in the accepted answer you referred calls $db->getConnection() from the bootstrap. This not recommended because it will defeat the purpose of lazy loading. You could also consider Zend_Application_Resource_Multidb, which is perhaps a more elegant approach:
http://framework.zend.com/manual/en/zend.application.available-resources.html#zend.application.available-resources.multidb
I am building a plugin for WordPress 3.3.1. In the code I define several shortcodes, class to support them, and a couple of admin pages. I am at a beginners level with php although I have 20+ years experience with programming, OOA&D, etc..
In the class methods, I make calls to a custom database not housed in the wp database. That is, the custom database is a separate schema, independent of the wp database.
Right now, I make the declaration in the methods that need the object. Works fine for dev but won't cut it in production. I am tempted to raise it to the class instance level. Here is where my question becomes clear. There are several classes that will need the connection. The plugin needs only one connection.
Where is the best place to put the database connection object declaration and initialization?
Given the answer to that, where is the proper place to destroy the db connection object instance?
I would make the database connection a static field. Which class to put it in depends on how you've structured your classes/code. Making it static will ensure that the same connection is shared throughout your script.
I recommend using PDO (connecting PDO to MySQL, etc) to connect to your database. PHP will automatically close the connection when the script ends.
What are the disadvantages of using a PHP database class as a singleton?
The disadvantages are the same as for any class that uses the Singleton pattern:
It is hard to test code that uses singletons.
If your DB class is built to only connect to a single database, you will have problems when you have a script that needs to connect to 2 two separate databases. However, you could build the singleton class to accept multiple server configurations, and then manage them within the singleton.
Otherwise, designing a database class as a singleton is a practice that makes a lot of sense, as you can maintain tight control over how many connections a script is making at any given time.
You can not use two database connections. You would want this because:
you have two databases.
you want to do something within a transaction when another transaction is already running on the 'current' database connection.
you want to use several mock database instances in your unit tests
It makes it hard to run unit tests against it and also makes it impossible to have multiple database connections. As we all know, global variables has lots of drawbacks and Singletons are no exception, only that they are a more "friendly" global variable.
I found a pretty good article about it and an old SO question as well.
We have a need to access a DB that only allows one connection at a time. This screams "singleton" to me. The catch of course is that the singleton connection will be exposed (either directly or indirectly) via a web-service (most probable a SOAP based web-service - located on a separate server from the calling app(s) ) - which means that there may be more than one app / instance attempting to connect to the singleton class.
In PHP, what is the best way to create a global singleton or a web-service singleton?
TIA
This screams "use a DB SERVER" to me. ;-), but...
You could create an SoapServer and use a semaphore to allow only 1 connection at a time
$s1 = sem_get(123, 1);
sem_acquire($s1);
// soapserver code here
sem_release($s1);
In PHP, there is no such thing as a "global" object that resides across all requests . In a java webserver, this would be called "application level data store". In php, the extent of the "global" scope (using the global keyword) is a single request. Now, there is also a cross session data store accessible via $_SESSION, but I'm trying to highlight that no variable in php is truly "global". Individual values emulate being global by being stored to a local file, or to a database, but for something like a resource, you are stuck creating it on each request.
Now, at the request level, you can create a Singleton that will return an initialized resource no matter which scope within the request you call it from, but again, that resource will not persist across or between requests. I know, it is a shortcoming of php, but on the other hand, the speed and stability of the individual requests help make up for this shortcoming.
Edit:
After reading over your question again, I realized you may not be asking for a singleton database access class, but rather something that can resource lock your database? Based on what you said, it sounds like the database may do the locking for you anyway. In other words, it won't allow you to connect if there is already another connection. If that is the case, it seems kind of like you have 2 options:
1) just let all your pages contend for the resource, and fail if they don't get it.
2) Create a queue service that can accept queries, run them, then cache the results for you for later retrieval.