Understanding Data_Handler snippet

amy_damnit

Below is a snippet from my book that is unclear to me...

// Class providing generic data access functionality
class database_handler {
    // Hold an instance of the PDO class
    private static $_mHandler;

// Private constructor to prevent direct creation of object
private function __construct()
{
}

// Return an initialized database handler
private static function GetHandler()
{
    // Create a database connection only if one doesn't already exist
    if (!isset(self::$_mHandler))
    {
        // Execute code catching potential exceptions
        try
        {
            // Create a new PDO class instance
            self::$_mHandler =
                new PDO(PDO_DSN, DB_USERNAME, DB_PASSWORD,
                    array(PDO::ATTR_PERSISTENT => DB_PERSISTENCY));
            // Configure PDO to throw exceptions
            self::$_mHandler->setAttribute(PDO::ATTR_ERRMODE,
                                           PDO::ERRMODE_EXCEPTION);
        }
        catch (PDOException $e)
        {
            // Close the database handler and trigger an error
            self::Close();
            trigger_error($e->getMessage(), E_USER_ERROR);
        }
    }

    // Return the database handler
    return self::$_mHandler;
}

Questions:
1.) "Hold an instance of the PDO class" is the same as saying "PDO Object", right??

2.) But private static $_mHandler; create a Static (Class) Variable and not an Object variable, right??

3.) // Create a database connection only if one doesn't already exist
if (!isset(self::$_mHandler))

Again, this conflicts with "Hold an instance of the PDO class" since $_mHandler is a Class Variable, right??

4.) // Create a database connection only if one doesn't already exist
if (!isset(self::$_mHandler))

Wouldn't you want to use Objects here??

If Bob and John and Sue and Alex all go to my website and need to perform queries, wouldn't they each need their own seperate Database Connections??

I am very new to OOP and while I understood the theory in my last book, applying the code from this new book is a little challenging!

Amy

laserlight

amy.damnit wrote:
1.) "Hold an instance of the PDO class" is the same as saying "PDO Object", right??

Yes, in the sense that "instance of the PDO class" means "PDO object".

amy.damnit wrote:
2.) But private static $_mHandler; create a Static (Class) Variable and not an Object variable, right??

It declares a class member variable as private. This variable can later contain a reference to an object.

amy.damnit wrote:
Wouldn't you want to use Objects here??

But you do 🙂

amy.damnit wrote:
If Bob and John and Sue and Alex all go to my website and need to perform queries, wouldn't they each need their own seperate Database Connections??

That is what will happen. The value of the class member variable is not necessarily the same for each run of the script.

amy_damnit

laserlight;10925229 wrote:

Originally Posted by amy.damnit
2.) But private static $_mHandler; create a Static (Class) Variable and not an Object variable, right??

It declares a class member variable as private. This variable can later contain a reference to an object.

Can it? That isn't how Class variables are typically used.

laserlight;10925229 wrote:

Originally Posted by amy.damnit
If Bob and John and Sue and Alex all go to my website and need to perform queries, wouldn't they each need their own seperate Database Connections??

That is what will happen. The value of the class member variable is not necessarily the same for each run of the script.

If you have 4 people connecting at basically the same time, then I don't see how you could use a Class variable to store things...

Identifying a variable as static means that it is shared among ALL instances of a class (i.e. "Class Variable"). If one instance changes the value of a static variable it changes for all instances. (This is in contrast to an "Instance Variable" where a new Variable exists for each newly instantiated Object.)

If you have 4 concurrent users, it would seem that you would need:

Obj1->$mHandler
Obj2->$mHandler
Obj3->$mHandler
Obj3->$mHandler

versus

Class::$_mHandler

Amy

laserlight

amy.damnit wrote:
Can it? That isn't how Class variables are typically used.

So, how are class member variables typically used? Why do you find it surprising that a class member variable can store an object (or rather, object reference) instead of say, an integer?

amy.damnit wrote:
Identifying a variable as static means that it is shared among ALL instances of a class (i.e. "Class Variable"). If one instance changes the value of a static variable it changes for all instances.

The static member variable is shared among all instances of a class, within the same run of the script. Different user, different run of the script.

If you want to take this "shared among ALL instances of a class" to the conclusion that you seem to be heading, then a static member variable in say, a widely distributed script, must be shared among all instances of that class, even if some instances are created on a web server running in a local intranet completely disconnected from the Internet. Obviously, that is not true.

amy_damnit

laserlight;10925233 wrote:
So, how are class member variables typically used? Why do you find it surprising that a class member variable can store an object (or rather, object reference) instead of say, an integer?

I suppose it could, but since Classes to Objects is One-to-Many, it wouldn't seem like storing the reference to one of many Objects in a Class variable would serve much purpose, but anything is possible.

The static member variable is shared among all instances of a class, within the same run of the script. Different user, different run of the script.

So if 4 people go and run a query against the database then the entire script runs 4 times?

If you want to take this "shared among ALL instances of a class" to the conclusion that you seem to be heading, then a static member variable in say, a widely distributed script, must be shared among all instances of that class, even if some instances are created on a web server running in a local intranet completely disconnected from the Internet. Obviously, that is not true.

Well, isn't that true?

I've seen Static Variables used in things like Iterators and Recursive Functions and the "value" of the Static Variable seems to persist?!

But back to my more real-world situation...

Based on the snippet I provided, if 4 people access the same PHP page and run separate queries against a database, then wouldn't I need 4 separate Database Handlers/Sessions for each person (i.e. Object Variables) versus one "global" Class Variable?? :queasy:

Amy

laserlight

amy.damnit wrote:
I suppose it could, but since Classes to Objects is One-to-Many, it wouldn't seem like storing the reference to one of many Objects in a Class variable would serve much purpose, but anything is possible.

Search the Web for "singleton pattern". This database_handler class is an instance of the singleton pattern.

amy.damnit wrote:
So if 4 people go and run a query against the database then the entire script runs 4 times?

Yes, even if the query is exactly the same. This is the reason why some heavily used PHP script that generate mostly static pages may be cached instead of being repeatedly interpreted to produce the same result.

amy.damnit wrote:
Based on the snippet I provided, if 4 people access the same PHP page and run separate queries against a database, then wouldn't I need 4 separate Database Handlers/Sessions for each person (i.e. Object Variables) versus one "global" Class Variable??

Yes, you would. The reason for the single class member variable is to avoid multiple such objects on the same run of the script, i.e., you only need one database connection for the various queries that will be run for the given user.

mpb001

Based on the snippet I provided, if 4 people access the same PHP page and run separate queries against a database, then wouldn't I need 4 separate Database Handlers/Sessions for each person (i.e. Object Variables) versus one "global" Class Variable??

The PHP web server spawns child processes for each connection and they are held in their own memory space. The difference between static and non static is a static member var has a memory address created at compile time of that class, that memory address is shared between all classes that extend of that one as well. A non static member var has a memory address created at instanciation time, hence no over instanciations in that child process has any knowledge of that value.

In other languages it can be a bit different as different languages can behave differently with different web servers. Using static for database connections is quite common as the amount of time a web site takes to render connectingand disconnecting all the time is wasteful. In a lot of projects they do use global instead of static( it can be done in a slightly different way in PHP4 as well ) but that is just probably due to lack of knowledge.

amy_damnit

laserlight;10925239 wrote:

Originally Posted by amy.damnit
Based on the snippet I provided, if 4 people access the same PHP page and run separate queries against a database, then wouldn't I need 4 separate Database Handlers/Sessions for each person (i.e. Object Variables) versus one "global" Class Variable??

Yes, you would. The reason for the single class member variable is to avoid multiple such objects on the same run of the script, i.e., you only need one database connection for the various queries that will be run for the given user.

So let me repeat all of this to make sure I understand...

1.) A given user only needs one Database Connection per session (assuming the same database is being accessed)?

So, whether Suzy-Q runs 1 or 1,000 queries on SomePage.php, she only needs one database connection?

2.) A Class (Static) Variable is used to ensure #1 happens, so that you don't get 1,000 Instance Variables/Database Connections?

3.) Each time a separate user accesses a given PHP page, the entire PHP page script runs?

So, if 293 users went to SomePage.php, then SomePage.php runs 293 times?

4.) Each separate user needs their own Database Connection?

5.) Even with #4, because of #3, you can "get by" with one Class (Static) Variable.

If 45 people access SomePage.php then SomePage.php runs 45 separate times and each of those 45 separate people gets one Class (Static) Variable for their respective Database Connection?

**NOTE: That last one is hard to believe, especially if we are talking about hundreds of concurrent users or thousands of visitors (e.g. Amazon.com), but if that is what you say then I guess I have to believe it! 🙂

Amy

laserlight

amy.damnit wrote:
1.) A given user only needs one Database Connection per session (assuming the same database is being accessed)?

So, whether Suzy-Q runs 1 or 1,000 queries on SomePage.php, she only needs one database connection?

Yes.

amy.damnit wrote:
2.) A Class (Static) Variable is used to ensure #1 happens, so that you don't get 1,000 Instance Variables/Database Connections?

A static member variable can be used for that purpose, yes.

amy.damnit wrote:
3.) Each time a separate user accesses a given PHP page, the entire PHP page script runs?

So, if 293 users went to SomePage.php, then SomePage.php runs 293 times?

You can understand it that way, yes.

amy.damnit wrote:
4.) Each separate user needs their own Database Connection?

Yes, though database connections can also be persistent and thus pooled.

amy.damnit wrote:
5.) Even with #4, because of #3, you can "get by" with one Class (Static) Variable.

Yes.

amy.damnit wrote:
**NOTE: That last one is hard to believe, especially if we are talking about hundreds of concurrent users or thousands of visitors (e.g. Amazon.com), but if that is what you say then I guess I have to believe it!

I am not sure how scalable PHP is for websites with such high volumes of traffic, but it will not be a single server box handling the requests by then, and things like compiling PHP scripts to bytecode can help.

NogDog

amy.damnit;10925241 wrote:
So let me repeat all of this to make sure I understand...

1.) A given user only needs one Database Connection per session (assuming the same database is being accessed)?

So, whether Suzy-Q runs 1 or 1,000 queries on SomePage.php, she only needs one database connection?

Actually, in some cases only one connection is needed even for multiple databases, as long as all the databases are on the same database host and accessible by the same database user.

2.) A Class (Static) Variable is used to ensure #1 happens, so that you don't get 1,000 Instance Variables/Database Connections?

Using a static variable to store an instance of a class (i.e. via the Singleton Pattern) is one way. Another way is to instantiate a database object, and then pass that same object to each other object that needs to use it (typically via each such object's constructor). There can be arguments for doing it either way.

3.) Each time a separate user accesses a given PHP page, the entire PHP page script runs?

So, if 293 users went to SomePage.php, then SomePage.php runs 293 times?

Yes.

4.) Each separate user needs their own Database Connection?

Yes (normally). It is possible with some database interfaces in PHP to use "persistent" connections, but they can bring their own pitfalls and problems, and generally don't gain you anything (and may lose you a lot) unless your database server and web server are configured correctly to take advantage of such connections.

However, while it's true that creating a database connection takes some time, remember that we are talking computer time: maybe milliseconds or hundredths of a second on a busy server, not seconds.

5.) Even with #4, because of #3, you can "get by" with one Class (Static) Variable.

If 45 people access SomePage.php then SomePage.php runs 45 separate times and each of those 45 separate people gets one Class (Static) Variable for their respective Database Connection?

**NOTE: That last one is hard to believe, especially if we are talking about hundreds of concurrent users or thousands of visitors (e.g. Amazon.com), but if that is what you say then I guess I have to believe it! 🙂

Rest assured that Amazon or Google are running on server farms with load-balancing software, so that each request is allocated to a separate server to be processed, rather than all of them loading down one machine.

amy_damnit

WHEW!!!

Okay, thanks guys (and gal!) for the thorough responses!

That was A LOT to digest, but I think I learned a lot! 🙂

At this rate someday I will be a very knowledgeable PHP developer like MPB001, NogDog and Laserlight, and by 2057 I should be done with this book and my e-commerce site! 😃

Oh well, quality over quantity!

Amy

NogDog

amy.damnit;10925246 wrote:
WHEW!!!

. . . thanks guys (and gal!) . . .

FYI, don't be fooled by LaserLight's avatar; he is not a she. (At least that's what s/he claims. 😉 )

amy_damnit

NogDog;10925258 wrote:
FYI, don't be fooled by LaserLight's avatar; he is not a she. (At least that's what s/he claims. 😉 )

Now I understand the "sassiness"!! LOL (Always thought "she" was a rather aggressive little dish!) :p

It's all good!

Amy

NogDog

amy.damnit;10925261 wrote:
Now I understand the "sassiness"!! LOL (Always thought "she" was a rather aggressive little dish!) :p
...

Nope, you're the only one here. :p 😃

laserlight

Heheh, my avatar strikes again! 😃

amy_damnit

laserlight;10925330 wrote:
Heheh, my avatar strikes again! 😃

Actually, it was more the fact that you call yourself a "PHP Witch".

"Witches" are females who practice black-magic, whereas "Warlocks" are their male counterparts.

But, hey, we accept all people here at PHP Builder... Even those with gender-identity issues! 😉

Amy

johanafm

lol :p