Kohana's ORM, by default, is not as smart as I wanted when it comes to recognizing which objects it has already loaded. It saves objects loaded through a relationship, for example:
$obj = $foo->bar; // hits the DB for bar
/* ... Later ... */
$obj = $foo->bar; // had bar preloaded, so uses that instead
But if there's more than one way to find bar, it doesn't see that. Let's say both foo and thing (we need more meta-syntactic variables) have a relationship with the same bar:
$obj = $foo->bar; // hits DB
$obj = $thing->bar // hits DB again, even though it's the same object
I've attempted to fix this by having a preloaded array of objects keyed by model and id. It works, but the problem is that it only works if I know the ID ahead of time. My overloaded ORM functions look like this:
public function find($id = NULL)
{
$model = strtolower(get_class($this));
if ($id != NULL) // notice I don't have to hit the db if it's preloaded.
{
$key = $model . '_' . $id;
if (array_key_exists($key, self::$preloaded)) return self::$preloaded[$key];
}
$obj = parent::find($id);
$key = $model . '_' . $obj->pk();
// here, I have to hit the DB even if it was preloaded, just to find the ID!
if (array_key_exists($key, self::$preloaded)) return self::$preloaded[$key];
self::$preloaded[$key] = $obj;
return $obj;
}
The purpose of this is so that if I access the same object in two different places, and there's a chance they're the same object, it won't incorrectly half-update two different instances, but correctly update the one preloaded instance.
The above find method hits the DB needlessly in cases where I find an object by something other than primary key. Is there anything I can do to prevent that, short of keying the preloaded objects by every imaginable criterion? The core issue here seems so basic that I'm surprised it's not part of the original ORM library. Is there a reason for that, or something I overlooked? What do most people do to solve this or get around it? Whatever the solution is, I'll be applying it further when I integrate memcache into my code, so it might help to keep that in mind.
Turn on per-request DB caching in config/database.php ('caching' param, its FALSE by default). This will allow you to use cached results for the same queries.
Related
I am trying to get a Produclist cache working in my plugin.
My ext_localconf.php
if (!is_array($GLOBALS['TYPO3_CONF_VARS']['SYS']['caching']['cacheConfigurations']['product_cache'])) {
$GLOBALS['TYPO3_CONF_VARS']['SYS']['caching']['cacheConfigurations']['product_cache'] = [];
}
if( !isset($GLOBALS['TYPO3_CONF_VARS'] ['SYS']['caching']['cacheConfigurations']['product_cache']['frontend'] ) ) {
$GLOBALS['TYPO3_CONF_VARS'] ['SYS']['caching']['cacheConfigurations']['product_cache']['frontend'] = 'TYPO3\\CMS\\Core\\Cache\\Frontend\\VariableFrontend';
}
And my Controller
$cache = GeneralUtility::makeInstance(\TYPO3\CMS\Core\Cache\CacheManager::class)->getCache('product_cache');
if(($products = $cache->get($cacheIdentifier)) === FALSE){
$objectManager = GeneralUtility::makeInstance(ObjectManager::class);
$productController = $objectManager->get(ProductController::class);
$productController->setSettings($this->settings);
$products = $productController->getEditedProducts($catId);
$cache->set($cacheIdentifier, $products, ['productajax'], 84600);
}
Normal content like string, int or array works fine, but when i try this with an DatabaseResultquerie than crashed by System with this error: Call to a member function map() on null
(only on get, set works fine)
You cannot cache this class, because that implies serializing it, and the class contains explicit methods that prevent certain properties from being included in the serialized string. In fact, the only property that is included, is query (the input query that caused the result).
You might be able to cache the QueryResult and then manually call the injection methods to add instances of DataMapper etc. - but even if you did, the serialized QueryResult would not contain the results and would fire again every time you attempt to load an entity from it.
The right way would be to extract to a not-QueryResult (array, iterator of own choosing) that you know will allow the results to be serialized.
See: https://github.com/TYPO3/TYPO3.CMS/blob/v8.7.26/typo3/sysext/extbase/Classes/Persistence/Generic/QueryResult.php#L250
In the case this is an Ajax-Controller you might want to cache the generated JSON response.
I have a strange problem with \Doctrine\ORM\UnitOfWork::getScheduledEntityDeletions used inside onFlush event
foreach ($unitOfWork->getScheduledEntityDeletions() as $entity) {
if ($entity instanceof PollVote) {
$arr = $entity->getAnswer()->getVotes()->toArray();
dump($arr);
dump($entity);
dump(in_array($entity, $arr, true));
dump(in_array($entity, $arr));
}
}
And here is the result:
So we see that the object is pointing to a different instance than the original, therefore in_array no longer yields expected results when used with stick comparison (AKA ===). Furthermore, the \DateTime object is pointing to a different instance.
The only possible explanation I found is the following (source):
Whenever you fetch an object from the database Doctrine will keep a copy of all the properties and associations inside the UnitOfWork. Because variables in the PHP language are subject to “copy-on-write” the memory usage of a PHP request that only reads objects from the database is the same as if Doctrine did not keep this variable copy. Only if you start changing variables PHP will create new variables internally that consume new memory.
However, I did not change anything (even the created field is kept as it is). The only operations that were preformed on entity are:
\Doctrine\ORM\EntityRepository::findBy (fetching from DB)
\Doctrine\Common\Persistence\ObjectManager::remove (scheduling for removal)
$em->flush(); (triggering synchronization with DB)
Which leads me to think (I might be wrong) that the Doctrine's change tracking method has nothing to do with the issue that I'm experiencing. Which leads me to following questions:
What causes this?
How to reliably check if an entity scheduled for deletion is inside a collection (\Doctrine\Common\Collections\Collection::contains uses in_array with strict comparison) or which items in a collection are scheduled for deletion?
The problem is that when you tell doctrine to remove entity, it is removed from identity map (here):
<?php
public function scheduleForDelete($entity)
{
$oid = spl_object_hash($entity);
// ....
$this->removeFromIdentityMap($entity);
// ...
if ( ! isset($this->entityDeletions[$oid])) {
$this->entityDeletions[$oid] = $entity;
$this->entityStates[$oid] = self::STATE_REMOVED;
}
}
And when you do $entity->getAnswer()->getVotes(), it does the following:
Load all votes from database
For every vote, checks if it is in identity map, use old one
If it is not in identity map, create new object
Try to call $entity->getAnswer()->getVotes() before you delete entity. If the problem disappears, then I am right. Of cause, I would not suggest this hack as a solution, just to make sure we understand what is going on under the hood.
UPD instead of $entity->getAnswer()->getVotes() you should probably do foreach for all votes, because of lazy loading. If you just call $entity->getAnswer()->getVotes(), Doctrine probably wouldn't do anytning, and will load them only when you start to iterate through them.
From the doc:
If you call the EntityManager and ask for an entity with a specific ID twice, it will return the same instance
So calling twice findOneBy(['id' => 12]) should result in two exact same instances.
So it all depends on how both instances are retrieved by Doctrine.
In my opinion, the one you get in $arr is from a One-to-Many association on $votes in the Answer entity, which results in a separate query (maybe a id IN (12)) by the ORM.
Something you could try is to declare this association as EAGER (fetch="EAGER"), it may force the ORM to make a specific query and keep it in cache so that the second time you want to get it, the same instance is returned ?
Could you have a look at the logs and post them here ? It may indicates something interesting or at least relevant to investigate further.
I was previously mostly scripting in PHP and now considering getting "more serious" about it :)
I am working on a hiking website, and I needed to put some values into an object that I then try to pass back to the calling code.
I tried doing this:
$trailhead = new Object ();
But the system sort of barfed at me.
Then I didn't declare the object at all, and started using it like this:
$trailhead->trailhead_name = $row['trailhead_name'];
$trailhead->park_id = $row['park_id'];
That seemed to work reasonably ok. But there are at least 3 problems with this:
Since that code gets executed when getting things from the database, what do I do in case there is more than one row?
When I passed the $trailhead back to the calling code, the variables were empty
I actually am maybe better off making a real object for Trailhead like this:
class Trailhead
{
private $trailhead_name;
private $park_id;
private $trailhead_id;
public function __construct()
{
$this->trailhead_name = NULL;
$this->park_id = NULL;
$this->trailhead_id = NULL;
}
}
What do people generally do in these situations and where am I going wrong in my approach? I know its more than one place :)
$trailheads[] = $trailhead;
I'd do a print_r() of $trailhead to check that it's what you expect it to be. The default object type in PHP is going to be stdClass.
Yes, that's going to be better, as it'll allow your Trailhead objects to have functions. The way you're currently doing it is basically taking advantage of none of PHP's object functionality - it's essentially an array with slightly different syntax.
I think you should get in "contact" with some of the basics first. Objects in PHP have sort of a history. They are a relative to the array and there are two sorts of objects:
data-objects
class objects
data objects are called stdClass and that's actually what you were initially looking for:
$trailhead = new Object();
in PHP is written as:
$trailhead = new stdClass;
(with or without brackets at the end, both works)
You then can dynamically add members to it, like you did in the second part without declaring the variable (that works, too in PHP):
$trailhead->trailhead_name = $row['trailhead_name'];
$trailhead->park_id = $row['park_id'];
If you want to more quickly turn $row into an object, just cast it:
$trailhead = (object) $row;
That works the other way, too:
$array = (array) $trailhead;
As you can see, those basic data based objects in PHP do not hide their relationship to the array data type. In fact you can even iterator over them:
foreach($trailhead as $key=>$value) {
echo $key, ' is ', $value, "<br>\n";
}
You can find lots of information about this in the PHP manual, it's a bit spread around, but worth to know the little basics before repeating a lot of names only to pass along the data that belongs together.
Next to these more or less stupid data objects, you can code complete classes that - next to what every stdClass can do - can have code, visibility, inheritance and all the things you can build nice stuff from - but this can be pretty complex as the topic is larger.
So it always depends on what you need. However, both type of objects are "real objects" and both should work.
class myClass {
function importArray(array $array) {
foreach($array as $key=>$value) {
if(!is_numeric($key)) $this->$key=$value;
}
}
function listMembers() {
foreach($this as $key=>$value) {
echo $key, ' is ', $value, "<br>\n";
}
}
}
$trailhead = new myClass();
$trailhead->importArray($row);
echo $trailhead->park_id;
Keep in mind that instead of creating a set of objects that merely does the same in each object (store data), you should just take one flexible class that is handling the job flexible (e.g. stdClass) because that will keep your code more clean.
Instead of coding a getter/setter orgy you can then spend the time thinking about how you can make the database layer more modular etc. .
Just pass back an array:
$trailhead = array(
'trailhead_name' => $row['trailhead_name'],
'park_id' => $row['park_id'],
'trailhead_id' => $row['trailhead_id'],
)
Then either access it like:
$trailhead['park_id'];
or use the nifty list() to read it into variables:
list($trailhead_name, $park_id, $trailhead_id) = $trailhead;
I'm looking for a way to prevent repeated calls to the database if the item in question has already been loaded previously. The reason is that we have a lot of different areas that show popular items, latest releases, top rated etc. and sometimes it happens that one item appears in multiple lists on the same page.
I wonder if it's possible to save the object instance in a static array associated with the class and then check if the data is actually in there yet, but then how do I point the new instance to the existing one?
Here's a draft of my idea:
$baseball = new Item($idOfTheBaseballItem);
$baseballAgain = new Item($idOfTheBaseballItem);
class Item
{
static $arrItems = array();
function __construct($id) {
if(in_array($id, self::arrItems)){
// Point this instance to the object in self::arrItems[$id]
// But how?
}
else {
// Call the database
self::arrItems[id] = $this;
}
}
}
If you have any other ideas or you just think I'm totally nuts, let me know.
You should know that static variables only exist in the page they were created, meaning 2 users that load the same page and get served the same script still exist as 2 different memory spaces.
You should consider caching results, take a look at code igniter database caching
What you are trying to achieve is similar to a singleton factory
$baseball = getItem($idOfTheBaseballItem);
$baseballAgain =getItem($idOfTheBaseballItem);
function getItem($id){
static $items=array();
if(!isset($items[$id])$items[$id]=new Item($id);
return $items[$id];
}
class Item{
// this stays the same
}
P.S. Also take a look at memcache. A very simple way to remove database load is to create a /cache/ directory and save database results there for a few minutes or until you deem the data old (this can be done in a number of ways, but most approaches are time based)
You can't directly replace "this" in constructor. Instead, prepare a static function like "getById($id)" that returns object from list.
And as stated above: this will work only per page load.
I realize the knee-jerk response to this question is that "you dont.", but hear me out.
Basically I am running on an active-record system on a SQL, and in order to prevent duplicate objects for the same database row I keep an 'array' in the factory with each currently loaded object (using an autoincrement 'id' as the key).
The problem is that when I try to process 90,000+ rows through this system on the odd occasion, PHP hits memory issues. This would very easily be solved by running a garbage collect every few hundred rows, but unfortunately since the factory stores a copy of each object - PHP's garbage collection won't free any of these nodes.
The only solution I can think of, is to check if the reference count of the objects stored in the factory is equal to one (i.e. nothing is referencing that class), and if so free them. This would solve my issue, however PHP doesn't have a reference count method? (besides debug_zval_dump, but thats barely usable).
Sean's debug_zval_dump function looks like it will do the job of telling you the refcount, but really, the refcount doesn't help you in the long run.
You should consider using a bounded array to act as a cache; something like this:
<?php
class object_cache {
var $objs = array();
var $max_objs = 1024; // adjust to fit your use case
function add($obj) {
$key = $obj->getKey();
// remove it from its old position
unset($this->objs[$key]);
// If the cache is full, retire the eldest from the front
if (count($this->objs) > $this->max_objs) {
$dead = array_shift($this->objs);
// commit any pending changes to db/disk
$dead->flushToStorage();
}
// (re-)add this item to the end
$this->objs[$key] = $obj;
}
function get($key) {
if (isset($this->objs[$key])) {
$obj = $this->objs[$key];
// promote to most-recently-used
unset($this->objs[$key]);
$this->objs[$key] = $obj;
return $obj;
}
// Not cached; go and get it
$obj = $this->loadFromStorage($key);
if ($obj) {
$this->objs[$key] = $obj;
}
return $obj;
}
}
Here, getKey() returns some unique id for the object that you want to store.
This relies on the fact that PHP remembers the order of insertion into its hash tables; each time you add a new element, it is logically appended to the array.
The get() function makes sure that the objects you access are kept at the end of the array, so the front of the array is going to be least recently used element, and this is the one that we want to dispose of when we decide that space is low; array_shift() does this for us.
This approach is also known as a most-recently-used, or MRU cache, because it caches the most recently used items. The idea is that you are more likely to access the items that you have accessed most recently, so you keep them around.
What you get here is the ability to control the maximum number of objects that you keep around, and you don't have to poke around at the php implementation details that are deliberately difficult to access.
It seems like the best answer was still getting the reference count, although debug_zval_dump and ob_start was too ugly a hack to include in my application.
Instead I coded up a simple PHP module with a refcount() function, available at: http://github.com/qix/php_refcount
Yes, you can definitely get the refcount from PHP. Unfortunately, the refcount isn't easily gotten for it doesn't have an accessor built into PHP. That's ok, because we have PREG!
<?php
function refcount($var)
{
ob_start();
debug_zval_dump($var);
$dump = ob_get_clean();
$matches = array();
preg_match('/refcount\(([0-9]+)/', $dump, $matches);
$count = $matches[1];
//3 references are added, including when calling debug_zval_dump()
return $count - 3;
}
?>
Source: PHP.net
I know this is a very old issue, but it still came up as a top result in a search so I thought I'd give you the "correct" answer to your problem.
Unfortunately getting the reference count as you've found is a minefield, but in reality you don't need it for 99% of problems that might want it.
What you really want to use is the WeakRef class, quite simply it holds a weak reference to an object, which will expire if there are no other references to the object, allowing it to be cleaned up by the garbage collector. It needs to be installed via PECL, but it really is something you want in every PHP installation.
You would use it like so (please forgive any typos):
class Cache {
private $max_size;
private $cache = [];
private $expired = 0;
public function __construct(int $max_size = 1024) { $this->max_size = $max_size; }
public function add(int $id, object $value) {
unset($this->cache[$id]);
$this->cache[$id] = new WeakRef($value);
if ($this->max_size > 0) && ((count($this->cache) > $this->max_size)) {
$this->prune();
if (count($this->cache) > $this->max_size) {
array_shift($this->cache);
}
}
}
public function get(int $id) { // ?object
if (isset($this->cache[$id])) {
$result = $this->cache[$id]->get();
if ($result === null) {
// Prune if the cache gets too empty
if (++$this->expired > count($this->cache) / 4) {
$this->prune();
}
} else {
// Move to the end so it is culled last if non-empty
unset($this->cache[$id]);
$this->cache[$id] = $result;
}
return $result;
}
return null;
}
protected function prune() {
$this->cache = array_filter($this->cache, function($value) {
return $value->valid();
});
}
}
This is the overkill version that uses both weak references and a max size (set it to -1 to disable that). Basically if it gets too full or too many results were expired, then it will prune the cache of any empty references to make space, and only drop non-empty references if it has to for sanity.
PHP 7.4 now has WeakReference
To know if $obj is referenced by something else or not, you could use:
// 1: create a weak reference to the object
$wr = WeakReference::create($obj);
// 2: unset our reference
unset($obj);
// 3: test if the weak reference is still valid
$res = $wr->get();
if (!is_null($res)) {
// a handle to the object is still held somewhere else in addition to $obj
$obj = $res;
unset($res);
}
I had a similar problem with the Incredibly Flexible Data Storage (IFDS) file format with trying to keep track of references to objects in an in-memory data cache. How I solved it was to create a ref-counting class that wrapped a reference to the underlying array. I generally prefer arrays over objects as PHP has traditionally tended to handle arrays better than objects with regards to unfortunate things like memory leaks.
class IFDS_RefCountObj
{
public $data;
public function __construct(&$data)
{
$this->data = &$data;
$this->data["refs"]++;
}
public function __destruct()
{
$this->data["refs"]--;
}
}
Since 'refs' is tracked as a regular value in the data, it is possible to know when the last reference to the data has gone away. Regardless of whether multiple variables reference the refcounting object or it is cloned, the refcount will always be non-zero until all references are gone. I don't need to care how many actual references there are internally in PHP as long as the value is correctly zero vs. non-zero. The IFDS implementation also tracks an estimated amount of RAM being used by each object (again, being exact isn't super important as long as it is in the ballpark), allowing it to prioritize writing and releasing unused objects that are occupying system resources first and then writing and releasing portions of still-referenced objects that are caching large quantities of DATA chunk information.
To get back to the topic/question, with this ref-counting class-based approach, it is, for example, mostly straightforward to prune to ~5,000 records in a cache upon hitting 10,000 records in the cache. General strategy is to not get rid of records still being referenced plus keep the most recently requested/used records that aren't being referenced because they are likely to be referenced again. Upon every new reference, unset() and then setting the item again will move the item to the end of the array so that the oldest probably unreferenced items appear first and the newest probably still referenced items appear last.
Weak references, as several people have suggested, won't solve every caching issue. They don't work in caching scenarios where you don't want to remove an item from the cache until the application is done working with it (i.e. deleting an item that the application later attempts to use) but also want to keep it around as long as RAM overhead permits even if the application stops referencing it temporarily but might need it again in a moment. Weak references are also incapable of working in scenarios where the item in the cache is holding onto unwritten data that may or may not be fine with staying unwritten even if there are no references to it in the application. In short, when there is a balancing act to maintain, weak references cannot be used.