4.2. The theory of caching
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## 4.2. The theory of caching

There are three key concepts in Zend_Cache. One is the unique indentifier (a string) that is used to identify cache records. The second one is the `'lifeTime'` directive as seen in the examples; it defines for how long the cached resource is considered 'fresh'. The third key concept is conditional execution so that parts of your code can be skipped entirely, boosting performance. The main frontend function (eg. `Zend_Cache_Core::get()`) is always designed to return false for a cache miss if that makes sense for the nature of a frontend. That enables end-users to wrap parts of the code they would like to cache (and skip) in `if(){ ... }` statements where the condition is a Zend_Cache method itself. On the end if these blocks you must save what you've generated, however (eg. `Zend_Cache_Core::save()`).

| Note
The conditional execution design of your generating code is not necessary in some frontends (`Function`, for an example) when the whole logic is implemented inside the frontend.

| Note
'Cache hit' is a term for a condition when a cache record is found, is valid and is 'fresh' (in other words hasn't expired yet). 'Cache miss' is everything else. When a cache miss happens, you must generate your data (as you would normally do) and have it cached. When you have a cache hit, on the other hand, the backend automatically fetches the record from cache transparently.

### 4.2.1. The `Zend_Cache` factory method

A good way to build a usable instance of a `Zend_Cache` Frontend is given in the following example :

    <?php

    # We "load" the Zend_Cache factory
    require 'Zend/Cache.php';

    # We choose a backend (for example 'File' or 'Sqlite'...)
    $backendName = '[...]';

    # We choose a frontend (for example 'Core', 'Output', 'Page'...)
    $frontendName = '[...]';

    # We set an array of options for the choosen frontend
    $frontendOptions = array([...]);

    # We set an array of options for the choosen backend
    $backendOptions = array([...]);

    # We make the good instance
    # (of course, the two last arguments are optional)
    $cache = Zend_Cache::factory($frontendName, $backendName, $frontendOptions, $backendOptions);

    ?>

In the following examples we will assume that the `$cache` variable holds a valid, instantiated frontend as shown and that you understand how to pass parametres to your chosen backends.

| Note
Always use `Zend_Cache::factory()` to get frontend instances. Instantiating frontends and backends yourself will not work as expected.

### 4.2.2. Tagging records

Tags are a way to categorize cache records. When you save a cache with the `save()` method, you can set an array of tags to apply for this record. Then you will be able to clean all cache records tagged with a given tag (or tags):

    <?php

    $cache->save($huge_data, 'myUniqueID', array('tagA', 'tagB', 'tagC'));

    ?>

### 4.2.3. Cleaning the cache

To remove/invalidate in particular cache id, you can use the `remove()` method :

    <?php

    $cache->remove('idToRemove');

    ?>

To remove/invalidate several cache ids in one operation, you can use the `clean()` method. For example to remove all cache records :

    <?php

    // clean all records
    $cache->clean(Zend_Cache::CLEANING_MODE_ALL);

    // clean only outdated
    $cache->clean(Zend_Cache::CLEANING_MODE_OLD);

    ?>

If you want to remove cache entries matching the tags 'tagA' and 'tagC':

    <?php

    $cache->clean(Zend_Cache::CLEANING_MODE_MATCHING_TAG, array('tagA', 'tagC'));

    ?>

Available cleaning modes are: `CLEANING_MODE_ALL`, `CLEANING_MODE_OLD`, `CLEANING_MODE_MATCHING_TAG` and `CLEANING_MODE_NOT_MATCHING_TAG`. The latter are, as their names suggest, combined with an array of tags in cleaning operations.

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