Keys for Cache (KFC)

This module helps you create singular cache keys which can be used as part of any cache strategy, but probably most commonly with Silverstripe's Partial Caching feature.

The overall aim of this module is twofold:

1. Make it easier for developers to create cache keys for our DataObjects (especially for those that have complex dependencies).
2. Increase the performance of our applications by reducing the number of, and the complexity of the cache keys that we calculate at the time of an end user's request.

How will we achieve these goals?
We will use and enhance existing config paradigms that Silverstripe developers are already familiar with.

We will move the cost of calculating cache keys to when the changes are made to our DataObjects, rather than at the time of an end user's request.

What this module is not:
This module is not here to provide a caching method, it is here to provide you with keys that you can use with your preferred caching method (EG: partial caching).

• Installation
• Why cache keys are difficult
• How we aim to solve these difficulties
• Setup and configuration
  • Has cache key
  • Cares
  • Touches
  • Global cares
  • Usage and Examples
• Performance impact/considerations
  • Queued jobs
• Case Studies
• Fluent support
• Unit testing
• License
• Maintainers
• Development and contribution

Installation

composer require silverstripe-terraformers/keys-for-cache

Requirements

• PHP ^8.1
• Silverstripe Framework ^5

Support for Silverstripe 4 is provided though our ^1 tagged releases.

Why cache keys are difficult

TL;DR: Cache keys need to be updated any time a related piece of information changes, and we often have structures of content (EG: Carousel blocks) that contain many levels of nested DataObjects.

The goal of any cache key is to have as low a cost as possible to calculate (as this must happen with every request), but also for it to invalidate at the appropriate times (IE: when a piece of relevant content has changed).

Consider the following:

• We have a Page with the Elemental module applied.
• One of the blocks available is a Carousel block. This block itself contains Carousel Items, and each Item contains an Image.
• When a change is made to the block itself, to one of its Items, or to any of the Images assigned to its Items, the cache key should be invalidated.

A cache key for a single Carousel block might look something like:

public function getCacheKey(): string
{
    $parts = [
        // Parts related to the block itself
        static::class,
        $this->ID,
        $this->LastEdited,
        // Parts related to the Items within the block
        $this->Items()->max('LastEdited'),
        $this->Items()->count(),
        // Parts related to the Images assigned to our Items
        Image::get()->filter('ID', $this->Items()->column('ImageID'))->max('LastEdited'),
        Image::get()->filter('ID', $this->Items()->column('ImageID'))->count(),
    ];

    return implode('-', $parts);
}

This can be optimised a bit, but the point is: It requires multiple queries each time it is performed, and it requires a decent chunk of cognitive load to understand (and to come up with in the first place).

In short, we want to achieve the same outcome as the example above, but with a simple configuration, like this:

App\Blocks\CarouselBlock:
    has_cache_key: true
    cares:
        - Items

App\Blocks\CarouselItem:
    cares:
        - Image

Setup and configuration

Preamble: When we talk about "changes to records", this includes all C.R.U.D. actions.

Relationship config: In order for this module to function, we often need to understand more about the relationships that your DataObjets have than perhaps Silverstripe ORM does. More on this here.

Has cache key

First, you need to tell us which DataObjects you would like cache keys to be generated for. For example, we might like a key for all pages and all Elements:

Page:
    has_cache_key: true

DNADesign\Elemental\Models\BaseElement:
    has_cache_key: true

By adding this configuration, you will have access to the getCacheKey() method on your DataObject, and the $CacheKey variable in your template when you have that DataObject in scope.

Next, in order to make sure that your cache keys invalidate when related DataObjects are updated, you need to define any dependencies that your model might have.

There are three important configurations to be aware of:

• Cares
• Touches
• Global cares

Cares

This configuration determines how $this DataObject will be affected when other (related) DataObjects are manipulated.

For example: We have requested that all Elements have cache keys. If you have a CarouselBlock that contains CarouselItems, then you will want to make sure that the cache key for the CarouselBlock is invalidated any time a CourselItem is updated.

You can do that by telling us that your CarouselBlock cares about its Items.

App\Blocks\CarouselBlock:
    cares:
        - Items

Or in your class:

class CarouselBlock extends BaseElement
{
    private static array $has_many = [
        'Items' => CarouselItem::class,
    ];

    // $owns is optional, but quite common in this use case. I've added it here simply to illustrate how $cares
    // follows the same paradigm
    private static array $owns = [
        'Items',
    ];

    private static array $cares = [
        'Items',
    ];
}

Take the original example where we also wanted our CarouselItem to include changes to Images as part of its cache key. We could now also add a cares config to our CarouselItem:

App\Blocks\CarouselItem:
    cares:
        - Image

Or in your class like so:

class CarouselItem extends DataObject
{
    private static array $has_one = [
        'Image' => Image::class,
    ];

    // $owns is optional, but quite common in this use case. I've added it here simply to illustrate how $cares
    // follows the same paradigm
    private static array $owns = [
        'Image',
    ];

    private static array $cares = [
        'Image',
    ];
}

Now whenever the linked Image is updated, it will also update the CarouselItem, and in turn the CarouselItem will update the linked Carousel. Taking this a step further all the way back to Page, we could also add the following:

# Our BlockPage cares about changes to its ElementalArea
App\Elemental\BlockPage:
    cares:
        - ElementalArea

# Our ElementalAreas care about any changes made to its Elements
DNADesign\Elemental\Models\ElementalArea:
    cares:
        Elements: BaseElement::class

Our BlockPage now cares about our ElementalArea, and our ElementalArea now cares about all of its blocks/elements. This now means that any time a change is made to any of our blocks (so long as we have configured them similarly to how we have shown with the Carousel), we will get a new cache key value on our Page.

Alternatively, you could achieve the same outcome by using Touches.

Important note: Your DataObject does not need to has_cache_key in order for it to care or touch other DataObjects. In fact, we very much rely on you providing us with the full relationship tree through cares/touches. We will only generate the cache key for DataObjects that has_cache_key, but we will continue to follow the paths you create until we run out of them.

Important note: Definitely consider the performance consideration of invalidating your Page cache any time an element is updated. It has been added above purely as an example of what it technically possible; it has not been added as a recommendation.

Touches

This configuration determines how $this DataObject will affect others when it is updated. EG: when $this DataObject is updated, it should "touch" some other DataObjects so that they too have their cache keys invalidated.

Using the example from above, if you have a CarouselBlock with CarouselItems then you could alternatively configure it like so (where the key is the field relationship name, and the value is the class that it relates to):

App\Blocks\CarouselItem:
    touches:
        - Parent

Or in your class like so:

class CarouselItem extends DataObject
{
    private static array $has_one = [
        'Parent' => CarouselBlock::class,
    ];

    private static array $touches = [
        'Parent',
    ];
}

This would mean that any time the CarouselItem is updated it would also update the cache key of the parent CarouselBlock.

Alternatively, you could achieve the same outcome by using Cares.

Important note: Your DataObject does not need to has_cache_key in order for it to care or touch other DataObjects. In fact, we very much rely on you providing us with the full relationship tree through cares/touches. We will only generate the cache key for DataObjects that has_cache_key, but we will continue to follow the paths you create until we run out of them.

Global cares

You might run into situations where you need your cache key to be updated when any DataObject of a certain class is changed. For example, we could have a "Recent updates block", which lists out the pages that have been most recently updated. For this, we would want to make it so that when any page is updated, it will also invalidate the cache keys for any DataObject that cares about global changes to our pages.

App\Blocks\RecentUpdates:
    global_cares:
        - SilverStripe\CMS\Model\SiteTree

Important note: These global updates won't use the touches/cares when they occur. For example, if Blocks\RecentUpdates had a touches of Link: SilverStripe\CMS\Model\SiteTree, The site tree wouldn't be updated. This is a mechanism of global updates to ensure we don't run into performance issues

Usage and examples

See: Usage and Examples

Performance impact/considerations

This will increase the queries to the database when DataObjects are updated, but so far there has been no noticeable increase to (eg) the publishing time of pages within the CMS, nor has there been any noticeable increase in processing time of cron jobs related to the manipulation of DataObjects.

That said:

• You should still be aware of what cares and touches configuration you enabled.
• If you start to notice performance issues with (eg) Publishing a page, then you might need to reconsider the scope of relationships that you cares or touches as part of your page and related DataObjects (EG: blocks).

Queued jobs

If you want to prevent content authors from getting slightly slower responses when editing in the CMS, you can queue a job to generate the cache updates by injecting over CacheKeyExtension and updating triggerEvent to create a job then call CacheRelationService::singleton()->processChange($this->DataObject) in the job.

Case studies

See: Case studies

Fluent support

See: Fluent support

Unit testing

See: Unit testing

License

See License

Maintainers

• Adrian Humphreys adrhumphreys@gmail.com
• Chris Penny chris.penny@gmail.com
• Adrian Jimson adrian.jimson@silverstripe.com

Development and contribution

If you would like to make contributions to the module please ensure you raise a pull request and discuss with the module maintainers.