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Persistence cache

SPI cache diagram

Layers

Persistence cache can best be described as an implementation of SPI\Persistence that decorates the main backend implementation, aka Storage Engine (currently: "Legacy Storage Engine").

As shown in the illustration, this is done in the exact same way as the SignalSlot feature is a custom implementation of API\Repository decorating the main Repository. In the case of Persistence Cache, instead of sending events on calls passed on to the decorated implementation, most of the load calls are cached, and calls that perform changes purge the affected caches. Cache handlers (Memcached, Redis, Filesystem, etc.) can be configured using Symfony configuration. For details on how to reuse this Cache service in your own custom code, see below.

Transparent cache

With the persistence cache, just like with the HTTP cache, eZ Platform tries to follow principles of transparent caching. This can shortly be described as a cache which is invisible to the end user (admin/editors) of eZ Platform where content is always returned fresh. In other words, there should be no need to manually clear the cache like it was frequently the case with eZ Publish 4.x. This is possible thanks to an interface that follows CRUD (Create Read Update Delete) operations per domain.

What is cached?

Persistence cache aims at caching most SPI\Persistence calls used in common page loads, including everything needed for permission checking and URL alias lookups.

Notes:

  • Cache tagging is used in order to allow clearing cache by alternative indexes. For instance tree operations or changes to Content Types are examples of operations that also need to invalidate content cache by tags.
  • Search is not defined as persistence and the queries themselves are not planned to be cached as they are too complex by design (full text, facets, etc.). Use Solr which caches this for you to improve scale/performance, and to offload your database.

For further details on which calls are cached or not, see details in the Symfony Web Debug Toolbar which has info on cache use in two places:

  • Symfony Cache tab: for Symfony Cache itself, the tab shows cache lookups to cache backends
  • eZ Platform tab: shows calls made to database back end, and if they are cached or not

To see where and how to contribute additional caches, refer to the source code.

Persistence cache configuration

Note

Current implementation uses Symfony cache. It technically supports the following cache backends: APCu, Array, Chain, Doctrine, Filesystem, Memcached, PDO & Doctrine DBAL, Php Array, Proxy, Redis. eZ Platform officially supports only using Filesystem for single server and Redis or Memcached for clustered setups.

Use of Memcached or Redis as shared cache back end is a requirement for use in clustering setup. For an overview of this feature, see Clustering. Filesystem adapters, for example, are not intended to be used over a shared filesystem.

Cache service

The underlying cache system is exposed as an ezpublish.cache_pool service, and can be reused by any other service as described in the Using Cache service section.

Configuration

By default, configuration uses the cache.tagaware.filesystem service to store cache files. The service is defined in app/config/cache_pool/cache.tagaware.filesystem.yml to use FilesystemTagAwareAdapter. This service is loaded through app/config/env/generic.php.

You can select a different cache backend and configure its parameters in the relevant file in the cache_pool folder.

Multi Repository setup

You can configure multisite to work with multiple repositories. Then, in ezplatform.yml you can specify which cache pool you want to use on a SiteAccess or SiteAccess group level.

The following example shows use in a SiteAccess group:

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# ezplatform.yml site group setting
ezpublish:
    system:
        # "site_group" refers to the group configured in site access
        site_group:
            # cache_pool is set to '%env(CACHE_POOL)%'
            # env(CACHE_POOL) is set to 'cache.tagaware.filesystem' (a Symfony service) by default, for more examples see app/config/cache_pool/*
            cache_service_name: '%cache_pool%'

One cache pool for each Repository

If your installation has several Repositories (databases), make sure every group of sites using different Repositories also uses a different cache pool.

In-Memory cache configuration

Persistence cache layer caches selected objects in-memory for a short time. It avoids loading repeatedly the same data from e.g. a remote Redis instance, which can take up to 4-5ms per call due to the network latency and Redis instance load. The cache is organized in 2 pools, one for metadata which is not updated frequently, and one for content related objects that is only meant as a short-lived burst cache. Limit is organized using a least frequently used (LFU) approach. It makes sure repeatedly used objects will stay in-memory until expired, and those seldom used will be bulk evicted from cache every time the maximum number of cache items is reached.

This in-memory cache will be purged (for the current PHP process) when clearing it using any of the mentioned methods below. For other processes, the object will be refreshed when it expires or evicted when it reaches the cache limits.

In-Memory cache is configured globally, and has the following default settings:

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parameters:
    # Config for metadata cache pool, here showing default config
    # ttl: Maximum number of  milliseconds objects are kept in-memory (3000ms = 3s)
    ezpublish.spi.persistence.cache.inmemory.ttl: 3000
    # limit: Maximum number of cache objects to place in-memory, to avoid consuming too much memory
    ezpublish.spi.persistence.cache.inmemory.limit: 100
    # enabled: Is the in-memory cache enabled
    ezpublish.spi.persistence.cache.inmemory.enable: true

    # Config for content cache pool, here showing default config
    ## WARNING: TTL is on purpose low to avoid getting outdated data in prod! For dev environment, you can safely increase it (e.g. by x3)
    ezpublish.spi.persistence.cache.inmemory.content.ttl: 300
    ezpublish.spi.persistence.cache.inmemory.content.limit: 100
    ezpublish.spi.persistence.cache.inmemory.content.enable: true

In-Memory cache is per-process

TTL and Limit need to have a low value. Setting limit high will increase memory use. High TTL value also increases exponentially risk for system acting on stale metadata (e.g. Content Type definitions). The only case where it is safe to increase these values is for dev environment with single concurrency on writes. In prod environment you should only consider reducing them if you have heavy concurrency writes.

Redis

Redis, an in-memory data structure store, is one of the supported cache solutions for clustering. Redis is used via Redis pecl extension.

See Redis Cache Adapter in Symfony documentation for information on how to connect to Redis.

Supported Adapters

There are two Redis adapters available out of the box that fit different needs.

Symfony\Component\Cache\Adapter\RedisTagAwareAdapter

Requirement: Redis server configured with eviction maxmemory-policy: volatile-ttl, volatile-lru or volatile-lfu (Redis 4.0+). Use of LRU or LFU is recommended. It is also possible to use noeviction, but it is usually not practical.

Pros: It is typically faster than RedisAdapter, because fewer lookups needed to cache backend.

Cons: Consumes much more memory. To avoid situations where Redis stops accepting new cache (warnings about Failed to save key), set aside enough memory for the Redis server.

Symfony\Component\Cache\Adapter\RedisAdapter

Pros: Uses a bit less memory than RedisTagAwareAdapter, so it eliminated the risk of stopping saving cache when there is not enough memory.

Cons: 1.5-2x more lookups to the back-end cache server then RedisTagAwareAdapter. Depending on the number of lookups and latency to cache server this might affect page load time.

Adjusting configuration

Out of the box in app/config/cache_pool/cache.redis.yml you'll find a default example that can be used.

eZ Platform Cloud

For eZ Platform Cloud/Platform.sh: This is automatically configured in app/config/env/platformsh.php if you have enabled Redis as rediscache Platform.sh service.

For anything else, you can enable it with environment variables detected automatically by app/config/env/generic.php. For instance, if you set the following environment variables export CACHE_POOL="cache.redis" CACHE_DSN="secret@example.com:1234/13", it will result in config like this:

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services:
    cache.redis:
        # NOTE: This optimized Redis Adapter is avaiable as of 2.5LTS via https://github.com/ezsystems/symfony-tools
        class: Symfony\Component\Cache\Adapter\TagAware\RedisTagAwareAdapter
        parent: cache.adapter.redis
        tags:
            - name: cache.pool
              clearer: cache.app_clearer
              provider: 'redis://secret@example.com:1234/13'
              # Default CACHE_NAMESPACE value, see app/config/cache_pool/cache.redis.yml for usage with e.g. multi repo.
              namespace: 'ez'

See app/config/default_parameters.yml and app/config/cache_pool/cache.redis.yml for further details on CACHE_POOL, CACHE_DSN and CACHE_NAMESPACE.

Clearing Redis cache

The regular php bin/console cache:clear command does not clear Redis persistence cache. Use a dedicated Symfony command to clear the pool you have configured: php bin/console cache:pool:clear cache.redis.

Redis Clustering

Persistence cache depends on all involved web servers, each of them seeing the same view of the cache because it's shared among them. With that in mind, the following configurations of Redis are possible:

  • Redis Cluster
  • Redis Sentinel
    • Provides high availability by providing one or several slaves (ideally 2 slaves or more, e.g. minimum 3 servers), and handle failover
    • Slaves are asynchronously replicated, so they can't be used for reads
    • Typically used with a load balancer (e.g. HAproxy) in the front in order to only speak to elected master
      • An alternative is that application logic itself speaks to Sentinel in order to always ask for elected master before talking to cache.

For best performance we recommend use of Redis Sentinel if it fits your needs. However different cloud providers have managed services that are easier to set up, and might perform better. Notable Services:

eZ Platform Cloud / Platform.sh usage

eZ Platform Cloud

If you use Platform.sh Enterprise you can benefit from the Redis Sentinel across three nodes for great fault tolerance. Platform.sh Professional and lower versions offer Redis in single instance mode only.

Memcached

Memcached, a distributed caching solution is a cache solution that is supported for clustering use, as an alternative to Redis.

See Memcached Cache Adapter in Symfony documentation for information on how to configure Memcached.

Supported Adapters

There is one Memcached adapter available out of the box.

Symfony\Component\Cache\Adapter\MemcachedAdapter

Pros: Memcached is able to handle much more concurrent load by design (multi threaded), and typically uses far less memory than Redis in general due to a simpler data structure.

Cons: 1.5-2x more lookups to the back-end cache server then RedisTagAwareAdapter. Depending on the number of lookups and latency to cache server this might affect page load time.

Adjusting configuration

Out of the box in app/config/cache_pool/cache.memcached.yml you'll find a default example that can be used.

eZ Platform Cloud

For eZ Platform Cloud/Platform.sh: This is automatically configured in app/config/env/platformsh.php if you have enabled Memcached as cache Platform.sh service.

For anything else, you can enable it with environment variables detected automatically by app/config/env/generic.php. For instance, if you set the following environment variables export CACHE_POOL="cache.memcached" CACHE_DSN="user:pass@localhost?weight=33", it will result in config like this:

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services:
    cache.memcached:
        parent: cache.adapter.memcached
        tags:
            - name: cache.pool
              clearer: cache.app_clearer
              provider: 'memcached://user:pass@localhost?weight=33'
              # Default CACHE_NAMESPACE value, see app/config/cache_pool/cache.redis.yml for usage with e.g. multi repo.
              namespace: 'ez'

See app/config/default_parameters.yml and app/config/cache_pool/cache.memcached.yml for further details on CACHE_POOL, CACHE_DSN and CACHE_NAMESPACE.

Clearing Memcached cache

The regular php bin/console cache:clear command does not clear Memcached persistence cache. Use a dedicated Symfony command to clear the pool you have configured: php bin/console cache:pool:clear cache.memcached.

Connection errors issue

If Memcached does display connection errors when using the default (ascii) protocol, then switching to binary protocol (in the configuration and Memcached daemon) should resolve the issue.

Note

Memcached must not be bound to the local address if clusters are in use, or user logins will fail. To avoid this, in /etc/memcached.conf take a look under # Specify which IP address to listen on. The default is to listen on all IP addresses

For development environments, change the address below this comment in /etc/memcached.conf to -l 0.0.0.0

For production environments, follow this more secure instruction from the Memcached man:

-l <addr>

Listen on <addr>; default to INADDR_ANY. <addr> may be specified as host:port. If you don't specify a port number, the value you specified with -p or -U is used. You may specify multiple addresses separated by comma or by using -l multiple times. This is an important option to consider as there is no other way to secure the installation. Binding to an internal or firewalled network interface is suggested.

Using Cache Service

Using the internal cache service allows you to use an interface and without caring whether the system is configured to place the cache in Memcached or on File system. And as eZ Platform requires that instances use a cluster-aware cache in Cluster setup, you can safely assume your cache is shared (and invalidated) across all web servers.

Note

Current implementation uses a caching library implementing TagAwareAdapterInterface which extends Psr\Cache\CacheItemPoolInterface, and therefore is compatible with PSR-6.

Use unique vendor prefix for Cache key

When reusing the cache service within your own code, it is very important to not conflict with the cache keys used by others. That is why the example of usage below starts with a unique myApp key. For the namespace of your own cache, you must do the same.

Get Cache service

Via Dependency injection

In your Symfony services configuration you can simply define that you require the cache service in your configuration like so:

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# yml configuration
    myApp.myService:
        class: '%myApp.myService.class%'
        arguments:
            - '@ezpublish.cache_pool'

This service is an instance of Symfony\Component\Cache\Adapter\TagAwareAdapterInterface, which extends the Psr\Cache\CacheItemPoolInterface interface with tagging functionality.

Via Symfony Container

Like any other service, it is also possible to get the cache service via container like so:

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// Getting the cache service in PHP

/** @var \Symfony\Component\Cache\Adapter\TagAwareAdapterInterface */
$pool = $container->get('ezpublish.cache_pool');

Using the cache service

Example usage of the cache service:

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// Example
$cacheItem = $pool->getItem("myApp-object-${id}");
if ($cacheItem->isHit()) {
    return $cacheItem->get();
}

$myObject = $container->get('my_app.backend_service')->loadObject($id)
$cacheItem->set($myObject);
$cacheItem->tag(['myApp-category-' . $myObject->categoryId]);
$pool->save($cacheItem);

return $myObject;

For more info on usage, see Symfony Cache's documentation.

Clearing Persistence cache

Persistence cache prefixes it's cache using "ez-". Clearing persistence cache can thus be done in the following ways:

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// To clear all cache (not recommended without a good reason)
$pool->clear();

// To clear a specific cache item (check source for more examples in eZ\Publish\Core\Persistence\Cache\*)
$pool->deleteItems(["ez-content-info-$contentId"]);

// Symfony cache is tag-based, so you can clear all cache related to a Content item like this:
$pool->invalidateTags(["content-$contentId"]);
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