What is Object Caching?

Object caching is a technique used to enhance the performance of a website by storing the results of database queries in memory. This way, when the same piece of data or query result is needed again, it can be quickly retrieved from the cache instead of being recreated from the database.

What are objects?

In the context of WordPress, an “object” refers to any data generated during the execution of WordPress, including query results, API requests, or complex calculations. Many things in WordPress can be considered an object, it’s a very broad term that covers everything from:

Objects Examples:

• the results of database queries
• pages, posts and user data
• site options
• data generated by plugins and themes
• API request
• & so on…

Object caching in WordPress helps reduce the load on the database server, speeding up WordPress by storing frequently accessed queries to be reused for subsequent page loads. This is especially useful for dynamic websites with heavy database interactions, as it can significantly decrease page load times and improve overall site performance.

How Does Object Caching Work?

Object caching involves storing data fetched from the database, APIs, or complex computations in memory to be reused for subsequent requests without the need to re-fetch or recalculate the data. This method significantly speeds up WordPress by reducing the load on the database and minimizing the execution of complex code. Here’s how it works:

1. Data Retrieval: When WordPress needs data, such as options, user information, or post content, it typically retrieves this data from the database. With object caching, the system first checks if this data is in the cache.
2. Cache Check: If the requested data is found in the cache (a cache hit), it is returned immediately, bypassing the database. This reduces page load times and database access overhead.
3. Fetching and Storing: If the data is not in the cache (a cache miss), WordPress retrieves the data from the database and then stores it in the cache for future requests. This involves serializing the data and storing it in memory.
4. Expiration and Eviction: Cached objects are usually stored with an expiration time. Once expired, they are evicted from the cache to make space for new data. In some setups, older or less frequently accessed data may also be evicted based on the caching algorithms used (like Least Recently Used, or LRU).
5. Persistent Caching: By default, WordPress object caching is non-persistent — it lasts only for the duration of the request. However, with the use of external object caching solutions such as Redis or Memcached, object caching can be made persistent across requests. This means that the cached data is available not just to subsequent operations in the same request but across different requests and even different users.

To enable persistent object caching in WordPress, you typically need to:

• Install and configure a caching system like Redis or Memcached on your server.
• Install a WordPress plugin that integrates with your caching system, and place a drop-in (a PHP file like object-cache.php) into your wp-content directory. This drop-in overrides WordPress’s default caching behavior.

Real World example

Imagine you run a busy e-commerce store on WordPress using WooCommerce. Your store features thousands of products, numerous categories, and receives high traffic, especially during sales or promotional events.

Every time a user visits a product page, WordPress needs to query the database for product details like price, descriptions, images, reviews, and availability. These queries are computationally expensive and slow down page loading, especially under heavy traffic.

Implementation of Object Caching: To address this, you implement object caching using a plugin like W3 Total Cache or set up a Redis object cache. Here’s how it works:

1. Initial Request:
   • A customer visits a product page for the first time.
   • WordPress performs database queries to retrieve all necessary product information.
   • This data is then stored in the object cache.
2. Subsequent Requests:
   • Another customer accesses the same product page shortly after.
   • Instead of querying the database again, WordPress retrieves the product data from the object cache.
   • This dramatically reduces the load on the database and speeds up page loading.

Benefits:

• Reduced Load Times: Customers experience faster browsing, enhancing user satisfaction and potentially increasing sales.
• Reduced Server Load: The database receives fewer queries, which reduces server load and can lower hosting costs.
• Scalability: During peak traffic times, such as a flash sale, the site remains responsive and can handle more concurrent visitors without performance degradation.

Maintenance:

• Cached objects need to be periodically refreshed, especially for dynamic data like stock levels.
• When products are updated, added, or removed, corresponding objects in the cache should be invalidated to ensure customers see the most current information.

Differernce Between Object Caching and Database Caching

Object caching is granular, caching individual data elements or objects. Page caching is broader, caching the entire content of a page at once.  Here are the key differences between Object Caching and Page Caching. 

Object Caching

Object caching involves storing database query results so that the next time a piece of data is needed, it can be fetched from the cache instead of being recalculated or retrieved from the database. This type of caching is beneficial for data that doesn’t change often but is requested frequently. Object caching can be implemented in memory with tools like Memcached or Redis to provide fast access to these cached objects across multiple requests and even users.

Key Points:

• Caches the results of complex queries and data fetch operations.
• Useful for data that is expensive to compute or retrieve.
• Often implemented using in-memory caching tools to speed up data retrieval across multiple page loads.

Database Caching

Database caching specifically targets the queries made to a database. When a query is run, its result set is stored in a cache. Subsequent identical queries can be served from this cache, significantly reducing the time spent accessing the database and executing the same queries repeatedly. This type of caching is particularly effective for reducing database load and improving the response time of websites that rely heavily on database operations.

Key Points:

• Caches the results of SQL queries.
• Reduces direct database access for fetching the same data, thereby decreasing database load.
• Can be directly managed by the database management system or via external caching systems.

Comparing Both

Object caching is broader as it can cache any data object in PHP, not just database queries. Database caching is specifically focused on caching the results of database queries. Both improve performance, but object caching can have a broader impact since it can cache more than just database queries. Implementing object caching typically requires more consideration regarding what to cache and how long to cache it since it involves multiple types of data, not just database queries.

While both caching types aim to reduce the load on resources and improve response times, they do so at different stages of data processing. Database caching is specific to SQL query results, whereas object caching has a wider application scope across any retrievable data within the application, enhancing flexibility but also requiring more careful management.

Object Caching Vs. Page Caching

Object Caching

Object caching involves storing data that results from database queries, calculations, or other data-fetch operations. When data is cached as objects, it can be reused by the application without the need to repeat the time-consuming processes used to generate them. This type of caching is particularly useful for data that is accessed frequently but changes infrequently. For example, settings, user profiles, or product catalog details might be stored in an object cache.

Key Points:

• Caches individual pieces of data or objects that the application uses.
• Helps reduce the workload on the database by avoiding repeated queries for the same data.
• Typically implemented using tools like Redis or Memcached.

Page Caching

Page caching can have a more immediate and noticeable impact on page load times for end-users because it bypasses all PHP processing and database querying. It refers to the process of storing the entire HTML output of a webpage. When a user requests a page, instead of WordPress generating the page through PHP and database queries each time, the server sends out a pre-rendered (cached) version of the page. This dramatically reduces the load time because the server does not need to process scripts or fetch multiple pieces of data from the database.

Key Points:

• Caches the complete HTML output of a page.
• Significantly reduces processing time for delivering pages because the entire page is served as a static file.
• Often managed directly by caching plugins or web server configurations.

To Sum it Up

Object caching is useful in dynamic environments where data needs to be fetched frequently and is the same across different pages, such as user settings or session data. Page caching is most effective on websites with static content where pages do not change often after being generated. In WordPress, using both caching types can provide optimal performance improvements, with object caching managing data operations efficiently and page caching ensuring quick page delivery to visitors.