Fixing Cache: New Pages Not Showing In Section Template

Introduction

Have you ever added a new page to a section on your website, only to find that it doesn't appear in the navigation or sidebar? This frustrating issue often stems from cache invalidation problems in template-based websites. When a template iterates through section pages, such as to generate a sidebar or navigation menu, adding a new file to that section should trigger a refresh of the cached content. However, sometimes the old section content continues to be served until a manual rebuild is initiated. This article dives deep into this problem, exploring the root causes and providing a suggested fix to ensure your website accurately reflects the latest content.

Understanding the Problem: Cache Not Invalidated

The core issue revolves around how website caching mechanisms handle updates to section pages within templates. Website caching is a technique used to store frequently accessed data, like the output of templates, to reduce server load and improve website loading times. When a user requests a page, the cached version is served instead of regenerating the page from scratch. This is particularly useful for dynamic websites where content is generated on the fly.

However, this caching strategy can lead to problems when content changes. If the cache isn't properly updated or invalidated when a new page is added to a section, the website will continue to display the old content, essentially hiding the new page from users. This can be confusing for visitors and frustrating for website administrators who expect changes to be immediately reflected. For instance, imagine a documentation website where new guides are frequently added. If the navigation sidebar, generated from a template that iterates through section pages, doesn't update automatically, users might miss the latest guides, leading to a poor user experience.

Reproducing the Issue: A Step-by-Step Guide

To better understand the cache invalidation problem, let's walk through a simple reproduction scenario. This will help you identify if you're experiencing the same issue on your website and provide a concrete example for testing the suggested fix.

1. Set up a Section with Multiple Pages: Begin by creating a section on your website, such as a /guide/ section, and populate it with several pages. These pages could be individual articles, tutorials, or any other content you want to group together.

2. Create a Template Iterating Through Section Pages: Next, design a template that iterates through the pages within the section. This is commonly used for generating sidebars, navigation menus, or lists of related articles. Here’s an example using Jinja, a popular templating language:

   {% for page in section.pages %}
     <a href="{{ page.permalink }}">{{ page.title }}</a>
   {% endfor %}
   

   This code snippet loops through each page in the section.pages list and creates a link to it, displaying the page title. Other templating languages might have slightly different syntax, but the underlying principle remains the same.

3. Add a New Markdown File to the Section Directory: Now, add a new markdown file (or any other content file format your website uses) to the section directory. This simulates adding a new page to the section.

4. Observe the Issue: The New Page Doesn't Appear: After adding the new file, refresh your website and check the section where the template is used (e.g., the sidebar). You'll likely find that the new page doesn't appear in the list. The cached template output is still being served, preventing the new page from being displayed.

By following these steps, you can clearly demonstrate the cache invalidation issue and verify that the problem exists on your website.

Expected Behavior: Automatic Updates

The expected behavior when adding a new file to a section is that any cached output that depends on that section's page list should be automatically invalidated. This means that the next time a user requests a page that uses the template, the template should be regenerated, incorporating the new page into the output. In our example, the navigation sidebar should update to include the link to the newly added page.

This automatic update is crucial for maintaining an accurate and up-to-date website. Without it, content creators have to manually trigger a rebuild or clear the cache every time they add a new page, which is time-consuming and prone to errors. A robust caching system should handle these updates seamlessly, ensuring that website visitors always see the latest content.

Root Cause Analysis: Tracking Dependencies

The root cause of this cache invalidation problem lies in the way template expansion accesses section data. When a template iterates through section.pages, it's essentially creating a dependency between the template's output and the list of pages in that section. However, these accesses aren't always tracked as dependencies by the caching system.

The dependency graph, which is used to determine which cached outputs need to be invalidated when content changes, doesn't capture the relationship between the template and the section's page list. In other words, the system doesn't realize that