Decoupling Logic from Layout: Mastering Chunks and Snippets in MODX

The Problem: Template Bloat and Redundant HTML

When building a site in MODX Revolution, it is tempting to build a comprehensive template for every page type. As the site grows, the same HTML structures like a footer, navigation menu, or product card get duplicated across templates. When a CSS class changes, every template must be hunted down and updated.

The solution is to move away from monolithic templates toward a component-based architecture using Chunks and Snippets. By separating static HTML Chunks from dynamic PHP logic Snippets, layout becomes agnostic of data fetching.

Chunks: The Static Blueprint

A Chunk is a reusable fragment of HTML stored in the MODX database. Think of it as a partial view. Instead of writing the same HTML for a Featured Article card in three places, write it once in a Chunk.

To make Chunks dynamic, MODX uses placeholders. These are markers like [[+title]] or [[+image]]. The Chunk does not know what these values are, it provides the slot where data will land. The same Chunk can be used for a blog post, team member profile, or product listing provided they share the same data keys.

Snippets: The Logic Engine

While Chunks handle look, Snippets handle how. A Snippet is PHP code that executes on the server, processes data often from the MODX database, and returns a string of content to the page.

The most powerful pattern in MODX is the Snippet-to-Chunk pipeline. Rather than having a Snippet return a giant string of hard-coded HTML, the Snippet fetches data and injects that data into a Chunk. This keeps PHP clean and allows a front-end developer to edit HTML in the Chunk manager without touching PHP code.

Worked Example: A Dynamic Staff List

Imagine you need a list of staff members. Logic handles sorting and filtering, HTML remains easily editable.

1. Create the Chunk named staffCard

<div class='staff-member'>
  <img src='[[+photo]]' alt='[[+name]]'>
  <h4>[[+name]]</h4>
  <p>[[+role]]</p>
</div>

2. Create the Snippet named getStaff

This example assumes pdoTools is installed, the standard for high-performance queries in MODX.

<?php
$pdo = $modx->getService('pdoTools');
$output = $pdo->runSnippet('pdoResources', [
    'parents' => 12,
    'tpl' => 'staffCard',
    'includeTVs' => 'photo,role',
    'outputSeparator' => '',
]);
return $output;
?>

3. Call the Snippet in your Template

Place this tag where you want the list to appear:

[[getStaff]]

Performance Trade-offs and Limitations

Modular design introduces parsing overhead. Every MODX tag must be resolved. Nesting Snippets inside Chunks which then call other Snippets creates a recursive loop that can slow page load times.

• Database Dependency: By default Snippets and Chunks are stored in the database. This makes Git version control difficult. For professional engineering consider a file-based provider.
• XSS Risks: When returning data from a Snippet ensure user-provided content is escaped. Use the MODX API filtering to prevent Cross-Site Scripting attacks.

Verifying the Implementation

1. Inspect the Source: View page source in your browser. Ensure HTML matches the Chunk exactly and placeholders like [[+name]] have been replaced by actual data.
2. Test Isolation: Change a CSS class inside staffCard Chunk. Refresh the page. If change appears across all lists using that Chunk decoupling is successful.
3. Check Query Efficiency: For large datasets use pdoTools and pdoResources instead of native getResources to reduce memory consumption.