---
title: 'Cache Flags: Targeted Invalidation'
description: 'Cache flags tag every MilliCache entry with labels like post:123, so WordPress content changes clear only affected pages instead of the whole cache.'
menu_order: 10
---

# Introduction to Cache Flags

Cache flags are one of MilliCache's most powerful features. 
They enable **targeted cache invalidation** — clearing only the cache entries that actually need updating, rather than wiping everything.

## The Challenge of Modern WordPress

Most caching plugins handle basic scenarios well — when a post is updated, they clear that post's cache and related archives. 
But WordPress is evolving rapidly with Gutenberg's modernization phases, creating new challenges.

### The Synced Pattern Problem

Consider **Synced Patterns** (reusable blocks) and **FSE Template Parts**. 
A single pattern might appear on dozens or hundreds of pages across your site:

- A promotional banner in your header template
- A newsletter signup block used across blog posts
- A pricing table embedded in multiple product pages

When you update that synced pattern, **which pages need their cache cleared?**

Traditional caching plugins have no way to know. 
Their only safe option is to **flush the entire site cache** — even if the pattern only appears on 5% of your pages.

### How Flags Solve This

MilliCache tags each cached page with **flags** — labels describing what content appears on that page. 
You can create flags for patterns, template parts, or any content relationship.

```mermaid
flowchart LR
    subgraph "Traditional: Synced Pattern Updated"
        A[Pattern Changed] --> B[Which pages use it?]
        B --> C[Unknown → Clear ALL]
    end
```

```mermaid
flowchart LR
    subgraph "MilliCache with Flags"
        D[Pattern Changed] --> E[Find entries with flag<br/><i>pattern:promo-banner</i>]
        E --> F[Clear only<br/>matching pages]
        F --> G[95% of cache<br/>stays warm]
    end
```

This becomes increasingly important as WordPress moves toward Full Site Editing, where template parts and patterns are shared across many pages.

## Real-World Analogy

Think of cache flags like **tags on photos** in your photo library:

- You tag photos with "vacation", "2024", "beach", "family"
- Later, you can find all "beach" photos instantly
- Deleting one tag doesn't affect others
- Photos can have multiple tags

Similarly, MilliCache pages can have multiple flags, and you can target specific flags for clearing.

## Why Single Pages Need Multiple Flags

A single URL can generate different cache entries based on:

- Query parameters (`?page=2`, `?sort=price`)
- Cookies (language preferences, currency)
- Other request variations

All these entries relate to the same content but have different cache keys. Flags group them logically:

| URL                        | Cache Key  | Flags                        |
|----------------------------|------------|------------------------------|
| `/product/shoe/`           | `abc123`   | `post:45`, `archive:product` |
| `/product/shoe/?color=red` | `def456`   | `post:45`, `archive:product` |
| `/product/shoe/reviews/`   | `ghi789`   | `post:45`, `archive:product` |

When the product is updated, clearing `post:45` removes all three entries — exactly what you need.

## Benefits of Flag-Based Invalidation

### Performance
- Cache stays warm for unaffected pages
- Less database load during updates
- Faster recovery after content changes

### Precision
- Update a post → only that post's pages cleared
- Update a category → only that category's archives cleared
- No collateral damage to unrelated content

### Flexibility
- Create custom flags for your specific needs
- Use wildcards for broad clearing (`post:*`)
- Integrate with your content workflow

## How Flags Flow Through MilliCache

```mermaid
sequenceDiagram
    participant V as Visitor
    participant M as MilliCache
    participant W as WordPress
    participant R as Redis/ValKey

    V->>M: Request /blog/my-post/
    M->>R: Check cache
    R-->>M: MISS
    M->>W: Load WordPress
    W-->>M: HTML + Flags [home, post:123]
    M-->>V: Response
    M->>R: Store with flags

    Note over V,R: Later: Post 123 is updated

    W->>M: Clear post:123
    M->>R: Find entries with flag
    R-->>M: 3 entries found
    M->>R: Delete entries
```

## Flags in MilliCache Pro

With [MilliCache Pro](https://www.millipress.com/millicache-pro/), flags become visible and reach further: the [Cache Entries Browser](https://www.millipress.com/docs/millicache-pro/02-modules/02-cache-entries/) shows the flags of every cached page right in the settings screen, and the [Edge Cache module](https://www.millipress.com/docs/millicache-pro/02-modules/07-edge-cache/) mirrors flag-based invalidation onto your CDN, so edge purges are as surgical as local ones.

## Next Steps

- [Built-in Flags](/docs/millicache/03-cache-flags/02-built-in-flags) — Flags automatically assigned by MilliCache
- [Custom Flags](/docs/millicache/03-cache-flags/03-custom-flags) — Create your own flags for advanced control
- [Cache Clearing](/docs/millicache/05-usage/20-cache-clearing) — Methods for clearing cache by flags