HTMX Synchronization — Complete Guide with Examples

In this tutorial, you'll learn about HTMX request synchronization. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.

HTMX synchronization with hx-sync coordinates multiple requests from different elements, preventing race conditions by queuing, dropping, or replacing requests to maintain data consistency.

What You'll Learn

By the end of this tutorial, you'll use hx-sync to queue requests, drop duplicates, replace in-flight requests, synchronize form submissions, and coordinate interdependent components.

Why It Matters

Without synchronization, multiple rapid requests can produce race conditions: a delete request completes before a create request, or two simultaneous updates overwrite each other. Synchronization ensures consistent server state.

Real-World Use

Durga Antivirus Pro's quarantine management uses hx-sync to prevent concurrent operations on the same file. When a user tries to restore and delete a file simultaneously, synchronization ensures one operation completes before the other starts.

Where This Fits in Your Learning Path

flowchart LR
    A["HTMX Swapping"] --> B["**Synchronization**"]
    B --> C["History & Indicators"]
    C --> D["Hyperscript & Extensions"]
    D --> E["Advanced HTMX"]
    style B fill:#3b82f6,stroke:#2563eb,color:#fff
    style A fill:#e2e8f0,stroke:#94a3b8
    style E fill:#e2e8f0,stroke:#94a3b8

Basic hx-sync

Synchronize multiple elements under a common parent.

<div hx-sync="this:replace">
  <button hx-post="/api/action1">Action 1</button>
  <button hx-post="/api/action2">Action 2</button>
  <button hx-post="/api/action3">Action 3</button>
</div>

Expected output: Only one request can be active at a time within the synchronized group. A new request replaces any in-flight request.

Queue Strategy

Queue requests instead of dropping or replacing them.

<div hx-sync="this:queue">
  <input hx-get="/api/search1" hx-trigger="input changed delay:200ms" placeholder="Search 1">
  <input hx-get="/api/search2" hx-trigger="input changed delay:200ms" placeholder="Search 2">
</div>

Expected output: Requests are queued and processed in order. Each request waits for the previous to complete.

Drop Strategy

Drop new requests while a request is in flight.

<div hx-sync="this:drop">
  <button hx-post="/api/submit-order">Submit Order</button>
  <button hx-post="/api/submit-order">Submit Order (ignored if first is in flight)</button>
</div>

Expected output: Clicking both buttons simultaneously only processes the first click. The second is dropped.

Replace Strategy

Cancel the current request and start the new one.

<div hx-sync="this:replace">
  <button hx-post="/api/save-draft">Save Draft</button>
  <button hx-post="/api/publish">Publish (cancels draft save)</button>
</div>

Expected output: Clicking Publish while Save Draft is in flight cancels the draft save and starts publishing.

Synchronizing Form Submissions

Prevent double form submissions.

<form hx-post="/api/signup" hx-sync="this:replace">
  <input name="email" type="email" required placeholder="Email">
  <input name="password" type="password" required placeholder="Password">
  <button type="submit">Sign Up</button>
</form>

Expected output: Clicking Submit multiple times only sends one request. Subsequent clicks are ignored until the first completes.

Cross-Element Synchronization

Synchronize elements that are not siblings.

<div>
  <button hx-post="/api/action" id="btn-a" hx-sync="#btn-b:replace">
    Button A
  </button>
  <button hx-post="/api/action" id="btn-b" hx-sync="#btn-a:replace">
    Button B
  </button>
</div>

Expected output: Button A cancels Button B's request and vice versa. Only one request is active at a time.

Common Mistakes

1. Forgetting to synchronize interdependent elements

When two elements modify the same resource, they need synchronization. Without it, updates may conflict.

2. Using the wrong queue strategy

Queue for sequential-dependent operations (form steps). Drop for idempotent operations (save). Replace for the latest-value operations (search).

3. Synchronizing too broadly

Over-synchronizing unrelated elements creates unnecessary bottlenecks. Only synchronize elements that modify shared state.

4. Not synchronizing forms with rapid double-clicks

Without hx-sync, double-clicking a submit button sends two requests. Use replace or drop to prevent duplicates.

5. Using queue for long-running operations

Queueing long requests (file uploads) blocks subsequent quick requests. Use replace or consider separate sync groups.

Practice Questions

1. What does hx-sync="this:queue" do? It queues requests from child elements. Each request waits for the previous to complete before starting.

2. What is the difference between drop and replace? drop ignores the new request while one is in flight. replace cancels the current request and starts the new one.

3. How do you prevent double form submissions? Add hx-sync="this:replace" to the form element.

4. Can hx-sync reference a specific element by ID? Yes. Use hx-sync="#other-element:queue" to synchronize with a specific element.

5. What happens if you use hx-sync without a strategy? The default strategy is replace.

Challenge

Build a multi-step form where each step's submission is queued. The next step cannot start until the current step's server validation completes.

FAQ

Mini Project

Build a synchronized dashboard with three widgets that update different parts of a shared data model. Use hx-sync to ensure that updates from any widget don't conflict with each other.

<div hx-sync="this:queue">
  <div id="widget-a" class="widget">
    <h3>Counter A</h3>
    <button hx-post="/api/counter/a/increment" hx-target="#widget-a">Increment A</button>
    <button hx-post="/api/counter/a/decrement" hx-target="#widget-a">Decrement A</button>
  </div>

  <div id="widget-b" class="widget">
    <h3>Counter B</h3>
    <button hx-post="/api/counter/b/increment" hx-target="#widget-b">Increment B</button>
    <button hx-post="/api/counter/b/decrement" hx-target="#widget-b">Decrement B</button>
  </div>

  <form hx-post="/api/counter/reset" hx-sync="this:replace">
    <button type="submit">Reset All Counters</button>
  </form>
</div>

What's Next

Manage browser history:

Related topics: race condition prevention, request queuing patterns.