The protocol

The following is a detailed spec of the Inertia protocol. Be sure to read the how it works page first for a high-level overview.

HTML responses

The very first request to an Inertia app is just a regular full-page browser request, with no special Inertia headers or data. For these requests, the server returns a full HTML document.

This HTML response includes the site assets (CSS, JavaScript) as well as a root <div> in the body. The root <div> serves as a mounting point for the client-side app, and includes a data-page attribute with a JSON encoded page object for the initial page. Inertia uses this information to boot your client-side framework, and display the initial page component.

Request
GET: http://example.com/events/80
Accept: text/html, application/xhtml+xml
Response
HTTP/1.1 200 OK
Content-Type: text/html; charset=utf-8
<html>
<head>
    <title>My app</title>
    <link href="/css/app.css" rel="stylesheet">
    <script src="/js/app.js" defer></script>
</head>
<body>

<div id="app" data-page='{"component":"Event","props":{"event":{"id":80,"title":"Birthday party","start_date":"2019-06-02","description":"Come out and celebrate Jonathan&apos;s 36th birthday party!"}},"url":"/events/80","version":"c32b8e4965f418ad16eaebba1d4e960f"}'></div>

</body>
</html>
While the initial response is HTML, Inertia does not server-side render the JavaScript page components.

Inertia responses

Once an Inertia app has been booted, all subsequent request to the site are made via XHR with a special X-Inertia header set to true. This header indicates that the request is being made by Inertia, and isn't a standard full-page visit.

When the server detects the X-Inertia header, instead of responding with a full HTML document, it instead returns a JSON response with an encoded page object.

Request
GET: http://example.com/events/80
Accept: text/html, application/xhtml+xml
X-Requested-With: XMLHttpRequest
X-Inertia: true
X-Inertia-Version: 6b16b94d7c51cbe5b1fa42aac98241d5
Response
HTTP/1.1 200 OK
Content-Type: application/json
Vary: Accept
X-Inertia: true
{
  "component": "Event",
  "props": {
    "event": {
      "id": 80,
      "title": "Birthday party",
      "start_date": "2019-06-02",
      "description": "Come out and celebrate Jonathan's 36th birthday party!"
    }
  },
  "url": "/events/80",
  "version": "c32b8e4965f418ad16eaebba1d4e960f"
}

The page object

Inertia shares data between the server and client via a page object. This object includes the necessary information required to render the page component, update the history state, and track the site's asset version. The page object includes the following four properties:

  1. component: The name of the JavaScript page component.
  2. props: The page props (data).
  3. url: The page url.
  4. version: The current asset version.

On standard full page visits, the page object is JSON encoded into the data-page attribute in the root <div>. On Inertia visits, the page object is returned as the JSON payload.

Asset versioning

One common challenge with single-page apps is refreshing site assets when they've been changed. Inertia makes this easy by optionally tracking the current version of the site's assets. In the event that an asset changes, Inertia will automatically make a hard (full) page visit instead of an XHR visit.

Included in the page object is a version identifier. This version identifier is set server-side, and can be a number, string, file hash, whatever. It doesn't matter, as long as it changes when the site's assets have been updated.

Whenever an Inertia request is made, Inertia will include the current asset version in the X-Inertia-Version header. When the server receives the request, it compares the asset version provided in the X-Inertia-Version header with the current asset version. This is typically handled in a middleware layer.

If the asset versions are the same, the request simply continues as expected. However, if they are different, the server immediately returns a 409 Conflict response, and includes the URL in a X-Inertia-Location header. This header is necessary, since server-side redirects may have occurred. This tells Inertia what the final intended destination URL is.

Note, 409 Conflict responses are only sent for GET requests, and not for POST/PUT/PATCH/DELETE requests. That said, they will be sent in the event that a GET redirect occurs after one of these requests.

Finally, in the event that flash session data exists when a 409 Conflict response occurs, the server will automatically reflash this data.

Request
GET: http://example.com/events/80
Accept: text/html, application/xhtml+xml
X-Requested-With: XMLHttpRequest
X-Inertia: true
X-Inertia-Version: 6b16b94d7c51cbe5b1fa42aac98241d5
Response
409: Conflict
X-Inertia-Location: http://example.com/events/80

Partial reloads

When making Inertia requests, the partial reloads option allows you to request a subset of the props (data) from the server on subsequent visits to the same page component. This can be a helpful performance optimization if it's acceptable that some page data becomes stale.

When a partial reload request is made, Inertia includes two additional headers with the request: X-Inertia-Partial-Data and X-Inertia-Partial-Component.

The X-Inertia-Partial-Data header is a comma separated list of the desired props (data) keys that should be returned.

The X-Inertia-Partial-Component header includes the name of the component that is being partially reloaded. This is necessary, since partial reloads only work for requests made to the same page component. If the final destination is different for whatever reason (eg. the user was logged out and is now on the login page), then no partial reloading will occur.

Request
GET: http://example.com/events
Accept: text/html, application/xhtml+xml
X-Requested-With: XMLHttpRequest
X-Inertia: true
X-Inertia-Version: 6b16b94d7c51cbe5b1fa42aac98241d5
X-Inertia-Partial-Data: events
X-Inertia-Partial-Component: Events
Response
HTTP/1.1 200 OK
Content-Type: application/json
{
  "component": "Events",
  "props": {
    "auth": {...},       // NOT included
    "categories": [...], // NOT included
    "events": [...]      // included
  },
  "url": "/events/80",
  "version": "c32b8e4965f418ad16eaebba1d4e960f"
}