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API Conditional requests

Conditional requests is an HTTP feature that lets a client request content that is newer than a known modification time, or different from a known content hash.

Unpoly uses conditional requests for reloading, cache revalidation and polling. By observing HTTP headers, your server can quickly produce an empty response for unchanged content. This saves CPU time and reduces the bandwidth cost for a request/response exchange to about 1 KB.

Important

Supporting conditional requests is entirely optional. If your backend does not honor conditional request headers, can still use features like polling or cache revalidation.

Requesting content changed from a known content hash

In this example the browser updates a fragment with a target from a URL /messages. For this it makes the following HTTP request:

GET /messages HTTP/1.1
X-Up-Target: .messages

The server responds with both the content and a hash over the underlying data. The hash can be any string that identifies the rendered objects, revision and representation. Here is an example response:

HTTP/1.1 200 OK
ETag: "x234dff"

<html>
  ...
  <div class='messages'>
    ...
  </div>
  ...  
</html>

Note

The double quotes are part of the ETag's format.

After rendering, Unpoly remembers the ETag headers in an [up-etag] attribute:

<div class='messages' up-etag='"x234dff"'>
...
</div>

When Unpoly reloads, revalidates or polls the fragment, it echoes the earlier content hash in an If-None-Match header:

GET /messages HTTP/1.1
If-None-Match: "x234dff"

The server can compare the ETag from the request with the ETag of the underlying data. If no more recent data is available, the server can skip rendering and respond with 304 Not Modified. No response body is required.

HTTP/1.1 304 Not Modified

Requesting content newer than a known modification time

In this example the browser updates a fragment with a target from a URL /messages. For this it makes the following HTTP request:

GET /messages HTTP/1.1
X-Up-Target: .messages

The server responds with both the content and its last modification time. The modification time can be any update timestamp for the data that was rendered. It should be sent as an Last-Modified header:

HTTP/1.1 200 OK
Last-Modified: Wed, 15 Nov 2000 13:11:22 GMT

<html>
  <div class='messages'>...</div>
</html>

After rendering, Unpoly remembers the Last-Modified header in an [up-time] attribute:

<div class='messages' up-time='Wed, 21 Oct 2015 07:28:00 GMT'>
...
</div>

When Unpoly reloads, revalidates or polls the fragment, it echoes the earlier modification time in an If-Modified-Since header:

GET /messages HTTP/1.1
If-Modified-Since: Wed, 15 Nov 2000 13:11:22 GMT

The server can compare the time from the request with the time of the last data update. If no more recent data is available, the server can skip rendering and respond with 304 Not Modified. No response body is required.

HTTP/1.1 304 Not Modified

Modification time or content hash?

Servers can use both Last-Modified and ETag, but ETag always takes precedence.

It's easier to mix in additional data into an ETag, e.g. the ID of the logged in user or the currently deployed commit hash.

Individual versions per fragment

A large response may contain multiple fragments that are later reloaded individually and should each have their own ETag or modification time. In this case the server may render each fragment with its own [up-etag] or [up-time] attribute:

<div class='messages' up-time='Wed, 21 Oct 2015 07:28:00 GMT'>
  <div class='message' id='message1'>...</div>
  <div class='message' id='message2'>...</div>
</div>

<div class='recent-posts' up-time='Thu, 3 Nov 2022 15:35:02 GMT'>
  <div class='post' id='post1'>...</div>
  <div class='post' id='post2'>...</div>
</div>

When a fragment is reloaded Unpoly will use the version from the closest [up-etag] or [up-time] attribute:

up.reload('.messages')     // If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMT
up.reload('.recent-posts') // If-Modified-Since: Thu, 3 Nov 2022 15:35:02 GMT
up.reload('#post1')        // If-Modified-Since: Thu, 3 Nov 2022 15:35:02 GMT

The server may send additional ETag or Last-Modified response headers. However this is optional and will only be used for fragments that don't have a close [up-etag] or [up-time] attribute.

Removing versions for a fragment

To prevent a fragment from inheriting a version from an ancestor, assign it an [up-etag=false] or [up-time=false] attribute.

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