Henning Koch  @triskweline

About Unpoly

Unpoly is an unobtrusive JavaScript framework for server-side web applications.
It enables fast and flexible frontends while keeping rendering logic on the server.

This presentation is for experienced Unpoly users
who want to learn about the major changes in Unpoly 3.

Unpoly 3 has been released on April 17th, 2023.


Fixing all the concurrency issues

  • User clicking faster than the server can respond
  • Multiple requests targeting the same fragment
  • Responses arrive in different order than requests
  • Forms where everything depends on everything else
  • Multiple users working on the same backend data (stale caches)
  • User losing connection while requests are in flight
  • Lie-Fi (spotty Wi-Fi, EDGE, tunnel)

Quality of life improvements

  • Optional targets
  • Idempotent up.hello()
  • More control over [up-hungry]
  • HTML5 data attributes
  • Extensive render callbacks
  • Strict target derivation
  • Log separated by user interaction
  • Foreign overlays

Fixing concurrency

Cache revalidation

Unpoly has always cached GET responses for a few minutes.
This allowed for instant navigation between pages we visited earlier, but had some issues:

  • Users were sometimes seeing stale content.
    Because of this many projects configured cache expiry down to a few seconds.
  • Every non-GET request cleared the entire cache.
    This caused unnecessary cache misses after submitting a form.

Unpoly 3 fixes all of this with cache revalidation.

Stale while revalidate

After rendering stale content from the cache, Unpoly 3 automatically reloads the fragment.

This process is called cache revalidation.

Unpoly renders twice:

  • A first render pass from the cache (which may be stale)
  • A second render pass from the server (which is always fresh)

This has many benefits:

  • We can have long cache eviction times, allowing instant navigation for 90 minutes.
  • We no longer need to clear the cache after a form submission.
    We just mark all cache entries as stale.
  • Because we always revalidate cached content, the user always sees fresh content


  • When cached content exceeds this age we perform cache revalidation
    (reload a rendered fragment)
  • This should only cover the time between preloading and rendering.
  • Defaults to 15 seconds (down from 5 minutes in Unpoly 2)


  • When cached content exceeds this age we delete it from the cache
  • This limit exists mostly to limit memory consumption.
  • Defaults to 90 minutes.

Controlling when to revalidate

You may configure what responses to revalidate, e.g. to exclude certain paths.

The default setting is:

// A response older than up.network.config.cacheExpireAge is expired
up.fragment.config.autoRevalidate = (response) => response.expired

You may also disable revalidation for an individual link:

<a href="/path" up-follow up-revalidate="false">

Does revalidation cause more requests?

With revalidation your app will make as many requests as a plain web app
or as an Unpoly 2 app with short cache expiry.

Remember that many of our projects have configured cache expiry to "fix" stale content.

Optionally your app can support conditional requests so reloading is effectively free for the server.

Conditional request support

Conditional requests is an old HTTP feature (RFC 7232).

It lets browser ask for content newer than a known modification time,
or content different from a known content hash.

Requesting content newer than a known modification time

Browser requests a URL for the first time:

GET /foo HTTP/1.1

Server responds with content and last modification time (e.g. #updated_at):

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

When the browser revisits the same URL is echoes the earlier modification time:

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

The server checks modification time and may respond without content:

HTTP/1.1 304 Not Modified

Requesting content changed from a known content hash

Browser requests a URL for the first time:

GET /foo HTTP/1.1

Server responds with content and a hash over the underlying data:

HTTP/1.1 200 OK
ETag: "x234dff"

When the browser revisits the same URL is echoes the earlier hash:

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

The server checks data hash and may respond without content:

HTTP/1.1 304 Not Modified

Unpoly 3 supports conditional requests when reloading

Unpoly remembers the Last-Modified and ETag headers a fragment was delivered with:

<div class='messages' up-time='Wed, 21 Oct 2015 07:28:00 GMT' etag='"x234dff"'>

When the fragment is reloaded, Unpoly echoes these values using standard HTTP headers:

GET /messages HTTP/1.1
X-Up-Target: .messages
If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMT
If-None-Match: "x234dff"

If no fresher data exists, the server may skip rendering and respond without content:

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 revision.

Benefits of implementing conditional requests

Conditional requests can improve cases where the user re-visit a page, in particular:

By sending 304 Not Modified for unchanged content you get:

  • Reduced server load (no server-side rendering required)
  • Fewer data transmitted (think slow connections, mobile data plans)
  • Prevent unnecessary DOM swaps of near-identical content

Note: Without conditional request support Unpoly will still discard identical responses when revalidating. You may define additional rules in up.fragment.config.skipResponse

Challenges of implementing conditional requests

  • Reloading the page during development may not always pick up changes
  • Pages that display many records must either
    (1) build ETags from multiple records or
    (2) propagate update timestamps through associations

Implementing conditional requests in Ruby on Rails

If you're not a Rails user, skip to a slide without the Rails logo in the corner ↗.

While you can manually manage and compare request.headers['ETag'] and response.headers['ETag'] in your controllers, Rails ships with helpers to help deal with ETags.

Controller example

class PostsController < AppplicationController

  def show
    @post = Post.find(params[:id])

    # Produces ETag from (1) class name (2) @post.id
    # (3) @post.updated_at (4) view template code (5) flashes.
    # Renders 304 Not Modified if ETag matches If-None-Match header.
    fresh_when @post
  def index
    @posts = Post.order(created_at: :desc)

    # Produces ETag from (1) class name (2) scope conditions
    # (3) @posts.maximum(:updated_at) (4) view template code (5) flashes.
    # Renders 304 Not Modified if ETag matches If-None-Match header.
    fresh_when @posts


Composing an ETag from multiple records

When associated records are rendered on the view they should (ideally) be included in the ETag:

class PostsController < ApplicationController

  etag { current_user&.id } # Produce different ETags for different users

  def show
    @post = Post.find(params[:id])

    # The show template also renders the posts's author.
    fresh_when [@post, @post.author]
  def index
    @posts = Post.order(created_at: :desc).preload(:author).to_a

    # The index template also renders the post authors.
    fresh_when [*@posts, *@posts.map(&:author)]


Default ETags in Rails

Even without fresh_when Rails produces a default ETag by hashing the response body.

❌ You still pay the rendering time.
✅ You won't transmit unchanged HTML.
✅ You don't need to care what goes into an ETag.

When you find it hard to produce a correct ETag for a complex view, you may find it easier to make the view fast (using fragment caching) and rely on the default ETag.

Tip: The default Rack::ETag middleware has issues with random tokens (CSRF token, CSP nonce), causing ETags to never match. Use our Rack::SteadyETag gem to address this.

Cache revalidation works without conditional requests

Unpoly's cache revalidation will work with or without conditional request support.

If your app does not support conditional requests, cache revalidation will cause just as many requests as an Unpoly 2 app with short cache expiry.

Concurrent updates to the same fragment

When two requests target main, what should happen?

The answer to that changed throughout Unpoly's history.

Unpoly 1: Do nothing

Responses would be rendered in whatever order they arrive.

Unpoly 2: Abort all pending requests when navigating

This mimics standard browser behavior, where clicking a link aborts earlier clicks.
Unfortunately this also aborted background requests, or requests for unrelated regions like side.

Unpoly 3: Abort requests within the targeted fragment only

Targeting a fragment will abort conflicting requests

Clicking this link will automatically cancel requests targeting .region or its descendants:

<a href="/path" up-target=".region">

Same when rendering programmatically:

up.render({ url: '/path', target: '.region' })

Aborting without rendering:


What about preloading?

Preloading never aborts targeted fragments.

Imperative preloading with up.link.preload() is no longer abortable by default in Unpoly 3.

You can use this to populate the cache while the user is navigating:

up.compiler('.main-nav', function(nav) {

Forms where everything depends on everything

A common challenge in development are forms here fields have many dependencies on other fields:

A form with many dependent fields

We can build this with [up-validate], but.….

<form method="post" action="/purchases">
  <select name="continent" up-validate="[name=country]">...</select>
  <select name="country" up-validate="[name=price]">...</select>
  <input name="weight" up-validate="[name=price]"> kg
  <output name="price">23 €</output>
  <button>Buy stamps</button>

This form has race conditions in Unpoly 2:

  • User changes continent
  • Request 1 targeting [name=country] starts
  • User changes weight
  • Request 2 targeting [name=price] starts
  • User changes continent again
  • Request 3 targeting [name=country] starts
  • Three responses arrive and render in random order 💥

Disabling form elements while loading

Forms with [up-disable] attribute disable all fields and buttons while submitting or validating.
This prevents user input while the form is loading:

<form up-submit up-disable>
  <input type="text" name="email"> <!-- will be disabled during submission -->
  <button>Submit</button>          <!-- will be disabled during submission -->

You can also only disable the submit button:

<form up-submit up-disable="button">

Or any given CSS selector:

<form up-submit up-disable="input[name=email]">

Form before user input

All fields are enabled.

A form with many dependent fields

User selects different country

All fields disable to prevent concurrent input.

A form is disabled while validating

Validation has completed

All fields are re-enabled.

A form is re-enabled after validation is done

Consistency without disabling

Sometimes we don't want to disable forms because of optics (gray fields)
or to not prevent user input.

Unpoly 3 has a second solution for forms with many [up-validate] dependencies
that does not require disabling.

Eventual consistency guarantee for [up-validate]

  • Multiple updates from [up-validate] or up.validate()
    are batched into a single render pass with multiple targets.
  • Duplicate or nested targets are consolidated.
  • Unpoly guarantees only one concurrent validation request per form.
    Additional validations are queued until the current validation request has loaded.
  • The form will eventually show a consistent state,
    regardless how fast the user clicks or how slow the network is.
<form method="post" action="/purchases">
  <select name="continent" up-validate="[name=country]">...</select>
  <select name="country" up-validate="[name=price]">...</select>
  <input name="weight" up-validate="[name=price]"> kg
  <output name="price">23 €</output>
  <button>Buy stamps</button>

Unpoly 2: Race conditions

  • User changes continent
  • Request 1 for [name=country] starts
  • User changes weight
  • Request 2 for [name=price] starts
  • User changes continent again
  • Request 3 for [name=country] starts
  • Responses arrive and render in random order

Unpoly 3: Eventual consistency

  • User changes continent
  • Request 1 for [name=country] starts
  • User changes weight
  • User changes continent again
  • Response 1 received and rendered
  • Request 2 for [name=price], [name=country] starts
  • Response 2 received and rendered

Field-specific watch options

Every form field can configure options that affect both validation ([up-validate]
and JavaScript watchers (up.observe(), now up.watch()).

For instance, we can tell a field to validate while the user is typing.


<form method="post" action="/purchases">
  <select name="continent" up-validate="[name=country]" up-watch-disable="[name=country]">
  <select name="country" up-validate="[name=price]">...</select>
  <input name="weight" up-validate="[name=price]" up-watch-event="input"> kg
  <output name="price">23 €</output>
  <button>Buy stamps</button>

The [up-watch-disable] attribute disables the country select while new countries are loading after a continent changes.

The [up-watch-event] attribute updates the price while the user is typing in the weight field (instead of waiting until the field is blurred).

Options can be set for a field, the entire form or any container element.


Which event triggers watch callbacks or validation (e.g. input or change).


How many milliseconds to wait after a change before a watch callback or validation is run.


Whether to disable this form (or any fragment) while an async watch callback or validation is running.


Whether to set .up-active and .up-loading classes while an async watch callback or validation is running.

Handling disconnects

Unpoly 3 lets you handle connection loss with an { onOffline } or [up-on-offline] callback:

<a href="..." up-on-offline="if (confirm('Retry?') event.retry()">Post bid</a>

You may also configure a global handler:

up.on('up:fragment:offline', function(event) {
  if (confirm('Retry?')) event.retry()

You may also do something other than retrying, like substituting content:

up.on('up:fragment:offline', function(event) {
  up.render(event.renderOptions.target, { content: "You are offline." })

Handling "Lie-Fi"

Often our device reports a connection, but we're effectively offline:

  • Smartphone in EDGE cell
  • Car drives into tunnel
  • Overcrowded Wi-fi with massive packet loss

Unpoly 3 handles Lie-Fi with timeouts:

  • All requests have a default timeout of 90 seconds (up.network.config.timeout)
  • Timeouts will now trigger onOffline() and use your offline handling
  • Customize timeouts per-request with { timeout }, [up-timeout] options

Expired pages remain accessible while offline

  • Cached content will remain navigatable for 90 minutes
  • Revalidation will fail, but not change the page and trigger onOffline()
  • Clicking uncached content will not change the page and trigger onOffline()


While Unpoly 3 lets you handle disconnects, it's not full "offline" support:

  • To fill up the cache the device must be online for the first part of the session (warm start)
  • The cache is still in-memory and dies with the browser tab

For a comprehensive offline experience (cold start) we recommend a service worker
or a canned solution like UpUp (no relation to Unpoly).

Quality of life

Optional targets

If you suffix a target selector with :maybe it will only be updated if there is a match in both the current page and server response (like [up-hungry]).


The following would require fragments matching .content and .navigation.
If .flashes is missing in either current page or server response, no error is thrown.

<a href="/post" up-target=".content, .flashes:maybe, .navigation">

up.hello() is now idempotent

You can call up.hello() on the same element tree multiple times without the fear of side effects.

Unpoly guarantees that each compiler only ever runs once for a matching elements.

Late compiler registrations

You can now register compilers after content was rendered.
Compilers registered after booting automatically run on current elements.

This makes it easier to split your compilers into multiple files that are then loaded as-needed.
We plan to work on this further. See RFC: Reconciliation of head elements.

More control over [up-hungry]

Elements with an [up-hungry] attribute are updated whenever the server sends a matching element, even if the element isn't targeted.

Unpoly 3 lets you control which updates [up-hungry] will piggy-back on.

Controlling which layer to watch

By default Unpoly only considers [up-hungry] fragments in the updating layer.

With [up-if-layer=any] a hungry fragment will be considered for updates in any layer.


A use case is notification elements in the application layout:

<div class="flashes" up-hungry up-if-layer="any">...</div>
<div class="unread-messages" up-hungry up-if-layer="any">...</div>

Updating only for history changes

By default Unpoly considers [up-hungry] fragments for any update in its layer.

With [up-if-history] a hungry fragment will only be updated when the history changes.


A use case is a canonical <link> element that should only update when we're updating history, but not when we update smaller fragments:

<link rel="canonical" href="..." up-hungry up-if-history />

Attaching data with HTML5 data attributes

Unpoly always had [up-data] to attach structured data to an element.

This is verbose when we're attaching simple key/value pairs:

<div class="user" up-data="<%= { name: @user.name }.to_json %>">

It would feel more natural to use HTML5 data attributes instead:

<div class="user" data-name="<%= @user.name %>">

In Unpoly 3 data can be attached with both [up-data] and HTML5 data attributes.

These three elements produce the same compiler data:

<div up-data='{ "foo": "one", "bar": "two" }'></div>

<div data-foo='one' data-bar='two'></div>

<div up-data='{ "foo": "one" }' data-bar='bar'></div>
up.compiler('div', function(element, data) {
  console.log(data.foo) // is always "one"
  console.log(data.bar) // is always "two"

Note: HTML5 data attributes are always flat objects with string values.
If you need to serialize something like an array of numbers, use [up-data].

Ruby on Rails users can now use the { data } option of any element helper
to pass data to compilers:

content_tag(:div, '...', data: { foo: 'one', bar: 'two' })

Highlighting the targeted fragment

Targeted fragments get an .up-loading class.

This lets you highlight the part of the screen that's loading.

A link targeting a fragment .target:

<a href="/path" up-target=".target">
<div class="target">old text</div>

While the request is loading the link gets .up-active and the target gets .up-loading:

<a href="/path" up-target=".target" class="up-active">
<div class="target" class="up-loading">old text</div>

Once the fragment is updated all feedback classes are feedback removed:

<a href="/path" up-target=".target">
<div class="target">new text</div>
A form is disabled while validating
.up-loading output {
  color: gray;

Log separated by user interaction

The log has a lot of debug information.
It's often hard to find where the relevant output begins and ends.

The new log shows which user interaction triggered an event chain.

New log

Playing nice with foreign overlays

Unpoly 2 sometimes clashes with overlays from other libraries ("foreign overlay")
like Bootstrap or TinyMCE:

  • Clicking a foreign overlay closes an Unpoly overlay
  • Unpoly steals focus from a foreign overlay

This happens when foreign overlays look "on top" visually (z-index: 99999999999), but their elements attach to the <body>. For Unpoly this looks like content on the root layer.

This could often be fixed by attaching the foreign overlay to the correct Unpoly layer (pseudo-code):

OtherOverlay.open({ content: 'foo', onOpen(overlay) { up.layer.element.attach(overlay) }})

However, the solution is custom to every library.

Making Unpoly aware of foreign overlays

You can push a selector into up.layer.config.foreignOverlaySelectors and Unpoly will no longer have layer-related opinions over that region. You no longer need to re-attach the foreign overlay element.

Example from unpoly-bootstrap5.js:


More control over the progress bar

Unpoly 2.1 has introduced a progress bar that shows while a request takes too long to load.

This may be unwanted for requests are loading in the background, or have longer load times in the best of cases (e.g. a large report).

Unpoly 3 gives you more control over if and when the progress bar shows.

Background requests

Pass { background: true } or [up-background] when rendering or making a request

Background requests never trigger the progress bar.
Background requests are also deprioritized.

Uses cases from Unpoly

  • Polling requests are background requests automatically
  • Preload requests are background requests automatically

Optimistic cache partitioning

Requests with the same URL and HTTP method, but different header values (e.g. X-Up-Target) now share the entry in Unpoly's client-side cache.

If a server optimizes its response, all request headers that influenced the response should be listed in a Vary response header:

if request.headers['X-Up-Target'] == '.card'
  response.headers['Vary'] = 'X-Up-Target'
  render 'card', layout: false
  render 'app'

A Vary header tells Unpoly to partition its cache for that URL so that each request header value gets a separate cache entries.

You can set a Vary header manually from your server-side code.
You may also be using a library like unpoly-rails that sets the Vary header automatically.

Extensive render callbacks

You may now pass callback functions to intervene at many points in the rendering lifecycle.

  url: '/path',
  onLoaded(event)        { /* Content was loaded from cache or server */ },
  focus(fragment, opts)  { /* Set focus */ },
  scroll(fragment, opts) { /* Set scroll positions */ },
  onRendered(result)     { /* Fragment was updated */ },
  onFailRendered(result) { /* Fragment was updated from failed response */ },
  onRevalidated(result)  { /* Stale content was re-rendered */ },
  onFinished(result)     { /* All finished, including animation and revalidation */ }
  onOffline(event)       { /* Disconnection or timeout */ },
  onError(error)         { /* Any error */ }

Strict target derivation

Unpoly often needs to guess a target selector that will match an element.
Some features that do this are [up-poll], up.reload(), [up-hungry].

up.reload(element) // Produces a target selector from the given element

To build the selector, Unpoly 2 uses the following element properties in decreasing order of priority:

  • The element's [up-id] attribute
  • The element's [id] attribute
  • The element's [name] attribute
  • The element's [class] names, ignoring up.fragment.config.badTargetClasses.
  • The element's tag name

When target derivation goes wrong

The target derivation in Unpoly 2 sometimes produces a weak selector that won't uniquely identify the element:

<link rel="stylesheet" href="...">
<link rel="canonical" href="..." up-hungry>

Here the [up-hungry] element would targets link, matching the stylesheet instead. 💥

New target derivation patterns

Unpoly 3 lets you configure patterns to use for target derivation.
The following patterns are configured by default:

up.fragment.config.targetDerivers = [
  '[up-id]',        // [up-id="foo"]
  '[id]',           // #foo
  'html',           // html
  'head',           // head
  'body',           // body
  'main',           // main
  '[up-main]',      // [up-main="root"]
  'link[rel]',      // link[rel="canonical"]
  'meta[property]', // link[rel="canonical"]
  '*[name]',        // input[name="email"]
  'form[action]',   // form[action="/users"]
  'a[href]',        // a[href="/users/"]
  '[class]',        // .foo (filtered by up.fragment.config.badTargetClasses)

Note that tag names are now only used for unique elements (like <body> or <main>).

You can also push a Function(Element): string? if your deriver can't be expressed in a pattern.

Derived target verification

  • Unpoly 3 verifies if a derived targets will actually match the element.
  • If another element is matched, the next pattern is tried.
  • If no pattern produces a matching target, an error is thrown.

This may throw an up.CannotTarget exception in existing apps with ambiguous selectors.
This means your app is updating the wrong fragments!
You should fix these bugs by setting an [id], [up-id] or [class] attribute.

Compilers can inspect the render pass

Compilers now accept an optional third argument with information about the current render pass:

up.compiler('.user', function(element, data, meta) {
  console.log(meta.response.text.length)        // => 160232
  console.log(meta.response.header('X-Course')) // => "advanced-ruby"
  console.log(meta.layer.mode)                  // => "root"
  console.log(meta.revalidating)                // => true

Dropped support for legacy technologies

IE11 is finally dead

Unpoly 3 will no longer boot on IE11 or legacy Edge.
If you need to support Internet Explorer 11, use Unpoly 2.

jQuery helpers are deprecated

jQuery helpers like up.$on() and up.$compiler() have been moved to unpoly-migrate.js.


Unpoly no longer ships with an version transpiled down to ES5 (unpoly.es5.js). Instead there is now a ES6 version (unpoly.es6.js).

Since most modern browsers now have great JavaScript support, we encourage you to try out the untranspiled distribution (unpoly.js), which has the smallest file size.

Many other features and fixes


New documentation guides

In our ongoing efforts to evolve Unpoly's documentation from an API reference to a guide, we have added many new documentation pages:


Expectations for upgrade effort

  • Upgrade from v2 to v3 will be much smoother than going from v1 to v2.
  • No changes in HTML or CSS provided by Unpoly.
  • Almost all are breaking changes are polyfilled by unpoly-migrate.js.
  • Unpoly 3 keeps aliases for deprecated APIs going back to 2016.
    You can upgrade from v1 to v3 (without going through v2).

Recommended upgrade workflow

  • Go through the CHANGELOG for 3.0.0 and review breaking changes marked with an ⚠️ icon.
  • Integrate unpoly-migrate.js
  • Run your test suite. Fix all deprecation warnings.
  • Remove unpoly-migrate.js.
  • Optional Remove user code no longer needed with Unpoly 3.

Code you can simplify in Unpoly 3

  • Workarounds to validate fields on input instead of change
  • Workarounds for [up-hungry] elements in another layer (e.g. notification flashes)
  • Workarounds to make foreign overlays play nice with Unpoly layers
  • Workarounds to prevent the display of stale caches

Example: Remove cache exemptions

Since cached content is now revalidated, you may want to remove any workarounds to prevent the display of stale content:

Remove short expiry times

up.network.config.cacheExpiry = 15_000
<a href="/path" up-follow up-cache="false">...</a>
up.render({ url: '/path', cache: false })

Remove global cache exemptions

up.network.config.autoCache = (request) => !request.url === '/dashboard' && ...

Example: Remove abort exemptions

Since Unpoly 3 only aborts conflicting requests when rendering a fragment, you may want to remove links that opt out of aborting everything:

<!-- Delete [up-abort] (former [up-solo]) attribute: -->
<a href="/path" up-follow up-abort="false">...</a>
// Delete { abort } (former { solo }) option:
up.render({ url: '/path', solo: false })
Henning Koch  @triskweline