A streaming implementation of XHP for HHVM.

v0.7.1 2022-01-16 15:55 UTC

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Last update: 2022-01-16 15:59:21 UTC


A streaming implementation of XHP for HHVM

Alternative library

This library is an alternative for xhp-lib. Xhp-lib has been open sourced by Facebook. If you have never used xhp before, you can find general xhp documentation here. This information describes how xhp (the underlying technology below xhp-lib and sgml-stream) works. It will often explain things from the perspective of xhp-lib. If you have a general understanding of xhp and the basics of xhp-lib. You can find sgml-stream specific documentation here. It will also help if you have a basic understanding of async and await in Hack.


You MUST enable the .hhconfig setting check_xhp_attribute when using sgml-stream. We don't validate @required at runtime when you access an attribute. We trust that the typechecker has made sure all required attributes are present. Xhp-lib does validate @required at runtime. If you do not turn this setting on, your @required attributes might be null when read, which is not typesafe.

Feature differences between xhp-lib and sgml-stream

Rendering model

Xhp-lib is an amazing library which renders a tree of nodes, scalars, and xhp-lib specific interfaces to a string. Xhp-lib will manage coordination of Awaitables in your tree. Sgml-stream was born from the realization that xhp-lib, although be plenty fast, pushes back the time at which you can start sending content back in your http response. Xhp-lib renders trees of all the types into a tree of primitives first. Once that process completes, it turns this tree of primitives into a (long) string. Sgml-stream does things differently. Instead of returning your content as a string, we feed smaller partial strings to a Consumer. You are able to decide how and when you want to flush these smaller strings over the network.

Let's illustrate with the following example:

  <head><link rel="stylesheet" href="..." /></head>
    <MyFastAsynchronousElement id="a" />
    <MySlowAsynchronousElement id="b" />
    <MyFastAsynchronousElement id="c" />

When rendering this tree with xhp-lib, all Awaitables fire at once. Once all the Awaitables finish, your tree gets turned into a string and returned from node->toStringAsync(): Awaitable<string>. However, everything before element a does not depend on the Awaitable inside of a resolving. Wouldn't it be nice if you could already stream this content to your users? They will discover required resources early and start loading your css immediately. As it turns out, MyFastAsynchronousElement renders itself to something that contains an image tag. So users would benefit greatly from getting this content as soon as possible. They could start downloading your image and get a partially rendered page sooner. In this example, everything until and including <body> can be sent immediately. a can be sent as soon as it is ready. b can be sent once it is ready and a has also completed. As the element name suggests, b is rather slow, so it will finish after a and stream immediately. If element c is done, b is not yet ready. This means that we can't stream it yet and we have to wait for b to complete. To read more about how we do this, see Streams, how do they work?.

Contexts v.s. Flow

Xhp-lib has a concept called contexts. It is essentially a dict<string, mixed> which is managed by xhp-lib and available to you when element->renderAsync() is called. You can call ->getContext() and ->setContext() to store values and retrieve them later. Sgml-stream does not implement contexts. Contexts were to difficult to get right when after we changed the rendering model.

Sgml-stream addresses this need in a different way. When your SimpleUserElement->compose(Flow $flow): Streamable method is called, you get access to a Flow. Flows support both constants and variables. The constant rules are relatively simple to explain. For a full explainer on Flow, including variables in flows, how variables and constants interact, and much more, see The intricacies of Flow.

A Flow is a representation of your scope. Your scope is a single element large. If an element has descendants, their scopes are sub scopes. They can read your constants, but you can't read theirs. A constant can be declared with WritableFlow->declareConstant(string $name, mixed $value): void. If this declaration succeeds, it will declare the constant with $name for your scope. If it fails, a RedeclaredConstantException is thrown and the value remains unchanged. You can read constants from your flow using Flow->get(string $name): mixed and Flow->getx(string $name): mixed. The method with the x suffix throws a ValueNotPresentException when the Flow doesn't know about $name. ->get() will return null when in this case. You can also query for the presence of $name using Flow->has(string $name): bool.

Let's illustrate with an example:

<Element id="a" data-comment="I declare C1 to be 'apple'">
  <Element id="b" data-comment="I declare C2 to be 'banana'">
    <Element id="c" />
  <Element id="d" data-comment="I declare C2 to be 'dragon fruit'">
    <Element id="e" data-comment="I declare C3 to be 'elder berry'" />
    <Element id="f" />

Element a declares a constant named C1 with the value 'apple'. a and all descendants of a can call ->getx('C1') to get 'apple'. C1 lives in the scope a and the sub scopes b, c, d, e, and f can read from the a scope.

Element b declares a constant named C2 with the value 'banana'. b and all descendants of b can call ->getx('C2') to get 'banana'. C2 lives in the scope b and the sub scope c can read from the b scope.

Element d declares a constant named C2. This constant was already declared in the b scope, but d is not a sub scope of b. Therefore, the call returns normally and d and all descendants of d can call ->getx('C2') to get 'dragon fruit'. This declaration would have failed if c attempted it, because c is a sub scope of b, so C2 would already exist. You are not allowed to redeclare a constant.

Element e declares a constant named C3. e has no descendants, so C3 is only visible to e. You might expect that f should be able to read from e. Do not confuse indentation level for scope. Even though e and f are at the same depth of the tree (they are siblings), they do not share a scope. This is to prevent strict order dependencies between e and f. If you want to have a constant be visible to both e and f, you should declare this constant in an ancestor.


Some may consider this a feature, others consider it a shortcoming of sgml-stream. Once an xhp open finishes. Which means that the <a href="...">...</a> object has been constructed, you can not modify the attributes, nor the children. This makes it easy to reason about the state of your xhp objects. ->appendChild() and ->setAttribute() (and friends) great tools when used correctly. We have been getting along fine without them, but there is a lot of xhp code out there that we do have visibility into. We might decide to weaken the immutability if enough issues for it get opened. See immutability why and how for more information.

How to get started

Sgml-stream does not come with any tags built-in. If you want to write html, you should also depend one or both of the following libraries:

They contain contain all the html tags from the WhatWG HTML specification. Xhp-lib comes with a couple more tags than those documented here. Namely: <x:frag>, <doctype>, <conditional_comment>, and some deprecated html tags. Here are some examples on how you could decide to implement them.

// This code is not checked by the typechecker.
// If this code does not work anymore, please open an issue or a PR.
namespace MyOwnNamespace;

use type XHPChild;
use type HTL\SGMLStream\RootElement;
use type HTL\SGMLStreamInterfaces\{FragElement, SnippetStream};

final xhp class conditional_comment extends RootElement {
  attribute string if @required;

  public function placeIntoSnippetStream(SnippetStream $stream): void {
    // This is unsafe, since the string passed for `->:if` could break out
    // of this comment and ruin your document. Be careful!
    $stream->addSafeSGML('<!--[if '.$this->:if.']>');

final xhp class doctype extends RootElement {
  public function placeIntoSnippetStream(SnippetStream $stream): void {
    $stream->addSafeSGML('<!DOCTYPE html>');

final xhp class frag extends RootElement implements FragElement {
  // `frag` is treated like x:frag because of this  ^^^^^^^^^^^
  public function getFragChildren(): vec<XHPChild> {
    return $this->getChildren();

  public function placeIntoSnippetStream(SnippetStream $stream): void {

As you can see from these examples, you get access to a dangerous method on SnippetStream, namely ->addSafeSGML(string $sgml): void. You probably don't want to write your own elements this way if you can compose yourself using other tags. Extending RootElement directly is wordy and encourages unsafe strings to be passed to ->addSafeSGML(). Most of the time, you should be using something else. This library includes base classes to hide the SnippetStream from you. DissolvableUserElement, SimpleUserElement, SimpleUserElementWithWritableFlow, AsynchronousUserElement, and AsynchronousUserElementWithWritableFlow. For a guide on how to choose between them, see What element type do I need? You can write your own too, since we haven't <<__Sealed>> RootElement off. If you decide to use a built-in base class, you'll implement a method with one of these signatures:

  • DissolvableUserElement->compose(): Streamable
  • SimpleUserElement->compose(Flow $flow): Streamable
  • SimpleUserElementWithWritableFlow->compose(WritableFlow $flow): Streamable
  • AsynchronousUserElement->composeAsync(Flow $flow): Awaitable<Streamable>
  • AsynchronousUserElementWithWritableFlow->composeAsync(WritableFlow $flow): Awaitable<Streamable>

The Flow is yours for as long as your Hack scope lasts. Either via return or throw. If you are async, the Flow stays yours until your Awaitable resolves. Don't try to hold on to a Flow after that. If we implement more optimizations in the future, we will not consider it a BC break if your code behaves differently if you keep the Flow around.

You return a Streamable from these methods. This will be an Element in most cases, but all other Streamables are also valid. So you can construct a markdown renderer, and return that. As long as you implement the Streamable interface on your markdown renderer, sgml-stream will understand what to do.

Unsafe strings

A common question: Why isn't a string Streamable?

Answer: Strings can not implement interfaces that are not in hhvm already (XHPChild, and the deprecated Stringish come to mind). If you have a string (pcdata) in your element's children, you don't need to do something special. We'll run it through htmlspecialchars() and stream it for you.

A common response: No, I want to stream the string, without escaping it. Please don't mess with my strings.

Sigh...: There is a way to get what you want, but be careful what you wish for. The interface ToSGMLStringAsync is the thing you are looking for. This interface is an escape hatch which introduces security risks which might come back to bite you. This interface bypasses all parts of sgml-stream that try to keep you safe. It streams your string directly with no escaping applied to your consumer. This interface is meant to be used very sparingly.