A simple approach to generating HAL-esque hypermedia responses based on resource definitions. Built to work best with autohost but should work with most Node.JS HTTP stacks with a little extra effort.
You can skip to the Using section but understanding the concepts behind should explain
The resource definition provides the metadata necessary for hyped to generate hypermedia based on the model you provide it. This declaritive approach takes advantage of the structure and property names you provide to make implicit associations. The trade-off is that the associations are made based on consistent naming across several properties and values and typos will break the expected outcome.
// resource
{
[resourceName]: the resource name (must be unique)
parent: (optional) the name of the resource that this "belongs" to
urlPrefix: (optional) a resource-level URL prefix for all actions in this resource
apiPrefix: (optional) an api-level URL prefix for all actions in this resource
resourcePrefix: (optional) defaults to true - guarantees URLs begin with resource name
actions: {
[actionName]: {
method: the http method
url: the URL for this action
authorize: a predicate to determine user access
render: override the resource/action that gets rendered from this action
include: property names array to include in the rendered response
exclude: property names array to exclude from rendered response
filter: predicate to determine if a property's key/value should be included
condition: returns true if the action is valid for the given model
transform: a map function that operates against the data
embed: defines which model properties are embedded resources and how to render them
links: provides alternate/compatible urls for activating this action
parameters: provide metadata to describe available query parameters for this action
hidden: a boolean indicating that this action should not be included in options
actions: allows control of which actions are shown in the hypermedia
}
},
hoist: {
[childResourceName]: [ ] // list of child actions to hoist to this resource's default hypermedia
},
versions: {
#: {
[actionName]: {
// provide any of the above properties to change how this
// version will differ from the previous version
// these changes are applied cumulatively in descending order
}
}
}
}
// render
// use case: when returning child items from a parent resource's action
{
resource: the resource that is actually being returned from this action
action: the action that should be used to render the resource
}
// embed
// use case: when returning a parent object that contains properties that should
// be rendered/treated like nested resources
{
[propertyName]: {
resource: name of the embedded resource
render: which action will be used to render the resource (usually "self")
actions: an array of the actions that should be included
}
}
// links
// use case: when an action has a limited set of parameter values that should be
// given a friendly name
{
[actionName]: url|generator
}
// parameters
// use case: when you want to define the available parameters for an action
// for use by the client
{
parameterName: {
[ range|choice|list|validate|invalidate ]: specification
}
}
This feature is to allow action aliases for known sets of query parameters.
</dd>
<dt>url</dt>
<dd>Works the same as the `url` for the action.</dd>
<dt>generator</dt>
<dd>A function that takes the data model and a context hash and optionally returns a url string. Returning an empty string or undefined will exclude the link from rendered results. See <a href="#rendering-api">Rendering API</a> for how the context is specified.
<div class="highlight highlight-javascript"> <pre>
"next-page": function( envelope, data ) {
// the envelope includes all incoming request information
// including query parameters which get merged onto the data property
var size = envelope.data.size;
var page = envelope.data.page;
if( page && size ) {
// results in a url like "/thing?page=1&size=10"
return "/thing?page=" + ( page + 1 ) + "&size=" + size;
}
}
</pre>
</div>
</dd>
<dt><h3>parameters</h3></dt>
<dd>A hash of parameter names and metadata about each parameter. Each parameter should specify what "type" it is along with a specification that the client can use to validate possible parameter values. Any of the specifications can be a generator function to allow for dynamic specification.</dd>
<dt>choice</dt>
<dd>An array of valid choices where the consumer is expected to provide a single value.</dd>
<dt>multi</dt>
<dd>An array of valid choices where the consumer can provide multiple values.</dd>
<dt>range</dt>
<dd>A two-element array consisting of a lower and upper bound for the parameter value.</dd>
<dt>validate</dt>
<dd>A regular expression used to determine if a parameter value is valid.</dd>
<dt>invalidate</dt>
<dd>A regular expression used to detect invalid parameter values.</dd>
<dt>required</dt>
<dd>A boolean indicating that this parameter is required.</dd>
<dt><h3>render</h3></dt>
<dd>Use this when the resource and action that should get rendered differs from the
resource and action currently being rendered.
</dd>
<dt>resource</dt>
<dd>The name of the resource that is being rendered.</dd>
<dt>action</dt>
<dd>The name of the action to use in rendering the resource.</dd>
<dt><h3>hidden</h3></dt>
<dd>Setting this property to true will exclude this action from hyperlinks when rendering options.</dd>
<dt><h3>actions</h3></dt>
<dd>Allows control of which actions appear as hyperlinks when rendering this action. When including a hoisted action, the child resource name should after the action name seperated by a dash. Ex. `"action-child"` to create a unique action name.</dd>
// account resource
{
name: "account",
actions: {
self: {
method: "get",
url: "/account/:id",
include: [ "id", "balance" ],
embed: {
transactions: {
resource: "transaction",
render: "self",
actions: [ "self", "detail" ]
}
}
},
withdraw: {
method: "POST",
url: "/account/:id/withdrawal",
condition: function( envelope, account ) {
return account.balance > 0;
},
include: [ "id", "accountId", "amount", "balance", "newBalance" ]
},
deposit: {
method: "POST",
url: "/account/:id/deposit",
include: [ "id", "accountId", "amount", "balance", "newBalance" ]
}
}
}
//transaction resource
{
name: "transaction",
parent: "account",
actions: {
self: {
method: "get",
url: "/transaction/:transaction.id",
include: [ "id", "amount", "date" ],
links: {
"details": "/transaction/:transaction.id?detail=true"
}
}
}
}
hyped generates an abstract hypermedia model based off a resource definition and data model. This hypermedia model is then passed to a "rendering engine" that produces a HTTP response body based on the requested media type.
Understanding the structure of the hypermedia model is important if you'd like to define your own rendering function for custom media types.
HyperModel Structure
{
// top level properties starting with a '_' are metadata
// all other properties are attached based on the resource definition
_origin: { href: "", method: "" },
_resource: "thing",
_action: "self",
_links: {
self: { href: "/thing/100", method: "get" },
child: { href: "/thing/100/child/:childId", method: "get", templated: true }
},
_embedded: {
children: [
// each embedded resource will follow the same pattern/format
]
}
}
<dt>_links</dt>
<dd>The actions available for the given resource.</dd>
<dt>_embedded</dt>
<dd>If a key in the <tt>embed</tt> section matches one on the data model provided, the resources contained will show up under the property matching the one on the data model. Each embedded resource will contain <tt>_origin</tt> and <tt>_links</tt> and may also include its own <tt>_embedded</tt> section.</dd>
Rendering engines are simply functions with the signature function( hyperModel ). These functions take the hyperModel and produce the correct response body for their given media type. The built in engine for "application/hal+json" looks like this:
function render( model ) {
return JSON.stringify( model );
}
Note: the engine for "application/json" is more complex since it has to effectively reduce and filter the hypermedia data structure to produce simple JSON.
The accept header is used to select an appropriate rendering engine (if one is available). If no engine matches the provided media type, hyped will respond to the client request with a 415 explaining that the requested media type is not supported.
The default mediaTypes supported are:
application/jsonapplication/hal+jsonNew versions are implemented as diffs that get applied, in order, against the baseline. Each version provides new values for properties that replace parts of the metadata for the resource. This will hopefully make it easy to see the differences between versions of a resource and reduce the amount of copy/pasted code between versions of a resource definition.
The default versioning strategies parses the version specifier out of the Accept header either as mimeType version specifiers or as a header parameter:
application/json.v2application/hal.v3+jsonapplication/json; version=2application/hal+json; version=3Note: hyped also supports the use of the keyword
latestin place of a numeric indicator.
Handlers behave a bit differently. When providing a new handle implementation for a version, that handle implementation will apply for the current version and any consecutive versions after that don't provide their own handle implementation.
Example: version 3 for the self action introduces a new handle implementation but no changes are made to the handle again until version 6. Versions 4 and 5 will use version 3's handle implementation.
hyped will attempt to replace path variables specified in two separate styles for two separate use cases:
Express style variables
:property:property.childPropertyBrace style variables
{property}{property.childProperty}Either style is valid when specifying the URL in the action, hyped will make sure that the correct form is used (Express style gets used server side for assigning routes while brace style is returned in all client responses).
The first form will be used to attempt to read a property directly from the returned model or incoming request. The second will attempt to read a nested property or a property name that combines the two in camel-case fahsion (i.e. propertyChildProperty). In either case, if no match is found, the variable will be left in tact. In the second case, the period is removed and the variable becomes camel-case.
Example:
{
name: "user",
actions: {
self: {
method: "get",
url: "/user/:user.name"
}
}
}
If the user model returned or envelope.data had a name property of "leroyJenkins", the response body would look like this:
{
"name": "leroyJenkins",
"_links": {
"self": { "href": "/user/leroyJenkins", "method": "GET" }
}
}
When a URL contains path variables that could not be replaced by a value in the model, the action/origin link will indicated this with a templated property set to true.
Note: the client will always see brace-style path variables
{
"name": "leroyJenkins",
"_links": {
"self": { "href": "/user/leroyJenkins", "method": "GET" },
"insult": { "href": "/user/leroyJenkins/{insult}", "method": "POST", "templated": true }
}
}
These examples show the bare minimum. You'll only get support for built in mediatypes - presently application/json and application/hal+json. This means if you don't provide your own engine for custom media types and a client sends an accept header for a media type hyped doesn't have an engine for, it will send back a 415 (unsupported media type) to the client rather than throwing an exception.
This example will add middleware to autohost that extends the envelope to integrate hyped's rendering. The correct version, rendering engine, resource and action are all determined during the hyped middleware you add to autohost so that when rendering a response, the only thing you must provide is the model.
Note: this approach only works with Autohost 0.4.0 or greater.
There may be times where you need to add routes or middleware that pre-empt hyped's. To do this, you can provide a preInit property in the configuration block (second argument to createHost). This function will be passed a handle to autohost and should either return a promise or invoke a callback when its finished.
{
preInit: function( host, cb ) {
host.http.middleware( "/", function( req, res, next ) {
// this would get called before all other middleware
// and for every request
next();
}, "global" );
}
}
index.js
var autohost = require( "autohost" );
var hyped = require( "hyped" )();
var host = hyped.createHost( autohost, {
// regular autohost configuration goes here
},
function() {
// callback gets invoked once all resources are loaded
// this is where you should call start on the host variable
host.start();
} );
resource.js
var databass = require( "myDAL" );
module.exports = function( host ) {
return {
name: "something",
actions: {
self: {
method: "get",
url: "/something/:id",
exclude: [],
handle: function( envelope ) {
var model = databass.getSomethingById( id );
return {
status: 200, // default
data: model
};
}
}
},
versions: {
2: {
self:
exclude: [ "weirdFieldWeShouldNotExpose" ]
}
}
}
};
};
express
// assumes you have a hash containing all your resource definitions and express set up
var hyped = require( "hyped" )( resources );
hyped.setupMiddleware( app );
app.get( "something/:id", function( req, res ) {
var id = req.param( "id" );
var model = databass.getSomethingById( id );
req.hyped( model ).status( 200 ).render();
} );
If you are using this library with Autohost, the only API you really need to know about is used for the initial setup. The format of the object literal returned from the handlers is also explained below.
You can skip passing resource list at all and provide the optional configuration to change default behavior.
The configuration hash can have the following properties:
Here's an example of the config block with defaults shown:
{
"defaultContentType": "application/json",
"defaultToNewest": false,
"includeChildrenInOptions": false
}
Note: the defaults in place were selected so that consumers unaware of hypermedia or your APIs versioning strategy can use your API like any other HTTP API they have likely encountered.
Adds the metadata for a particular resource.
Adds multiple resources at once. Intended for internal use.
Creates an autohost instance with the configuration provided and returns it. The callback is invoked once all resources have been processed.
Note: the callback is most likely only needed during integration testing
var autohost = require( "autohost" );
var hyped = require( "hyped" )();
var host = hyped.createHost( autohost, {
// regular autohost configuration goes here
} );
Registers a rendering function with one or more mediaTypes.
// this is a ridiculous example. don't do it.
hyped.registerEngine( "text/plain", function( model ) {
return model.toString();
} );
Note: this call should only be used with express. Use the new
createHostcall to setup autohost.
This call can take either a reference to the autohost lib or express's app instance. It will register both the hypermediaMiddleware and the optionsMiddleware with the HTTP library in use.
Refer to the usage examples above to see this call in use.
Changes hyped's default versioning detection approach. This is experimental and not recommended. The versionFn is a function that takes the request object and returns a version number.
// this isn't great, but if you have to ...
hyped.versionWith( req ) {
return req.headers[ "x-version" ] || 1;
}
Note: the rendering API is deprecated for use with autohost - see response format for details.
The hypermedia middleware extends the underlying request object with a set of fluent calls to help with the construction of a hypermedia response. The response object can then be used to send a response via express.
Keep in mind that normally, you will only use the hyped, status and render calls. The middleware should correctly detect the version, mediatype, resource and action to be rendered.
// an approach to rendering the resulting response
function respond( res, response ) {
var code = response.status || 200;
if ( response.headers ) {
_.each( response.headers, function( v, k ) {
res.set( k, v );
} );
}
if ( response.cookies ) {
_.each( response.cookies, function( v, k ) {
res.cookie( k, v.value, v.options );
} );
}
res.status( code ).send( response.data );
}
...
// from within an express route
var response = req.hyped( myData ).status( 200 ).render();
respond( res, response );
You provide the data model that the resource will render a response based on. The resources are designed to work with models that may have a great deal more information than should ever be exposed to the client.
Another way to provide context to any link generators for this action.
Sets cookies on the response generated.
Sets headers on the response generated.
If omitted, this is always 200. Be good to your API's consumers and use proper status codes.
Returns the response object literal.
The response literal has the following structure:
{
status: 200, // default
data: , // the data to pass to the rendering engine
headers: {}, // defaults to content-type header only
cookies: {}, // defaults to empty
resource: , // defaults to the resource that received the request
action: , // defaults to the action that received the request
}
The resource and action properties of the response returned from your action handle control what resource and action metdata will be used when rendering the top-level response.
If the data property contains an array, the rendering engine will assume that you are returning a list of the resource that owns the list action and use the "self" action to determine how to render each item.
To change the resource or action that is used to render the items in the list, you can use the render property on the action and provide resource and/or action properties which will change what resource and action metadata are being used to render each item in the list.
The property on the response containing the rendered items will be the pluralized form of the resource used to render each item. To control this yourself, you can provide a _alias property on data.
If the data property is an object with a _list property containing an array, then the array will be rendered as before but with the additional properties from data included as part of the body.
In an autohost handler, this literal will be processed by hyped first. The data property will be replaced by the output of the appropriate rendering engine. If only the data is returned from the handler, then that will be used to generate the data property of the literal with defaults used for all other properties.
This literal format is what an express handle can expect to get back from the render call.
I think HAL is pretty awesome. We"ve added a few minor extensions and may continue. This is where we"ll describe them.
_versions and _mediatypes properties have been added to the payload returned from the OPTIONS call to the API root. They will list the available versions and mediatypes currently supported by the server.
OPTIONS response structure
{
"_links": {
"resource:action": { href: "", method: "", templated: true }
}
}
All actions will be returned under the top-level _links property. The action names returned from OPTIONS will be namespaced by the resource name delimited by a colon.
Provides a data structure identical to the link that would be called to produce the response. This is especially useful within embedded resources as it allows the client to see what link action was used to produce the embedded resource included in the payload.
We added this to make it easier for clients to know which representation they have in memory for the sake of both local caching and requesting updated versions. We recommend including etag or last-modified data along with resources to further improve efficiency.
{
"id": 1,
"_origin": { "href": "/item/1", "method": "GET" }
...
}
We also expose the resource and action name as part of each resource (including embedded resources).
{
"id": 1,
"_origin": { "href": "/item/1", "method": "GET" }
"_resource": "item",
"_action": "self"
}
HAL does not include the HTTP method used for an link"s href. Our version does include this information in the link using the method property. As in our URL example from above:
{
"name": "leroyJenkins",
"_links": {
"self": { "href": "/user/leroyJenkins", "method": "GET" }
}
}
While the links property allows you to provide actions with the parameters already attached to the URL, there will be times when defining every possible combination of parameters as a specific action won't make sense.
In those cases, we believe hypermedia should include metadata about the available parameters. This at least reduces the amount of implementation details exposed to API consumers.
{
"_links": {
"self": {
"href": "/thing/100",
"method": "GET",
"parameters": {
"arg1": {
"range": [ 0, 100 ]
},
"arg2": {
"choice": [ 4, 8, 15, 16, 23, 42 ]
},
"arg3": {
"multi": [ "a", "b", "c", "d" ]
},
"arg4": {
"validate": "/^starts with.*/",
"invalidate": "/.*ends with$"
}
}
}
}
}
We've also added a _version property so that the effective version of the API call which generated the response is included in the response body:
{
"id": 1,
"_origin": { "href": "/item/1", "method": "GET" }
"_resource": "item",
"_action": "self",
"_version": 1
}
Note: if no version has ever been specified in your resources, the default is always
1.
The tests are a bit of a contrived mess at the moment but do exercise the features and known use cases. If submitting a PR, please do the following:
None of this is guaranteed but here are some items that would be nice/great to have.
FAQs
Hypermedia response generation engine
We found that hyped demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 2 open source maintainers collaborating on the project.
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