Hypertext Application Language (HAL)

The REST API provides some resources in an additional media type. The HAL media type application/hal+json describes a format which contains links and information about other resources. This allows us to embed the process definition or assignee of a task directly into the response, which in turn reduces the number of necessary requests to gather all information about a single task or a list of tasks.

In order to interact with HAL, you have to set application/hal+json as Accept header. The response of a HAL request has always the following structure:

{
  "_links" : {},
  "_embedded" : {}
}

The property _links contains relational links that give an easy way to navigate between associated resources. A _links property contains at least a self relational link. The property _embedded includes other linked resources in the representing resource. Each embedded resource will be structured as a HAL resource.

Example: Resource

Request

GET /task/a-task-id

Request Header:

Accept: application/hal+json

Response

{
  "_links" : {
    "self": {
      "href": "/task/a-task-id"
    },
    "assignee": {
      "href": "/user/demo"
    },
    ...
  },
  "_embedded" : {
    "group" : [{
      "_links" : {
        "self" : {
          "href" : "/group/management"
        }
      },
      "_embedded" : null,
      "id" : "management",
      ...
    }],
    "processDefinition" : [ {...}, {...} ],
    ...
  },
  "id" : "a-task-id",
  "name": "Assign Approver",
  "assignee": "demo",
  ...
}

Example: Collection

Request

GET /task

Request Header:

Accept: application/hal+json

Response

{
  "_links" : {
    "self": {
      "href": "/task"
    }
  },
  "_embedded" : {
    "assignee" : [{
      "_links" : {
        "self" : {
          "href" : "/user/demo"
        }
      },
      "_embedded" : null,
      id: "demo",
      ...
    }],
    "processDefinition" : [ {...} ],
    "task" : [{
      "_links" : {
        "self": {
          "href": "/task/a-task-id"
        },
        "assignee": {
          "href": "/user/demo"
        },
        ...
      },
      "_embedded" : {
        "variable" : [ {...}, {...} ]
      },
      "id" : "a-task-id",
      "name": "Assign Approver",
      "assignee": "demo",
      ...
    }, {
      ...
    }]
  },
  "count" : 2
}

Caching of HAL Relations

During the generation of a HAL response, linked resources are resolved to embed them. Some of these resolved resources, like process definitions or users, are rarely modified. Also, if user information is stored in an external system (such as LDAP), every request will access this external system which is an unnecessary overhead. To reduces such expensive requests, the REST API can be configured to use a cache to temporary store such relations.

This caching can be configured in the web.xml file of the REST API (or the Camunda Web Application in case the REST API is embedded into the Camunda Web Application).

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">

  <!-- ... -->

  <listener>
    <listener-class>org.camunda.bpm.engine.rest.hal.cache.HalRelationCacheBootstrap</listener-class>
  </listener>

  <context-param>
    <param-name>org.camunda.bpm.engine.rest.hal.cache.config</param-name>
    <param-value>
      {
        "cacheImplementation": "org.camunda.bpm.engine.rest.hal.cache.DefaultHalResourceCache",
        "caches": {
          "org.camunda.bpm.engine.rest.hal.user.HalUser": {
            "capacity": 100,
            "secondsToLive": 900
          },
          "org.camunda.bpm.engine.rest.hal.group.HalGroup": {
            "capacity": 100,
            "secondsToLive": 900
          },
          "org.camunda.bpm.engine.rest.hal.processDefinition.HalProcessDefinition": {
            "capacity": 100,
            "secondsToLive": 600
          }
        }
      }
    </param-value>
  </context-param>

  <!-- ... -->

</web-app>

The HalRelationCacheBootstrap Listener

To bootstrap the caching, the HalRelationCacheBootstrap context listener is used:

<listener>
  <listener-class>org.camunda.bpm.engine.rest.hal.cache.HalRelationCacheBootstrap</listener-class>
</listener>

It is configured by the context parameter org.camunda.bpm.engine.rest.hal.cache.config. The configuration is provided as JSON and consists of two properties:

Property Description
cacheImplementation The class which is used as cache. The class has to implement the org.camunda.bpm.engine.rest.cache.Cache interface. A simple default implementation is provided by the org.camunda.bpm.engine.rest.hal.cache.DefaultHalResourceCache class.
caches A JSON object to specify which HAL relations should be cached. Every HAL relation cache is configured separately and identified by the HalResource class to cache. The possible configuration parameters depend on the cache implementation and have to be available as setters on the implementation class.

DefaultHalResourceCache Configuration Options

The simple default cache implementation DefaultHalResourceCache provides following configuration options:

Property Description
capacity The number of maximal cache entries.
secondsToLive The number of seconds a cache entry is valid. If a cache entry is expired, it is removed and resolved again.

List of Resources Which Support Caching

  • Case Definition: org.camunda.bpm.engine.rest.hal.caseDefinition.HalCaseDefinition
  • Group: org.camunda.bpm.engine.rest.hal.group.HalGroup
  • Identity Links (of a Task): org.camunda.bpm.engine.rest.hal.identitylink.HalIdentityLink
  • Process Definition: org.camunda.bpm.engine.rest.hal.processDefinition.HalProcessDefinition
  • Task: org.camunda.bpm.engine.rest.hal.task.HalTask
  • User: org.camunda.bpm.engine.rest.hal.user.HalUser

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