HTTP API concepts

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Ditto’s HTTP API follows some concepts which are documented on this page.

The entry point into the HTTP API is:

http://localhost:8080/api/<apiVersion>

API versioning

Ditto’s HTTP API is versioned in the URL: /api/<apiVersion>. Currently, Ditto only provides API version 2.
API version 1 was deprecated and deleted as of Ditto version 2.0.0

The API version is a promise that no HTTP resources (the static ones defined by Ditto itself) are modified in an incompatible/breaking way. As the HTTP resources reflect the JSON structure of the Thing entity, that also applies for this entity.

In API 2 the Thing structure contains a Policy where the authorization information is managed.

Endpoints

In the HTTP API, some endpoints are static and can be seen as the “schema” of Ditto. They are in sync with the JSON representation of the model classes, e.g. Thing for the layout of the /things endpoint and Policy for the layout of the /policies endpoint.

API version 2

In API version 2, a Thing contains a policyId, which points to a Policy managed as another entity. Its API endpoint is /policies.

`/things` in API 2

The base endpoint for accessing and working with Things.
A Thing in API 2 has the following JSON structure:

{
  "thingId": "{thingId}",
  "policyId": "{policyId}",
  "definition": "{definition}",
  "attributes": {
  },
  "features": {
  }
}

This maps to the following HTTP API endpoints:

/things/{thingId}: accessing a complete specific thing
/things/{thingId}/policyId: accessing the policy ID of the specific thing
/things/{thingId}/definition: accessing the definition of the specific thing
/things/{thingId}/attributes: accessing the attributes of the specific thing
/things/{thingId}/features: accessing the features of the specific thing

`/things` in API 2 - dynamic part

Additionally, to that “static part” of the HTTP API which is defined by Ditto, the API is dynamically enhanced by the JSON structure of the Thing.

Note: This automatically turns each small aspect of a digital twin into an API endpoint.

For example for a Thing with following content:

{
  "thingId": "{thingId}",
  "policyId": "{policyId}",
  "definition": "{definition}",
  "attributes": {
    "manufacturer": "ACME corp",
    "complex": {
      "some": false,
      "serialNo": 4711
    }
  },
  "features": {
    "lamp": {
      "properties": {
        "on": false,
        "color": "blue"
      }
    }
  }
}

The following additional API endpoints are automatically available:

/things/{thingId}/attributes/manufacturer: accessing the attribute manufacturer of the specific thing
/things/{thingId}/attributes/complex: accessing the attribute complex of the specific thing
/things/{thingId}/attributes/complex/some: accessing the attribute complex/some of the specific thing
/things/{thingId}/attributes/complex/serialNo: accessing the attribute complex/serialNo of the specific thing
/things/{thingId}/features/lamp: accessing the feature lamp of the specific thing
/things/{thingId}/features/lamp/properties: accessing all properties of the feature lamp of the specific thing
/things/{thingId}/features/lamp/properties/on: accessing the on property of the feature lamp of the specific thing
/things/{thingId}/features/lamp/properties/color: accessing the color properties of the feature lamp of the specific thing

`/policies` in API 2

The base endpoint for accessing and working with Policies.
A Policy in API 2 has the following JSON structure:

{
  "policyId": "{policyId}",
  "entries": {
    "{entryLabel-1}": {
      "subjects": {
        "{subjectId1}": {
        }
      },
      "resources": {
        "{resource1}": {
        }
      }
    }
  }
}

This maps to the following HTTP API endpoints:

/policies/{policyId}: accessing complete Policy
/policies/{policyId}/entries: accessing the Policy entries
/policies/{policyId}/entries/{entryLabel-1}: accessing a single Policy entry with the label {entryLabel-1}
/policies/{policyId}/entries/{entryLabel-1}/subjects: accessing the subjects of a single Policy entry with the label {entryLabel-1}
/policies/{policyId}/entries/{entryLabel-1}/resources: accessing the resources of a single Policy entry with the label {entryLabel-1}

`/connections` in API 2

The base endpoint for accessing and working with Connections.

It has the following HTTP API endpoints:

/connections: accessing all connections or create a new connection
/connections/{connectionId}: accessing a specific Connection entry
/connections/{connectionId}/command: send a command to a specific Connection
/connections/{connectionId}/status: retrieve status of a specific Connection
/connections/{connectionId}/metrics: retrieve metrics of a specific Connection
/connections/{connectionId}/logs: retrieve logs of a specific Connection

Partial updates

As a benefit of the above-mentioned mechanism that an API is automatically available based on the JSON structure, the “partial update” pattern can be applied when modifying data.

The benefit of this is a reduction in payload to be transferred. Further, it is beneficial because other parts of the Thing are not overwritten with a potentially outdated value - only the actually changed data part can be modified.

So instead of modifying a complete Thing only a specific part is affected.

Given, the on property of lamp should be changed to true.

Instead of

PUT .../things/{thingId} with the complete payload:

{
  "thingId": "{thingId}",
  "policyId": "{policyId}",
  "definition": "{definition}",
  "attributes": {
    "manufacturer": "ACME corp",
    "complex": {
      "some": false,
      "serialNo": 4711
    }
  },
  "features": {
    "lamp": {
      "properties": {
        "on": true,
        "color": "blue"
      }
    }
  }
}

we can use a smarter request
PUT .../things/{thingId}/features/lamp/properties/on with a minimal payload:

true

Partial requests

Similar to the partial updates from above, the HTTP API can also be used to retrieve a single value instead of a complete Thing.

Again, the benefit is a reduction in response payload and that the caller can directly use the returned data value (for example expect it to be a boolean and treat it accordingly).

For example, we can request
GET .../things/{thingId}/features/lamp/properties/on and get as response:

true

With field selector

A further mechanism in the API for partial requests is using a so-called field selector. This is useful when the JSON structure of the Thing or other entity should be kept intact, but not all information is relevant.

The field selector is passed as an HTTP query parameter fields and contains a comma separated list of fields to include in the response.

It is possible to use the wildcard operator ‘*’ as feature ID and retrieve a property of multiple features. For details see the example below.

Given, you have the following Thing:

{
  "thingId": "{thingId}",
  "policyId": "{policyId}",
  "definition": "{definition}",
  "attributes": {
    "manufacturer": "ACME corp",
    "complex": {
      "some": false,
      "serialNo": 4711,
      "misc": "foo"
    }
  },
  "features": {
    "lamp": {
      "properties": {
        "on": true,
        "color": "blue"
      }
    },
    "infrared-lamp": {
      "properties": {
        "on": false,
        "color": "red"
      }
    }
  }
}

Field selector examples

The following GET request examples with field selectors show how you can retrieve only the parts of a thing which you are interested in:

GET .../things/{thingId}?fields=attributes
Response:

{
  "attributes": {
    "manufacturer": "ACME corp",
    "complex": {
      "some": false,
      "serialNo": 4711,
      "misc": "foo"
    }
  }
}

GET .../things/{thingId}?fields=attributes/manufacturer
Response:

{
  "attributes": {
    "manufacturer": "ACME corp"
  }
}

GET .../things/{thingId}?fields=attributes/complex/serialNo
Response:

{
  "attributes": {
    "complex": {
      "serialNo": 4711
    }
  }
}

GET .../things/{thingId}?fields=attributes/complex/some,attributes/complex/serialNo
Response:

{
  "attributes": {
    "complex": {
      "some": false,
      "serialNo": 4711
    }
  }
}

GET .../things/{thingId}?fields=attributes/complex(some,serialNo)
Response:

{
  "attributes": {
    "complex": {
      "some": false,
      "serialNo": 4711
    }
  }
}

GET .../things/{thingId}?fields=attributes/complex/misc,features/lamp/properties/on
Response:

{
  "attributes": {
    "complex": {
      "misc": "foo"
    }
  },
  "features": {
    "lamp": {
      "properties": {
        "on": true
      }
    }
  }
}

Field selector with wildcard

GET .../things/{thingId}?fields=features/*/properties/on
Response:

{
  "features": {
    "lamp": {
      "properties": {
        "on": true
      }
    },
    "infrared-lamp": {
      "properties": {
        "on": false
      }
    }
  }
}

Merge updates

Merge updates can be used to update multiple parts of a Thing in a single request e.g. multiple properties of different features or a feature property and an attribute value. Merge updates are applied by using the HTTP PATCH method with the payload in JSON merge patch (RFC-7396) format. The content-type of the request must be set to application/merge-patch+json.

RFC-7396 specifies how a set of modifications is applied to an existing JSON document:

A JSON merge patch document describes changes to be made to a target JSON document using a syntax that closely 
mimics the document being modified. Recipients of a merge patch document determine the exact set of changes being  
requested by comparing the content of the provided patch against the current content of the target document.
If the provided merge patch contains members that do not appear within the target, those members are added. If the 
target does contain the member, the value is replaced.  Null values in the merge patch are given special meaning to 
indicate the removal of existing values in the target.

Note: Please note the special meaning of null values. When using PATCH a null value is interpreted as delete in contrast to PUT requests where null values have no special meaning.

Like PUT requests, PATCH requests can be applied at any level of the JSON structure of a thing, e.g. patching a complete thing at root level or patching a single property value at property level.

Removing fields in a merge update with a regex

Note: This is an addition to the JSON merge patch (RFC-7396), enhancing using null values for deleting certain parts of JSON objects specified with a regular expression before applying new fields to it.

The merge patch functionality in Ditto solves a common problem to the JSON merge patch (RFC-7396): whenever a JSON object shall be patched, all the old json fields are merged with all the new json fields, unless the exact field names are specified in the patch with "<field>": null.
This would however require to know all existing fields upfront, which for a merge patch can not be assumed.

The solution is a little enhancement to Ditto’s merge patch functionality: The ability to delete arbitrary parts from JSON objects using a regular expression before applying all other patch values.

The syntax for this function is rather specific (so that no “normally” occurring JSON keys match the same syntax):

{
  "{{ /.*/ }}": null
}

When such a {{ /<regex>/ }} with the value null is detected in the merge patch, the content between the 2 / is interpreted as regular expression to apply for finding keys to delete from the target object.
As a result, using "{{ /.*/ }}": null would delete all the values inside a JSON object before applying the new values provided in the patch.

Example:
Assuming that inside a JSON object every month some aggregated data is stored with the year and month:

{
  "thingId": "{thingId}",
  "policyId": "{policyId}",
  "features": {
    "aggregated-history": {
      "properties": {
        "2022-11": 42.3,
        "2022-12": 54.3,
        "2023-01": 80.2,
        "2023-02": 99.9
      }
    }
  }
}

Then the data from “last year” could be purged with the following patch, while adding a new value to the existing ones of this year:

{
  "features": {
    "aggregated-history": {
      "properties": {
        "{{ /2022-.*/ }}": null,
        "2023-03": 105.21
      }
    }
  }
}

The resulting Thing JSON after applying the patch would then look like:

{
  "thingId": "{thingId}",
  "policyId": "{policyId}",
  "features": {
    "aggregated-history": {
      "properties": {
        "2023-01": 80.2,
        "2023-02": 99.9,
        "2023-03": 105.21
      }
    }
  }
}

Permissions required for merge update

To successfully execute merge update the authorized subject needs to have WRITE permission on all resources affected by the provided JSON merge patch. If the permission is missing for one of the affected resources the whole merge patch is rejected, i.e. the merge update is executed as a whole or not at all.

Merge update example

Given an existing thing with the JSON structure:

{
  "thingId": "{thingId}",
  "policyId": "{policyId}",
  "attributes": {
    "location": {
      "longitude": 47.682170,
      "latitude": 9.386372
    },
    "serialNo": "0000000"
  },
  "features": {
    "temperature": {
      "properties": {
        "value": 25.43,
        "unit": "°C"
      }
    },
    "pressure": {
      "properties": {
        "value": 1013.25,
        "unit": "hPa"
      }
    }
  }
}

Assuming a single request should:

add the manufacturer attribute
update the existing serialNo attribute to the value of 23091861
remove the existing location attribute
set the existing property value of feature temperature to the value of 26.89
remove the existing property unit of feature pressure
add a new feature humidity

This can be achieved using a PATCH .../things/{thingId} with the request payload of

{
  "attributes": {
    "location": null,
    "manufacturer": "Bosch",
    "serialNo": "23091861"
  },
  "features": {
    "temperature": {
      "properties": {
        "value": 26.89
      }
    },
    "pressure": {
      "properties": {
        "unit": null
      }
    },
    "humidity": {
      "properties": {
        "value": 55,
        "unit": "%"
      }
    }
  }
}

The resulting JSON representation of the updated thing after applying the PATCH is:

{
  "thingId": "{thingId}",
  "policyId": "{policyId}",
  "attributes": {
    "manufacturer": "Bosch",
    "serialNo": "23091861"
  },
  "features": {
    "temperature": {
      "properties": {
        "value": 26.89,
        "unit": "°C"
      }
    },
    "pressure": {
      "properties": {
        "value": 1015
      }
    },
    "humidity": {
      "properties": {
        "value": 55,
        "unit": "%"
      }
    }
  }
}

Conditional Requests

The HTTP API for Things and Policies partially supports Conditional Requests as defined in RFC-7232.

ETag

A successful response on a thing or policy resource provides an ETag header.

For read responses, it contains the current entity-tag of the resource.
For write responses, it contains the entity-tag after successful write.

The ETag has a different format for top-level resources and sub-resources.

Top-level resources (e.g. .../things/{thingId}): The entity-tag contains the revision of the entity which is addressed by the resource in the format "rev:<revision>", e.g. "rev:2".
Sub-resources (e.g. .../things/{thingId}/features/{featureId}): The entity-tag contains a hash of the current value of the addressed sub-resource in the format "hash:<calculated-hash>", e.g. "hash:87192253740". Note that this format may change in the future.

Conditional Headers

The following request headers can be used to issue a conditional request:

If-Match:
- Read or write the resource only
  - if the current entity-tag matches at least one of the entity-tags provided in this header
  - or if the header is * and the entity exists
- The response will be:
  - in case of a match, the same response as if the header wouldn’t have been specified
  - in case of no match, status 412 (Precondition Failed) with an error response containing detail information and the current entity-tag of the resource as ETag header
If-None-Match:
- Read or write the resource only
  - if the current entity-tag does not match any one of the entity-tags provided in this header
  - or if the header is * and the entity does not exist
- The response will be:
  - in case of no match, the same response as if the header wouldn’t have been specified
  - in case of a match:
    - for write requests, status 412 (Precondition Failed) with an error response containing detail information and the current entity-tag of the resource as ETag header
    - for read requests, status 304 (Not Modified) without response body, with the current entity-tag of the resource as ETag header
if-equal:
- The if-equal header can take the values 'update' (which is the default if omitted) or 'skip'
- Write the resource only
  - in case if-equal: 'update' is defined - even if the entity is equal before the update.
  - in case if-equal: 'skip' is defined, the entity will not be updated if it is equal before the update.
    In this case a ‘Not Modified’ 304 status is returned.

Note that the Ditto HTTP API always provides a strong entity-tag in the ETag header, thus you will never receive a weak entity-tag (see RFC-7232 Section 2.1). If you convert this strong entity-tag to a weak entity-tag and use it in a Conditional Header, Ditto will handle it according to RFC-7232. However, we discourage the usage of weak entity-tags, because in the context of Ditto they only add unnecessary complexity.

In addition to the ETag header Ditto supports conditional requests with a condition header. For further information see Conditional Requests

Exempted fields

Assuming you have a thing with an associated policy. When querying the thing with

GET .../things/{thingId}?fields=_policy

you will get the thing containing its revision and associated policy.

If you now modify the associated policy, the revision of the thing will not change! This could lead to an inconsistent state if the thing is getting refetched by using the If-None-Match header, because this would return a 304 Not Modified, even if the policy has changed.

To tackle this, Ditto has the following list of exempted fields which automatically bypass the precondition header check:

_policy

Examples

The following examples show several scenarios on a top-level (Thing) resource. Nevertheless, these scenarios can also be applied on any sub-resource in the same way.

Create: Write only if the resource does not exist

The following example request shows, how you can make sure that a PUT request does not overwrite existing data, i.e. how you can enforce that the Thing can only be created by the request.

PUT .../things/{thingId}
If-None-Match: *

{
  "policyId": "{policyId}",
  "attributes": {
    "manufacturer": "ACME crop",
    "otherData": 4711
  }
}

You will get one of the following responses:

201 (Created) in case the creation was successful, i.e. the Thing did not yet exist.
412 (Precondition Failed) in case the creation failed, i.e. a Thing with the exactly same {thingId} already exists.

Update: Write only if the resource already exists

The following example request shows how you can make sure that a PUT request does not create the resource, i.e. how you can enforce that the Thing can only be updated by the request, but you do not generate a duplicate by mistake.

PUT .../things/{thingId}
If-Match: *

{
  "attributes": {
    "manufacturer": "ACME crop",
    "otherData": 4711
  }
}

You will get one of the following responses:

204 (No Content) in case the update was successful, i.e. the Thing already existed.
412 (Precondition Failed) in case the update failed, i.e. the Thing does not yet exist.

Optimistic Locking

First, GET the Thing in order to retrieve both: the current data and the entity-tag:

GET .../things/{thingId}:

Response:

ETag: "rev:2"

{
  "thingId": "{thingId}",
  "policyId": "{policyId}",
  "definition": "{definition}",
  "attributes": {
    "manufacturer": "ACME crop",
    "otherData": 4711
  }
}

Assume that you have detected the typo in the manufacturer attribute (“ACME crop”) and want to fix this with a top-level Thing PUT. You want to make sure, that no one else has modified the Thing in the meantime, because otherwise his changes would be lost. (You could also achieve this with a PUT on the concrete attribute, but for this example we assume that you want to use a top-level Thing PUT.)

PUT the Thing with the changed data and the entity-tag from the preceding GET response in the If-Match header.

PUT .../things/{thingId}
If-Match: "rev:2"

{
  "attributes": {
    "manufacturer": "ACME corp",
    "otherData": 4711
  }
}

You will get one of the following responses:

204 (No Content) in case the update was successful, i.e. no one else has changed the Thing in the meantime.
412 (Precondition Failed) in case the update was not successful, i.e. the Thing has been changed by someone else in the meantime.

Tags:

API versioning

Endpoints

API version 2

/things in API 2

/things in API 2 - dynamic part

/policies in API 2

/connections in API 2

Partial updates

Partial requests

With field selector

Field selector examples

Field selector with wildcard

Merge updates

Removing fields in a merge update with a regex

Permissions required for merge update

Merge update example

Conditional Requests

ETag

Conditional Headers

Exempted fields

Examples

Create: Write only if the resource does not exist

Update: Write only if the resource already exists

Optimistic Locking

`/things` in API 2

`/things` in API 2 - dynamic part

`/policies` in API 2

`/connections` in API 2