CRUD Operations in YANG-API

YANG-API	NETCONF
OPTIONS	none
HEAD	none
GET	<get-config>, <get>
POST	<edit-config> (operation="create")
PUT	<edit-config> (operation="replace")
PATCH	<edit-config> (operation="merge")
DELETE	<edit-config> (operation="delete")

The NETCONF "remove" operation attribute is not supported by the HTTP DELETE method. The resource must exist or the DELETE operation will fail.

This section defines the YANG-API protocol usage for each HTTP method.

3.1. OPTIONS

The OPTIONS method is sent by the client to discover which methods are supported by the server for a specific resource, or field within a resource. It is supported for all media types. Note that implementation of this operation is part of HTTP, and this section does not introduce any additional requirements.

The request MUST contain a request URI that contains at least the entry point component.

The server will return a "Status‑Line" header containing "204 No Content". and include the "Allow" header in the response. This header will be filled in, based on the target resource media type. Other headers MAY also be included in the response.

Example 1:

A client might request the methods supported for a data resource called "library"

   OPTIONS /yang-api/datastore/jukebox/library HTTP/1.1
   Host: example.com
	    

The server might respond (for a config=true list):

   HTTP/1.1 204 No Content
   Date: Mon, 23 Apr 2012 17:01:00 GMT
   Server: example-server
   Allow: OPTIONS,HEAD,GET,POST,PUT,PATCH,DELETE
	    

Example 2:

A client might request the methods supported for a non-configuration leaf within a data resource:

   OPTIONS /yang-api/datastore/jukebox/library/
       song-count HTTP/1.1
   Host: example.com
	    

The server might respond:

   HTTP/1.1 204 No Content
   Date: Mon, 23 Apr 2012 17:02:00 GMT
   Server: example-server
   Allow: OPTIONS,HEAD,GET
	    

Example 3:

A client might request the methods supported for an operation resource called "play":

   OPTIONS /yang-api/operations/play HTTP/1.1
   Host: example.com
	    

The server might respond:

   HTTP/1.1 204 No Content
   Date: Mon, 23 Apr 2012 17:02:00 GMT
   Server: example-server
   Allow: POST
	    

3.2. HEAD

The HEAD operation is sent by the client to retrieve just the headers that would be returned for the comparable GET operation, without the response body. The HTTP HEAD method is used for this operation. It is supported for all resource types, except operation resources.

The request MUST contain a request URI that contains at least the entry point component.

The same query parameters supported by the GET operation are supported by the HEAD operation. For example, the "select" query parameter can be used to specify a field within the target resource.

The access control behavior is enforced as if the method was GET instead of HEAD. The server MUST respond the same as if the method was GET instead of HEAD, except that no response body is included.

Example:

The client might request the response headers for the default (JSON) representation of the "library" resource:

   HEAD /yang-api/datastore/jukebox/library HTTP/1.1
   Host: example.com
	    

The server might respond:

   HTTP/1.1 200 OK
   Date: Mon, 23 Apr 2012 17:02:40 GMT
   Server: example-server
   Content-Type: application/vnd.yang.data+json
   Cache-Control: no-cache
   Pragma: no-cache
   ETag: a74eefc993a2b
   Last-Modified: Mon, 23 Apr 2012 11:02:14 GMT
	    

3.3. GET

The GET operation is sent by the client to retrieve data and meta-data for a resource or field within a resource. The HTTP GET method is used for this operation. It is supported for all resource types, except operation resources. The request MUST contain a request URI that contains at least the entry point component.

The following query parameters are supported by the GET operation:

GET Query Parameters

Name	Section	Description
config	3.8.1	Request either configuration or non-configuration data
depth	3.8.2	Control the depth of a retrieval request
format	3.8.3	Request either JSON or XML content in the response
select	3.8.6	Specify a field within the target resource

The server MUST NOT return any data resources or fields within any data resources for which the user does not have read privileges.

If the user is not authorized to read any portion of the target resource, an error response containing a "403 Forbidden" Status-Line is returned to the client.

If the user is authorized to read some but not all of the target resource, the unauthorized content is omitted from the response message body, and the authorized content is returned to the client.

Example:

The client might request the response headers for a JSON representation of the "library" resource:

   GET /yang-api/datastore/jukebox/library/artist/
     1/album HTTP/1.1
   Host: example.com
	    

The server might respond:

   HTTP/1.1 200 OK
   Date: Mon, 23 Apr 2012 17:02:40 GMT
   Server: example-server
   Content-Type: application/vnd.yang.data+json
   Cache-Control: no-cache
   Pragma: no-cache
   ETag: a74eefc993a2b
   Last-Modified: Mon, 23 Apr 2012 11:02:14 GMT

   {
     "album" : {
       "name" : "Wasting Light",
       "genre" : "example-jukebox:Alternative",
       "year" : 2011
     }
   }
	    

3.4. POST

The POST operation is sent by the client for various reasons. The HTTP POST method is used for this purpose. The request MUST contain a request URI that contains a target resource that identifies one of the following resource types:

Resource Types that Support POST

Type	Description
Data	Create a configuration data resource
Operation	Invoke protocol operation
Transaction	Create a new transaction

The following query parameters are supported by the POST operation:

POST Query Parameters

Name	Section	Description
insert	3.8.4	Specify where to insert a resource
point	3.8.5	Specify the insert point for a resource

If the POST operation succeeds, a "200 OK" Status-Line is returned if there is no response message body, and a "204 No Content" Status-Line is returned if there is a response message body.

If the user is not authorized to invoke the target (operation) resource, or create the target resource, an error response containing a "403 Forbidden" Status-Line is returned to the client. All other error responses are handled according to the procedures defined in Section 6.

3.5. PUT

The PUT operation is sent by the client to replace the target resource.

The HTTP PUT method is used for this purpose. The request MUST contain a request URI that contains a target resource that identifies the data resource to replace.

The following query parameters are supported by the PUT operation:

PUT Query Parameters

Name	Section	Description
insert	3.8.4	Specify where to move a resource
point	3.8.5	Specify the move point for a resource

If the PUT operation succeeds, a "200 OK" Status-Line is returned, and there is no response message body.

If the user is not authorized to replace the target resource an error response containing a "403 Forbidden" Status-Line is returned to the client. All other error responses are handled according to the procedures defined in Section 6.

3.6. PATCH

The PATCH operation uses the HTTP PATCH method defined in [RFC5789] to provide a "merge" editing mode for data resources. Instead of replacing all or part of the target resource, the supplied values are merged into the target resource.

If the PATCH operation succeeds, a "200 OK" Status-Line is returned, and there is no response message body.

If the user is not authorized to alter the target resource an error response containing a "403 Forbidden" Status-Line is returned to the client. All other error responses are handled according to the procedures defined in Section 6.

3.7. DELETE

The DELETE operation uses the HTTP DELETE method to delete the target resource.

If the DELETE operation succeeds, a "200 OK" Status-Line is returned, and there is no response message body.

If the user is not authorized to delete the target resource then an error response containing a "403 Forbidden" Status-Line is returned to the client. All other error responses are handled according to the procedures defined in Section 6.

3.8. Query Parameters

Each YANG-API operation allows zero or more query parameters to be present in the request URI. Refer to Section 3 for details on the query parameters used in the definition of each operation.

Query parameters can be given in any order. Each parameter can appear zero or one time. A default value may apply if the parameter is missing.

This section defines all the YANG-API query parameters.

3.8.1. "config" Parameter

The "config" parameter is used to specify whether configuration or non-configuration data is requested.

This parameter is only supported for the GET and HEAD methods. It is also only supported if the target resource is a data resource.

     syntax: config= true | false
     default: true
	    

Example:

This example request by the client would retrieve only the non-configuration data nodes that exist within the second-level "library" resource.

   GET /yang-api/datastore/jukebox/library?config=false HTTP/1.1
   Host: example.com
   Accept: application/vnd.yang.data+xml
	    

The server might respond:

   HTTP/1.1 200 OK
   Date: Mon, 23 Apr 2012 17:01:30 GMT
   Server: example-server
   Cache-Control: no-cache
   Pragma: no-cache
   Content-Type: application/vnd.yang.data+json

   {
     "library" : {
        "artist-count" : 42,
        "album-count" : 59,
        "song-count" : 374
     }
   }
	    

3.8.2. "depth" Parameter

The "depth" parameter is used to specify the number of nest levels returned in a response for a GET operation. A nest-level consists of the target resource and any child nodes which are optional data nodes (anyxml, leaf, or leaf-list). A non-presence container is transparent when determining the nest level. A child node (which is not a non-presence container) within a non-presence container is used to determine the nest-level.

The start level is determined by the target resource for the operation.

     syntax: depth=<range: 1..max> | unbounded
     default: 1
	    

Example:

This example operation would retrieve 2 levels of configuration data nodes that exist within the top-level "jukebox" resource.

   GET /yang-api/datastore/jukebox?depth=2 HTTP/1.1
   Host: example.com
   Accept: application/vnd.yang.data+json
	    

The server might respond:

   HTTP/1.1 200 OK
   Date: Mon, 23 Apr 2012 17:11:30 GMT
   Server: example-server
   Cache-Control: no-cache
   Pragma: no-cache
   Content-Type: application/vnd.yang.data+json

   {
     "jukebox" : {
       "library" : {
         "artist" : {
           "index" : 1,
           "name" : "Foo Fighters"
         }
       },
       "player" : {
         "gap" : 0.5
       }
     }
   }
	    

3.8.3. "format" Parameter

The "format" parameter is used to specify the format of any content returned in the response. Note that the "Accept" header MAY be used instead of this parameter to identify the format desired in the response. For example:

   GET /yang-api/datastore/routing HTTP/1.1
   Host: example.com
   Accept: application/vnd.yang.data+xml
	    

This example request would retrieve only the configuration data nodes that exist within the top-level "routing" resource, and retrieve them in XML encoding instead of JSON encoding.

The "format" parameter is only supported for the GET and HEAD methods. It is supported for all YANG-API media types.

     syntax: format= xml | json
     default: json
	    

Example:

   GET /yang-api/datastore/routing?format=xml HTTP/1.1
   Host: example.com
	    

This example URI would retrieve only the configuration data nodes that exist within the top-level "routing" resource, and retrieve them in XML encoding instead of JSON encoding.

3.8.4. "insert" Parameter

The "insert" parameter is used to specify how a resource should be inserted (or moved) within the user-ordered list or leaf-list data resource.

This parameter is only supported for the POST and PUT methods. It is also only supported if the target resource is a data resource, and that data represents a YANG list or leaf-list that is ordered by the user, not the system.

If the values "before" or "after" are used, then a "point" parameter for the insertion parameter MUST also be present.

     syntax: insert= first | last | before | after
     default: last
	    

Example:

 Request from client:

   POST /yang-api/datastore/jukebox/library/artist/1/album
      /Wasting%20Light/song?insert=first HTTP/1.1
   Host: example.com
   Content-Type: application/vnd.yang.data+json

   {
     "song" : {
        "name" : "Bridge Burning",
        "location" : "/media/bridge_burning.mp3",
        "format" : "MP3",
        "length" : 286
     }
   }

 Response from server:  201 status

   HTTP/1.1 201 Created
   Date: Mon, 23 Apr 2012 13:01:20 GMT
   Server: example-server
   Last-Modified: Mon, 23 Apr 2012 13:01:20 GMT

   Location: http://example.com/yang-api/datastore/jukebox
      /library/artist/1/album?Wasting%20Light/song/1
   ETag: eeeada438af
	    

3.8.5. "point" Parameter

The "point" parameter is used to specify the insertion point for a data resource that is being created or moved within a user ordered list or leaf-list. It is ignored unless the "insert" query parameter is also present, and has the value "before" or "after".

This parameter contains the instance identifier of the resource, or field within a resource, to be used as the insertion point for a POST or PUT operation. It is encoded according to the rules defined in Section 5.3.1. There is no default for this parameter.

   syntax: point= <instance-identifier of insertion point node>
	    

Example:

In this example, the client is moving an existing "song" resource within an "album" resource after another song. The request URI is split for display purposes only.

 Request from client:

   PUT /yang-api/datastore/jukebox/library/artist/1/album/
     Wasting%20Light/song/2?insert=after
     &point=/yang-api/datastore/jukebox/library/artist/1/
     album/Wasting%20Light/song/4   HTTP/1.1
   Host: example.com

 Response from server:

   HTTP/1.1 204 No Content
   Date: Mon, 23 Apr 2012 13:01:20 GMT
   Server: example-server
   Last-Modified: Mon, 23 Apr 2012 13:01:20 GMT
   ETag: abcada438af
	    

3.8.6. "select" Parameter

The "select" query parameter is used to specify an expression which can represent a subset of all data nodes within the target resource. It contains a relative path expression, using the target resource as the context node.

It is supported for all resource types except operation resources. The contents are encoded according to the "api‑select" rule defined in Section 5.3.1. This parameter is only allowed for GET and HEAD operations.

[FIXME: the syntax of the select string is still TBD; XPath, schema-identifier, regular expressions, something else]

Refer to Section 1.4.2 for example request messages using the "select" parameter.

3.9. Protocol Operations

The YANG-API also allows data-model specific protocol operations to be invoked using the POST method. The media type "vnd.yang.operation+xml" or "vnd.yang.operation+json" MUST be used in the "Content‑Type" field in the message header.

Data model specific operations are supported. The syntax and semantics of these operations exactly correspond to the YANG "rpc" statement definition for the operation.

Any input for a protocol operation is encoded in an element called "input", which corresponds to the <input> element in a NETCONF message. The child nodes of the "input" element are encoded according to the data definition statements in the input section of the "rpc" statement.

Any output for a protocol operation is encoded in an element called "output", which corresponds to the <rpc‑reply> element in a NETCONF message. The child nodes of the "output" element are encoded according to the data definition statements in the output section of the "rpc" statement.

4. Messages

This section describes the messages that are used in the YANG-API protocol.

4.1. Request URI Structure

Resources are represented with URIs following the structure for generic URIs in [RFC3986].

A YANG-API operation is derived from the HTTP method and the request URI, using the following conceptual fields:

     <OP> /yang-api/<path>?<query>#<fragment>
	    

      ^      ^       ^        ^         ^   
      |      |       |        |         |   
    method entry  resource  query    fragment

      M       M        O        O         I


    M=mandatory, O=optional, I=ignored
	    

    <text> replaced by client with real values
	    

• method: the HTTP method identifying the YANG-API operation requested by the client, to act upon the target resource specified in the request URI. YANG-API operation details are described in Section 3.
• entry: the well-known YANG-API entry point ("/yang‑api").
• resource: the path expression identifying the resource that is being accessed by the operation. If this field is not present, then the target resource is the API itself, represented by the media type "vnd.yang.api".
• query: the set of parameters associated with the YANG-API message. These have the familiar form of "name=value" pairs. There is a specific set of parameters defined, although the server MAY choose to support additional parameters not defined in this document.
• fragment: This field is not used by the YANG-API protocol.

The client SHOULD NOT assume the final structure of a URI path for a resource. Instead, existing resources can be discovered with the GET operation. When new resources are created by the client, a "Location" header is returned, which identifies the path of the newly created resource. The client MUST use this exact path identifier to access the resource once it has been created.

The "target" of an operation is a resource. The "path" field in the request URI represents the target resource for the operation.

4.2. Message Headers

There are several HTTP header lines utilized in YANG-API messages. Messages are not limited to the HTTP headers listed in this section.

HTTP defines which header lines are required for particular circumstances. Refer to each operation definition section in Section 3 for examples on how particular headers are used.

There are some request headers that are used within YANG-API, usually applied to data resources. The following tables summarize the headers most relevant in YANG-API message requests:

YANG-API Request Headers

Name	Description
Accept	Response Content-Types that are acceptable
Content-Type	The media type of the request body
Host	The host address of the server
If-Match	Only perform the action if the entity matches ETag
If-Modified-Since	Only perform the action if modified since time
If-Range	Only retrieve range if resource unchanged
If-Unmodified-Since	Only perform the action if un-modified since time
Range	Specify a range of data resource entries

The following tables summarize the headers most relevant in YANG-API message responses:

YANG-API Response Headers

Name	Description
Allow	Valid actions when 405 error returned
Content-Type	The media type of the response body
Date	The date and time the message was sent
ETag	An identifier for a specific version of a resource
Last-Modified	The last modified date and time of a resource
Location	The resource identifier for a newly created resource

4.3. Message Encoding

YANG-API messages are encoded in HTTP according to RFC 2616. The "utf‑8" character set is used for all messages. YANG-API message content is sent in the HTTP message body.

Content is encoded in either JSON or XML format.

XML encoding rules for data nodes are defined in [RFC6020]. The same encoding rules are used for all XML content. XML attributes are not used and will be ignored if present in an XML-encoded message.

JSON encoding rules are defined in [I-D.lhotka-netmod-json]. Special encoding rules are needed to handle multiple module namespaces and provide consistent data type processing.

Request input content encoding format is identified with the Content-Type header. This field MUST be present if message input is sent by the client.

Response output content encoding format is identified with the Accept header, the "format" query parameter, or if neither is specified, the request input encoding format is used. If there was no request input, then the default output encoding is JSON. File extensions encoded in the request are not used to identify format encoding.

4.4. Return Status

Each message represents some sort of resource access. An HTTP "Status‑Line" header line is returned for each request. If a 4xx or 5xx range status code is returned in the Status-Line, then the error information will be returned in the response, according to the format defined in Section 6.1.

4.5. Message Caching

Since the datastore contents change at unpredictable times, responses from a YANG-API server generally SHOULD NOT be cached.

The server SHOULD include a "Cache‑Control" header in every response that specifies whether the response should be cached. A "Pragma" header specifying "no‑cache" MAY also be sent in case the "Cache‑Control" header is not supported.

Instead of using HTTP caching, the client SHOULD track the "ETag" and/or "Last‑Modified" headers returned by the server for the datastore resource (or data resource if the server supports it).

A retrieval request for a resource can include headers such as "If‑None‑Match" or "If‑Modified‑Since" which will cause the server to return a "304 Not Modified" Status-Line if the resource has not changed.

The client MAY use the HEAD operation to retrieve just the message headers, which SHOULD include the "ETag" and "Last‑Modified" headers, if this meta-data is maintained for the target resource.

5. Resources

The resources used in the YANG-API protocol are identified by the "path" component in the request URI. Each operation is performed on a target resource.

5.1. API Resource (/yang-api)

The API resource contains the state and access points for the YANG-API features.

It is the top-level resource and has the media type "application/vnd.yang.api+xml" or "application/vnd.yang.api+json". It is accessible through the well-known URI "/yang‑api".

This resource has the following fields:

YANG-API Resource Fields

Field Name	Description
datastore	Link to "datastore" resource
modules	YANG module capability URIs
operations	Data-model specific operations

5.1.1. /yang-api/datastore

This mandatory resource represents the running configuration datastore and any non-configuration data available. It may be retrieved and edited directly. It cannot be created or deleted by the client. This resource type is defined in Section 5.2.

5.1.2. /yang-api/modules

This mandatory field contains the identifiers for the YANG data model modules supported by the server. There MUST be exactly one instance of this field.

The server MUST maintain a last-modified timestamp for this field, and return the "Last‑Modified" header when this field is retrieved with the GET or HEAD methods.

5.1.3. /yang-api/operations

This optional field provides access to the data-model specific protocol operations supported by the server. The server MAY omit this field if no data-model specific operations are advertised.

Any data-model specific operations defined in the YANG modules advertised by the server SHOULD be available as child nodes of this field.

5.1.3.1. /yang-api/modules/module

This mandatory field contains one URI string for each YANG data model module supported by the server. There MUST be an instance of this field for every YANG module that is accessible via an operation resource or a data resource.

The server MAY maintain a last-modified timestamp for each instance of this resource, and return the "Last‑Modified" header when this resource is retrieved with the GET or HEAD methods. If not supported then the timestamp for the parent "modules" field MUST NOT be used instead.

The contents of this field are encoded with the "uri" derived type from the "ietf‑iana‑types" modules in [RFC6021].

There are additional encoding requirements for this field. The URI MUST follow the YANG module capability URI formatting defined in section 5.6.4 of [RFC6020].

5.1.3.2. Retrieval Example

In this example the client is retrieving the modules field from the server in the default JSON format:

   GET /yang-api?select=modules HTTP/1.1
   Host: example.com
   Accept: application/vnd.yang.api+json
	    

The server might respond as follows. Note that the content below is split across multiple lines for display purposes only:

   HTTP/1.1 200 OK
   Date: Mon, 23 Apr 2012 17:01:00 GMT
   Server: example-server
   Cache-Control: no-cache
   Pragma: no-cache
   Last-Modified: Sun, 22 Apr 2012 01:00:14 GMT
   Content-Type: application/vnd.yang.api+json

   {
     "yang-api": {
       "modules": {
         "module": [
           "example.com?module=foo&revision=2012-01-02",
           "example.com?module=bar&revision=2011-10-10"
           "example.com?module=itf&revision=2011-10-10
               &feature=restore"
         ]
       }
     }
   }
	    

5.1.4. /yang-api/version

This mandatory field identifies the specific version of the YANG-API protocol implemented by the server.

The same server-wide response MUST be returned each time this field is retrieved. It is assigned by the server when the server is started. The server MUST return the value "1.0" for this version of the YANG-API protocol.

This field is encoded with the rules for an "enumeration" data type, using the following leaf definition:

   leaf version {
     config false;
     type enum {
       enum "1.0" {
         description 
           "Version 1.0 of the YANG-API protocol.";
       }
     }
   }
	    

5.2. Datastore Resource

A datastore resource represents the conceptual root of a tree of data resources.

The server MUST maintain a last-modified timestamp for this resource, and return the "Last‑Modified" header when this resource is retrieved with the GET or HEAD methods. Only changes to configuration data resources within the datastore affect this timestamp.

The server SHOULD maintain a resource entity tag for this resource, and return the "ETag" header when this resource is retrieved with the GET or HEAD methods. The resource entity tag SHOULD be changed to a new previously unused value if changes to any configuration data resources within the datastore are made.

A datastore resource can be retrieved with the GET operation, to retrieve either configuration data resources or non-configuration data resources within the datastore. The "config" query parameter is used to choose between them. Refer to Section 3.8.1 for more details.

The depth of the subtrees returned in retrieval operations can be controlled with the "depth" query parameter. The number of nest levels, starting at the target resource, can be specified, or an unlimited number can be returned. Refer to Section 3.8.2 for more details.

A datastore resource cannot be written directly with any edit operation. Only the configuration data resources within the datastore resource can be edited.

5.3. Data Resource

A data resource represents a YANG data node that is a descendant node of a datastore resource.

For configuration data resources, the server MAY maintain a last-modified timestamp for the resource, and return the "Last‑Modified" header when it is retrieved with the GET or HEAD methods.

For configuration data resources, the server MAY maintain a resource entity tag for the resource, and return the "ETag" header when it is retrieved as the target resource with the GET or HEAD methods. The resource entity tag SHOULD be changed to a new previously unused value if changes to the resource or any configuration field within the resource is altered.

A data resource can be retrieved with the GET operation, to retrieve either configuration data resources or non-configuration data resources within the target resource. The "config" query parameter is used to choose between them. Refer to Section 3.8.1 for more details.

The depth of the subtrees returned in retrieval operations can be controlled with the "depth" query parameter. The number of nest levels, starting at the target resource, can be specified, or an unlimited number can be returned. Refer to Section 3.8.2 for more details.

A configuration data resource can be altered by the client with some of all of the edit operations, depending on the target resource and the specific operation. Refer to Section 3 for more details on edit operations.

5.3.1. Encoding YANG Instance Identifiers in the Request URI

In YANG, data nodes are named with an absolute XPath expression, from the document root to the target resource. In YANG-API, URL friendly path expressions are used instead.

The YANG "instance‑identifier" (i-i) data type is represented in YANG-API with the path expression format defined in this section.

YANG-API instance-identifier Type Conversion

Name	Comments
point	Insertion point is always a full i-i
path	Request URI path is a full or partial i-i

The "path" component of the request URI contains the absolute path expression that identifies the target resource. The "select" query parameter is used to optionally identify the requested data nodes within the target resource to be retrieved in a GET operation.

A predictable location for a data resource is important, since applications will code to the YANG data model module, which uses static naming and defines an absolute path location for all data nodes.

A YANG-API data resource identifier is not an XPath expression. It is encoded from left to right, starting with the top-level data node, according to the "api‑path" rule in Section 5.3.1.1. The node name of each ancestor of the target resource node is encoded in order, ending with the node name for the target resource.

If the "select" is present, it is encoded, starting with a child node of the target resource, according to the "api‑select" rule defined in Section 5.3.1.1.

If a data node in the path expression is a YANG list node, then the key values for the list (if any) are encoded according to the "key‑value" rule. If the list node is the target resource, then the key values MAY be omitted, according to the operation. For example, the POST operation to create a new data resource for a list node does not allow the key values to be present in the request URI.

The key leaf values for a data resource representing a YANG list MUST be encoded as follows:

• The value of each leaf identified in the "key" statement is encoded in order.
• All the components in the "key" statement MUST be encoded. Partial instance identifiers are not supported.
• Each value is encoded using the "key‑value" rule in Section 5.3.1.1, according to the encoding rules for the data type of the key leaf.
• An empty string can be a valid key value (e.g., "/top/list/key1//key3").
• The "/" character MUST be URL-encoded (i.e., "%2F").
• All whitespace MUST be URL-encoded.
• A "null" value is not allowed since the "empty" data type is not allowed for key leafs.
• The XML encoding is defined in [RFC6020].
• The JSON encoding is defined in [I-D.lhotka-netmod-json].
• The entire "key‑value" MUST be properly URL-encoded, according to the rules defined in [RFC3986].

Notifications are not supported by YANG-API because they are not supported by HTTP. YANG notification statements are ignored by a YANG-API server.

Examples:

  /yang-api/datastore/jukebox/library/artist/17&select=name

  /yang-api/datastore/newlist/17&select=nextlist/22/44/myleaf

  /yang-api/datastore/somelist/fred%20and%20wilma

  /yang-api/datastore/somelist/fred%20and%20wilma/address
	    

5.3.1.1. ABNF For Data Resource Identifiers

The following ABNF syntax is used to construct YANG-API path identifiers:

    api-path = "/" api-identifier
                    0*("/" (api-identifier | key-value ))

    [FIXME: the syntax for the select string is still TBD]
    api-select = api-identifier
                    0*("/" (api-identifier | key-value ))

    api-identifier = [module-name ":"] identifier

    module-name = identifier

    key-value = string 

    ;; An identifier MUST NOT start with 
    ;; (('X'|'x') ('M'|'m') ('L'|'l'))
    identifier  = (ALPHA / "_")
                  *(ALPHA / DIGIT / "_" / "-" / ".")

    string = <an unquoted string>
	    

5.3.2. Identifying YANG-defined Data Resources

The data resources used in YANG-API are defined with YANG data definition statements.

Not every data node defined in a YANG module should be treated as a resource. The YANG-API needs to know which YANG data nodes are resources, and which are fields within a resource.

For data resources, YANG-API uses a simple algorithm for defining resource boundaries, within the conceptual sub-trees described by YANG data definition statements.

All top-level data nodes are considered to be resources. For nodes within a top-level resource:

• a presence container starts a new resource
• a list starts a new resource
• an optional terminal node (anyxml, leaf, or leaf-list) starts a new resource
• a data node of type "anyxml" cannot have any sub-resources

A non-configuration data node cannot be a separate resource from its parent. Only top-level data nodes are considered to be resources (which only support retrieval methods).

5.3.3. Identifying Optional Keys

It is sometimes useful to have the server assign the key(s) for a new resource. The "Location" header will indicate the key value(s) that the server selected, so the client does not need to provide all the key leaf values.

It is useful to identify in the YANG data model module which key leafs are optional to provide, and which are not. The YANG extension statement "optional‑key" is provided to indicate that the leaf definition represents an optional key.

The client MAY provide a value for a key leaf in a POST operation. Refer to Section 8 for details on the "optional‑key" extension. Refer to Section 13 for usage examples of this YANG extension statement.

5.3.4. Data Resource Retrieval

There are four types of filtering for retrieval of data resources. This section defines each mode.

5.3.4.1. Conditional Retrieval

The HTTP headers (such as "If‑Modified‑Since" and "If‑Match") can by used in for a request message for a GET operation to check a condition within the server state, such as the last time the datastore resource was modified, or the resource entity tag of the target resource.

If the condition is met according to the header definition, a "200 OK" Status-Line and the data requested is returned in the response message. If the condition is not met, a "304 Not Modified" Status-Line is returned in response message instead.

5.3.4.2. Data Classification Retrieval

The "config" query parameter can be used with the GET operation to specify whether configuration or non-configuration data is requested. Refer to Section 3.8.1 for more details on the "config" query parameter.

5.3.4.3. Subset Retrieval

The "Range" header is used to request a specific subset of the instances of a list or leaf-list data resource that are returned by the server for a retrieval operation. Normally, if the target resource in a request message does not specify an instance, then all instances are returned.

The YANG-API protocol uses the token "entries" instead of "bytes" as the range units.

The entries are numbered starting from "0". A list or leaf-list can change order between requests so the client needs to be aware of the data model semantics, and whether the list contents are stable enough to use the subset retrieval mechanism.

If the requested range cannot be returned because the range specification includes index values for entries that do not exist, then an error occurs, and the server MUST return a "416 Requested range not satisfiable" Status-Line.

If the range request can be satisfied, then a "200 OK" Status-Line is returned, and the response MUST include a "Content‑Range" header indicating which entries are returned. The response message body contains the data for the requested range of entries.

Example:

In this example, the client is requesting 5 "artist" resource entries, starting with the 10th entry:

 Request from client:

   GET /yang-api/datastore/jukebox/library/artist HTTP/1.1
   Host: example.com
   Accept: application/vnd.yang.data+json
   Range: entries 10-14

 Response from server:

   HTTP/1.1 200 OK
   Date: Mon, 23 Apr 2012 13:01:20 GMT
   Cache-Control: no-cache
   Pragma: no-cache
   Content-Type: application/vnd.yang.data+json
   Content-Range: entries 10-14
   Server: example-server
   Last-Modified: Mon, 23 Apr 2012 02:12:20 GMT
   ETag: abcada438af

   {
     "artist" : {
         // content removed for brevity
     }
   }
	    

5.3.4.4. Filtered Retrieval

The "select" query parameter is used to specify a filter that should be applied to the target resource to request a subset of all possible descendant nodes within the target resource.

The format of the "select" parameter string is defined in Section 3.8.6. The set of nodes selected by the filter expression is applied to each context node identified by the target resource.

5.4. Operation Resource

An operation resource represents an protocol operation defined with the YANG "rpc" statement.

All operation resources share the same module namespace as any top-level data resources, so the name of an operation resource cannot conflict with the name of a top-level data resource defined within the same module.

If 2 different YANG modules define the same "rpc" identifier, then the module name MUST be used in the request URI. For example, if "module‑A" and "module‑B" both defined a "reset" operation, then invoking the operation from "module‑A" would be requested as follows:

   POST /yang-api/operations/module-A:reset HTTP/1.1
   Server example.com
	    

Any usage of an operation resource from the same module, with the same name, refers to the same "rpc" statement definition. This behavior can be used to design protocol operations that perform the same general function on different resource types.

If the "rpc" statement has an "input" section, then a message body MAY be sent by the client in the request, otherwise the request message MUST NOT include a message body. If the "rpc" statement has an "output" section, then a message body MAY be sent by the server in the response. Otherwise the server MUST NOT include a message body in the response message, and MUST send a "204 No Content" Status-Line instead.

5.4.1. Encoding Operation Input Parameters

If the "rpc" statement has an "input" section, then the "input" node is provided in the message body, corresponding to the YANG data definition statements within the "input" section.

Example:

The following YANG definition is used for the examples in this section.

    rpc reboot {
      input {
        leaf delay {
          units seconds;
          type uint32;
          default 0;
        }
        leaf message { type string; }
        leaf language { type string; }
      }
    }
	    

The client might send the following POST request message:

   POST /yang-api/datastore/operations/reboot HTTP/1.1
   Host: example.com
   Content-Type: application/vnd.yang.data+json

   {
     "input" : {
       "delay" : 600,
       "message" : "Going down for system maintenance",
       "language" : "en-US"
     }
   }
	    

The server might respond:

   HTTP/1.1 204 No Content
   Date: Mon, 25 Apr 2012 11:01:00 GMT
   Server: example-server
	    

5.4.2. Encoding Operation Output Parameters

If the "rpc" statement has an "output" section, then the "output" node is provided in the message body, corresponding to the YANG data definition statements within the "output" section.

Example:

The following YANG definition is used for the examples in this section.

    rpc get-reboot-info {
      input {
        leaf reboot-time {
          units seconds;
          type uint32;
        }
        leaf message { type string; }
        leaf language { type string; }
      }
    }
	    

The client might send the following POST request message:

   POST /yang-api/datastore/operations/get-reboot-info HTTP/1.1
   Host: example.com
	    

The server might respond:

   HTTP/1.1 200 OK
   Date: Mon, 25 Apr 2012 11:10:30 GMT
   Server: example-server
   Content-Type: application/vnd.yang.data+json

   {
     "output" : {
       "reboot-time" : 30,
       "message" : "Going down for system maintenance",
       "language" : "en-US"
     }
   }
	    

5.4.3. Identifying YANG-defined Operation Resources

The operation resources used in YANG-API are defined with YANG "rpc" statements. All "rpc" statements within a YANG module that are supported by the server are available as operation resources.

6. Error Reporting

HTTP Status-Lines are used to report success or failure for YANG-API operations. The <rpc‑error> element returned in NETCONF error responses contains some useful information. This error information is adapted for use in YANG-API, and error information is returned for "4xx" class of status codes.

The following table summarizes the return status codes used specifically by YANG-API operations:

HTTP Status Codes used in YANG-API

Status-Line	Description
100 Continue	POST accepted, 201 should follow
200 OK	Success with response body
201 Created	POST to create a resource success
202 Accepted	POST to create a resource accepted
204 No Content	Success without response body
304 Not Modified	Conditional operation not done
400 Bad Request	Invalid request message
403 Forbidden	Access to resource denied
404 Not Found	Resource target or resource node not found
405 Method Not Allowed	Method not allowed for target resource
409 Conflict	Resource or lock in use
413 Request Entity Too Large	too-big error
414 Request-URI Too Large	too-big error
415 Unsupported Media Type	non YANG-API media type
416 Requested range not satisfiable	If-Range error
500 Internal Server Error	operation-failed
501 Not Implemented	unknown-operation
503 Service Unavailable	Recoverable server error

Since an operation resource is defined with a YANG "rpc" statement, a mapping between the NETCONF <error‑tag> value and the HTTP status code is needed. The specific error condition and response code to use are data-model specific and might be contained in the YANG "description" statement for the "rpc" statement.

Mapping from error-tag to status code

<error‑tag>	status code
in-use	409
invalid-value	400
too-big	413
missing-attribute	400
bad-attribute	400
unknown-attribute	400
bad-element	400
unknown-element	400
unknown-namespace	400
access-denied	403
lock-denied	409
resource-denied	409
rollback-failed	500
data-exists	409
data-missing	409
operation-not-supported	501
operation-failed	500
partial-operation	500
malformed-message	400

6.1. Error Response Message

When an error occurs for a request message on a data resource or an operation resource, and a "4xx" class of status codes (except for status code "403"), then the server SHOULD send a response body containing the information described by the following YANG data definition statement:

    container errors {
      config false;

      list error {
        reference "RFC 6241, Section 4.3";
        leaf error-type { 
          mandatory true;
          type enumeration {
            enum transport;
            enum rpc;
            enum protocol;
            enum application;
          }
        }
        leaf error-tag { 
          mandatory true;
          type string;
        }
        leaf error-app-tag { 
          type string;
        }
        leaf error-path { 
          type string;  // YANG-API encoded instance-identifier
        }
        leaf error-message { 
          type string;
        }
        container error-info { 
           // anyxml content here
        }
      }
    }
	    

Example:

The following example shows an error returned for an "lock‑denied" error on a datastore resource.

   POST /yang-api/operations/lock-datastore HTTP/1.1
   Host: example.com
	    

The server might respond:

   HTTP/1.1 409 Conflict
   Date: Mon, 23 Apr 2012 17:11:00 GMT
   Server: example-server
   Content-Type: application/vnd.yang.api+json

   {
     "errors": {
       "error": {
         "error-type": "protocol",
         "error-tag": "lock-denied",
         "error-message": "Lock failed, lock is already held",
       }
     }
   }
	    

7. RelaxNG Grammar

TBD

8. YANG-API module

RFC Ed.: update the date below with the date of RFC publication and remove this note.

<CODE BEGINS> file "ietf-yang-api@2012-11-30.yang"

module ietf-yang-api {
  namespace "urn:ietf:params:xml:ns:yang:ietf-yang-api";
  prefix "api";

  organization
    "IETF NETCONF (Network Configuration) Working Group";

  contact
    "Editor:   Andy Bierman
               <mailto:andy@yumaworks.com>

     Editor:   Martin Bjorklund
               <mailto:mbj@tail-f.com>";
  
  description
    "This module contains a collection of YANG language extensions
     to describe REST API Resources using YANG data definition 
     statements.

     Copyright (c) 2012 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject
     to the license terms contained in, the Simplified BSD License
     set forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents
     (http://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC XXXX; see
     the RFC itself for full legal notices.";

  // RFC Ed.: replace XXXX with actual RFC number and remove this
  // note.

  // RFC Ed.: remove this note
  // Note: extracted from draft-bierman-netconf-yang-api-01.txt

  // RFC Ed.: update the date below with the date of RFC publication
  // and remove this note.
  revision 2012-11-30 {
    description
      "Initial revision.";
    reference
      "RFC XXXX: YANG-API Protocol.";
  }
  
  /*
   * Extensions
   */ 

  extension optional-key {
    description 
      "This extension is used to allow the client to create
       a new instance of a resource without providing a
       value for the key leaf containing this statement.
       This extension is ignored for NETCONF, and only
       applies to YANG-API resources and fields.
       This extension is ignored unless it appears
       directly within a 'leaf' data definition statement.";
  }

}

	    

<CODE ENDS>

9. IANA Considerations

This document registers one URI in the IETF XML registry [RFC3688]. Following the format in RFC 3688, the following registration is requested to be made.

     URI: urn:ietf:params:xml:ns:yang:ietf-yang-api
     Registrant Contact: The NETMOD WG of the IETF.
     XML: N/A, the requested URI is an XML namespace.
	    

This document registers one YANG module in the YANG Module Names registry [RFC6020].

  name:         ietf-yang-api
  namespace:    urn:ietf:params:xml:ns:yang:ietf-yang-api
  prefix:       api
  reference:    RFC XXXX
	    

10. Security Considerations

TBD

11. Change Log

    -- RFC Ed.: remove this section before publication.  
	    

11.1. 00-01

• expanded introduction
• removed transactions
• removed capabilities
• simplified editing model
• removed global protocol operations from ietf-yang-api.yang
• changed RPC operation terminology to protocol operation
• updated JSON draft reference
• updated open issues section
• updated IANA section

12. Open Issues

• Which WG should do this work? NETCONF? NETMOD? It is not clear since YANG-API builds on concepts and standards from documents owned by both working groups.
• Resource creation order and other dependencies between resources are not well identified in YANG. YANG has leafrefs and instance-identifiers, which can be used to identify some order dependencies. Are any new mechanisms needed in YANG-API needed to identify resource creation order and other dependency requirements?
• There is no "message‑id" field in a YANG-API message. Is a message identifier needed? If so, should either the "Message‑ID" or "Content‑ID" header from RFC 2392 be used for this purpose?
• Should sessions be used or not? Should "reusable sessions" be used? Better for auditing? How does locking of the /yang-api/datastore resource work for multiple edits if a session is 1 operation? When does the server release the lock and decide it has been abandoned or client was disconnected?
• What syntax should be used for the "select" query parameter?
• Should the "/yang‑api/modules" field within the API resource be a separate resource, with its own timestamp? Currently the API timestamp is coupled to any changes to the list of loaded modules. Should the API resource be static and cacheable?
• What to do about no REMOVE operation, just DELETE? The effect is local to the request; in a NETCONF edit-config it is worse, since the netconf request might create/delete/modify many nodes
• Should every YANG data node be a data resource and every YANG RPC statement an operation resource? Is a YANG extension needed to allow data modeler control of resource boundaries?
• Encoding of leafrefs? Is there some additional meta-data needed? Do leafref nodes need to be identified in responses (RFC 5988) or is the YANG module definition sufficient to provide this meta-data?
• What should the default algorithm be for defining data resources? Should the default for an augment from another namespace be to start a new resource? Top-level data node defaults as a resource OK?
• Is the token "entries" legal in the YANG-API usage of Range? What units should be used? "bytes" is the only token defined by HTTP.
• Are all header lines used by YANG-API supported by common application frameworks, such as FastCGI and WSGI? If not, then should query parameters be used instead, since the QUERY_STRING is widely available to WEB applications?
• Should the <errors> element returned in error responses be a separate media type?
• How should additional datastores be supported, which may be added to the NETCONF/NETMOD framework in the future?

13. Example YANG Module

module example-jukebox {

   namespace "http://example.com/ns/example-jukebox";
   prefix "jbox";

   import ietf-yang-api { prefix api; }

   organization "Example, Inc.";
   description "Example Jukebox Data Model Module";
   revision "2012-05-30";


   identity genre {
     description "Base for all genre types";
   }

   // abbreviated list of genre classifications
   identity Alternative {
     base genre;
   }
   identity Blues {
     base genre;
   }
   identity Country {
     base genre;
   }
   identity Jazz {
     base genre;
   }
   identity Pop {
     base genre;
   }
   identity Rock {
     base genre;
   }

   container jukebox {
     presence 
       "An empty container indicates that the jukebox
        service is available";

     container library {
       list artist {
         key index;
         unique name;

         leaf index {
           api:optional-key;
           type uint32;
           description
             "Optional key used instead of natural key for
              example. Also rare but possible artists with
              the same name are really different entities.";
         }
         leaf name {
           type string;
         }

         list album {
           key name;
           leaf name {
             type string {
               length "1 .. max";
             }
           }
           leaf genre {
             type identityref { base genre; }
           }
           leaf year {
             type uint16 {
               range "1900 .. max";
             }
           }
           list song {
             api:optional-key;
             key index;
             ordered-by user;
             leaf index {
               type uint32;
             }
             leaf name {
               mandatory true;
               type string;
             }
             leaf location {
               mandatory true;
               type string;
             }
             leaf format {
               type string;
             }
             leaf length {
               units "seconds";
               type uint32;
             }
           }
         }
       }
       leaf artist-count {
          config false;
          type uint32;
          units "songs";
          description "Number of artists in the library";
       }
       leaf album-count {
          config false;
          type uint32;
          units "albums";
          description "Number of albums in the library";
       }
       leaf song-count {
          type uint32;
          units "songs";
          description "Number of songs in the library";
       }
     }
  
     list playlist {
       description 
         "Example configuration data resource";
       key name;
       leaf name {
         type string;
       }
       leaf description {
         type string;
       }
       list song {
         description 
           "Example nested configuration data resource";
         ordered-by user;
         key index;
         leaf index {
           api:optional-key;
           type uint32;
         }
         leaf id {
           mandatory true;
           type instance-identifier;
           description 
             "Song identifier. Must identify an instance of
              /jukebox/library/artist/album/song.

              The id is not the key to allow duplicates
              in a playlist";
         }
       }
     }
  
     container player {
       leaf gap {
         description "Time gap between each song";
         units "tenths of seconds";
         type decimal64 {
           fraction-digits 1;
           range "0.0 .. 2.0";
         }
       }
     }
   }

   rpc play {
     description "Control function for the jukebox player";
     input {
       leaf playlist {           
         type string;
         mandatory true;
         description "playlist name";
       }
       leaf song-number {
         type uint32;
         mandatory true;
         description "Song number in playlist to play";
       }
     }
   }         
}

	    

14. Normative References

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616]	Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3688]	Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, January 2004.
[RFC3986]	Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.
[RFC5789]	Dusseault, L. and J. Snell, "PATCH Method for HTTP", RFC 5789, March 2010.
[RFC6241]	Enns, R., Bjorklund, M., Schoenwaelder, J. and A. Bierman, "Network Configuration Protocol (NETCONF)", RFC 6241, June 2011.
[RFC6020]	Bjorklund, M., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, October 2010.
[RFC6021]	Schoenwaelder, J., "Common YANG Data Types", RFC 6021, October 2010.
[RFC6536]	Bierman, A. and M. Bjorklund, "Network Configuration Protocol (NETCONF) Access Control Model", RFC 6536, March 2012.
[I-D.lhotka-netmod-json]	Lhotka, L., "Modeling JSON Text with YANG", Internet-Draft draft-lhotka-netmod-yang-json-00, October 2012.

Authors' Addresses