JSON Merge Patch
draft-ietf-appsawg-json-merge-patch-02

Abstract

This specification defines the JSON merge patch format and processing rules.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 22, 2014.

Copyright Notice

Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 2. Processing Merge Patch Documents
• 3. Example
• 4. IANA Considerations
• 5. Security Considerations
• 6. Normative References
• Appendix A. Example Test Cases
• Appendix B. Example JavaScript Implementation
• Author's Address

1. Introduction

This specification defines the JSON merge patch document format, processing rules, and associated MIME media type identifier. The merge patch format is primarily intended for use with the HTTP PATCH method [RFC5789] as a means of describing a set of modifications to a subset of target resource's content.

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.

For example, given the following original JSON document:

  {
    "a": "b",
    "c": {
      "d": "e",
      "f": "g"
    }
  }

Changing the value of "a" and removing "f" can be achieved by sending:

  PATCH /target HTTP/1.1
  Host: example.org
  Content-Type: application/merge-patch+json
  
  {
    "a":"z",
    "c": {
      "f": null
    }
  }

When applied to the target resource, the value of the "a" member is replaced with "z" and "f" is removed, leaving the remaining content untouched.

This design means that merge patch documents are suitable for describing modifications to JSON documents that primarily use objects for their structure and do not make use of explicit null values. The merge patch format is not appropriate for all JSON syntaxes.

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119].

2. Processing Merge Patch Documents

JSON merge patch documents describe, by example, a set of changes that are to be made to a target resource. Recipients of merge patch documents are responsible for comparing the merge patch with the current content of the target resource to determine the specific set of change operations to be applied to the target.

The following rules MUST be applied to determine what changes are to be made:

1. If the roots of either the merge patch or target resource documents are JSON Arrays, the target resource is replaced, in whole, by the merge patch document. Any null member contained in the merge patch MUST be ignored and treated as if those members are undefined.
2. If, however, the roots of the merge patch or target resource documents are Objects, iterate through each member of merge patch object and determine the following:
   • If an equivalent member in the target resource is currently undefined, and the given value in the merge patch is not null, the member is added to the target.
   • If the value given by the merge patch is null and the equivalent member is defined in the target resource, the existing member is removed.
   • If the value given by the merge patch is either a non-null JSON primitive or Array, and the equivalent member is defined in the target, the existing value is replaced with the given value. Any null member contained in a provided array MUST be ignored and treated as if the member was undefined, even within array or object members within the array.
   • If the value given by the merge patch is a JSON object, and the equivalent member is defined in the target resource with a JSON primitive or Array value, that existing value is replaced in whole by the object provided. Any null member in arrays or objects within the provided data MUST be ignored and treated as if the member was undefined.
   • If the value given by the merge patch is a JSON object, and the equivalent member is currently defined in the target resource with an Object value, then recursively apply Rule #2 to the two objects.
   • Any member currently defined in the target resource that does not explicitly appear within the merge patch is to remain untouched and unmodified.

Once the set of intended modifications is derived from the merge patch document, the recipient is free to determine the appropriateness of the modification based on it's own understanding of the target resource. If the recipient is unable to apply any individual modification described by the merge patch document, it MUST NOT apply any of the changes and MUST stop processing the modification.

If the merge patch document attempts to remove a member from the target resource that does not currently exist, the recipient SHOULD NOT consider the request to be in error. The requested removal SHOULD be ignored as the final modified state of the target will still accurately reflect the merge documents original intent.

3. Example

For example, given the following example JSON document:

  {
    "title": "Goodbye!",
    "author" : {
      "givenName" : "John",
      "familyName" : "Doe"
    },
    "tags":[ "example", "sample" ],
    "content": "This will be unchanged"
  }
      

A user-agent wishing to change the value of the "title" member from "Goodbye!" to the value "Hello!", add a new "phoneNumber" member, remove the "familyName" from the "author" object, and remove the word sample from the "tags" Array, would send the following request:

  PATCH /my/resource HTTP/1.1
  Host: example.org
  Content-Type: application/merge-patch+json; charset="UTF-8"
  
  {
    "title": "Hello!",
    "phoneNumber": "+01-123-456-7890",
    "author": {
      "familyName": null
    },
    "tags": [ "example" ]
  }

The resulting JSON document would be:

  {
    "title": "Hello!",
    "author" : {
      "givenName" : "John"
    },
    "tags": [ "example" ],
    "content": "This will be unchanged",
    "phoneNumber": "+01-123-456-7890"
  }

4. IANA Considerations

This specification registers the following additional MIME Media Types:

• Type name: application
• Subtype name: merge-patch+json
• Required parameters: None
• Optional parameters: "charset" : Specifies the character set encoding. If not specified, a default of "UTF-8" is assumed.
• Encoding considerations: Resources that use the "application/merge-patch+json" media type are required to conform to the "application/json" Media Type and are therefore subject to the same encoding considerations specified in Section 6 [RFC4627].
• Security considerations: As defined in this specification
• Published specification: This specification.
• Applications that use this media type: None currently known.
• Additional information:
  • Magic number(s): N/A
  • File extension(s): N/A
  • Macintosh file type code(s): TEXT
• Person & email address to contact for further information: James M Snell <jasnell@gmail.com>
• Intended usage: COMMON
• Restrictions on usage: None.
• Author: James M Snell <jasnell@gmail.com>
• Change controller: IESG

5. Security Considerations

The "application/merge-patch+json" Media Type allows user agents to indicate their intention that the server determine the specific set of change operations to be applied to a target resource. As such, it is the server's responsibility to determine the appropriateness of any given change as well as the user agent's authorization to request such changes. How such determinations are made is considered out of the scope of this specification.

All of the the security considerations discussed in Section 5 [RFC5789] apply to all uses of the HTTP PATCH method with the "application/merge-patch+json" Media Type.

6. Normative References

Appendix A. Example Test Cases

  ORIGINAL          PATCH          RESULT
  ------------------------------------------
  {"a":"b"}       {"a":"c"}       {"a":"c"}
    
  {"a":"b"}       {"b":"c"}       {"a":"b",
                                   "b":"c"}
  
  {"a":"b"}       {"a":null}      {}
  
  {"a":"b",       {"a":null}      {"b":"c"}
   "b":"c"}
   
  {"a":["b"]}     {"a":"c"}       {"a":"c"}
  
  {"a":"c"}       {"a":["b"]}     {"a":["b"]}
  
  {"a": {         {"a": {         {"a": {
    "b": "c"}       "b": "d",       "b": "d"
  }                 "c": null}      }
                  }               }
                  
  {"a": [         {"a": [1]}      {"a": [1]}
    {"b":"c"}
   ]
  }
  
  ["a","b"]       ["c","d"]       ["c","d"]
  
  {"a":"b"}       ["c"]           ["c"]
  
  [1,2]           {"a":"b",       {"a":"b"}
                   "c":null}

  {"e":null}      {"a":1}         {"e":null,
                                   "a":1}

  {}              {"a": {         {"a": {
                    "bb": {         "bb": {
                     "ccc":null   }}}
                  }}}
              
  {"a":"foo"}     {"b": [         {"a":"foo",
                    3,             "b": [3, {}]
                    null,         }
                    {"x":null}
                   ]
                  }
                  
  [1,2]           [1,null,3]      [1,3]
  
  [1,2]           [1,null,2]      [1,2]
  
  {"a":"b"}       {"a": [         {"a": [
                    {"z":1,         {"z":1}
                     "b":null      ]
                    }             }
                   ]
                  }

  {"a":"foo"}     null            Invalid Patch

  {"a":"foo"}     "bar"           Invalid Patch  

Appendix B. Example JavaScript Implementation

The following example implementation is provided as is, without warranty. It is provided in the public domain. Note that this example is provided strictly for illustrative purposes and has not been optimized for performance or reliability in any way.

// Apply the patch to the original, return the 
// modified object... this will mutate the 
// passed in object in place as well...
function apply(orig, patch) {
  if (patch == null) 
    return orig;
  else if (patch instanceof Array)
    orig = purge_nulls(patch);
  else if (is_primitive(patch))
    orig = patch;
  else if (patch instanceof Object) {
    if (orig instanceof Array) {
      orig = purge_nulls(patch);
    } else {
      for (m in patch) {
        if (orig.hasOwnProperty(m)) {
          if (patch[m] == null)
            delete orig[m];
          else {
            if (is_primitive(patch[m])) 
              orig[m] = patch[m];
            else {
              if (orig[m] instanceof Array)
                orig[m] = purge_nulls(patch[m]);
              else 
                orig[m] = apply(orig[m],patch[m]);
            }
          }
        } else if (patch[m] != null)
          orig[m] = purge_nulls(patch[m]);
      }
    }
  }
  return orig;
}

function is_primitive(val) {
  var m = typeof val;
  return m == 'string'  || 
         m == 'number'  ||
         m == 'boolean';
}

function purge_nulls(obj) {
  var ret = obj;
  if (!is_primitive(obj)) {
    if (obj instanceof Array) {
      var ret = [];
      for (m in obj)
        if (obj[m] != null) 
          ret.push(purge_nulls(obj[m]));
    } else if (obj instanceof Object) {
      var ret = {};
      for (m in obj) {
        if (obj[m])
          ret[m] = purge_nulls(obj[m]);
      }
    }
  }
  return ret;    
}

// Define the original object...
var orig = {
  "a": "b",
  "c": {
    "d": [1,2,3],
    "e": {
      "f": 1
    }
  }
}
  
// Define the patch...
var patch = {
  "c": {
    "d": [1,2],
    "e": {
      "f": null
    }
  }
}
  
// Apply the patch...
var modified = apply(orig,patch);

Author's Address