JSON Diff and Patch

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Introduction to JSON Diff and Patch (JSON-DP)

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Key topics

Use JSON-DP with a TerminusDB client

JSON-DP operations

JSON-DP examples using curl

The uses of JSON-DP

JSON objects are a common way of representing data for software development. The serialization of JSON is simple and facilitates communication via networks and storage in databases. Almost all modern programming languages support JSON objects natively.

When objects are modified in distributed systems, it is useful to compare versions of an object to see what has changed. This is where diff and patch come in.

Diff

A diff takes two JSON objects and presents any differences between them. Diff has several uses. A key use is displaying a clear summary of differences between large objects, enhancing the visibility of changes. This enables manual or user-interface assisted action to resolve changes. Actions include retaining the original object, changing to the new (or latest) version of the object, or creating an entirely new version of the object.

Patch

A patch applies a diff to two objects to obtain a new object with any differences highlighted. A patch is applied individually or in bulk to a patch endpoint that will apply the patch to the specified data product.

Use JSON-DP with a TerminusDB client

Use JSON-DP with a TerminusDB JavaScript or Python client to find and handle changes in TerminusDB schemas and documents, JSON schemas, and other document databases such as MongoDB. See JSON-DP client tutorials for use cases, including connectivity with MongoDB.

Prerequisites

Install TerminusDB JavaScript client

Install TerminusDB Python client

Get started

Get started with the simple steps below.

If using TerminusX with Python, connect to your TerminusX cloud instance first - see Connect with WOQLClient for instructions if required.

1. Create an endpoint

2. Apply a diff to obtain a patch

3. Review the patch

4. Apply the patch

Create an endpoint

Create a client endpoint with WOQLClient.

JavaScript

const TerminusClient = require("@terminusdb/terminusdb-client");

var client = new TerminusClient.WOQLClient("http://127.0.0.1:6363")

Python

from terminusdb_client import WOQLClient

client = WOQLClient("http://localhost:6363/")

Apply a diff to obtain a patch

Get the difference/s between two hypothetical documents - Doc1 and Doc2.

JavaScript Use getDiff

let result_patch = await client.getDiff(Doc1, Doc2)

Python Usediff

result_patch = client.diff(Doc1, Doc2)

Review the patch

Print the contents of a patch.

JavaScript

console.log(result_patch)

Python Example usespprint (from pprint import pprint)

pprint(result_patch.content)

Apply the patch

Apply the patch to Doc1.

JavaScript

let after_patch = await client.patch(Doc1, result_patch);

Python

after_patch = client.patch(Doc1, result_patch)

JSON-DP operations

The available JSON-DP operations with some examples using curl.

Copy Diff

Copy is implicit. All properties which are not specifically mentioned will be considered part of an implicit copy. This will make patches more compressed and easier to specify by hand.

Mandatory Diff

@before/@after instructions contain objects specified as tightly as required to obtain ids, or as ids.

{ '@id' : "Person/jim",
  'date_of_birth' : { '@op' : 'SwapValue',
                      '@before' : "1928-03-05",
                      '@after' : "1938-03-05"
                    }}

Optional Diff

Optional diffs also contain @before/@after designations, but potentially null fields to describe missing elements.

{ '@id' : "Object/my_object",
  'name' : { '@op' : 'SwapValue',
             '@before' : null,
             '@after' : "Jim" }}

Set Diff / Cardinality Diff

Set requires the ability to explicitly remove or add elements - we can do this by maintaining a @before/@after with a list of those which exist only on the left, and only on the right.

List Diff

The list diff requires swaps at a position. We use, @copy, @swap and @keep.

Copy List

Copy the previous list from From_Position to To_Position.

{ "@op" : "CopyList",
  "@to" : To_Position,
  "@rest" : Diff }

Swap List

Swap out the list starting from the current point from Previous to Next. This can be used to extend, or drop elements as well as do full replacement.

{ "@op" : "SwapList",
  "@before" : Previous,
  "@after" : Next,
  "@rest" : Diff }

Patch List

Patch the list starting from the current point with the patch list in "@patch". The patch must be less than or equal to the length of the list.

{ "@op" : "PatchList",
  "@patch" : Patch,
  "@rest" : Diff }

Example:

var Patch =
{ '@id' : "TaskList/my_tasks",
  'tasks' : { '@op' : "CopyList",                      % Replace List
              '@to' : 2,
              '@rest' : { '@op' : "PatchList",
                          '@patch' : [{ '@op' : "SwapValue",
                                        '@before' : "Task/shopping",
                                        '@after' : "Task/climbing"},
                                      { '@op' : "SwapValue",
                                        '@before' : "Task/cleaning",
                                        '@after' : "Task/dining"},
                                      { '@op' : "SwapValue",
                                        '@before' : "Task/fishing",
                                        '@after' : "Task/travelling"}],
                          '@rest' : { '@op' : "KeepList" } } }}
var Before =
{ '@id' : "TaskList/my_tasks",
  'tasks' : ["Task/driving", "Task/reading", "Task/shopping",
             "Task/cleaning","Task/fishing", "Task/arguing"] }
var After =
{ '@id' : "TaskList/my_tasks",
  'tasks' : ["Task/driving", "Task/reading", "Task/climbing",
             "Task/dining", "Task/travelling", "Task/arguing"] }

Array Diff

Arrays will allow index swapping or "shrink" and "grow".

Force Diff

A "Force Diff" will set the value of a location regardless of the current read-state. This is a potentially unsafe operation as there is no guarantee we are seeing the object state version we think we are.

{ '@id' : "Employee/012" ,
  'name' : { '@op' : 'ForceValue',
             '@after' : "Jake" }}

Table Diff

A Table diff specifies the differences and similarities between the two tables. These tables need not have the same dimensions. In order to describe these differences, we use a ModifyTable patch. The ModifyTable patch is comprised of copies, deletes, inserts and moves.

copies give the sections of the table which can be copied verbatim. deletes gives all segments which are to be removed from the original. inserts gives all segments which are to be inserted into the new table.

moves specifies segments that are the same in both tables, but have moved location. This is particularly useful as moving rows and columns is a typical operation in a table (such as a CSV or Excel document).

Example Table

Given the following table:

[['Job Title','Company','Location','Company Size','Company Industry'],
 ['Sr. Mgt.','Boeing','USA','Large','Aerospace'],
 ['Data Architect','Airbus','France','Large','Aerospace'],
 ['Founder','Ellie Tech','Sweden','Startup','AI'],
 ['Platform Engineer','Adidas','Germany','Large','Apparel']]

And a sorted version of the same (sorting on the first column):

[['Job Title','Company','Location','Company Size','Company Industry'],
 ['Data Architect','Airbus','France','Large','Aerospace'],
 ['Founder','Ellie Tech','Sweden','Startup','AI'],
 ['Platform Engineer','Adidas','Germany','Large','Apparel'],
 ['Sr. Mgt.','Boeing','USA','Large','Aerospace']]

We have the following patch resulting from the diff:

{'@op':"ModifyTable",
 dimensions:{'@after':[5,5],'@before':[5,5]},
 deletes:[],
 inserts:[],
 copies:[{'@at':{'@height':1,'@width':5,'@x':0,'@y':0},'@value':[['Job Title','Company','Location','Company Size','Company Industry']]}],
 moves:[{'@from':{'@height':1,'@width':5,'@x':0,'@y':1},
         '@to':{'@height':1,'@width':5,'@x':0,'@y':4},
         '@value':[['Sr. Mgt.','Boeing','USA','Large','Aerospace']]},
        {'@from':{'@height':1,'@width':5,'@x':0,'@y':2},
         '@to':{'@height':1,'@width':5,'@x':0,'@y':1},
         '@value':[['Data Architect','Airbus','France','Large','Aerospace']]},
        {'@from':{'@height':1,'@width':5,'@x':0,'@y':3},
         '@to':{'@height':1,'@width':5,'@x':0,'@y':2},
         '@value':[['Founder','Ellie Tech','Sweden','Startup','AI']]},
        {'@from':{'@height':1,'@width':5,'@x':0,'@y':4},
         '@to':{'@height':1,'@width':5,'@x':0,'@y':3},
         '@value':[['Platform Engineer','Adidas','Germany','Large','Apparel']]}]}

Diff and Patch Endpoints

The Patch and Diff endpoints expose endpoints for clients to obtain diffs or patches of data.

Diff

The diff endpoint takes a POST of two JSON documents, before, and after. This endpoint then returns a 200 and a patch which takes before to after if applied using the patch interface. The payload is structured as a JSON document, with two keys, "before" and "after", pointing to the documents you would like to diff.

An example of the payload:

{ "before" : { "@id" : "Person/Jane", "@type" : "Person", "name" : "Jane"},
  "after" :  { "@id" : "Person/Jane", "@type" : "Person", "name" : "Janine"}}

Which would result in the following patch:

{ "name" : { "@op" : "SwapValue", "@before" : "Jane", "@after": "Janine" }}

Diff examples using curl

$ curl -X POST -H "Content-Type: application/json" 'http://127.0.0.1:6363/api/diff' -d \
  '{ "before" : [{ "asdf" : "foo"}], "after" : [{ "asdf" : "bar"}]}'
# Output: [ {"asdf": {"@after":"bar", "@before":"foo", "@op":"SwapValue"}} ]

$ curl -X POST -H "Content-Type: application/json" 'http://127.0.0.1:6363/api/diff' -d \
  '{ "before" : [0,1,2], "after" : [0,1,2,3]}'

# Output:
{
  "@op":"CopyList",
  "@rest": {
    "@after": [3 ],
    "@before": [],
    "@op":"SwapList",
    "@rest": {"@op":"KeepList"}
  },
  "@to":3
}

$ curl -X POST -H "Content-Type: application/json" 'http://127.0.0.1:6363/api/diff' -d \
  '{ "before" : { "asdf" : { "fdsa" : "quux"}}, "after" : { "asdf" : { "fdsa" : "quuz" }}}'

# Output:
{
  "asdf": {"fdsa": {"@after":"quuz", "@before":"quux", "@op":"SwapValue"}}
}

Patch

Patch takes a POST with a before document and a patch and produces an after document.

{ "before" : { "@id" : "Person/Jane", "@type" : "Person", "name" : "Jane"}
  "patch" : {"name" : { "@op" : "ValueSwap", "@before" : "Jane", "@after": "Janine" }}}

Resulting in the following document:

{ "@id" : "Person/Jane", "@type" : "bahPerson", "name" : "Janine"}

Patch examples using curl

$ curl -X POST -H "Content-Type: application/json" 'http://127.0.0.1:6363/api/patch' -d \
   '{ "before" : { "alpha" : 1, "asdf" : { "fdsa" : "quux"}}, "patch" : {
      "asdf": {"fdsa": {"@after":"quuz", "@before":"quux", "@op":"SwapValue"}}
}}'
# Output: {"alpha":1, "asdf": {"fdsa":"quuz"}}

$ curl -X POST -H "Content-Type: application/json" 'http://127.0.0.1:6363/api/patch' -d '
{ "before" : [0,1,2], "patch" : {
  "@op":"CopyList",
  "@rest": {
    "@after": [3 ],
    "@before": [],
    "@op":"SwapList",
    "@rest": {"@op":"KeepList"}
  },
  "@to":3
}}'
#Output: [0, 1, 2, 3 ]

See Also

JSON-DP client tutorials

Tutorials for using JSON-DP with a TerminusDB JavaScript or Python client, including the use of JSON-DiffPatch with MongoDB.

JSON-DP JavaScript client reference guide

JSON-DP functions in the JavaScript client reference guide.

JSON-DP Python client reference guide

JSON-DP functions in the Python client reference guide.