HDFS-13190. Document WebHDFS support for snapshot diff

Signed-off-by: Akira Ajisaka <aajisaka@apache.org>
Signed-off-by: Xiaoyu Yao <xyao@apache.org>

hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/WebHDFS.md

@@ -50,6 +50,7 @@ The HTTP REST API supports the complete [FileSystem](../../api/org/apache/hadoop
     * [`CHECKACCESS`](#Check_access) (see [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).access)
     * [`GETALLSTORAGEPOLICY`](#Get_all_Storage_Policies) (see [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).getAllStoragePolicies)
     * [`GETSTORAGEPOLICY`](#Get_Storage_Policy) (see [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).getStoragePolicy)
+    * [`GETSNAPSHOTDIFF`](#Get_Snapshot_Diff)
 *   HTTP PUT
     * [`CREATE`](#Create_and_Write_to_a_File) (see [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).create)
     * [`MKDIRS`](#Make_a_Directory) (see [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).mkdirs)
@@ -1266,6 +1267,21 @@ See also: [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).deleteSna
 
 See also: [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).renameSnapshot
 
+### Get Snapshot Diff
+
+* Submit a HTTP GET request.
+
+        curl -i GET "http://<HOST>:<PORT>/webhdfs/v1/<PATH>?op=GETSNAPSHOTDIFF
+                           &oldsnapshotname=<SNAPSHOTNAME>&snapshotname=<SNAPSHOTNAME>"
+
+    The client receives a response with a [`SnapshotDiffReport` JSON object](#SnapshotDiffReport_JSON_Schema):
+
+        HTTP/1.1 200 OK
+        Content-Type: application/json
+        Transfer-Encoding: chunked
+
+        {"SnapshotDiffReport":{"diffList":[],"fromSnapshot":"s3","snapshotRoot":"/foo","toSnapshot":"s4"}}
+
 Delegation Token Operations
 ---------------------------
 
@@ -2043,6 +2059,82 @@ A `BlockStoragePolicies` JSON object represents an array of `BlockStoragePolicy`
 }
 ```
 
+### SnapshotDiffReport JSON Schema
+
+```json
+{
+  "name": "SnapshotDiffReport",
+  "type": "object",
+  "properties":
+  {
+    "SnapshotDiffReport":
+    {
+      "type"        : "object",
+      "properties"  :
+      {
+        "diffList":
+        {
+          "description": "An array of DiffReportEntry",
+          "type"        : "array",
+          "items"       : diffReportEntries,
+          "required"    : true
+        },
+        "fromSnapshot":
+        {
+          "description": "Source snapshot",
+          "type"        : "string",
+          "required"    : true
+        },
+        "snapshotRoot":
+        {
+          "description" : "String representation of snapshot root path",
+          "type"        : "string",
+          "required"    : true
+        },
+        "toSnapshot":
+        {
+          "description" : "Destination snapshot",
+          "type"        : "string",
+          "required"    : true
+        }
+      }
+    }
+  }
+}
+```
+
+#### DiffReport Entries
+
+JavaScript syntax is used to define `diffReportEntries` so that it can be referred in `SnapshotDiffReport` JSON schema.
+
+```javascript
+var diffReportEntries =
+{
+  "type": "object",
+  "properties":
+  {
+    "sourcePath":
+    {
+      "description" : "Source path name relative to snapshot root",
+      "type"        : "string",
+      "required"    : true
+    },
+    "targetPath":
+    {
+      "description" : "Target path relative to snapshot root used for renames",
+      "type"        : "string",
+      "required"    : true
+    },
+    "type":
+    {
+      "description" : "Type of diff report entry",
+      "enum"        : ["CREATE", "MODIFY", "DELETE", "RENAME"],
+      "required"    : true
+    }
+  }
+}
+```
+
 HTTP Query Parameter Dictionary
 -------------------------------