AWS documentdb documentation change

Service: documentdb · 2026-03-31 · Documentation low

File: documentdb/latest/developerguide/reduce.md

Summary

Added comprehensive documentation for the $reduce aggregation operator in Amazon DocumentDB, including syntax, parameters, MongoDB shell example, and code examples in Node.js and Python

Security assessment

This change adds standard documentation for a database aggregation operator ($reduce) with code examples. The examples include TLS connection parameters (tls=true, tlsCAFile) which are standard security practices for database connections, but this is not new security documentation - it's just standard secure connection setup that was likely already documented elsewhere. There is no evidence this addresses a specific security vulnerability or weakness.

Diff

diff --git a/documentdb/latest/developerguide/reduce.md b/documentdb/latest/developerguide/reduce.md
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+[](/pdfs/documentdb/latest/developerguide/developerguide.pdf#reduce "Open PDF")
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+[Documentation](/index.html)[Amazon DocumentDB](/documentdb/index.html)[Developer Guide](what-is.html)
+
+Example (MongoDB Shell)Code examples
+
+# $reduce
+
+The `$reduce` aggregation operator in Amazon DocumentDB is used to apply a function of two arguments cumulatively to the elements of an array to reduce the array to a single value. This operator is particularly useful for performing complex calculations or transformations on array data within the aggregation pipeline.
+
+**Parameters**
+
+  * `input`: The array to be reduced.
+
+  * `initialValue`: The initial value to be used in the reduction operation.
+
+  * `in`: The expression to be evaluated on each element of the `input` array. This expression should return a value that will be used in the next iteration of the reduction.
+
+
+
+
+## Example (MongoDB Shell)
+
+The following example demonstrates how to use the `$reduce` operator to calculate the sum of all elements in an array.
+
+**Create sample documents**
+    
+    
+    db.orders.insertMany([
+      { "_id": 1, "items": [1, 2, 3, 4, 5] },
+      { "_id": 2, "items": [10, 20, 30] },
+      { "_id": 3, "items": [5, 15, 25, 35] },
+      { "_id": 4, "items": [100, 200] }
+    ])
+
+**Query example**
+    
+    
+    db.orders.aggregate([
+      {
+        $project: {
+          total: {
+            $reduce: {
+              input: "$items",
+              initialValue: 0,
+              in: { $add: ["$$value", "$$this"] }
+            }
+          }
+        }
+      }
+    ])
+
+**Output**
+    
+    
+    [
+      { "_id": 1, "total": 15 },
+      { "_id": 2, "total": 60 },
+      { "_id": 3, "total": 80 },
+      { "_id": 4, "total": 300 }
+    ]
+
+The `$reduce` operator iterates over the `items` array, adding each element to the `initialValue` of 0. The result is the sum of all elements in the array.
+
+## Code examples
+
+To view a code example for using the `$reduce` command, choose the tab for the language that you want to use:
+
+Node.js
+    
+
+Here's an example of using the `$reduce` operator in a Node.js application:
+    
+    
+    const { MongoClient } = require("mongodb");
+    
+    async function main() {
+      const client = await MongoClient.connect("mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false");
+      const db = client.db("test");
+      const orders = db.collection("orders");
+    
+      const result = await orders.aggregate([
+        {
+          $project: {
+            total: {
+              $reduce: {
+                input: "$items",
+                initialValue: 0,
+                in: { $add: ["$$value", "$$this"] }
+              }
+            }
+          }
+        }
+      ]).toArray();
+    
+      console.log(result);
+      await client.close();
+    }
+    
+    main();
+
+Python
+    
+
+Here's an example of using the `$reduce` operator in a python application:
+    
+    
+    from pymongo import MongoClient
+    
+    def main():
+        client = MongoClient("mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false")
+        db = client["test"]
+        orders = db["orders"]
+    
+        result = list(orders.aggregate([
+            {
+                "$project": {
+                    "total": {
+                        "$reduce": {
+                            "input": "$items",
+                            "initialValue": 0,
+                            "in": { "$add": ["$$value", "$$this"] }
+                        }
+                    }
+                }
+            }
+        ]))
+    
+        print(result)
+        client.close()
+    
+    if __name__ == "__main__":
+        main()
+
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+
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