DOCSP-32718: add CodeWhisperer comments to transactions code snippets

source/code-snippets/transactions/txn-core.js

@@ -1,12 +1,14 @@
+/* Performs multiple write operations in a transaction */
+
 const { MongoError, MongoClient } = require('mongodb');
 
-// drop collections
+// Drop the "customers", "inventory", and "orders" collections from the "testdb" database
 async function cleanUp(client) {
   await Promise.all( ['customers', 'inventory', 'orders'].map(async c => {
     try {
       const coll = client.db('testdb').collection(c);
       await coll.drop();
-    } catch(e) {}
+    } catch(e) {} // Ignore any exceptions
   }));
 }
 
@@ -15,17 +17,21 @@ async function setup(client) {
     const customerColl = client.db('testdb').collection('customers');
     const inventoryColl = client.db('testdb').collection('inventory');
 
+    // Insert order data for customer "98765" in the customers collection
     await customerColl.insertOne({ _id: 98765, orders: [] });
 
+    // Insert inventory data for "sunblock" and "beach towel"
     await inventoryColl.insertMany([
       { name: 'sunblock', sku: 5432, qty: 85 },
       { name: 'beach towel', sku: 7865, qty: 41 },
     ]);
   } catch (e) {
+    // Print the exception if one was thrown
     console.log('Unable to insert test data: ' + e);
   }
 }
 
+// Print all documents in the "customers", "inventory", and "orders" collections
 async function queryData() {
   const uri = process.env.MONGODB_URI;
   const client = new MongoClient(uri);
@@ -36,23 +42,29 @@ async function queryData() {
     }, client));
 
   } finally {
+    // Close the database connection
     client.close();
   }
 }
 
 // start placeOrder
 async function placeOrder(client, cart, payment) {
+  // Specify readConcern, writeConcern, and readPreference transaction options
   const transactionOptions = {
     readConcern: { level: 'snapshot' },
     writeConcern: { w: 'majority' },
     readPreference: 'primary'
   };
 
+  // Start the session
   const session = client.startSession();
   try {
+    // Start the transaction in the session, specifying the transaction options
     session.startTransaction(transactionOptions);
 
     const ordersCollection = client.db('testdb').collection('orders');
+    // Within the session, insert an order that contains information about the
+    // customer, items purchased, and the total payment.
     const orderResult = await ordersCollection.insertOne(
       {
         customer: payment.customer,
@@ -63,21 +75,26 @@ async function placeOrder(client, cart, payment) {
     );
 
     const inventoryCollection = client.db('testdb').collection('inventory');
+
+    // Within the session, for each item purchased, decrement the purchased quantity in the "inventory" collection.
+    // Cancel the transaction when you have insufficient inventory or if the item SKU does not exist.
     for (let i=0; i<cart.length; i++) {
       const item = cart[i];
 
-      // Cancel the transaction when you have insufficient inventory
+      // Retrieve the inventory information for the item
       const checkInventory = await inventoryCollection.findOne(
         {
           sku: item.sku,
           qty: { $gte: item.qty }
         },
         { session }
       )
+      // Throw an exception if the item lacks sufficient quantity or SKU does not exist.
       if (checkInventory === null) {
         throw new Error('Insufficient quantity or SKU not found.');
       }
 
+      // Decrement the inventory of the item by the amount specified in the order.
       await inventoryCollection.updateOne(
         { sku: item.sku },
         { $inc: { 'qty': -item.qty }},
@@ -86,15 +103,23 @@ async function placeOrder(client, cart, payment) {
     }
 
     const customerCollection = client.db('testdb').collection('customers');
+
+    // Within the session, add the order details to the "orders" array of the customer document.
     await customerCollection.updateOne(
       { _id: payment.customer },
       { $push:  { orders: orderResult.insertedId }},
       { session }
     );
+
+    // Commit the transaction to apply all updates performed within it
     await session.commitTransaction();
     console.log('Transaction successfully committed.');
 
   } catch (error) {
+    /*
+      Handle any exceptions thrown during the transaction and end the
+      transaction. Roll back all the updates performed in the transaction.
+    */
     if (error instanceof MongoError && error.hasErrorLabel('UnknownTransactionCommitResult')) {
       // add your logic to retry or handle the error
     }
@@ -105,28 +130,41 @@ async function placeOrder(client, cart, payment) {
     }
     await session.abortTransaction();
   } finally {
+    // End the session so that no further calls can be made on it
     await session.endSession();
   }
 }
 // end placeOrder
 
+
+// Run the full transaction example
 async function run() {
   const uri = process.env.MONGODB_URI;
   const client = new MongoClient(uri);
 
+  // Call a method that removes data from prior runs of this example
   await cleanUp(client);
+
+  // Call a method that creates sample inventory data for this example
   await setup(client);
 
+  // Create sample data for a customer's shopping cart that includes "sunblock" and "beach towel" items
   const cart = [
     { name: 'sunblock', sku: 5432, qty: 1, price: 5.19 },
     { name: 'beach towel', sku: 7865, qty: 2, price: 15.99 }
   ];
+
+  // Create sample data for a customer's payment, calculated from the contents of their cart
   const payment = { customer: 98765, total: 37.17 };
 
   try {
+    // Call the method that updates the customer and inventory in a transaction
     await placeOrder(client, cart, payment);
   } finally {
+    // Call a method that removes data from prior runs of this example
     await cleanUp(client);
+
+    // Close the database connection
     await client.close();
   }
 }

source/fundamentals/transactions.txt

@@ -16,7 +16,7 @@ Overview
 In this guide, you can learn how to use the
 {+driver-short+} to perform **transactions**. Transactions allow you
 to run a series of operations that do not change any data until the
-entire transaction is committed. If any operation in the transaction fails, the 
+entire transaction is committed. If any operation in the transaction fails, the
 driver aborts the transaction and discards all data changes before they
 ever become visible. This feature is called **atomicity**.
 
@@ -247,16 +247,16 @@ shows how you can perform the following:
 .. literalinclude:: /code-snippets/transactions/txn-core.js
    :language: javascript
    :linenos:
-   :emphasize-lines: 2-6,8,10,19,32,41,49,51,55-59,63,65
+   :emphasize-lines: 3-7,13,24,40,51,61,65,73-74,76-77,81,84
    :start-after: start placeOrder
    :end-before: end placeOrder
 
 You must pass the session object to each CRUD operation that
 you want to run on that session.
 
 .. important:: Use a Session with the Client That Started It
-   
-   Starting in version 6.0 of the {+driver-short+}, the driver 
+
+   Starting in version 6.0 of the {+driver-short+}, the driver
    throws an error if you provide a session from one ``MongoClient``
    instance to a different client instance.
 
@@ -267,7 +267,7 @@ you want to run on that session.
 
    .. code-block:: js
       :emphasize-lines: 2
-      
+
       const session = client1.startSession();
       client2.db('myDB').collection('myColl').insertOne({ name: 'Jane Eyre' }, { session });