Adds transaction with watched key example script. (#2297)

* transction example improved

* readme fixed

* delay added for watched key changes

* pooling error fixed on recursion

* Minor comment update.

Co-authored-by: Ajay <ajay.markana@yudiz.com>
Co-authored-by: Simon Prickett <simon@redis.com>

Closes #2280.

examples/README.md

@@ -3,7 +3,7 @@
 This folder contains example scripts showing how to use Node Redis in different scenarios.
 
 | File Name                   | Description                                                                                                                                         |
-|-----------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------|
+| --------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `blocking-list-pop.js`      | Block until an element is pushed to a list                                                                                                          |
 | `bloom-filter.js`           | Space efficient set membership checks with a [Bloom Filter](https://en.wikipedia.org/wiki/Bloom_filter) using [RedisBloom](https://redisbloom.io)   |
 | `command-with-modifiers.js` | Define a script that allows to run a command with several modifiers                                                                                 |
@@ -12,7 +12,7 @@ This folder contains example scripts showing how to use Node Redis in different
 | `count-min-sketch.js`       | Estimate the frequency of a given event using the [RedisBloom](https://redisbloom.io) Count-Min Sketch                                              |
 | `cuckoo-filter.js`          | Space efficient set membership checks with a [Cuckoo Filter](https://en.wikipedia.org/wiki/Cuckoo_filter) using [RedisBloom](https://redisbloom.io) |
 | `get-server-time.js`        | Get the time from the Redis server                                                                                                                  |
-| `hyperloglog.js`            | Showing use of Hyperloglog commands [PFADD, PFCOUNT and PFMERGE](https://redis.io/commands/?group=hyperloglog)                                |
+| `hyperloglog.js`            | Showing use of Hyperloglog commands [PFADD, PFCOUNT and PFMERGE](https://redis.io/commands/?group=hyperloglog)                                      |
 | `lua-multi-incr.js`         | Define a custom lua script that allows you to perform INCRBY on multiple keys                                                                       |
 | `managing-json.js`          | Store, retrieve and manipulate JSON data atomically with [RedisJSON](https://redisjson.io/)                                                         |
 | `pubsub-publisher.js`       | Adds multiple messages on 2 different channels messages to Redis                                                                                    |
@@ -26,10 +26,11 @@ This folder contains example scripts showing how to use Node Redis in different
 | `time-series.js`            | Create, populate and query timeseries data with [Redis Timeseries](https://redistimeseries.io)                                                      |
 | `topk.js`                   | Use the [RedisBloom](https://redisbloom.io) TopK to track the most frequently seen items.                                                           |
 | `stream-consumer-group.js`  | Reads entties from a [Redis Stream](https://redis.io/topics/streams-intro) as part of a consumer group using the blocking `XREADGROUP` command      |
+| `transaction-with-watch.js` | An Example of [Redis transaction](https://redis.io/docs/manual/transactions) with `WATCH` command on isolated connection with optimistic locking    |
 
 ## Contributing
 
-We'd love to see more examples here. If you have an idea that you'd like to see included here, submit a Pull Request and we'll be sure to review it!  Don't forget to check out our [contributing guide](../CONTRIBUTING.md).
+We'd love to see more examples here. If you have an idea that you'd like to see included here, submit a Pull Request and we'll be sure to review it! Don't forget to check out our [contributing guide](../CONTRIBUTING.md).
 
 ## Setup
 
@@ -47,21 +48,21 @@ $ npm install
 
 When adding a new example, please follow these guidelines:
 
-* Add your code in a single JavaScript or TypeScript file per example, directly in the `examples` folder
-* Do not introduce other dependencies in your example
-* Give your `.js` file a meaningful name using `-` separators e.g. `adding-to-a-stream.js` / `adding-to-a-stream.ts`
-* Indent your code using 2 spaces
-* Use the single line `//` comment style and comment your code
-* Add a comment at the top of your `.js` / `.ts` file describing what your example does
-* Add a comment at the top of your `.js` / `.ts` file describing any Redis commands that need to be run to set up data for your example (try and keep this minimal)
-* Use semicolons
-* Use `async` and `await`
-* Use single quotes, `'hello'` not `"hello"`
-* Use [template literals](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals) when embedding expressions in strings
-* Unless your example requires a connection string, assume Redis is on the default localhost port 6379 with no password
-* Use meaningful example data, let's not use `foo`, `bar`, `baz` etc!
-* Leave an empty line at the end of your `.js` file
-* Update this `README.md` file to add your example to the table
+- Add your code in a single JavaScript or TypeScript file per example, directly in the `examples` folder
+- Do not introduce other dependencies in your example
+- Give your `.js` file a meaningful name using `-` separators e.g. `adding-to-a-stream.js` / `adding-to-a-stream.ts`
+- Indent your code using 2 spaces
+- Use the single line `//` comment style and comment your code
+- Add a comment at the top of your `.js` / `.ts` file describing what your example does
+- Add a comment at the top of your `.js` / `.ts` file describing any Redis commands that need to be run to set up data for your example (try and keep this minimal)
+- Use semicolons
+- Use `async` and `await`
+- Use single quotes, `'hello'` not `"hello"`
+- Use [template literals](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals) when embedding expressions in strings
+- Unless your example requires a connection string, assume Redis is on the default localhost port 6379 with no password
+- Use meaningful example data, let's not use `foo`, `bar`, `baz` etc!
+- Leave an empty line at the end of your `.js` file
+- Update this `README.md` file to add your example to the table
 
 Use [connect-as-acl-user.js](./connect-as-acl-user.js) as a guide to develop a well formatted example script.
 
@@ -87,5 +88,4 @@ await client.connect();
 // Add your example code here...
 
 await client.quit();
-
 ```

examples/transaction-with-watch.js

@@ -0,0 +1,40 @@
+import { createClient, WatchError } from 'redis';
+
+const delay = (ms) => new Promise((resolve) => setTimeout(resolve, ms));
+const client = createClient();
+await client.connect();
+
+function restrictFunctionCalls(fn, maxCalls) {
+  let count = 1;
+  return function (...args) {
+    return count++ < maxCalls ? fn(...args) : false;
+  };
+}
+
+const fn = restrictFunctionCalls(transaction, 4);
+
+async function transaction() {
+  try {
+    await client.executeIsolated(async (isolatedClient) => {
+      await isolatedClient.watch('paymentId:1259');
+      const multi = isolatedClient
+        .multi()
+        .set('paymentId:1259', 'Payment Successfully Completed!')
+        .set('paymentId:1260', 'Refund Processed Successfully!');
+      await delay(5000); // Do some changes to the watched key during this time...
+      await multi.exec();
+      console.log('Transaction completed Successfully!');
+      client.quit();
+    });
+  } catch (error) {
+    if (error instanceof WatchError) {
+      console.log('Transaction Failed Due To Concurrent Modification!');
+      fn();
+    } else {
+      console.log(`Error: ${error}`);
+      client.quit();
+    }
+  }
+}
+
+transaction();