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Preventing and Debugging Memory Leaks in Node.js

Stanley Ulili
Updated on October 11, 2023

Memory leaks can be a big problem in all applications regardless of the programming languages. Whether leaks happen incrementally or in smaller chunks, the application will come to a point where it will start getting slow and eventually crash. This can leave a bad impression on users, so it's important to be prudent and avoid writing code that can introduce memory leaks.

In this tutorial, we will look at what memory leaks are and their causes. We will also look at the best practices on how to prevent them, and strategies you can use to temporarily fix memory leaks as you debug the application for memory leaks. Finally, we look at how to monitor an application using Prometheus and configure it to send email alerts.

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Prerequisites

Before proceeding with this tutorial ensure that you have a recent version of Node.js  and npm installed. You should also have Google Chrome or a Chromium-based browser installed as we'll be making heavy use of the DevTools for debugging a memory leak in a sample program.

Understanding the memory lifecycle in Node.js

Let's begin by examining the life cycle of memory to understand what memory leaks are:

  1. Allocation of memory: When a program starts, the operating system allocates memory to the Node.js process to store values.
  2. Memory usage: The process utilizes the allocated memory to read and write values.
  3. Releasing memory: When the program completes its execution, the operating system frees the memory allocated to the Node.js process, making it available to other processes that require it.

If an application's memory consumption exceeds the memory allocated to it by the operating system, it will be terminated. Increasing the V8 memory limit can provide temporary relief, but eventually, you may run out of memory or be forced to pay more for server resources. Therefore, it's critical to understand how to prevent memory leaks to make the best use of the allocated memory.

Understanding memory storage in JavaScript

This section will explain how data is stored in memory by the JavaScript engine utilized by the Node.js runtime. When memory is allocated to a program, the JavaScript engine stores data in two primary storage areas which are discussed below:

1. Stack

The stack is a Last-In-First-Out (LIFO) data structure that stores data with a fixed and known size, such as numbers or strings. JavaScript categorizes fixed-sized values into primitive types, which include string, number, boolean, undefined, symbol, and null. These data types are stored on the stack and are immutable, as shown in the following example:

 
let name = "Stanley";
let num = 15;
let isLogged = true;
let check = null;

The variables and their values are allocated on the stack, with the name variable added first and the check variable added last, as illustrated in the illustration below:

Diagram showing variables on a stack

2. Heap

The heap is a dynamic memory location that stores elements with an unknown size, such as arrays, functions, and objects. The heap can expand if more memory is required, or it can shrink if objects are deleted.

JavaScript stores arbitrarily sized objects on the heap, including functions, arrays, and objects. These objects' sizes are typically unknown and can change dynamically, such as when elements are added to or removed from an array. Adding elements requests more memory from the heap, while removing elements frees up memory.

The following code demonstrates examples of an object, function, and an array, typically stored on the heap:

 
const user = {
  name: "Stanley",
  email: "user1@mail.com",
};

function printUser() {
  console.log(`name is ${name}`);
}

const interests = ["bikes", "motorcycles"];

We can visualize them as follows:

diagram of a heap

In this example, the variable names are stored on the stack, but the values they reference are placed on the heap instead.

Understanding how JavaScript objects hold memory

A Garbage Collector (GC) automatically manages the allocation and freeing of memory in JavaScript. The GC goes through the heap and deletes all objects that are no longer needed.

Objects occupy memory in the heap in two ways:

  1. Shallow size: The amount of heap memory allocated to store the object itself.
  2. Retained size: The amount of memory allocated to the object, including the size of all objects referenced by it.

Let's examine how an object creates references using the following example:

 
var user = {
  name: "Stanley",
  email: "user1@mail.com",
};

When you define the user variable, the global object in Node.js references the object stored on the heap. We can represent this using a graph data structure:

diagram showing the root node reference references the user object

If later in the code, you set user to null:

 
user = null

The reference from the root is lost, and the object in the heap becomes unreferenced and garbage.

Diagram showing the object having no references

As your codebase grows and more objects are stored in the heap, you may end up with complex references:

Diagram of a memory graph

Now that you understand how objects consume memory in JavaScript, let's learn about how the garbage collector works in the next section.

Understanding how the JavaScript Garbage Collector works

In languages like C or C++, programmers manually allocate or free memory on the heap. However, this is not the case in Node.js whose V8 engine contains a garbage collector. The GC automatically removes objects that are no longer required in the heap. It starts from the root node, traverses all object references, and deletes the ones that don't have any references.

In the following diagram, the garbage collector identifies two nodes (objects) that don't have any references and are no longer needed:

diagram showing nodes that have no references and can be classified as garbage

Objects that are not referenced are classified as garbage, and their deletion frees up memory from the heap.

As your program executes, the garbage collector periodically pauses the application to remove unreferenced objects from the heap. You don't control when the garbage collector runs; it runs as it sees fit or when it detects a shortage of free memory.

The Mark-and-Sweep Algorithm

The garbage collector uses the mark-and-sweep algorithm to eliminate garbage data and free up space. The algorithm functions in the following manner:

  1. Mark phase: the GC traverses from the root (global) and marks all referenced objects that are reachable from the root.

  2. Sweep phase: next, the GC examines all memory from start to finish and removes all unmarked objects. This frees up memory in the heap.

Now that we've covered how the GC works, we can proceed to learn about how memory leaks occur.

What are memory leaks?

As discussed earlier, the garbage collector deletes all objects that are not reachable from the root node. However, sometimes objects that are no longer required in the program are still referenced from the root node or another node reachable from the root. As a result, the garbage collector assumes that these objects are still required due to the references. So, every time the garbage collector runs, the garbage objects survive each garbage collection phase, causing the program's memory usage to continue growing until it runs out of memory. This is known as a memory leak.

Causes of Memory Leaks

This section will discuss some of the most common causes of memory leaks in a Node.js application.

1. Global variables

Global variables are directly referenced by the root node, and they remain in memory for the entire duration of your application. The garbage collector does not clean them up.

Consider the following example:

 
const express = require("express");
const app = express();
const PORT = 3000;

const data = [];
app.get("/", (req, res) => {
  data.push(req.headers);
  res.status(200).send(JSON.stringify(data));
});

In this snippet, you have a global variable data, which is initially empty. However, every time a user visits the / route, the request headers object is appended to the data array. If the app receives 1000 requests, the data array will grow to 1000 elements, and the memory will persist as long as the app runs. As the app receives more requests, it will eventually exhaust all allocated memory and crash.

While this memory leak is easy to identify, it is possible to accidentally introduce global variables in Node.js that cause memory leaks. For example:

 
function setName() {
  name = "Stanley";
}
setName();
console.log(name);

In the setName() function, a name variable is assigned the value Stanley. Although it might appear to be a local variable of the function, it is a global variable in non-strict mode. The variable is attached to the global object and remains in memory as long as your app runs.

If you run the program, the console.log() method logs the value of the name variable in the console, even after the setName() memory has been destroyed.

Output
Stanley

2. Closures

Another common cause of memory leaks in Node.js is closures. A closure is a function that is returned from another function and retains the memory of the parent (outer) function. When the closure is returned and invoked, the data it holds in memory is not destroyed, and it can be accessed in the program at any time, leading to a memory leak.

Consider the following example, which has a function that returns an inner function:

 
function outer(elementCount) {
  // Create an array containing numbers from 0 to elementCount value
  let elements = Array.from(Array(elementCount).keys());

  return function inner() {
    // return a random number
    return elements[Math.floor(Math.random() * elements.length)];
  };
}

In the outer() function, an array is created with numbers ranging from 0 to the value of the elementCount parameter. The function then returns an inner() function that randomly selects a number from the elements array and returns it. The inner() function is a closure because it retains access to the scope of the outer() function.

To execute the closure, you can call it as follows:

 
let getRandom = outer(10000);
console.log(getRandom());
console.log(getRandom());

Here, the outer() function is invoked with an argument of 10000, and it returns the inner() function. The getRandom() function then retrieves a random number from the elements array. You can call getRandom() as many times as you want, and it will always return a different result.

Output
300
8

However, once you have finished using the inner() function, you might assume that its memory has been cleaned up. Unfortunately, the closure retains the memory of the outer() function, and it persists in the heap even after you have finished using it. The garbage collector will not clean it up because it assumes that the closure is still required and that you might use it later. This can result in a memory leak.

3. Forgotten Timers

Node.js comes with timers such as setTimeout() and setInterval(). The former executes a callback function after a specified delay, while the latter executes a callback function repeatedly with a fixed delay between each execution. These timers can cause memory leaks, especially when used with closures.

Consider this example:

 
function increment() {
  const data = [];
  let counter = 0;

  return function inner() {
    data.push(counter++); // data array is now part of the callback's scope
    console.log(counter);
    console.log(data);
  };
}

setInterval(increment(), 1000);

In this example, the setInterval() method runs the inner() function returned by the increment() function, repeatedly adding an element to the data array each time it runs. Since the data array is part of the closure created by inner(), it remains in memory after each call to increment(), even though it's no longer needed. As a result, the heap keeps growing over time until the application runs out of memory.

To avoid this issue, you can clear the timer when it's no longer needed, for example by calling clearInterval() or clearTimeout(). It's also a good practice to avoid using closures in timer callbacks unless necessary, to reduce the risk of memory leaks.

Preventing Memory Leaks

In this section, we will discuss best practices to prevent memory leaks in your Node.js applications.

1. Reduce global variables usage

While it may be challenging to eliminate global variables completely, it is essential to avoid using them whenever possible. If you must use global variables, set them to null once you are done using them, so the garbage collector can clean them up.

 
const data = [];
// do some stuff

data = null;

Avoid using global variables solely because it is easier to pass them around your codebase. Group functionality that constantly references a variable in a class or use ES modules. Use functions as much as possible so that variables can be locally scoped and destroyed after the function finishes executing.

2. Avoid accidental global variables

To avoid creating accidental global variables, use ES modules  in Node.js or the browser. ES modules run in strict mode by default. Therefore, running the following code will trigger an error:

 
function setName() {
  name = "Stanley";
}
setName();
console.log(name);
Output
ReferenceError: name is not defined

To use ES Modules, add the following line in your package.json file:

package.json
{
  ...
"type": "module"
}

If you cannot switch to ES modules right now, add "use strict" to the top of each file in your project:

 
"use strict"
...

Or use the --use-strict flag when running a Node.js program:

 
node --use-strict program.js

In addition, make a habit of using ES6's const and let to define variables, which are block-scoped:

 
function setName() {
const name = "Stanley";
} setName(); console.log(name);
Output
Uncaught ReferenceError: name is not defined

3. Clearing timers

As discussed earlier, timers can cause memory leaks if not handled properly. To prevent such leaks, it's important to clear the timers when they are no longer needed.

In the following example, we used the setInterval() method to repeatedly execute a function that adds a new item to an array every second:

 
function increment() {
  const data = [];
  let counter = 0;

  return function inner() {
  ...
  };
}
const timerId = setInterval(increment(), 1000);
// Clear the timer after 10 seconds
setTimeout(() => {
clearInterval(timerId);
}, 10000);

In the code above, the ID of the timer returned by setInterval() is stored in the timerId variable, and setTimeout() is used to clear the timer after 10 seconds by passing the timerId to clearInterval() to ensure that the timer stops running after the specified duration.

Remember that the same principles can be applied to other types of timers, such as setTimeout(), as well as event listeners or EventEmitters . Always clear the timers and listeners when they are no longer needed to prevent memory leaks.

Coping with memory leaks in production

Finding and fixing memory leaks in a large application can be challenging and time-consuming. While investigating the root cause of the issue, it helps to deploy some temporary measures to prevent the memory leak from getting out of hand.

One common strategy is to configure a process manager to auto-restart the application process when the memory reaches a pre-configured threshold. This approach helps to clear the process memory, including the heap, allowing the application to start afresh with an empty memory.

Here's an example that configures PM2 to auto restart a node application when it exceeds a certain limit (400 Megabytes in this case):

ecosystem.config.js:
module.exports = {
  apps : [{
    name: 'app_name',
    script: 'app.js',
max_memory_restart: '400M'
... }] }

With this in place, pm2 will automatically check memory usage every 30 seconds and restart the application when the memory limit 400M has been reached. You can also use the --max-memory-restart option when starting the application:

 
pm2 start app.js --max-memory-restart 400M

Once you have such auto restart strategy in place, you can focus on debugging and fixing the memory leak, and that's what we will focus on in the rest of this article.

Debugging Node.js memory leaks

In this section, you will learn how to debug the application to identify the memory leak source and fix it permanently. Starting with Node.js v11.13.0 or higher, you can use writeHeapSnapshot() method of the v8  module to take a heap snapshot as your application is running. If you are using a Node.js version lower than v11.13, use the heapdump  package instead.

Once the snapshots have been taken, you can load them in the Chrome DevTools. The DevTools have a memory panel that allows you to load heap snapshots, compare them, and give you a summary of the memory usage.

To debug a memory leak, first, create a project directory and move into it:

Before we begin to describe the process of debugging memory leaks, let's create a Node.js project that contains a memory leak first. Start by creating and changing into a new project directory using the command below:

 
mkdir memoryleak_demo && cd memoryleak_demo

Next, initialize the project with a package.json file:

 
npm init -y

Afterward, install Express  and loadtest  packages in your project directory. The former is for creating a simple Node.js server, while the latter is for sending traffic to the server.

 
npm i express loadtest

Once the dependencies are installed, open a new index.js file in your text editor, then add the following code:

index.js
const express = require("express");
const app = express();
const PORT = 3000;

const headersArray = [];
app.get("/", (req, res) => {
  headersArray.push({ userAgentUsed: req.get("User-Agent") });
  res.status(200).send(JSON.stringify(headersArray));
});

app.listen(PORT, () => {
  console.log(`Server listening on http://localhost:${PORT}/`);
});

In the above snippet, a memory leak is introduced through the use of the headersArray global variable. Upon each request to the / route, the route handler pushes an object that contains the visitor's user agent to the headersArray array.

Next, modify the code to create a heap snapshot when the SIGUSR2 signal is sent to the server:

index.js
const v8 = require("v8");
const express = require("express"); const app = express(); const PORT = 3000; const headersArray = []; app.get("/", (req, res) => { headersArray.push({ userAgentUsed: req.get("User-Agent") }); res.status(200).send(JSON.stringify(headersArray)); });
process.on('SIGUSR2', () => {
const fileName = v8.writeHeapSnapshot();
console.log(`Created heapdump file: ${fileName}`);
});
app.listen(PORT, () => { console.log(`Server listening on http://localhost:${PORT}/`); });

In this snippet, an event listener is attached to the Node.js process to listen for the SIGUSR2 signal which is sent when the kill command is executed with the -SIGUSR2 option in Linux. Once this event is detected, the callback function is executed and a heap snapshot file is created in the current directory.

Once you are done adding the new lines, save the file and execute the program to start the server on port 3000:

 
node index.js
Output
Server listening on http://localhost:3000/

The output confirms that the server is listening on port 3000.

Now, let's create our first heap snapshot.

Open another terminal window (or tab) and ensure the application server is still running in the first terminal. In the second terminal, run the following command to get the process ID of the Node application that is running on port 3000:

 
ss -lptn 'sport = :3000'

The output will look similar to the following(though the process ID will differ):

 
State           Recv-Q          Send-Q                     Local Address:Port                     Peer Address:Port          Process
LISTEN          0               511                                    *:3000                                *:*              users:(("node",pid=13446,fd=21))

Copy the process ID in the process column, which is the number after pid=. Substitute it on the following command in the second terminal:

 
kill -SIGUSR2 <the_process_id>

Here, you provided the -SIGUSR2 option to the kill command to send a SIGUSR2 signal to the Node.js process instead of terminating it. This causes a new heap snapshot file to the current directory.

Note that creating a heap snapshot is a synchronous operation that will block the event loop until it is finished, and the time taken to create the snapshot depends on the size of the heap. Therefore, it's a good idea to create heap snapshots only when the application traffic is low.

Return to the first terminal window to see the confirmation that the heap dump has been created:

Output
Server listening on http://localhost:3000/
Created heapdump file: Heap.20230226.163127.14272.0.001.heapsnapshot

Now that the first snapshot has been created, you will use the loadtest package to simulate 7000 HTTP requests to the Node.js app server. In the second terminal, run the following command:

 
npx loadtest -n 7000 -c 1 -k http://localhost:3000/

You will observe a output that looks similar to this:

Output
INFO Requests: 0 (0%), requests per second: 0, mean latency: 0 ms
INFO Requests: 1817 (26%), requests per second: 364, mean latency: 2.8 ms
INFO Requests: 3367 (48%), requests per second: 310, mean latency: 3.1 ms
INFO Requests: 4689 (67%), requests per second: 264, mean latency: 3.7 ms
INFO Requests: 5802 (83%), requests per second: 223, mean latency: 4.4 ms
INFO Requests: 6779 (97%), requests per second: 195, mean latency: 5 ms
INFO
INFO Target URL:          http://localhost:3000/
INFO Max requests:        7000
INFO Concurrency level:   1
INFO Agent:               keepalive
INFO
INFO Completed requests:  7000
INFO Total errors:        0
INFO Total time:          26.827927721000002 s
INFO Requests per second: 261
INFO Mean latency:        3.7 ms
INFO
INFO Percentage of the requests served within a certain time
INFO   50%      3 ms
INFO   90%      4 ms
INFO   95%      5 ms
INFO   99%      8 ms
INFO  100%      436 ms (longest request)

In the output, Completed requests: 7000 confirms that the requests have been successfully sent. To regain the prompt in the second terminal so that you can enter more commands, press CTRL+ C.

Next, run the kill command again to create a second heap snapshot that you will use soon for comparison:

 
kill -SIGUSR2 <your_process_id>

In the first terminal, you will see confirmation that the second snapshot file has been created:

Output
Server listening on http://localhost:3000/
Created heapdump file: Heap.20230226.163127.14272.0.001.heapsnapshot
Created heapdump file: Heap.20230226.163444.14272.0.002.heapsnapshot

At this point, you have created two snapshot files. One when the application just started running, and the other after some load has been sent to the server:

Next, open Chrome and visit http://localhost:3000/ to get an idea of the data stored in the headersArray:

screenshot of the data shown on route "/"

Due to the simulated 7000 visits to the server using loadtest, the headersArray has 7000 objects with the userAgentUsed property set to the loadtest/5.2.0 value.

Analyzing Node.js heap snapshots using the Chrome DevTools

After creating the snapshot files, you need to analyze them using Chrome DevTools to locate the memory leak. Therefore, open the Chrome DevTools in your browser tab and switch to the memory panel:

Screenshot of the memory panel

Click the Load button to open your operating system's file picker. Locate the first heap dump in your application directory and select it:

Screenshot of the "load" button marked

Repeat the process once again to load the second heap dump file. You will now see the two heap dumps loaded:

Heap dumps loaded in Chrome

Now, click the second heap dump file and select Statistics. The panel will give you an idea of what kind of data is taking space in the heap:

Screenshot of showing the memory usage of code, strings, arrays, etc

If you observe closely, you will notice that Strings is using most of the memory in the heap, which is 3046kb. It is followed by the Code, which includes your application code, as well as all the code in the node_modules directory.

Observing the statistics give you a hint of the objects you need to investigate to find the memory leak. We already know that we have over 7000 objects in the headersArray. This can mislead you into thinking that JS arrays should be the one using the most memory since headersArray is an array. The best thing you can do for now is to trust the data you are looking at on the chart and take note of what is taking the most memory, which is the strings here.

Next, you will compare the differences between the two heap snapshots by selecting the Comparison option:

Screenshot showing the Comparison option selected

When you select this option, the objects in the heap of the first snapshot will be compared with the ones in the second snapshot.

Let's go over some of the columns in the table and what they mean:

  • Constructor: Shows the type of Constructor used.
  • New: The new objects that have been added to the heap.
  • Deleted: The objects that have been deleted.
  • Delta: The number of objects created and deleted in the heap. If a number is prefixed with +, it represents the number of objects added. When prefixed with -, it represents the number of objects deleted.
  • Alloc. Size: The amount of memory allocated to the constructor
  • Freed Size: The amount of memory freed after deleting objects.

Our focus will be on the Delta column. First, click on the Delta column header twice to sort the column values from highest to lowest.

Screenshot of the delta column sorted

If you look at the column, you will see that over 7000 new objects have been added for the constructor Object and (string) also has close to 7000 objects. This confirms that there is a memory leak. Usually, when there is no memory leak, the column shows you negative values, 0, or smaller positive values.

To investigate the source of the memory leak, we will need to expand the (string) object. This is easier to do in the Summary panel. To switch to the panel, choose the Summary option, and then select the Objects allocated between snapshot 1 and snapshot 2 option.

Screenshot of the summary panel

Once you are in the Summary panel, double-click the Shallow Size panel to sort the column from highest to lowest.

Screenshot of the Summary panel with the **Shallow Size** field marked to sort the column from highest to lowest

Let's briefly go over over the columns in this panel:

  • Constructor: Shows the type of Constructor used for the data in the heap.
  • Distance: The number of references between the root and the object.
  • Shallow size: The amount of heap memory allocated to store the object itself.
  • Retained size: The amount of memory allocated to the object, including the size of all the objects it references.

Following this, looking closely at the (string) and Object constructor rows:

Screenshot showing that **(string)** and **Object** constructors are taking a lot of space in the heap

You should now be able to see that both (string) and Object constructors show that they have over 7000 objects. In the Shallow Size and Retained Size columns, they are also taking a lot of memory in bytes more than the constructors below. This further confirms what we have seen from the Statistics and the Comparison panel. So we are on the right track.

If you recall the Statistics panel showed that String is taking more objects. So let's expand (string):

Screenshot of (string) expanded

Next, scroll down until you see rows that look like the following:

Screenshot of the memory leak strings found

This matches with what we saw when we visited http://localhost:3000/ earlier. So it is a good place to stop and investigate further.

Next, click on the first or any of the strings containing loadtest/5.2.0, which is the user agent of the loadtest library.

Screenshot showing the source of the memory leak

If you don't see the expanded objects, drag up the Retainers panel.

In the screenshot, there is a lot of important information Chrome is providing. For starters, userAgentUsed has shown up, which is the first hint of where the string is getting a reference. Second, [282] in Array, tells you that the element userAgentUsed resides in an array. Next, you'll see headersArray in system, which tells you the name of the array.

You can use this information to go back to the source code and investigate how the program is interacting with the array. For our program, we already know that the source is the headersArray global variable, but if we didn't know, this would have given us a hint. Of course, most memory leaks investigation won't be a bit straightforward as this. You would have to click on multiple objects or expand them.

Fixing the memory leak

You have now found the source of the memory leak, and it is the headersArray. Every time a user visits the / endpoint, an object is pushed to the headersArray with no mechanism in place to clear the array.

To fix the memory leak, the following are some of options you can use:

  • Store the user agent objects in the file system instead of storing them in the headersArray. You can write the data to a JSON, CSV, or text file.
  • You can also store the data in a database system, which includes SQlite , MySQL , or Postgres .

Once you have made the changes, you can create two heap snapshots as you have done earlier in the article and load them in the Chrome Devtools.

When you switch to the Comparison panel, you will see that fewer objects have been added.

Screenshot of the delta column showing that there are few objects being added

In the first comparison, you had the constructors Object and (string) at the top with close to 7000 objects. That is no longer the case, proving that the memory leak has been fixed.

That takes care of the memory leaks. Next, you will look at the tools you can use to detect memory leaks.

Monitoring memory usage in Node.js with Prometheus

Memory monitoring tools track the memory usage of your application and give you insights into how your application is using memory through reports/graphs. You can usually configure such tools to alert you when memory usage is too high. In this section, you will monitor memory usage with Prometheus  and configure it to alert you when a specified memory threshold is reached.

Before you can proceed, you must download Prometheus  and install it on your machine. You may follow this tutorial  to download and install Prometheus on Linux, and to get it up and running.

After Prometheus is installed, ensure that it is running before proceeding:

 
sudo systemctl status prometheus
Output
prometheus.service - Prometheus
     Loaded: loaded (/etc/systemd/system/prometheus.service; enabled; vendor preset: enabled)
     Active: active (running) since Thu 2023-03-02 11:41:28 CAT; 36s ago
   Main PID: 19530 (prometheus)
      Tasks: 9 (limit: 9302)
     Memory: 22.3M
        CPU: 287ms
     CGroup: /system.slice/prometheus.service
             └─19530 /usr/local/bin/prometheus --config.file /etc/prometheus/prometheus.yml --storage.tsdb.path /var/lib/prometheus/ --web.co>

In the output, if you see Active: active (running), then Prometheus is running.

Return to your terminal and install the prom-client  package in the application directory. It is a Prometheus client for Node.js applications.

 
npm install prom-client

We'll reuse the original example in the last section that has a memory leak:

index.js
const v8 = require("v8");
const express = require("express");
const app = express();
const PORT = 3000;

const headersArray = [];
app.get("/", (req, res) => {
  headersArray.push({ userAgentUsed: req.get("User-Agent") });
  res.status(200).send(JSON.stringify(headersArray));
});

process.on("SIGUSR2", () => {
  const fileName = v8.writeHeapSnapshot();
  console.log(`Created heapdump file: ${fileName}`);
});

app.listen(PORT, () => {
  console.log(`Server listening on http://localhost:${PORT}/`);
});

Add the highlighted lines to set the /metrics endpoint that Prometheus will scrap later:

index.js
const v8 = require("v8");
const client = require("prom-client");
const express = require("express"); const app = express(); const PORT = 3000;
const register = new client.Registry();
client.collectDefaultMetrics({ register });
const headersArray = []; app.get("/", (req, res) => { headersArray.push({ userAgentUsed: req.get("User-Agent") }); res.status(200).send(JSON.stringify(headersArray)); });
app.get("/metrics", async (req, res) => {
res.setHeader("Content-Type", register.contentType);
res.send(await register.metrics());
});
process.on("SIGUSR2", () => { const fileName = v8.writeHeapSnapshot(); console.log(`Created heapdump file: ${fileName}`); }); app.listen(PORT, () => { console.log(`Server listening on http://localhost:${PORT}/`); });

The prom-client module is imported and used to instantiate the registry to collect metrics for Prometheus. Next, the /metrics endpoint is created to exposes all the metrics collected by Prometheus.

When you're finished, save the file and start the server again:

 
node index.js

Return to Chrome and visit http://localhost:3000/metrics. You will see the following page:

Screenshot of the metrics that Prometheus will scrap

Now that the endpoint is working, you should keep the server running so that when we configure Prometheus, it should be able to scrap this endpoint.

Prometheus uses a configuration file to define the scraping targets, which are running instances. The memoryleak_domo app instance runs on port 3000. For Prometheus to scrap it, you need to define it as the target.

Open the Prometheus configuration file and add the highlighted code to add an entry for the memoryleak_demo app:

/etc/prometheus/prometheus.yml
global:
  scrape_interval: 15s

scrape_configs:
  - job_name: 'prometheus'
    scrape_interval: 5s
    static_configs:
      - targets: ['localhost:9090']
- job_name: 'memoryleak_demo'
scrape_interval: 5s
static_configs:
- targets: ['localhost:3000']

In the preceding configuration file, Prometheus will scrap two targets:

  • prometheus: A scrapping job of itself that is scrapped every 5 seconds as defined with the scrape_interval property.
  • memoryleak_demo: A scrapping job for the memoryleak_demo app you created earlier in the section. It will be scrapped every 5 seconds as well.

At this point, restart Prometheus to ensure that the new changes take effect:

 
sudo systemctl restart prometheus

Next, visit http://localhost:9090/targets to view the targets that Prometheus is currently scrapping. You will see that Prometheus recognizes the memoryleak_demo job. It has detected the http://localhost:3000/metrics endpoint in the Endpoint field and that the instance is running(UP) in the State field.

Screenshot of Prometheus confirming that it can monitor the endpoint of your app

Next, visit http://localhost:9090/graph to view the Prometheus console which allows you to enter queries. Enter the expression nodejs_external_memory_bytes to check the memory usage. Following that, press Execute and switch the Graph tab:

Screenshot showing a memory usage graph

Prometheus plots a graph that shows the current application's memory usage, which is around 1MB.

In a second terminal, simulate the traffic to the app:

 
npx loadtest -n 7000 -c 1 -k http://localhost:3000/

Return to the Prometheus graph page, and press Execute once again. You will observe that the memory usage has grown to over 5MB:

Screenshot of the Prometheus Graph 5MB memory usage

Send alerts on high memory usage

Now that you can observe your application's memory usage via the Prometheus interface, the next step is to configure it to alert you when the memory usage reaches a specific threshold.

You can use the Prometheus Alertmanager  to send alerts to your preferred channel which could be Email, Slack, and any service that provides a webhook receiver.

In this tutorial, we will configure Alertmanager to use Gmail to send email notifications. First, you need to to install the program on your machine. You can do this by following this tutorial up until step 7 .

Once you've installed Alertmanager, make sure that it is running on your system:

 
sudo systemctl status alertmanager

You will receive output that looks like this:

Output
alertmanager.service - Prometheus Alert Manager Service
     Loaded: loaded (/etc/systemd/system/alertmanager.service; enabled; vendor preset: enabled)
     Active: active (running) since Tue 2023-02-28 21:12:22 CAT; 6s ago
   Main PID: 24277 (alertmanager)
      Tasks: 9 (limit: 9302)
     Memory: 13.2M
        CPU: 205ms
     CGroup: /system.slice/alertmanager.service
             └─24277 /usr/local/bin/alertmanager/alertmanager --config.file=/usr/local/bin/alertmanager/alertmanager.yml

The output confirms that the Alertmanager service is active. Visit http://localhost:9093/ and you will see the following page, further confirming that it works:

screenshot of the Alertmanager homepage

At this stage, you should configure an app password for your Gmail account so that you can use it to send emails through Alertmanager. You can do this by heading to Google My Account → Security , and enabling 2-Step Verification.

Enable Google 2fa

Afterward, find the App passwords  section, and create a new app password. Choose the Other (custom name) option and type Alertmanager in the resulting text field. Once done, click the GENERATE button.

App passwords

Copy the password presented in the popup dialog and paste it somewhere safe. You won't be able to see it again.

Copy app password screen

Now, return to your terminal and open the alertmanager.yml config file in your text editor:

 
sudo nano /etc/alertmanager/alertmanager.yml
/etc/alertmanager/alertmanager.yml

global:
  resolve_timeout: 1m

route:
  group_by: ['alertname', 'cluster']
  group_wait: 30s
  group_interval: 5m
  repeat_interval: 3h
  receiver: 'gmail-notifications'

receivers:
- name: 'gmail-notifications'
  email_configs:
  - to: <example2@gmail.com>
    from: <example@gmail.com>
    smarthost: smtp.gmail.com:587
    auth_username: <example@gmail.com>
    auth_identity: <example@gmail.com>
    auth_password: <app_password>
    send_resolved: true

In the Alertmanager config, replace all example@gmail.com with the Gmail account that Alertmanager should use to send emails. Update the to property with the receiver's email address. In theauth_password property, replace <app_password> with the app password you generated with your Google account.

Next, add the following in the alerts.yml file to define the rules that should trigger the alert:

 
sudo nano /etc/prometheus/alerts.yml
/etc/prometheus/alerts.yml
groups:
- name: memory leak
  rules:
  - alert: High memory Usage
    expr: avg(nodejs_external_memory_bytes / 1024) > 2000
    for: 1m
    annotations:
      severity: critical
      description: memory usage high

In the preceding code, you configure Prometheus to send an alert when memory usage for the Node.js application is greater than 2000 KB (2 MB) for 1 minute. The expression avg(nodejs_external_memory_bytes / 1024) > 2000 checks if memory usage is over 2 MB, and for is set to 1m (1 minute).

Now that you have defined the rules, create a reference to the alerts.yml file and add an entry for the Alertmanager in the Prometheus config:

 
sudo nano /etc/prometheus/prometheus.yml
/etc/prometheus/prometheus.yml
global:
  scrape_interval: 15s

rule_files:
- "/etc/prometheus/alerts.yml"
alerting:
alertmanagers:
- static_configs:
- targets:
- localhost:9093
scrape_configs: - job_name: 'prometheus' scrape_interval: 5s static_configs: - targets: ['localhost:9090'] - job_name: 'memoryleak_demo' scrape_interval: 5s static_configs: - targets: ['localhost:3000']

Restart Alertmanager to reflect the new changes:

 
sudo systemctl restart alertmanager

Also, restart Prometheus:

 
sudo systemctl restart prometheus

Next, let's do a final load test to trigger the alert:

 
npx loadtest -n 7000 -c 1 -k http://localhost:3000/

Visit http://localhost:9093/#/alerts. It might take a while to see something like this:

Screenshot of Alertmanager confirming that there is an alert

Next, visit http://localhost:9090/alerts?search=, which is the Prometheus alerts page. You should observe that an alert is firing:

Screenshot of Prometheus comfirming an alert has been fired

After a few minutes, you should also receive an email that looks like this:

Screenshot of the email alert received on Gmail

At this point, you have successfully monitored the application using Prometheus, and configured Alertmanager to send email notifications when memory usage is high.

Final thoughts and next steps

In this article, you have gained an understanding of how memory leaks can be introduced into a codebase, and explored techniques for both preventing and temporarily fixing such leaks. Furthermore, you have learned how to debug a memory leak by identifying its source and implementing a solution. Finally, you have discovered how to monitor an application using Prometheus, and how to configure it to send email alerts via Gmail.

To continue your journey of memory profiling with DevTools, you can visit the Chrome documentation  for more information. Also, if you're interested in delving deeper into how JavaScript manages memory, the Memory Management tutorial on the Mozilla Developer Network  is a great resource. Lastly, to further enhance your knowledge of monitoring applications using Prometheus, you can explore the Prometheus docs  for a comprehensive overview.

Thanks for reading!

Author's avatar
Article by
Stanley Ulili
Stanley is a freelance web developer and researcher from Malawi. He loves learning new things and writing about them to understand and solidify concepts. He hopes that by sharing his experience, others can learn something from them too!
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