What is client side tracking? A simple explanation

This page is for anyone curious about the basics of Client Side Tracking, also known as Client Side Tagging or clientside tracking. I understand that distinguishing between Server Side and Client Side, along with terms like Tagging and Tracking, can be confusing. Don’t worry—my goal is to shed light on these concepts and break them down as simply as possible. 

Why does tracking even matter?

Maybe you’re wondering, “Why should I even care about Client Side Tracking? Why is it important?” After all, why do we need tracking and different methods to track data?

In the past, there was no internet, yet competition existed—it just looked different from today’s landscape of ads, SEO content, and other strategies. Advertising used to rely on billboards or word of mouth. Plus, the number of competitors you were up against was far lower than it is now.

Fast-forward a few years, and the internet entered our lives. It started out rough and pixelated, but by the early 2000s, it was transforming. Websites emerged as companies' digital front doors, becoming more colorful, engaging, and sophisticated over time. With the internet’s accessibility, the number of competitors grew too. A pizza shop owner, for example, now competes not only with the pizzeria down the block but with every pizza place in town.

Why this little history lesson? Simply put, today, it’s more critical than ever to stand out from the competition. It's no longer enough just to excel at what you do—you also need effective tracking, and for that, data is essential. Data is the gold of the digital age. In today’s information flood, collecting the right data on website visitors is crucial for drawing valuable insights and making informed decisions. Decisions about your online presence and content should always be data-driven rather than blind guesses. Sure, you could invest a lot of money in your marketing efforts and only measure sales as a result. But often the reach is far greater than the number of people who ultimately take action.

That’s why we need web tracking. It’s not just about knowing who buys but understanding why users don’t. This is where models like the Customer Journey and Funnels come in, as they help us map a user’s path to making a purchase.

Figure: Phases of a Customer Journey

Now that we’re aware of how crucial web tracking is in digital marketing, let’s take a closer look at what Client Side Tracking actually means.

What is Client Side Tracking?

Client Side Tracking can be seen as the classic approach to web tracking. Imagine a user decides to pick up their phone, open a browser, and search for sewing machines on Google. Perhaps you happen to sell sewing machines and have a great website with a range of products. Now, this user lands on your site to browse your offerings. They might be looking for a specific feature in a sewing machine and clicking through different products in your catalog to read and compare details. They might even click on a button to add a sewing machine to their cart. Let’s pause here; you get the idea. Throughout this session, many interactions occur between the user and your website. You’re tracking these interactions—that is, sending them to analytics tools like Google Analytics for later review.

How are these data collected?

Now, this is where things get a bit technical, but I’ll keep it simple. Tracking scripts and tags run directly in a user’s browser. These scripts contain information about how users engage with your website. For example, they might record page views (of your site), the pages they click through, how far they scroll on a particular page, or which buttons they click (like adding an item to the cart). In general, tracking data like page views, scroll depth, clicks, or other interactions are collected directly in the browser and then sent to relevant third-party providers, such as Google Analytics.

So, what exactly is Client Side Tracking?

In essence, it’s just what I’ve described. Client Side Tracking is a tracking method where all tracking scripts and tags are executed directly in the user’s browser. It’s called "Client-Side" because “browser” is referred to as the “client” in English. Client Side Tracking typically uses a Tag Management System, like Google Tag Manager (GTM), which is a popular tool for Client Side Tracking. With GTM, or any Web Tag Manager, you can define specific events that represent significant interactions you’d like to track. This means that beyond the default reports in Google Analytics, you can track custom events, such as varying scroll depths, which are valuable for understanding user engagement.

Figure: The Client-Side Tracking Process

The sewing machine example above simplifies a common process that starts when a user visits a website. This user’s browser initiates a request that’s sent to the web server. The server then returns the integration code for the Web Tag Manager. This Tag Manager script loads in the browser, which then loads further tracking scripts, such as Google Analytics. Essentially, all tags are executed directly in the browser, and the collected tracking data is sent to third-party providers like Google Analytics.

Note: Every step of communication occurs between the user’s browser and third-party providers, meaning the entire data processing happens on the client side, that is, within the user’s browser.

What are the Benefits of Client Side Tracking?

A major advantage of Client Side Tracking lies in its simplicity of implementation and management. Most tag manager solutions, like Google Tag Manager, offer user-friendly interfaces that allow for configuring tags and triggers without needing deep technical knowledge. Tags define which data should be collected, while triggers determine when these tags are activated—such as on a page load or after scrolling to a specific point. This flexibility in capturing specific events and user actions makes Client Side Tracking very popular.

What are the Downsides of Client-Side Tracking?

However, there are also drawbacks: Since tracking occurs in the browser, scripts are vulnerable to browser extensions like ad blockers, which can prevent tracking scripts from loading. Additionally, modern browser privacy features, like Safari’s and Firefox’s Intelligent Tracking Prevention (ITP), may significantly impact the effectiveness of Client Side Tracking by limiting or blocking cookie storage.

What Alternative Method exists besides Client Side Tracking? 

Server Side Tracking, also known as Server Side Tagging

Another, albeit more complex, tracking method is called Server Side Tracking, also known as serverside tracking. You might have come across the term “Server Side Tagging” in your research and wondered about the difference between Server Side Tracking and Server Side Tagging. I haven't found any significant difference between the two, which leads me to believe they are essentially the same process. Let me explain further.

No matter which term you search on Google, all providers of this method discuss the integration of a server using a Server Tag Manager, specifically the Google Tag Manager (GTM). I assume that Google coined the term “Server Side Tagging” because it uses the word “Tag,” though they could have referred to it as “Tracking” as well. In essence, we can distinguish between two tracking methods: Client Side Tracking and Server Side Tracking. Whether you call them “tracking” or “tagging” is not particularly important. What matters is understanding that both methods utilize Google’s Tag Management systems—the Google Tag Manager (GTM). With Client Side Tracking, a Web Tag Manager loads a web container, while with Server Side Tracking, a Server Tag Manager loads a server container. Both management systems are part of Google Tag Manager (GTM).

Note: Server Side Tagging is the same as Server Side Tracking, and it always involves the Server Tag Manager within Google Tag Manager (GTM).

What is the difference between Server Side and Client Side Tracking?

With Server Side Tracking, the processing of tracking data no longer occurs in the user’s browser (the client) but on a separate server. The collection, processing, and forwarding of data is managed via Google’s Server Tag Manager.

Figure: The Server Side Tracking Process

The process begins similarly to Client Side Tracking: When a user visits a website, the Tag Manager script loads. However, with Server Side Tracking, the tracking data is not processed directly in the browser and sent to third parties. Instead, it is first sent to the company’s own server, where the Server Tag Manager is running. This server then processes the tracking data and forwards it to the relevant third-party providers, such as Google Analytics. This way, communication between the user and third-party providers is minimized, as all tracking data first passes through the company’s server.

Note: Server Side Tracking does not occur in the user's browser (client) as it does with Client Side Tracking; instead, it happens on a dedicated server.

What are the Advantages of Server Side Tracking?

Enhanced Data Control and Quality with improved Data Security

A significant advantage of Server Side Tracking is the increased control over collected data. Since all tracking data is processed on the company’s own server, businesses can decide which data to forward to third-party providers and which to withhold. This provides considerable benefits in terms of data security and GDPR compliance, as sensitive user data—such as IP addresses or email addresses—can be anonymized or removed on the company’s server before being sent to external services. Overall, Server Side Tracking enables businesses to broaden their data set because no data is lost within the user’s browser (see below). Furthermore, even if a user rejects the cookie banner, data can still be retained in an anonymized or pseudonymized form.

Figure: Key Advantages of Server Side Tracking

Bypassing Browser Tracking Prevention Mechanisms (e.g., Safari's ITP)

Another advantage is bypassing browser tracking prevention mechanisms. Browser extensions like ad blockers or the privacy features in modern browsers often block or limit Client Side Tracking because browsers are configured to prohibit third-party tracking. Since Server Side Tracking primarily involves communication between your own server and third-party providers, it is less susceptible to such blocking mechanisms.

Remember: Processing no longer occurs in the browser, so these mechanisms do not identify tracking as a third-party service. Additionally, Server Side Tracking can enhance website performance, as fewer scripts need to be loaded directly in the browser, reducing page load times. You can read more about bypassing browser tracking prevention mechanisms and the resulting benefits here.

What Are the Drawbacks of Server Side Tracking?

Complex Implementation and Technical Expertise

However, implementing Server Side Tracking is more complex than Client Side Tracking. It requires technical knowledge and the provision of a server to handle data processing. The costs associated with running a server and setting up and maintaining the infrastructure can be higher. For this reason, it is often advisable to consult experts to ensure that the server runs reliably and meets all legal data protection requirements.

The server acts as the central hub between your website and third-party providers like Google Analytics. Errors in setup can result in no data being sent to third-party providers—unlike with Client Side Tracking, where only certain events may not be tracked. With Server Side Tracking, however, you lose all tracking data, which cannot be recovered.

In summary, Client Side Tracking is the conventional method in which tracking scripts and data processing occur in the user's browser. This approach is easy to implement and flexible, but it can be affected by browser restrictions and ad blockers. Companies looking to capture tracking data effectively and in compliance with GDPR should consider adopting Server Side Tracking to avoid these limitations and gain greater control over collected data. Despite its complexity and higher costs, Server Side Tracking is particularly worthwhile for companies prioritizing data security and accurate data capture.