12min read

When to Make the Leap from Client- to Server-Side Testing

As companies start out their experimentation journey, they may find themselves experimenting with small changes on their website such as the design of a CTA and other small changes to explore how they perform and their impact on key KPIs such as conversion rate and transaction.

However, as an experimentation program matures, companies are more likely to want to delve deeper into more sophisticated types of testing which require more expertise and a more advanced tool.

This is the point when many companies are ready to take the plunge from client- to server-side experimentation.

In this article, we will explore when is the right time to make the shift to server-side testing and discuss the importance of running these types of tests by outlining the various scenarios where server-side experiments are more appropriate. 

Client-  vs server-side testing

Before we go deeper into server-side testing, we will quickly point out the differences between client- and server-side tests to understand why you need both types of testing as your experimentation and optimization program evolves.

In client-side testing is where experimentation occurs on the client side through Javascript that runs in the browser. Therefore, client-side tools enable you to create variations of your pages by changing the content sent by your server to users in the web browser. The user then gets one variation of your altered content based on your targeting rules.

Put simply, all the work happens at the level of the browser thanks to Javascript. Because of this, client-side testing is usually best for surface-level changes such as layout, design and colors to measure their performance and impact on key KPIs.

Meanwhile, all the work in server-side testing happens at the server level rather than the browser. In other words, it’s your server that randomly sends a user the modified variation. As a result, the experimentation tool works on the server instead of inside your users’ browsers. 

Perhaps one of the biggest benefits of client-side tests is that it’s easy to implement and no special expertise is required to run these tests on the front-end of the website.

However, because of its advanced capabilities, server-side tests require technical expertise  and coding skills and so developers are usually the ones running these tests on the back-end.  

What are the benefits of server-side testing?

Before we go into when you should be using server-side tests, we will outline some of their benefits in order to better understand their use cases.

Omnichannel experimentation

Client-side solutions are usually limited to devices with web browsers, whether that’s on desktop, mobile or tablet. This means you cannot run experiments on mobile apps or connected devices.

Server-side solutions, for their part, allow you to experiment across multiple channels including mobile apps which significantly widens your playing field and opens up numerous opportunities to A/B test beyond web browsers so you can cover all your bases.

Enhanced performance

Perhaps one of the biggest advantages of server-side tests is the absence of the flicker effect, which is often a major downside associated with client-side solutions.  

The flicker effect occurs when the original page loads and is replaced by the test variation and is usually seen by the user.

Unlike client-side tools that require adding a Javascript tag to your pages, you don’t need to add this tag in server-side tools. That’s because experiments are rendered on the server before being pushed to the client and so all the major work occurs at the server level- it’s not noticeable on the client side. 

In other words, during a server-side test, the variation is retrieved from the server and delivered to the user’s browser. As a result, no modifications take place on the front-end or browser, so there’s no flickering effect

Endless testing opportunities

Your teams have a lot more options to work with when it comes to server-side testing as it enables you to modify all aspects of your site.

As server-side tests are rendered from the back-end server, you can test more complex dynamic content unlike with client-side tests where it’s not easy to test such content and may damage the user experience if done on the client-side. 

In other words, you can build tests that are much more complex that go deeper within your tech stack beyond the scope of UI or cosmetic changes to unlock a whole new world of experimentation. 

With such deep experimentation capabilities, teams can thoroughly test all facets of a product to validate its functionality such as evaluating its underlying features, algorithms and back-end logic. 

The next section will go over these various use cases where you should consider running server-side testing instead.

When does it make sense to move to server-side testing?

As we’ve mentioned, server-side tests are usually used to run more advanced types of tests and experiment deeply within your tech stack to thoroughly explore how a product works.

Put simply, server-side experimentation solutions allow teams to conduct more robust and secure experiments that are focused on modifying a product’s functions. 

Here are some use cases where server side testing is recommended over client-side testing:

  • Run experiments on your mobile app

As already mentioned, one of the key advantages of server-side testing is enabling omnichannel and cross-platform experimentation.

As client-side solutions rely on Javascript and cookies, it’s not possible to use them to test on native mobile apps and you’re limited to devices that have a default web browser.

This means to run experiments on your mobile app, you will need a more advanced server-side testing solution to handle mobile app technologies, which are more complex and vastly different from web technologies. 

Moreover, because server-side testing works on both web applications and mobile apps, you can run the same test serving the same variations irrespective of which channel they’re using. This allows you to compare data from each channel and optimize the user experience across all the different touch points accordingly. 

Finally, if you use feature flags to conduct server-side testing on mobile apps, you can bypass the tedious and time-consuming app store approval. Put simply, feature flags enable you to turn functionality on or off remotely without redeploying code to app stores and waiting for its approval or having to wait for all changes to be ready at the same time to release your own changes.

  • Test your search algorithms

Server-side A/B tests are efficient for testing deeper level modifications related to the backend and the architecture of your website.

This is the case with search algorithms which require modification to your existing code. E-commerce websites usually turn to server-side testing to ensure that customers can easily browse through their website and find the product they’re looking for. 

Thus, search algorithms are key in getting customers to find the product they’re looking for by providing a seamless search experience that eventually gets them to purchase.

For example, you can test what is displayed to customers in their search results, whether this should be based on pricing, popularity or reviews or whether you need to prioritize products based on what customers purchased/liked/viewed in the past. 

Server-side testing allows you to create such complex scenarios and rules to provide customers with more personalized reocmmendations and optimize their search experience on your site. These are more difficult to test through client-side solutions as search pages are based on the search query and so they are rendered dynamically.

Thus, server-side testing offers more comprehensive testing and allows you to experiment with multiple algorithms by modifying the existing code.

  • Optimize your product recommendations

Similarly, with server-side tools, you can test multiple product recommendation algorithms typically found at the bottom of product pages to measure which types of layouts generate the most sales or increase average order value. For example, is it better to promote similar products, the most popular products or those that were recently viewed?

Such recommendations are based on a variety of criteria, like browsing history and on your own PIM (Product Information Management) system and other data sources. Server-side testing will allow you to test these multiple algorithms to uncover the best performing selection of recommended products.

  • Test data-sensitive pages

Server-side testing is great for testing on pages where data security is vital as it ensures that the data remains safe internally within the server without worrying about a security threat.

This is ideal to test on e-commerce payment pages or for banking applications, for example, or any other web pages or apps that contain sensitive data.

You can also use feature flags to test out a new payment method on a subset of users and see how they adapt before rolling it out to everyone else.

  • Determine the ideal form length

This is especially important for SaaS businesses that rely on free trial and request demo forms to gather information from their visitors. Thus, they need to determine the best length for such forms without risking drop-offs but still being able to gather all the necessary information about a prospect. 

Server-side testing is the way to go in this scenario as your forms are directly linked to how your database is structured. If a field is obligatory, you will not be able to hide it using JavaScript because the form’s validation will fail server-side.

As a result, testing the length and complexity of a form that has the highest positive impact on conversion rates should be done on the server-side.

This also applies to other websites that utilize such forms from visitors such as hotel reservation or insurance sites. Note that different sectors will require different and more or less information depending on the type and purpose of the information being gathered. 

  • Test the limit for free shipping

E-commerce businesses should be able to determine the order value over which free shipping is possible. This is important as shipping costs are one of the major causes for cart abandonment.

Therefore, since shipping cost is one of the determining factors in a customer’s purchase decision, companies should test out various cart value thresholds to find out the optimal limit for free shipping to improve transaction rates.  

Since shipping costs are usually rendered dynamically from the back-end server, you will need to test them on the server-side. Any modifications made should have an impact on all of the following steps, and should be managed server-side. 

  • Validate your features

Server-side tests allow you to conduct feature testing to validate your product features by rolling out variations of your feature to different groups of users to evaluate its performance before going for a general release. 

With the help of feature flags, you can run server-side A/B tests and segment your users by directing them to one variation or the other. If anything goes wrong with a variation, you can easily roll it back by disabling the flag before it further impacts the user experience. 

Finally, according to the feedback generated from these users, you can then optimize your features and release them to all your users with the confidence that it matches customers requirements. 

Server-side tests and feature flags

The best way to run server-side tests is through feature flags. By decoupling deployment from release, you can conduct server-side A/B tests by rolling out new features to a small group of users. You can then measure their performance on this group of users before rolling out to everyone else.

While server-side testing requires technical expertise and coding skills, it’s not only relevant to tech teams. Often, non-technical staff team up with product teams to define experiments, which are then executed by the engineers. Once implemented, an experiment can usually be controlled, monitored, and analyzed via a dashboard.

With the right feature management solution, all teams across an organization can run server-side tests with the help of an easy-to-use dashboard without facing any technical hassles. 

Client- vs server-side testing: Context matters

In the end, it’s important to note that it’s not a question of whether server-side is better than client-side. They’re both complementary approaches and whichever one a company chooses depends on which is better suited for its optimization and larger business goals.

In other words, one type of testing doesn’t replace the other. It’s a matter of looking at the type of experiment you want to run and which type is more suited in that particular context and which teams are looking to run the experiment- marketers tend to favor and use client-side testing more often while product managers and developers usually opt for server-side testing for their experimentation needs. It also depends on the resources businesses have at hand and the maturity of their optimization programs.

To ensure your website is optimized and provides a seamless user experience, having both testing techniques at hand is key to surviving in a competitive digital world. 

Both types of testing are indispensable to help you build great products that will bring in maximum revenue. The key is to use both together to heighten productivity and achieve maximum impact.


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