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Rich, complex rules for advanced load balancing


7 min read

Load Balancing — functionality that’s been around for the last 30 years to help businesses leverage their existing infrastructure resources. Load balancing works by proactively steering traffic away from unhealthy origin servers and — for more advanced solutions — intelligently distributing traffic load based on different steering algorithms. This process ensures that errors aren’t served to end users and empowers businesses to tightly couple overall business objectives to their traffic behavior.

What’s important for load balancing today?

We are no longer in the age where setting up a fixed amount of servers in a data center is enough to meet the massive growth of users browsing the Internet. This means that we are well past the time when there is a one size fits all solution to suffice the needs of different businesses. Today, customers look for load balancers that are easy to use, propagate changes quickly, and — especially now — provide the most feature flexibility. Feature flexibility has become so important because different businesses have different paths to success and, consequently, different challenges! Let’s go through a few common use cases:

  • You might have an application split into microservices, where specific origins support segments of your application. You need to route your traffic based on specific paths to ensure no single origin can be overwhelmed and users get sent to the correct server to answer the originating request.
  • You may want to route traffic based on a specific value within a header request such as “PS5” and send requests to the data center with the matching header.
  • If you heavily prioritize security and privacy, you may adopt a split-horizon DNS setup within your network architecture. You might choose this architecture to separate internal network requests from public requests from the rest of the public Internet. Then, you could route each type of request to pools specifically suited to handle the amount and type of traffic.

As we continue to build new features and products, we also wanted to build a framework that would allow us to increase our velocity to add new items to our Load Balancing solution while we also take the time to create first class features as well. The result was the creation of our custom rule builder!

Now you can build complex, custom rules to direct traffic using Cloudflare Load Balancing, empowering customers to create their own custom logic around their traffic steering and origin selection decisions. As we mentioned, there is no one size fits all solution in today's world. We provide the tools to easily and quickly create rules that meet the exact requirements needed for any customer's unique situation and architecture. On top of that, we also support ‘and’ and ‘or’ statements within a rule, allowing very powerful and complex rules to be created for any situation!

Load Balancing by path becomes easy, requiring just a few minutes to enter the paths and some boolean statements to create complex rules. Steer by a specific header, query string, or cookie. It’s no longer a pain point. Leverage a split horizon DNS design by creating a rule looking at the IP source address and then routing to the appropriate pool based on the value. This is just a small subset of the very robust capabilities that load balancing custom rules makes available to our users and this is just the start! Not only do we have a large amount of functionality right out of the box, but we’re also providing a consistent, intuitive experience by building on our Firewall Rules Engine.

Let’s go through some use cases to explore how custom rules can open new possibilities by giving you more granular control of your traffic.

High-volume transactions for ecommerce

For any high-volume transaction business such as an ecommerce or retail store, ensuring the transactions go through as fast and reliably as possible is a table stakes requirement. As transaction volume increases, no single origin can handle the incoming traffic, and it doesn’t always make sense for it to do so. Why have a transaction request travel around the world to a specifically nominated origin for payment processing? This setup would only add latency, leading to degraded performance, increased errors, and a poor customer experience. But what if you could create custom logic to segment transactions to different origin servers based on a specific value in a query string, such as a PS5 (associated with Sony’s popular PlayStation 5)? What if you could then couple that value with dynamic latency steering to ensure your load balancer always chooses the most performant path to the origin? This would be game changing to not only ensure that table-stakes transactions are reliable and fast but also drastically improve the customer experience. You could do this in minutes with load balancing custom rules:

For any requests where the query string shows ‘PS5’, then route based on which pool is the most performant.

Load balance across multiple DNS vendors to support privacy and security

Some customers may want to use multiple DNS providers to bolster their resiliency along with their security and privacy for the different types of traffic going through their network. By utilizing  two DNS providers, customers can not only be sure that they remain highly available in times of outages, but also direct different types of traffic, whether that be internal network traffic across offices or unknown traffic from the public Internet.

Without flexibility, however, it can be difficult to easily and intelligently route traffic to the proper data centers to maintain that security and privacy posture. Not anymore! With load balancing custom rules, supporting a split horizon DNS architecture takes as little as five minutes to set up a rule based on the IP source condition and then overwriting which pools or data centers that traffic should route to.

This can also be extremely helpful if your data centers are spread across multiple areas of the globe that don’t align with the 13 current regions within Cloudflare. By segmenting where traffic goes based on the IP source address, you can create a type of geo-steering setup that is also finely tuned to the requirements of the business!

How did we build it?

We built Load Balancing rules on top of our open-source wirefilter execution engine. People familiar with Firewall Rules and other products will notice similar syntax since both products are built on top of this execution engine.

By reusing the same underlying engine, we can take advantage of a battle-tested production library used by other products that have the performance and stability requirements of their own. For those experienced with our rule-based products, you can reuse your knowledge due to the shared syntax to define conditionals statements. For new users, the Wireshark-like syntax is often familiar and relatively simple.

DNS vs Proxied?

Our Load Balancer supports both DNS and Proxied load balancing. These two protocols operate very differently and as such are handled differently.

For DNS-based load balancing, our load balancer responds to DNS queries sent from recursive resolvers. These resolvers are normally not the end user directly requesting the traffic nor is there a 1-to-1 ratio between DNS query and end-user requests. The DNS makes extensive use of caching at all levels so the result of each query could potentially be used by thousands of users. Combined, this greatly limits the possible feature set for DNS. Since you don’t see the end user directly nor know if your response is going to be used by one or more users, all responses can only be customized to a limited degree.

Our Proxied load balancing, on the other hand, processes rules logic for every request going through the system. Since we act as a proxy for all these requests, we can invoke this logic for all requests and access user-specific data.

These different modes mean the fields available to each end up being quite different. The DNS load balancer gets access to DNS-specific fields such as “” (the query name) while our Proxied load balancer has access to “http.request.method” (the HTTP method used to access the proxied resource). Some more general fields — like the name of the load balancer being used — are available in both modes.

How does it work under the hood?

When a load balancer rule is configured, that API call will validate that the conditions and actions of the rules are valid. It makes sure the condition only references known fields, isn’t excessively long, and is syntactically valid. The overrides are processed and applied to the load balancers configuration to make sure they won’t cause an invalid configuration. After validation, the new rule is saved to our database.

With the new rule saved, we take the load balancer’s data and all rules used by it and package that data together into one configuration to be shipped out to our edge. This process happens very quickly, so any changes are visible to you in just a few seconds.

While DNS and proxied load balancers have access to different fields and the protocols themselves are quite different, the two code paths overlap quite a bit. When either request type makes it to our load balancer, we first load up the load balancer specific configuration data from our edge datastore. This object contains all the “static” data for a load balancer, such as rules, origins, pools, steering policy, and so forth. We load dynamic data such as origin health and RTT data when evaluating each pool.

At the start of the load balancer processing, we run our rules. This ends up looking very much like a loop where we check each condition and — if the condition is true — we apply the effects specified by the rules. After each condition is processed and the effects are applied we then run our normal load balancing logic as if you have configured the load balancer with the overridden settings. This style of applying each override in turn allows more than one rule to change a given setting multiple times during execution. This lets users avoid extremely long and specific conditionals and instead use shorter conditionals and rule ordering to override specific settings creating a more modular ruleset.

What’s coming next?

For you, the next steps are simple. Start building custom load balancing rules! For more guidance, check out our developer documentation.

For us, we’re looking to expand this functionality. As this new feature develops, we are going to be identifying new fields for conditionals and new options for overrides to allow more specific behavior. As an example, we’ve been looking into exposing a means to creating more time-based conditionals, so users can create rules that only apply during certain times of the day or month. Stay tuned to the blog for more!

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