What is an application delivery controller (ADC)?

An ADC can be a hardware appliance or software program and is generally placed in the demilitarized zone (DMZ) of an enterprise network, between the firewall and one or more application servers. ADCs serve as reverse proxies that receive, decrypt, validate and route client requests before sending encrypted responses back to the user.

ADCs help businesses modernize network applications, accelerate connections between client machines and web servers, streamline global server load balancing (GSLB) and increase overall app availability for end users.

The evolution of application delivery controllers is closely linked to the development of web applications, data centers and the broader field of network management.

In the late 1990s and early 2000s, as the internet commercialized and web traffic increased in kind, businesses needed a way to ensure that websites and apps could handle high-volume application traffic efficiently. The primary challenge was finding a way to distribute incoming requests across multiple servers to guarantee high availability and network resilience.

The first generation of devices to address the challenge were called load balancers. Initially, they were simple and operated at the transport layer. Load balancers focused on distributing incoming traffic across several servers to balance loads and prevent them from overwhelming any single server.

However, as apps and websites became more complex, legacy load balancers fell out of favor for their inability to read and make decisions based on the content of user requests (not just IP addresses and ports). This led developers to upgrade to layer 7 load balancing solutions, which could inspect message content at the application layer.

Advanced load balancers could route traffic based on factors like HTTP headers, cookies or even specific page requests, facilitating more sophisticated traffic management strategies, like session persistence (or “stickiness”) and content-based routing. Over time, as developers continued to add functionalities, these load balancers would become the intelligent controllers we now call ADCs.

Modern application delivery controllers are vital components of resilient, highly available IT infrastructures, particularly network data centers. They manage app traffic by using a series of complex processes and features that ensure efficient, secure data flow between client devices and backend servers. Those features include:

The shift toward cloud-native applications has demanded that application delivery controllers evolve to support containerized environments and microservices architectures. Though hardware- and software-based ADCs are still effective in many ways, traditional ADCs were designed for monolithic apps and often struggle to keep up with the dynamic nature of today’s cloud-native applications.

To address this, service providers like Citrix, VMWare, Amazon Web Services (AWS), Microsoft Azure and IBM, among others, have developed cloud-native ADCs (in the form of IaaS, PaaS and SaaS) that offer advanced agility, scalability and automation capabilities. Modern ADCs are designed to integrate seamlessly with cloud provider APIs and container orchestration tools (like Kubernetes) to deliver superior service discovery, autoscaling and traffic management capabilities.

Despite their origin as basic load balancers, application delivery controllers have consistently adapted to meet the dynamic needs of IT infrastructures and application delivery solutions. Today’s ADCs are multifaceted app delivery platforms that offer application performance, security and delivery optimization across complex, multicloud environments.

Any enterprise operating a large-scale, complex or distributed content delivery network (CDN) can use ADCs to ensure that network applications are consistently available and perform well.

ADCs can help businesses with:

ADCs can redirect network traffic to server clusters in data centers across the network. Specifically, they can collaborate with other ADCs to optimize the path that traffic takes through the internet, allowing each ADC to route requests to the data center closest to the client and minimize latency and round-trip time.

ADCs handle fast and slow clients by adjusting timeouts and buffer sizes accordingly. Controllers can also terminate connections to free up unneeded resources and multiplex connections for greater network efficiency.

ADCs continuously monitor the real-time health of app servers to ensure that traffic is only routed to responsive and available servers. They can ping web servers, attempt to establish TCP connections and make app-level requests to verify that a server is functioning optimally.

If a server fails or starts to function suboptimally, the ADC can redirect traffic to other servers and even initiate server restarts and other recovery actions (assuming they’re configured to do so).

Multitenancy designs allow different groups within an organization (DevOps, marketing and sales or specific application teams like customer service, inventory and payments) to share the same application infrastructure. Consolidating more services onto fewer devices—whether virtual or physical—helps organizations increase network agility and reduce costs related to administration, acquisition and ongoing support.

Edge computing involves processing data near the source (at the edge of the network), instead of relying solely on a central data center. ADCs deployed at the network edge can optimize local traffic, manage local security and compliance policies and ensure that apps work efficiently, even in distributed environments. This is particularly useful for IoT applications and services that require real-time processing capabilities.

ADCs have evolved to be more programmable and more easily integrated with SDN architectures, which separate the network's control layer from the data layer for more centralized, flexible resource management. Now, ADCs can be dynamically configured and managed with software controllers, enabling more agile responses to network traffic fluctuations and app demands.

Artificial intelligence (AI) and ML technologies can help ADCs predict traffic patterns, detect potential security threats and automate network management tasks, improving efficiency and reducing the need for human interaction with the network.

Application delivery controllers help businesses optimize traffic routing and maximize network availability, especially for enterprises that handle high-volume web traffic, host mission-critical applications or require high levels of data security.

ADCs offer businesses: