CDN Monitoring: Strategy, Benefits & Best Practices

CDN Monitoring

A content delivery network (CDN) is a network of web servers distributed globally to deliver content faster.

All the assets of a web application, including scripts, images, videos, stylesheets, and base HTML, must be delivered to the client server as quickly and efficiently as possible. A CDN utilizes geographically distributed Points of Presence (PoP) to bring content closer to the end user and reduce latency. A CDN’s distributed network reduces the reliance on a central server to respond to all user requests.

CDN services have gained prominence over the last couple of decades and have become an integral part of the application delivery chain.

CDNs are used by service providers across verticals, from e-commerce and social media to advertising and streaming services. Popular CDN providers include Akamai, Fastly, Cloudflare, Level3, MaxCDN, Limelight, and Microsoft Azure.

Fine-tuning application performance involves optimizing the different components that make up the application, and using a CDN is one such critical component. The role of a CDN in the application delivery chain also makes it an important factor in defining the digital end-user experience. CDN performance can directly impact the user experience, making CDN monitoring crucial to any monitoring strategy.

What are the benefits of using a CDN?

CDNs have become a critical extension of modern IT infrastructure. CDN providers typically leverage their expertise and platforms to act as partners in enabling infrastructure and helping make business-related decisions. There are many advantages to using a CDN. Investing in a CDN service will help:

• Improve load time: Application load time has improved significantly because content is served from a CDN server closest to the user’s location, reducing latency. A faster application means lower bounce rates and higher user engagement.
• Reduce bandwidth costs: CDNs offer optimization services, such as caching and image optimization, which can help reduce bandwidth usage on the hosting server.
• Better availability: The distributed network can serve more requests, so a CDN can handle traffic better during a sudden surge.
• Additional security: CDNs can provide DDoS mitigation and other critical security optimizations.

A continuous CDN monitoring strategy that specifically targets key features and deliverables of the CDN, such as network performance, web performance, CDN infrastructure, origin-to-edge stability, and content integrity validation, is essential for achieving an organization’s digital experience goals.

Definitions of common CDN terminology

Origin server

An origin server hosts the application, processes, and responds to all incoming client requests. The distance between the client and the origin server increases the time it takes for the requested resource to load.

Edge server

An edge server is placed on the network “edge” to quickly process requests. These servers are within a CDN at each Internet Exchange Point (IxP) in the network, which reduces transit time to a resource location.

Caching

Caching involves replicating resources and temporarily storing these files in a “cache” that can be easily accessed when requested. Scripts, HTML files, and images are usually cached to save download time and reduce latency. CDN servers store cache to optimize content delivery.

Anycast

Anycast is a method of network routing that routes traffic to a specific location via multiple paths, allowing each router to choose an optimal path based on distance, cost, latency, and network congestion. CDNs use Anycast to route traffic to the closest data center, helping manage network congestion and mitigate DDoS attacks.

How does a CDN work?

A CDN is essentially a network of servers linked together that share the same content and deliver it securely and as fast as possible when requested. CDN providers have servers in strategic points between different networks. These IXPs connect major Internet providers and enable traffic to flow between these networks. The PoPs located on these interconnected network highways have access to high-speed data paths, enabling CDNs to deliver content faster.

The data centers maintained by CDN providers across the globe offer redundancy, making it easier to handle hardware failures or sudden traffic surges and to prevent outages caused by server overload.

The above diagram illustrates how end users are routed to the nearest CDN server. In the absence of an edge server, the end user is routed to the origin, reducing latency and the time required to process each server request.

Why is CDN monitoring critical?

A CDN monitoring strategy that tracks the CDN’s key features and deliverables will ensure its services are utilized effectively. Reducing the time to detect problems and drastically improving mean time to resolution are obvious outcomes of this monitoring strategy. Many organizations are now adopting a multi-CDN approach to ensure performance worldwide. Increasingly, organizations are also utilizing geo-specific CDNs and real-time performance-based CDN routing, adding greater complexity to the content delivery path. These changing conditions make CDN monitoring an indispensable tool for regularly assessing CDN performance and evaluating new CDN vendors to address potential gaps.

Monitoring CDN and origin performance helps determine how much your key metrics have improved based on the presence of the CDN. Additionally, CDN monitoring offers:

• The ability to benchmark CDN performance. The data aggregated over time helps identify performance patterns and areas for optimization.
• The ability to hold the CDN vendor accountable in case of an SLA breach. IT teams can leverage aggregated performance data to ensure their CDN vendor is delivering the best possible service.
• The ability to quickly detect and fix performance issues at the edge or origin. Monitoring origin and edge servers is key to delivering a superior end-user experience.
• The ability to better manage performance incidents, quickly detect performance issues, and lower the mean time to resolution.

A continuous CDN monitoring strategy that specifically targets key features and deliverables of the CDN, such as network performance, web performance, CDN infrastructure, origin-to-edge stability, and content integrity validation, is essential for achieving an organization’s digital experience goals.

Building an effective CDN monitoring strategy

CDNs have sophisticated infrastructures. There are multiple interdependent components that can create bottlenecks and performance issues. As stated, CDNs add to the existing complexity of the application architecture; for example, the use of cache and optimized routing between CDN PoPs can potentially cause latency issues. Monitoring all the different processes and network paths is vital to maintaining optimal CDN performance.

When building a CDN monitoring strategy, what should you monitor and where should you start?

Check DNS resolutions

DNS resolution is a lengthy process with multiple server hops. Introducing a CDN to the equation can add additional points of failure. Using synthetic monitoring to monitor DNS resolution allows you to emulate a client DNS query. This makes it easier to detect and resolve DNS issues quickly, for example, misconfigured nameservers in the DNS resolution chain.

When a Managed DNS is in use, synthetic monitoring helps analyze performance and availability and identify issues across the levels of DNS resolution. Monitoring DNS will also help identify configuration issues and serve as the first line of defense against a DDoS attack.

CDNs use DNS redirection to route requests to the appropriate edge servers, enabling webpage resources to load faster. There are instances where CDN nameservers are slow to respond, resulting in performance degradation. It is important to monitor such cases and the best way to do so is by using synthetic monitoring for DNS and the webpage itself. The scatterplot graph below highlights DNS failures. In this scenario, authoritative nameservers on the same subnet IP address fail to respond when requests come from a specific ISP.

You should also monitor DNS performance between the CDN and the origin. The chart shows a trend line of DNS resolution time between the CDN and the origin, and we can correlate the dip in availability with the spike in CDN DNS response time.

Check CDN mapping

CDN providers identify and map optimal paths for the requested content based on the end user’s location. CDN monitoring will identify mapping anomalies and sub-optimal peering policies. It will also help verify if the end user is being served from the nearest edge server.

Consider this example of a synthetic test to monitor CDNs. Most CDNs either provide a CNAME Resource Record (e.g., *akamaiedge.com) or provide a set of A records, which you need to map the domain name to. When setting up tests to evaluate CDNs, you should ideally have two sets of tests running:

1. A set of tests that hits the domain names mapped to the CDN.
2. Another set of tests, which hit the domain names, overriding the IP addresses to those of the origin servers.

Running these synthetic monitoring tests from multiple locations will provide useful performance data you can use to compare CDN and origin performance across regions.

Additionally, analyzing traceroute data will help compare and contrast the network round-trip times, that is, the number of hops required to reach a server when using a CDN versus otherwise. The same use case is also relevant when comparing one CDN versus another.

With synthetic monitoring, you can run tests against a domain served by a specific CDN. Overriding the DNS will allow you to test origin performance. The chart below shows a comparison of origin vs. CDN; the webpage response time (the total time to load the full page) for CDN was lower than for the origin.

We can also analyze different performance metrics to compare CDN performance against the origin to determine optimal content mapping and routing.

Check cache hit ratio

Cache hit ratio compares the content requests the CDN cache can process successfully to the total number of requests it handles. An efficient CDN will have a high cache hit ratio. In contrast, a higher cache miss means requests are routed back to the origin as the cache was unable to deliver the requested content.

Synthetic monitoring can provide a detailed view of CDN cache vs. CDN origin, for instance, in the chart below, you can see exactly how cache hits and cache misses measure against one another in terms of latency.

Advanced monitoring capabilities allow you to compare the origin vs. cache KPIs per city. You can use a number of metrics to compare performance, including average ping round-trip times, average response, average connect, and so on.

Check latency

CDN monitoring measures end-user-to-edge location latency to track performance degradation between the end user and a specific edge server or across multiple edge servers. Edge-to-origin data center latency is also important to measure, especially if the enterprise has multiple origin data centers. In the chart below, we can identify the CDN origin latency trend (the correlation between wait and response times).

It is also important to identify the bottleneck on the page once the hosts on the page have been segregated into first-party, CDN, third-party, etc. How much bottleneck, for example, is created by a specific zone, and how does it impact the overall performance of the page? Metrics to consider include webpage response, bottleneck time, and availability.

CDNs use load balancing to distribute requests across available servers, routing traffic optimally without taxing any single server. This process is called Global Server Load Balancing (GSLB). The traffic is distributed across a set of connected servers worldwide to improve reliability and reduce latency. CDN monitoring will ensure optimal load balancing and trigger an alert if there is an unusual surge in traffic.

Check image optimization

Images often account for most of the bytes downloaded on a web page and can occupy a significant amount of visual space. As a result, optimizing images can often yield some of the largest byte savings and performance improvements for your website. The fewer the bytes a browser has to download, the lower the bandwidth consumption, allowing the resource to load faster.

CDN vendors offer image optimization as an additional feature – from simple compression to advanced optimization, the image is delivered based on end-user connectivity, device, and other factors. It is important to ensure these additional services are functioning and delivering an improved end-user experience. Synthetic monitoring allows you to capture and compare metrics relevant to image optimization.

Here is an example using Akamai Image Manager. You can set up synthetic tests that will allow you to compare performance before and after optimization.

Enhance CDN monitoring with synthetic monitoring

Proactive monitoring is necessary to catch potential incidents early and resolve them quickly before they impact the end-user experience. Synthetic monitoring provides all the insights needed to optimize and maintain consistent CDN performance. With synthetic monitoring tools, you can:

• Monitor the last-mile network to verify optimal CDN performance and ensure it maps end users to the relevant PoP.
• Track performance across multiple devices, networks, and locations to ensure consistent performance.
• Use the performance data to optimize the application, including content, code, and the user journey.
• Identify any bottlenecks, latency, or availability issues within the application infrastructure.
• Perform A/B tests on content to evaluate how these changes impact the end-user experience.
• Benchmark performance of CDNs in a multi-CDN environment.
• Track CDN performance for SLA breaches.

Conclusion

Content Delivery Networks are used by organizations as part of their continuous effort to optimize application performance and deliver a consistent end-user experience. Deploying a CDN can yield a host of benefits, including improved site availability, reduced latency, optimized content delivery, and the ability to meet performance expectations when expanding into new markets.

A robust CDN monitoring strategy will allow organizations to monitor the performance of their chosen CDN, from the evaluation phase through to deployment. Synthetic monitoring tools offer a range of performance metrics that enable faster detection of potential performance issues. With a proactive approach to CDN monitoring, you can track CDN performance and ensure content is delivered efficiently with minimal latency, without compromising the digital experience.

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