What CDN Usage Does for SharePoint Online (SPO) Performance

The (Not Entirely Obvious) TL;DR Answer

The Basis For That Answer: Introduction

Knowing that some folks simply want the answer up-front, I hope that I’ve satisfied their curiosity. The rest of this post is dedicated to explaining content delivery networks (CDNs), how they operate, and how you can easily enable them for use within your SharePoint Online (SPO) sites.

Let me first address a misconception that I sometimes encountered among SPO administrators and developers (including some MVPs) – that being that CDNs don’t really “do a whole lot” to help site and/or page performance. Sure, usage of a CDN is recommended … but a common misunderstanding is that a CDN is really more of a “nice-to-have” than “need-to-have” element for SPO sites. Of the people saying such things, oftentimes that judgment comes without any real research, knowledge, or testing. Skeptics typically haven’t read the documentation (the “non-RTFM crowd”) and haven’t actually spent any time profiling and troubleshooting the performance of SPO sites. Since I enjoy addressing perf. problems and challenges, I’ve been fortunate to experience firsthand the benefits that CDNs can bring. By the end of this post, I hope I’ll have made converts of a CDN skeptic or two.

What Is A CDN?

A CDN is a Content Delivery Network. There are a lot of (good) web resources that describe and illustrate what CDNs are and how they generally operate (like this one and this one), so I’m not going to attempt to “add value” with my own spin. I will simply call attention to a couple of the key characteristics that we really care about in our use of CDNs with SPO.

1. A CDN, at its core, can be thought of as a system of distributed (typically geographically so) servers for caching and offloading of SPO content. Rather than needing to go to the Microsoft network and data center where your tenant is located in order to fetch certain files from SPO, your browser can instead go to a (geographically) closer CDN server to get those same files.
2. By virtue of going to a closer CDN instead of the Microsoft network, the chance that you’ll have a “bigger pipe” with more bandwidth – and less latency/delay – are greater. This usually translates directly to an improvement in performance.
3. In addition to giving us the opportunity to download certain SPO files faster and with less delay, CDNs can do other things to improve the experience for the SPO files they serve. For instance, CDN servers can pass files back to the browser with cache-control headers that allow browsers to re-serve downloaded files to other users (i.e, to users who haven’t actually download the files), store downloaded files locally (to avoid having to download them again for a period of time), and more.

If you didn’t know about CDNs prior to this post, or didn’t understand how they could help you, I hope you’re beginning to see the possibilities!

The Arrival Of The Office 365 CDN

It wasn’t all that long ago that Microsoft was a bit more “modest” in its use of CDNs. Microsoft certainly made use of them, but prior to the implementation of its own content delivery networks, Microsoft frequently turned to a company called Akamai for CDN support.

When I first started presenting on SharePoint and its built-in caching mechanisms, I often spoke about Akamai and their edge network when talking about BLOB caching and how the max-age cache-control header could be configured and misconfigured. Back then, “Akamai” was basically synonymous with “CDN,” and that’s how many of us thought about the company. They were certainly leading the pack in the CDN service space.

Back then, if you were attempting to download a large file from Microsoft (think DVD images, ISO files, etc.), then there was a good change that the download link your browser would receive (from Microsoft’s servers) would actually point to an Akamai edge node near your location geographically instead of a Microsoft destination.

Fast forward to today. In addition to utilizing third-party CDNs like those deployed by Akamai, Microsoft has built (and is improving) their own first-party CDNs. There are a couple of benefits to this. First, many data regulations you may be subject to that prevent third-party housing of your data (yes, even in temporary locations like a CDN) can be largely avoided. In the case of CDNs that Microsoft is running, there is no hand-off to a third party and thus much less practical concern regarding who is housing your data.

Second, with their own CDNs, Microsoft has a lot more latitude and ability to extend the specifics of CDN configuration and operation its customers. And that’s what they’ve done with the Office 365 CDN.

Set Up The O365 CDN For Tenant’s Use

Now we’re talking! This next part is particularly important, and it’s what drove the creation of this post. It’s also the one bit of information that I promised Scott Stewart at Microsoft that I would try to get “out in the wild” as quickly and as visibly as possible.

So, if you remember nothing else from this post,please remember this:

Set-SPOTenantCdnEnabled -CdnType Public -Enable $true

That is the line of PowerShell that needs to be executed (against your SPO tenant, so you need to have a connection to your tenant established first) to enable transparent CDN support for public files. Run that, and non-sensitive files of public origin from SPO will begin getting cached in a CDN and served from there.

The line of PowerShell I shared goes through the SharePoint Online Management Shell – something most organizations using SPO (and their admins in particular) have installed somewhere.

It is also possible to enable CDN support if you’re using the PNP PowerShell module, if that’s your preference, by executing the following PowerShell:

Set-PnPTenantCdnEnabled -CdnType Public -Enable $true

No matter how you enable the CDN, it should be noted that the PowerShell I’ve elected to share (above) enables CDN usage for files of public origin only. It is easy enough to alter the parameters being passed in our PowerShell command so as to cover all files, public and private, by switching -CdnType to Both (with the SPO management shell) or executing another line of PowerShell after the first that swaps –type Public with –type Private (in the case of the SharePointPnP PowerShell module).

The reason I chose only public enablement is because your organization may be bound by restrictions or policies that prohibit or limit CDN use with private files. This is discussed a bit in the O365 CDN post originally cited, but it’s best to do your own research.

Enabling CDN support for public files, however, is considered to be safe in general.

What Sort Of Improvements Can I Potentially See?

The example that is called out in the link I just shared involves offloading of the jQuery JavaScript library from SPO to CDN. The real world numbers that were captured reduced fetch-and-load time from just over 1.5 seconds to less than half a second (<500ms). That is no small change … and that’s for just one file!

The Other (Secret) Benefit Of CDNs

I guess “Secret” is technically the wrong choice of term here. A more accurate description would be to say that I seldom hear or see anyone talking about another CDN benefit I consider to be very important and significant. That benefit, quite simply, involves improving file fetching and retrieval parallelism when a web page and associated assets (CSS, JS, images, etc.) are requested for download by your browser. In plain English: CDNs typically improve file downloading by allowing the browser to issue a greater number of concurrent file requests.

To help with this concept and its explanation, I’ve created a couple of diagrams that I’ll share with you. The first one appears below, and it is meant to represent the series of steps a browser might execute when retrieving everything needed to show a (SharePoint/SPO) page. As we’ve talked about, what is commonly thought of as a single page in a SharePoint site is, more accurately, a page containing all sorts of dependent assets: image files, JavaScript files, cascading style sheets, and a whole bunch more.

A request for a SharePoint page housed at http://www.thesite.com might start out with one request, but your browser is going to need all of the files referenced within the context of that page (default.aspx, in our case) to render correctly. See below:

To get what’s needed to successfully render the example SharePoint page without CDN support, we follow the numbers:

1. Your browser issues an HTTP request for the page you want to load – http://www.thesite.com/default.aspx in the case of example above.
2. That page request goes to (and is served by) the web server/front-end that can return the page.
3. Our page needs other files to render properly, like styling.css, logo.png, functions.js, and more. These get queued-up and returned according to some rules – more on this in a minute.
4. In step four (4), files get returned to the browser. Notice I say “no more than six at a time” in the illustration. That’s important and will come into play once we start introducing CDN support to the page/site.

You might be wondering, “Only six files at a time? Really? Why the limitation?” Well, I should start by saying the limit is probably six … maybe a bit more, perhaps a bit less. It depends on the browser you’re using what the specific number is. There was a good summary answer on StackOverflow to a related (but slightly different) question that provides some additional discussion.

Section eight (8) of the HTTP specification (RFC 2616) specifically addresses HTTP connections, how they should be handled, how proxies should be negotiated, etc. For our purposes, the practical implementation of the HTTP specification by modern browsers generally limits the number of concurrent/active connections a browser can have to any given host or URL to six (6).

Notice how I worded that last sentence. Since you folks are smart cookies, I’ll bet you’re already thinking “Wait a minute. CDNs typically have different URLs/hosts from the sites they cache” and you’re imaging what happens (or can happen) when a new source (i.e., different host/URL) is introduced.

This illustration roughly outlines the fetch process when a CDN is involved:

Steps one (1) through four (4) of the fetch process with a CDN are basically still the same as was illustrated without a CDN a bit earlier. When the page is served-up in step three (3) and returned in step four (4), though, there are some differences and additional activity taking place:

5. Since at least one CDN is in-use for the SPO environment, some of the resource links within the page that is returned will have different URLs. For instance, whereas styling.css was previously served from the SPO environment in the non-CDN example, it might now be referenced through the CDN host shown as http://cdn.source.com/styling.css
6. The requested file is retrieved, and …
7. Files come back to the client browser from the CDN at the same time they’re being passed-back from the SPO environment.

Since we’re dealing with two different URLs/hosts in our CDN example (http://www.thesite.com and cdn.source.com), our original six (6) file concurrent download limitation transforms into a 12 file limitation (two hosts serving six files a time, 2 x 6 = 12).

Whether or not the CDN-based process is ultimately faster than without a CDN depends on a great many factors: your Internet bandwidth, the performance of your computer, the complexity/structure of the page being served-up, and more. In the majority of cases, though, at least some performance improvement is observed. In many cases, the improvement can be quite substantial (as referenced and discussed earlier).

Conclusion

So, to sum-up: enable CDN use within your SPO tenant. The benefits are compelling!

References

1. Microsoft Docs: Use The Office 365 Content Delivery Network (CDN) With SharePoint Online
2. Imperva: What Is A CDN?
3. Akamai: What Does CDN Stand For?
4. MDN Web Docs: Cache-Control
5. Company: Akamai
6. Presentations: Caching-In For SharePoint Performance
7. Akamai: Download Delivery
8. Microsoft Docs: Configure Cache Settings For A Web Application In SharePoint Server
9. Blog Post: Do You Know What’s Going To Happen When You Enable The SharePoint BLOB Cache?
10. LinkedIn: Scott Stewart
11. Microsoft Docs: Enabling O365 CDN support for public origin files.
12. Microsoft Docs: Get Started With SharePoint Online Management Shell
13. Microsoft Docs: PnP PowerShell Overview
14. Microsoft Docs: Set Up And Configure The Office 365 CDN By Using PnP PowerShell
15. Microsoft Docs: What Performance Gains Does A CDN Provide?
16. Push Technologies: Browser Connection Limitations
17. StackOverflow: How many maximum number of simultaneous Chrome connections/threads I can start through Selenium WebDriver?
18. W3.org: RFC 2616, Section 8: Connection