What Ops Needs to Know about APIs and Compression

I’ve been reading up on APIs cause, coolness. And in particular I really enjoyed reading Best Practices for Designing a Pragmatic RESTful API because it had a lot of really good information and advice.

And then I got to the part about compressing your APIs.

Before we go too far let me first say I’m not saying you shouldn’t compress your API or app responses. You probably should. What I am saying is that where you compress data and when are important considerations.

That’s because generally speaking no one has put their web server (which is ultimately what tends to serve up responses, whether they’re APIs or objects, XML or JSON) at the edge of the Internet. You know, where it’s completely vulnerable. It’s usually several devices back in the networking gauntlet that has be run before data gets from the edge of your network to the server.

This is because there are myriad bad actors out salivating at the prospect of a return to an early aughts data center architecture in which firewalls, DDoS protection, and other app security services were not physically and logically located upstream from the apps they protect today.

Cause if you don’t have to navigate the network, it’s way easier to launch an attack on an app.

Today, we employ an average of 11 different services in the network, upstream from the app, to provide security, scale, and performance-enhancing services. Like compression.  

Now, you can enable compression on the web server. It’s a standard thing in HTTP and it’s little more than a bit to flip in the configuration. Easy peasy performance-enhancing change, right?

Except that today that’s not always true.

The primary reason compression improves performance is because when it reduces the size of data it reduces the number of packets that must be transmitted. That reduces the potential for congestion that causes a Catch-22 where TCP retransmits increase congestion that increases packet loss that increases… well, you get the picture. This is particularly true when mobile clients are connecting via cellular networks, because latency is a real issue for them and the more round trips it takes, the worse the application experience.

Suffice to say that the primary reason compression improves performance is that it reduces the amount of data needing to be transmitted which means “faster” delivery to the client. Fewer packets = less time = happier users.

That’s a good thing. Except when compression gets in the way or doesn’t provide any real reduction that would improve performance.

What? How can that be, you ask.

Remember that we’re looking for compression to reduce the number of packets transmitted, especially when it has to traverse a higher latency, lower capacity link between the data center and the client.

It turns out that sometimes compression doesn’t really help with that.

Consider the aforementioned article and its section on compressing. The author ran some tests, and concluded that compression of text-based data produces some really awesome results:

Let's look at this with a real world example. I've pulled some data from GitHub's API, which uses pretty print by default. I'll also be doing some gzip comparisons:

$ curl https://api.github.com/users/veesahni > with-whitespace.txt
$ ruby -r json -e 'puts JSON JSON.parse(STDIN.read)' < with-whitespace.txt > without-whitespace.txt
$ gzip -c with-whitespace.txt > with-whitespace.txt.gz
$ gzip -c without-whitespace.txt > without-whitespace.txt.gz

The output files have the following sizes:

• without-whitespace.txt - 1252 bytes
• with-whitespace.txt - 1369 bytes
• without-whitespace.txt.gz - 496 bytes
• with-whitespace.txt.gz - 509 bytes

In this example, the whitespace increased the output size by 8.5% when gzip is not in play and 2.6% when gzip is in play. On the other hand, the act of gzipping in itself provided over 60% in bandwidth savings. Since the cost of pretty printing is relatively small, it's best to pretty print by default and ensure gzip compression is supported!

To further hammer in this point, Twitter found that there was an 80% savings (in some cases)when enabling gzip compression on their Streaming API. Stack Exchange went as far as to never return a response that's not compressed!

Wow! I mean, from a purely mathematical perspective, that’s some awesome results. And the author is correct in saying it will provide bandwidth savings.

What those results won’t necessarily do is improve performance because the original size of the file was already less than the MSS for a single packet. Which means compressed or not, that data takes exactly one packet to transmit. That’s it. I won’t bore you with the mathematics, but the speed of light and networking says one packet takes the same amount of time to transit whether it’s got 496 bytes of payload or 1396 bytes of payload. The typical MSS for Ethernet packets is 1460 bytes, which means compressing something smaller than that effectively nets you nothing in terms of performance. It’s like a plane. It takes as long to fly from point A to point B whether there are 14 passengers or 140. Fuel efficiency (bandwidth) is impacted, but that doesn’t really change performance, just the cost.

Furthermore, compressing the payload at the web server means that web app security services upstream have to decompress if they want to do their job, which is to say scan responses for sensitive or excessive data indicative of a breach of security policies. This is a big deal, kids. 42% of respondents in our annual State of Application Delivery survey always scan responses as part of their overall security strategy to prevent data leaks. Which means they have to spend extra time to decompress the data to evaluate it and then recompress it, or perhaps they can’t inspect it at all.

Now, that said, bandwidth savings are a good thing. It’s part of any comprehensive scaling strategy to consider the impact of increasing use of an app on bandwidth. And a clogged up network can impact performance negatively so compression is a good idea. But not necessarily at the web server. This is akin to carefully considering where you enforce SSL/TLS security measures, as there are similar impacts on security services upstream from the app / web server.

That’s why the right place for compression and SSL/TLS is generally upstream, in the network, after security has checked out the response and it’s actually ready to be delivered to the client. That’s usually the load balancing service or the ADC, where compression can not only be applied most efficiently and without interfering with security services and offsetting the potential gains by forcing extra processing upstream.

As with rate limiting APIs, it’s not always a matter of whether or not you should, it’s a matter of where you should.

Architecture, not algorithms, are the key to scale and performance of modern applications.