Of Client-Server Communication

We’ve recently been having some interesting conversations, both internally and with customers, about the challenges inherent in client-server software interaction, aka Web Services or Web APIs. The relatively baked state of web browsers and servers has shielded us from most of the issues that come with getting computers to talk to other computers.

It didn’t happen over-night, but today’s web browsing world rides on top of a well vetted pipeline of technology to give us good browsing (client-side) experiences. However, there are a lot of assumptions and moving parts behind our browser windows that get uncovered when working with web services (servers). There are skeletons in the closet unfortunately.

End-users’ web browsing demands eventually forced ports 80 and 443 (SSL) open across all firewalls and ISPs and we now take their availability for granted. When was the last time you heard someone ask “is port 80 open?” It’s probably been awhile. By 2000, server-side HTTP implementations (web servers) started solidifying and at the HTTP-level client and server tier there was relatively little incompatibility. Expectations around socket timeouts and HTTP protocol exchanges were clear, and both sides of the connection adhered to those expectations.

Enter the world of web-services/APIs.

We’ve been enjoying the stable client-server interaction that web browsing has provided over the past 15 years, but web services/APIs thrust the ugly realities that lurk beneath into view. When we access modern web services through lower-level software (e.g. something other than the browser), we have to make assumptions and implementation/configuration choices that the browser otherwise makes for us. Among them…

  • port to use for the socket connection
    • the browser assumes you always want ’80’ (HTTP) or ‘443’ (HTTPS)
    • the browser provides built-in encryption handling of HTTPS/SSL
  • URL parsing
    • the browser uses static rules for interpreting and parsing URLs
  • HTTP request methods
    • browsers inherently know when to use GET vs. POST
  • HTTP POST bodies.
    • browsers pre-define how POST bodies are structured, and never deviate from this methodology
  • HTTP header negotiation (this is the big one).
    • browsers handle all of the following scenarios out-of-the-box
    • Request
      • compression support (e.g. gzip)
      • connection duration types (e.g. keep-alive)
      • authentication (basic/oauth/other)
      • user-agent specification
    • Response
      • chunked responses
      • content-types. the browser has a pre-defined set of content types that it knows how to handle internally.
      • content-encoding. the browser knows how to handle various encoding types (e.g. gzip compression), and does so by default
      • authentication (basic/oauth/other)
  • HTTP Response body formats/character sets/encodings
    • browsers juggle the combination between content-encoding, content-type, and charset handling to ensure their international audience can see the information as its author intended.

Web browsers have the luxury of being able to lock down all of the above variables and not worry about changes in these assumptions. Having built browsers (Netscape/Firefox) in the past for a living, it’s still a very difficult task but at least the problem is constrained (e.g. ensure the end user can view the content within the browser). Web service consumers have to understand, and make decisions around, each of those points. Getting just one of them wrong can lead to issues in your application. These issues can range from being connectivity- or content handling-related to service authentication and can lead to long guessing games off “what went wrong?”

To further complicate the API interaction pipeline, many IT departments prevent abnormal connection activity from occurring. This means that while your application may be “doing the right thing” (TM), a system that sits between your application and the API with which it is trying to interact may prevent the exchange from occurring as you intended.

What To Do?

First off, you need to be versed not only in the documentation of the API you’re trying to use. Documentation is often outdated and doesn’t reflect actual implementations or account for bugs and behavioral nuances inherent in any API, so you also need to engage with its developer community/forums. From there, you need to ensure your HTTP client accounts for the assumptions I outline above and adheres to the API you’re interacting with. If you’re experiencing issues you’ll need to ensure your code is establishing the connection successfully, receiving the data it’s expecting, and parsing the data correctly. Never underestimate using a packet sniffer to view the raw HTTP exchange between your client and the server; debugging HTTP libraries at the code-level (even with logging) often don’t yield the truth behind what’s being sent to the server and received.

The Power of cURL

This is an entire blog post in and of itself, but the swiss army knife of any web service developer is cURL. In the right hands, cURL allows you to easily construct HTTP requests to test interaction with a web service. Don’t underestimate the translation of your cURL test to your software however.

  • Eric Marcoullier

    Jud — great overview post. I can’t wait to see the posts that come next as you drill down into each of these posts.

  • http://crisismaven.wordpress.com crisismaven

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