Monday, 7 August 2017

Generating self-signed HTTPS certificates with subjectAltNames

We provide online services via a bunch of different websites, using federated authentication so that if you sign in to our authentication server, you get a * cookie that’s sent to any other server on our domain.

We use local wildcard DNS, so there’s a * record that resolves everything to, and for each developer machine we create a  * record that’s an alias for hostname, so you can browse to<machine> to see code running on another developer’s workstation, or to view your own local development code.

This works pretty well, but getting a local development system set up involves running local versions of several different apps – and since Google Chrome now throws a security error for any HTTPS site whose certificate doesn’t include a “subject alternative name” field, getting a bunch of local sites all happily sharing the same cookies over HTTPS proved a bit fiddly.

So… here’s a batch file that will spit out a bunch of very useful certificates, adapted from this post on

How it works

  1. Get openssl.exe working - I use the version that's shipped with Cygwin, installed into C:\Windows\Cygwin64\bin\ and added to my system path.
  2. Run makecert.bat. If you don't want to specify a password, just provide a blank one (press Enter). This will spit out three files:
    • local_and_hostname.crt
    • local_and_hostname.key
    • local_and_hostname.pfx
  3. Double-click the local_and_hostname.crt file, click "Install Certificate", and use the Certificate Import Wizard to import it. Choose "Local Machine" as the Store Location, and "Trusted Root Certification Authorities" as the Certificate Store.
  4. Open IIS, select your machine, open "Server Certificates" from the IIS snapin, click "Import..." in the Actions panel
  5. Select the local_and_hostname.pfx certificate created by the batch file. If you used a password when exporting your PKCS12 (.pfx) file, you'll need to provide it here
  6. Finally, set up your IIS HTTPS bindings to use your new certificate.

Yay! Security! 

Monday, 31 July 2017

Deployment Through the Ages

Vanessa Love just posted this intriguing little snippet on Twitter:

And I got halfway through sticking some notes into the Google doc, and then thought actually this might make a fun blog post. So here’s how deployment has evolved over the 14 years since I first took over the hallowed mantle of [email protected].

2003: Beyond Compare (maybe?)

The whole site was classic ASP – no compilation, no build process, all connection credentials and other settings were managed as application variables in the global.asa file. On a good day, I’d get code running on my workstation, test it in our main target browsers, and deploy it using a visual folder comparison tool. It might have been Beyond Compare; it might have been something else. I honestly can’t remember and the whole thing is lost in the mists of time. But that was basically the process – you’d have the production codebase on one half of your screen and your localhost codebase on the other half, and you’d cherry-pick the bits that needed to be copied across.

Of course, when something went wrong in production, I’d end up reversing the process – edit code directly on live (via UNC share), normally with the phone wedged against my shoulder and a user on the other end; fix the bug, verify the user was happy, and then do a file sync in the other direction to get everything from production back onto localhost. Talk about a tight feedback loop – sometimes I’d do half-a-dozen “deployments” in one phone call. It was a simpler time, dear reader. Rollback plan was to hammer Ctrl-Z until it’s working again; disaster recovery was tape backups of the complete source tree and database every night, and the occasional copy’n’paste backup of wwwroot before doing something ambitious.

Incidentally, I still use Beyond Compare almost daily – I have it configured as my merge tool for fixing Git merge conflicts. It’s fantastic.

2005: Subversion

Once we hired a second developer (hey Dan!) the Beyond Compare approach didn’t really work so well any more, so we set up a Subversion server. You’d get stuff running on localhost, test it, maybe share an link (hooray for local wildcard DNS) so other people could see it, and when they were happy, you’d do an svn commit, log into the production web server (yep, the production web server – just the one!) and do an svn update. That would pull down the latest code, update everything in-place. There was still the occasional urgent production bugfix. One of my worst habits was that I’d fix something on production and then forget to svn commit the changes, so the next time someone did a deployment (hey Dan!) they’d inadvertently reintroduce whatever bug had just been fixed and we’d get upset people phoning up asking why it was broken AGAIN.

2006: FinalBuilder

This is where we start doing things with ASP.NET in a big way. I still dream about OnItemDataBound sometimes… and wake up screaming, covered in sweat. Fun times. The code has all long since been deleted but I fear the memories will haunt me to my grave.

Anyway. By this point we already had the Subversion server, so we had a look around for something that would check out and compile .NET code, and went with FinalBuilder. It had a GUI for authoring build pipelines and processes, some very neat features, and could deploy .NET applications to IIS servers. This was pretty sophisticated for 2006. 

2008: test server and msdeploy

After one too many botched FinalBuilder deployments, we decided that a dedicated test environment and a better deployment process might be a good idea. Microsoft had just released a preview of a new deployment tool called MSDeploy, and it was awesome. We set up a ‘staging environment’ – it was a spare Dell PowerEdge server that lived under my desk, and I’d know when somebody accidentally wrote an infinite loop because I’d hear the fans spin up. We’d commit changes to Subversion, FinalBuilder would build and deploy them onto the test server, we’d give everything a bit of a kicking in IE8 and Firefox (no Google Chrome until September 2008, remember!) and then – and this was magic back in 2008 – you’d use msdeploy.exe to replicate the entire test server into production! Compared to the tedious and error-prone checking of IIS settings, application pools and so on, this was brilliant. Plus we’d use msdeploy to replicate the live server onto new developers’ workstations, which was a really fast, easy way to get them a local snapshot of a working live system. For the bits that still ran interpreted code, anyway.

2011: TeamCity All The Things!

By now we had separate dev, staging and production environments, and msdeploy just wasn’t cutting it any more. We needed something that can actually build different deployments for each environments – connection strings, credentials, and so on. And there’s now support in Visual Studio for doing XML configuration transforms, so you create a different config file for every environment, check those into revision control, and get different builds for each environment. I can’t remember exactly why we abandoned FinalBuilder for TeamCity, but it was definitely a good idea – TeamCity has been the backbone of our build process ever since, and it’s a fantastically powerful piece of kit.

2012: Subversion to GitHub

At this point, we’d grown from me, on my own doing webmaster stuff, to a team of about six developers. Even Subversion is starting to creak a bit, especially when you’re trying to merge long-lived branches and getting dozens of merge conflicts, so we start moving stuff across to GitHub. It takes a while – I’m talking months – for the whole team to stop thinking of Git as ‘unnecessarily complicated Subversion’ and really grok the workflow, but we got there in the end.

Our deployment process at this point was to commit to the Git master branch, and wait for TeamCity to build the development version of the package. This would get built and deployed. Once it was tested, you’d use TeamCity to build and deploy the staging version – and if that went OK, you’d build and deploy production. Like very step on this journey, it was better than anything we’d had before, but had some obvious drawbacks. Like the fact we had several hundred separate TeamCity jobs and no consistent way of managing them all.

2013: Octopus Deploy and Klondike

When we started migrating from TeamCity 6 to TeamCity 7, it became rapidly apparent that our “build everything several times” process… well, it sucked. It was high-maintenance, used loads of storage space and unnecessary CPU cycles, and we needed a better system.

Enter Octopus Deploy, whose killer feature for us was the ability to compile a .NET web application or project into a deployment NuGet package (an “octopack”), and then apply configuration settings during deployment. We could build a single package, and then use Octopus to deploy and configure it to dev, staging and live. This was an absolute game-changer for us. We set up TeamCity to do continuous integration, so that every commit to a master branch would trigger a package build… and before long, our biggest problem was that we had so many packages in TeamCity that the built-in NuGet server started creaking.

This started life as an experimental build of themotleyfool/NuGet.Lucene – which we actually deployed onto a server we called “Klondike” (because klondike > gold rush > get nuggets fast!) – and it worked rather nicely. Incidentally, that NuGet.Lucene library is now the engine behind themotleyfool/Klondike, a full-spec NuGet hosting application – and I believe our internal hostname was actually the inspiration for their project name. That was a lot of fun for the 18 months or so that Klondike existed but we were still running the old NuGet.Lucene codebase on a server called ‘klondike’. It’s OK, we’ve now upgraded it and everything’s lovely.

It was also in 2013 that we started exploring the idea of automatic semantic versioning – I wrote a post in October 2013 explaining how we hacked together an early version of this. Here’s another post from January 2017 explaining how it’s evolved. We’re still working on it. Versioning is hard.

And now?

So right now, our build process works something like this.

  1. Grab the ticket you’re working on – we use Pivotal Tracker to manage our backlogs
  2. Create a new GitHub branch, with a name like 12345678_fix_the_microfleems – where 12345678 is the ticket ID number
  3. Fix the microfleems.
  4. Push your changes to your branch, and open a pull request. TeamCity will have picked up the pull request, checked out the merge head and built a deployable pre-release package (on some projects, versioning for this is completely automated)
  5. Use Octopus Deploy to deploy the prerelease package onto the dev environment. This is where you get to tweak and troubleshoot your deployment steps.
  6. Once you’re happy, mark the ticket as ‘finished’. This means it’s ready for code review. One of the other developers will pick it up, read the code, make sure it runs locally and deploys to the dev environment, and then mark it as ‘delivered’.
  7. Once it’s delivered, one of our testers will pick it up, test it on the dev environment, run it past any business stakeholders or users, and make sure we’ve done the right thing and done it right.
  8. Finally, the ticket is accepted. The pull request is merged, the branch is deleted. TeamCity builds a release package. We use Octopus to deploy that to staging, check everything looks good, and then promote it to production.

And what’s on our wishlist?

  • Better production-grade smoke testing. Zero-footprint tests we can run that will validate common user journeys and scenarios as part of every deployment – and which potentially also run as part of routine monitoring, and can even be used as the basis for load testing.
  • Automated release notes. Close the loop, link the Octopus deployments back to the Pivotal tickets, so that when we do a production deployment, we can create release notes based on the ticket titles, we can email the person who requested the ticket saying that it’s now gone live, that kind of thing.
  • Deployments on the dashboards. We want to see every deployment as an event on the same dashboards that monitor network, CPU, memory, user sessions – so if you deploy a change that radically affects system resources, it’s immediately obvious there might be a correlation.
  • Full-on continuous deployment. Merge the PR and let the machines do the rest.

So there you go – fourteen years worth of continuous deployments. Of course, alongside all this, we’ve moved from unpacking Dell PowerEdge servers and installing Windows 2003 on them to running Chef scripts that spin up virtual machines in AWS and shut them down again when nobody’s using them – but hey, that’s another story.

Thursday, 27 July 2017

Securing Blogger with CloudFlare and HTTPS

As you may have read, life is about to get a whole lot harder for websites without HTTPS. Now this site is hosted on Blogger – I used to run my own MovableType server, but I realised I was spending way more time messing around with the software than I was actually writing blog posts, so I shifted the whole thing across to Blogger about a decade ago and never really looked back.

One of the limitations of Blogger is that it doesn’t support HTTPS if you’re using custom domains – there’s no way to install your own certificate or anything. So, since Chrome’s about to crank up the warnings for any websites that don’t use HTTPS, I figured I ought to set something up. Enter CloudFlare, who are really rather splendid.

First, you sign up. (bonus points for them NOT forcing you to choose a password that contains a lowercase letter, an uppercase letter, a number, a special character, the poo emoji and the Mongolian vowel separator).

Second, you tell them which domain you want to protect:


They scan all your DNS records, which takes about a minute – and not only is there a nice real-time progress bar keeping you in the loop, they use this opportunity to play a really short video explaining what's going on. I think this is absolute genius.


Finally, after checking it's picked up all your DNS records properly (it had), you tell your domain registrar to update the nameservers for your domain to CloudFlare's DNS servers, give it up to 24 hours, and you're done. Zero downtime, zero service interruption – the whole thing was smooth, simple, and completely free-as-in-beer.

Yes, I realise this does not encrypt content end-to-end. For what we're doing here, this is absolutely fine. It'll secure your traffic against dodgy hotel wi-fi and unscrupulous internet service providers - and if anyone's genuinely intercepting HTTP traffic between CloudFlare and Google, I'm sure they can think of more exciting things to do with it than mess around with my blog posts.

Having done that, I then had to use the Google Chrome console to track down the resources – photos and the odd bit of script – that were being hosted via HTTP, and update them to be HTTPS. The only thing I couldn't work out how to fix was the search bar that's embedded in Blogger's default page layout – it's injected by JavaScript, it's hosted by Google's CDN (so I can't use any of CloudFlare's clever rewriting tricks to fix it), it's stuck inside an IFRAME, and it points to – see the plain HTTP with no S?


After an hour or so of messing around with CSS, I gave up, posted a question on the ProWebmasters Stack Exchange, and – of course, immediately found the solution; go into Blogger, Layout, find the Navbar gadget, click Edit, and there's an option to switch the nav off entirely.

So there you go. Thanks to CloudFlare, now has a green padlock on it. I don't know about you, but I take comfort in that.


Friday, 21 July 2017

Summer 2017 .NET Community Update

Summer here in the UK is normally pretty quiet, but this year there's so much going on around .NET and the .NET community that I thought this would be a great opportunity to do a bit of a round-up and let you all know about some of the great stuff that's going on.

First, there's the news of two new .NET user groups starting up in southern England. Earlier this week, I was down in Bournemouth speaking at the first-ever meetup of the new .NET Bournemouth group, and thoroughly enjoyed it. Three speakers – Stuart Blackler, Tommy Long and me – with talks on leadership, agile approaches to information security, and an updated version of my "happy code" talk I've done at a few conferences already this year. The venue and A/V setup worked flawlessly, there was a strong turnout, and some really good questions and discussion after each of the talks – I think it's going to turn out to be a really engaging group, so if you're in that part of the world, stop by and check them out. Their next few meetup dates are on already.

Next month, Steve Gordon is starting a new .NET South East group based in Brighton, who will be kicking off with their inaugural meetup on August 22nd with Steve talking about Docker for .NET developers.

Brighton based .NET South East user group logo

There's a great post on Steve's blog explaining what he's doing and what he's hoping to get out of the group, and they're also on (and I have to say, they've done an excellent job of branding the Meetup site – nice work!)

It's an exciting time for .NET – between the cross-platform stuff that's happening around Xamarin and .NET Core, new tooling like JetBrains Rider and Visual Studio Code, and the growing number of cloud providers who are supporting C# and .NET Core for building serverless cloud applications, we've come a long, long way from the days of building Windows Forms and databinding in Visual Studio .NET.

If you're interested in really getting to grips with the future of .NET, join us at the Progressive.NET Tutorials here in London in September. With a great line-up of speakers including Julie Lerman, Jon Skeet, Jon Galloway, Clemens Vasters and Rachel Appel – plus Carl Franklin and Rich Campbell from DotNetRocks, and a few familiar faces you might recognise from the London.NET gang – it promises to be a really excellent event. It goes a lot deeper than most conferences – with one day of talks and two days of hands-on workshops, the idea is that attendees don't just go away with good ideas, they actually leave with running code, on their laptops, that they can refer back to when they take those ideas back to the office or to their own projects. Check out the programme, follow #ProgNET on Twitter, and hopefully see some of you there. 

Then on Saturday 16th September – the day after Progressive.NET – is the fourth DDD East Anglia community conference in Cambridge. Their call for speakers is now closed, but voting is open until July 29th – so sign up, vote on the sessions you want to see – or just vote for mine if you can't make your mind up ;) - and hopefully I'll see some of you in Cambridge.

t_shirt_logo_thumb[23]Finally, just in case any readers of this blog DON'T know about the London .NET User Group… yep, we have .NET User Group! In London! I know, right? We're on, and on Twitter as @LondonDotNet, and we meet every month at SkillsMatter's CodeNode building near Moorgate.

Our next meetup is on August 8th, with Ana Balica talking about the history and future of HTTP and HTTP/2, and Steve Gordon – and on September 12th we've got Rich Campbell joining us for a Progressive.NET special meetup and presenting the History of .NET as you've never heard it before.

New people, new meetups, new platforms and new ideas. Like I said, it's a really exciting time to be part of the .NET community – join us, come to a meetup, follow us online, and let's make good things happen.

Tuesday, 4 July 2017

Use Flatscreens

This started life as a lightning talk for PubConf after NDC in Sydney, back in August 2016… and after quite a lot of tweaking, editing and learning to do all sorts of fun things with Adobe AfterEffects and Premiere, it's finally on YouTube. The inspiration is, of course, "Wear Sunscreen", Baz Luhrmann's 1999 hit song based on an essay written by Mary Schmich. Video footage and stock photography is all credited at the end of the clip, and the music, vocals, video, audio and, well, basically everything else is by me. Happy listening - and don't forget to use flatscreens :)

Ladies and gentlemen of the class of 2017… use flat screens. If I could offer you only one tip for the future, flat screens would be it. The benefits of flat screens have been proved by Hollywood, whereas the rest of my advice has no basis more reliable than my own meandering experience.
I will dispense this advice... now.

Enjoy the confidence and optimism of greenfield projects. Oh, never mind. You will not appreciate the confidence and optimism of greenfield until everything starts going to hell. But trust me, when you finally ship, you'll look back at the code you wrote and recall in a way you can't grasp now how simple everything seemed, and how productive you really were. Your code is not as bad as you imagine.

Don't worry about changing database providers. Or worry, but know that every company who ever used an OR/M in case they needed to switch databases never actually did it. The real problems in your projects are the dependencies you don't control; the leaking air conditioner that floods your data centre at 5pm on the Thursday before Christmas.

Learn one thing every day that scares you.


Don't reformat other people's codebases; don't put up with people who reformat yours.


Don't get obsessed with frameworks. Sometimes they help, sometimes they hurt. It's the user experience that matters, and the user doesn't care how you created it.

Remember the retweets you receive; forget the flame bait. If you succeed in doing this, tell me how.

Keep your old hard drives. Throw away your old network cards.


Don't feel guilty if you don't understand f#. Some of the most productive junior developers I've worked with didn't know F#. Some of the best systems architects I know still don't.

Write plenty of tests.

Be kind to your keys; you'll miss them when they're gone.

Maybe you have a degree; maybe you don't. Maybe you have an open source project; maybe you won't. Maybe you wrote code that flew on the Space Shuttle; maybe you worked on Microsoft SharePoint. Whatever you do, keep improving, and don't worry where your next gig is coming from. There's a big old world out there, and they're always going to need good developers.

Look after your brain. Don't burn out, don't be afraid to take a break. It is the most powerful computer you will ever own.

Launch, even if you have no users but your own QA team.

Have a plan, even if you choose not to follow it.

Do NOT read the comments on YouTube : they will only make you feel angry.

Cache your package dependencies; you never know when they'll be gone for good.

Read your log files. They're your best source of information, and the first place you'll notice if something's starting to go wrong.

Understand that languages come and go, and that it's the underlying patterns that really matter. Work hard to fill the gaps in your knowledge, because the wiser you get,  the more you'll regret the things you didn't know when you were young.

Develop in 86 assembler once, but stop before it makes you smug; develop in Visual Basic once, but stop before it makes you stupid.


Accept certain inalienable truths. Your code has bugs, you will miss your deadlines, and you, too, might end up in management. And when you do, you'll fantasize that back when you were a developer, code was bug-free, deadlines were met, and developers tuned their database indexes.

Tune your database indexes.

Don't deploy your code without testing it. Maybe you have a QA team. Maybe you have integration tests. You never know when either one might miss something.

Don't mess too much with your user interface, or by the time you ship, it will look like a Japanese karaoke booth.

Be grateful for open source code, but be careful whose code you run. Writing good code is hard, and open source is a way of taking bits from your projects folder, slapping a readme on them, and hoping if you put them on GitHub somebody else will come along and fix your problems.

But trust me on the flat screens.

Wednesday, 28 June 2017

Interview with Channel 9 at NDC Oslo

I was in Oslo earlier this month, where – as well as doing the opening keynote, a couple of talks, a workshop on hypermedia systems and PubConf – I had the chance to chat with Seth Juarez from Channel 9 about code, culture, speaking at conferences, and… all kinds of things, really.

The interview's here, or you can watch it over on - thanks Seth and co for taking the time to put this together!

Saturday, 13 May 2017

Interview with about HTTP APIs in .NET

Next week I'll be in Russia, where I'm speaking about HTTP APIs and REST at the DotNext conference in Saint Petersburg. As part of this event, I've done an interview with the Russian tech site Хабрахабр about the history and future of API development on the web and in Microsoft.NET. The interview's available on their site (in Russian), but for readers who are interested but can't read Russian, here's the original English version.

Cathedral in Saint Petersburg, Russia. / Photo by DEZALB.

Q: What kind of APIs are you designing? Where does API design fit into software development?

That’s kind of an interesting question, because I think one of the biggest misconceptions in software is that designing APIs is an activity that happens separately to everything else. Sure, there are certain kinds of API projects – particularly things like HTTP APIs which are going to be open to the public – where it might make sense to consider API design as a specific piece of work. But the truth is that most developers are actually creating APIs all the time – they just don’t realise they’re doing it. Every time you write a public method on one of your classes or choose a name for a database table, you’re creating an interface – in the everyday English sense of the word – that will end up being used by other developers at some point in the future. Other people on your team will use your classes and methods. Other teams will use your data schema or your message format.

What’s interesting, though, is that once developers realize that the code they’re working on will probably form part of an API, they tend to go too far in the other direction. They’ll implement edge cases and things that they don’t actually need, just in case somebody else might need it later. I think there’s a very fine balance, and I think the key to that balance is to be very strict about only building the features that you need right now, but to make those things as reusable and self-explanatory as you can. There’s a great essay by Pieter Hintjens, Ten Rules for Good API Design, that goes into more detail about these kinds of ideas.

The biggest API project I’m working on at the moment is a thing I’m building at Spotlight in the UK, where I work. It’s a hypermedia API exposing information about professional actors, acting jobs in film and television, and various other kinds of data used in the casting industry. We’re building it in the architectural style known as REST – and if you’re not sure what REST is, you should come to my talk at DotNext in Saint-Petersburg and learn all about it. There’s lots of different patterns for creating HTTP APIs – there’s REST, there’s GraphQL, there’s things like SOAP and RPC – but for me, the biggest appeal of REST is that I think the constraints of the RESTful style lead to a natural decoupling of the concepts and operations that your API needs to support, which makes it easier to change things and evolve the API design over time.

Q: One of the most famous applications that was "killed" by backward compatibility is IE. The problem of this browser was that it has too large number of applications for which it was required to have backward compatibility. Problem was solved by adding new application Edge, which is updatable and supports all new standards. Can you give an piece of advice on how not to get caught by that backward-compatibility trap? As an example could it be a modularity which doesn't have layers? May be there is a way to replace API with RESTful API, Service Oriented Architecture or something else?

I’ve been building web applications for a long, long time – I wrote my first HTML page a couple of years before Internet Explorer even existed, back when the only browsers were NCSA Mosaic and Erwise. It’s fascinating to look back at the history of the web, and how the web that exists today has been shaped and influenced by things like the evolution of Internet Explorer – and you’re absolutely right; one of the reasons why Microsoft has introduced a completely new browser, Edge, in the latest versions of Windows is that Internet Explorer’s commitment to backwards-compatibility has made it really difficult to implement support for modern web standards alongside the existing IE codebase.

Part of the reason why that backwards compatibility exists is that, around the year 2000, there was a massive shift in the way that corporate IT systems were developed. There are countless corporations who have bespoke applications for doing all sorts of business operations – stock control, inventory, HR, project management, all kinds of things. Way back in the 1980s and early 1990s, most of them used a central mainframe system and employees would have to use something like a terminal emulator to connect to that central server, but after the first wave of the dotcom boom hit in the late 1990s, companies realised that most of their PCs now had a web browser and an network connection, and so they could replace their old mainframe terminal applications with web applications. Windows had enormous market share at the time, and Internet Explorer was the default browser on most Windows PCs, so lots of organizations built intranet web applications that only had to work on a specific version of Internet Explorer. Sometimes they did this to take advantage of specific features, like ActiveX support; more often I think they just did it save money because it meant they didn’t have to do cross-browser testing. This happened with some pretty big commercial applications as well; as late as 2011, Microsoft Dynamics CRM still offered no support for any browser other than Internet Explorer.

So you’ve got all these companies who have invested lots of time and money in building applications that only work with Internet Explorer. Those applications aren’t built using web standards or progressive enhancement or with any notion of ‘forward compatibility’ – they’re explicitly targeting one version of one browser running on one operating system. And so when Microsoft releases a new version of Internet Explorer, those applications fail – and the companies don’t want to invest in upgrading their legacy intranet applications, so they blame the browser. So we end up with this weird situation where here in 2017, Microsoft are still shipping Internet Explorer 11, which has a compatibility mode where it switches back to the IE9 rendering engine but sends a user agent string claiming that it's IE7. Meanwhile, everyone I know uses Google Chrome or Safari for all their web browsing - but still has an IE shortcut on their desktop for when they have to log in to one of those legacy systems. .

So… to go back to the original question: is there anything Microsoft could have done to avoid this trap? I think there’s a lot of things they could have done. Building IE from the ground up with a modular rendering engine, so that later versions could selectively load the appropriate engine for rendering a particular website or application. They could have made more effort to embrace the web standards that existed at the time, instead of implementing ad-hoc support for things like the MARQUEE tag and ActiveX plugins, which would have avoided the headache of having to support these esoteric features in later versions. The point is, though, none of this mattered at the time. Their focus – the driving force behind the early versions of Internet Explorer – was not to create a great application with first-class support for web standards. They were trying to kill Netscape Navigator and win market share – and it worked.

Q: Let’s imagine someone is going to introduce an API. So they collect some requirements, propose a version and gets feedback. That’s rather simple and straightforward thing. But if there are any hidden obstacles there down the road?

Always! Requirements are going to change – in fact, one of the biggest mistakes you can make is to try and anticipate those changes and make your design ‘future-proof’. Sometimes that pays off, but what mostly happens is that you end up with a much more complicated design purely because you’re trying to anticipate those future changes. Those obstacles are often things outside your control. There’s a change to the law that means you need to expose certain data in a different way. There’s a change to one of the other systems in your organization, or one of your cloud hosting providers announces that they’re deprecating a particular feature that you were relying on.

The best thing to do is to identify something simple and usable, ship it, and get as quickly as you can to a point where your API is stable, there’s no outstanding technical debt, and your team is free to move on to the next thing. That way when you do encounter one of those ‘hidden obstacles’, you have a stable codebase to use as a basis for your solution, and you have a team who have the time and the bandwidth to deal with it. And if by some stroke of luck you don’t hit any hidden obstacles, then you just move on to the next thing on your backlog.

Q: Continue with API design. We’ve released the v1.0 of our API and now v1.1 is approaching. I believe many of us noticed and or something. What are the best practices (a couple of points) you can think of that can help a developer to design a good v1.1 API in respect to v1.0?

It’s worth reading up on the concept of semantic versioning, (SemVer) and taking the time to really understand the distinction between major, minor and patch versions. SemVer says that you shouldn’t introduce any breaking changes between version x.0 and version x.1, so the most important thing is to understand what would constitute a breaking change for your particular API.

If you’re working with HTTP APIs that return JSON, for example, a typical non-breaking change would be adding a new data field to one of your resources. Clients that are using version 1.1 and are expecting to see this additional field can take advantage of it, whereas clients that are still using version 1.0 can just discard the unrecognised property.

There’s a related question about how you should manage versioning in your APIs. One very common solution is to expose URLs via routing – as opposed to – but if you’re adhering to the constraints of a RESTful system, you really need to understand whether the change in version represents a change in the underlying resource or just the representation. Remember that a URI is a Uniform Resource Identifier, and so we really shouldn’t be changing the URI that we’re using to refer to the same resource.

For example – if we have a resource We could request that resource as a JPEG (Accept: image/jpeg), or as a PNG (Accept: image/png), or ask the server if it has a plain-text representation of the resource (Accept: text/plain) – but they’re just different representations of the same underlying resource, and so they should all use the same URI.

If – say – you’ve completely replaced the CRM system used by your organization, and so “version 1” of a customer represents a record used in the old CRM system and “version 2” represents that same customer after they’ve been migrated onto a completely new platform, then it probably makes sense to treat them as separate resources and give them different URIs.

Versioning is hard, though. The easiest thing to do is never change anything

Q: .NET Core - what do you think about its API?

When .NET Core was first announced in 2015, back when it was going to be be called .NET Core 5.0, it was going to be a really stripped-down, lightweight alternative to the .NET Framework and the Common Language Runtime. That was an excellent idea in terms of making it easier to port .NET Core to different platforms, but it also left a sizable gap between the API exposed by .NET Core, and the ‘standard’ .NET/CLR API that most applications are built against.

I believe – and this is just my interpretation based on what I’ve read and people I’ve talked to – that the idea was that .NET Core would provide the fundamental building blocks. It would provide things like threading, filesystem access, network access, and then a combination of platform vendors and the open source community would develop the modules and packages that would eventually match the level of functionality offered by something like the Java Class Library or the .NET Framework. That’s a great idea in principle, but it also creates a chicken-and-egg situation: people won’t build libraries for a platform with no users, but nobody wants to use a platform that doesn’t have any libraries.

So, the decision was made that cross-platform .NET needed a standard API specification that would provide the libraries that users and application developers expected to be available on the various supported platforms. This is .NET Standard 2.0, which is already fully supported by the .NET Framework 4.6.1 and will be supported in the next versions of .NET Core and Xamarin. Of course, .NET Core 1.1 is out, and works just fine, and you can use it right now to build web apps in C# regardless of whether you’re running Windows or Linux or macOS, which is pretty awesome – but I think the next release of .NET Core is going to be the trigger for a lot of framework and package developers to migrate their projects across to .NET Core, which in turn should make it easier for developers and organizations to migrate their own applications.

Tram on Moscow Gate Square in Saint Petersburg. /  Photo by Dinamik.

API flexibility VS. API precision. One can design a method API so it can accept many different types of values. It’s flexibility. We also can design a method API with lots of rules on input parameters. Both ways are correct. Where is the boundary across these approaches? When should I make a “strict” API and when should I make a more “flexible” design? Don’t forget that you should take backward-compatibility into account.

By implementing an API where the method signatures are flexible, all you’re doing is pushing the complexity to somewhere else in your stack. Say we’re building an API for finding skiing holidays, and we have a choice between

DoSearch(SearchCriteria criteria)


DoSearch(string resortName, string countryCode, int minAltitude, int maxDistanceToSkiList)

One of those methods is pretty easily extensible, because we can extend the definition of the SearchCriteria object without changing the method signature – but we’re still changing the behaviour of the system, we’re just not changing that particular method. By contrast, we could add new arguments to our DoSearch method signature, but if we’re working in a language like C# where you can provide default argument values, you won’t break anything by doing that as long as you provide sensible defaults for the new arguments.

At some point, though, you need to communicate to the API consumers what search parameters are accepted by your API, and there’s lots of ways to accomplish that. If you’re building a .NET API that’s installed as a NuGet package and used from within code, then using XML comments on your methods and properties is a great way to explain to your users what they need to specify when making calls to your API. If your API is an HTTP service, look at using hypermedia and formats like SIREN to define what parameter values and ranges are acceptable.

I should add that I think within the next decade, we’re going to start seeing a whole different category of APIs powered by machine learning systems, where a lot of the conventional rules of API design won’t apply. It wouldn’t surprise me if we got an API for finding skiing holidays where you just specify what you want in natural language, and so there’s not even a method signature – you just call something like DoSearch(“ski chalet, in France or Italy, 1400m or higher, that sleeps 12 people in 8 bedrooms, available from 18-25 January 2018”) – and the underlying system will work it all out for us. Those sorts of development in machine learning are hugely exciting, but they’re also going to create a lot of interesting challenges for the developers and designers trying to incorporate them into our products and applications. 

Thanks to Alexej Sommer for taking the time to set this up (and for translating my answers into Russian – Спасибо!), and if you're at DotNext next week and want to chat about APIs, hypermedia or any of the stuff in the interview, please come and say hi!