Still getting my head around Continuous Deployment
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In a webinar on CD, Kent Beck explored a fundamental mismatch between rapid cycling in design and construction, and then getting stuck when we are ready to deploy. He argues that that queuing theory and experience show that there is more value in a system when all of the pipes are the same size, and follow the same cycle times. Ideally, there should be a smooth flow from ideas to design and development and to deployment, and then information from real use fed back as soon as possible to ideas. Instead we have a choke point at deployment.
Then there is the ROI argument that we can get faster return on money spent if we deploy something that we have done as soon as it is ready.
Kent Beck also explained that based on his experience at one company the constraints of deploying immediately make people more careful and thoughtful: that the practice becomes self-reinforcing, that developers stop taking risks because they don’t have time to. Essentially problems become simpler because they have to be.
Timothy Fitz presented a Deployment Equation:
If Information Value + Direct Value > Deployment Risk then Deploy
The idea is that Continuous Deployment increases information value by giving us information earlier. He talked about ways to reduce risk:
- Rolling out larger changes slowly to customers, through dark launching (hiding the changes from the front-end until ready: not exactly a new idea) and enabling features for different sets of users.
- Extensive automated testing, supplemented with manual exploratory testing before exposing dark-launched features.
- Ensuring that you can detect problems quickly and correct them through production monitoring, looking for leading indicators of problems, and instant production roll back.
- An architecture that supports stability through isolation. Follow the patterns in Release It! to minimize the chance of “stupid take the cluster out” errors.
- Locking down core infrastructure, preventing changes from certain parts of the system without additional checks.
Jez Humble at ThoughtWorks presented on Continuous Delivery: building on top of Continuous Integration to automate and optimize further downstream packaging and deployment activities. Continuous Deployment is effectively an extension of Continuous Delivery. It was mostly a re-hash of another presentation that I had already seen from ThoughtWorks, and of course there will be a book coming out soon on all of this.
Some questions on Continuous Delivery and Continuous Deployment
Me: Continuous Delivery is based on the assumption that you can get immediate feedback: from automated tests, from post-deployment checks, from customers. How do you account for problems that don't show up immediately, by which time you have deployed 50 or 100 or more changes?
Answer from Timothy Fitz: The first time, you revert and re-push. Then you post-mortem and figure out how to catch faster by looking for a leading indicator. Performance issues can be caught by dark launching, in which case turning off or reverting the functionality will have 0 visible effect. Frontend issues are usually caught by A/B tests, where you can mitigate risk by not running them at 100% of all traffic (have 80% control, 20% hypothesis, etc)
Me: Followup on my question about handling problems that show after 50 or 100 changes. The answer was to revert and re-push - but revert what? A problem may not show itself immediately. How do you know which changes or changes to rollback?
Answer from Timothy Fitz: If it took 50-100 changes, then you'll be finding the change manually. It turns out to be fairly easy even if it's been 48-96 hours, you're only looking through a few hundred very small commits most of which are in isolated areas unrelated to your problem.
Me: How to you handle changes to data (contents and/or schema) on a continuous basis?
Answer: not answered. Jez Humble talked about writing code that could work with multiple different database versions (which would make design and testing nasty of course), and how to automate some database migration tasks with tools like DBDeploy, but admitted that “databases were not optimized for Continuous Delivery”. There were no good answers on how to handle expensive data conversions.
Me: My team has obligations to ensure that the software we deliver is secure, so we follow secure SDLC checks and controls before we release. In Continuous Delivery I can see how this can be done along the pipeline. But secure Continuous Delivery?
Answer from Jez Humble: Ideally you'd want to run those checks against every version. If you can't do that, do it as often as you can.
[I didn’t expect a meaningful answer on this one, and I didn’t get one]
Somebody else’s question: Do you find users struggling to keep up and adapt to the constant changes?
Answer from Kent Beck: In practice it doesn't seem to be a problem usually because each change is small--a new widget, a new menu item, a new property page that's similar to existing pages. A wholesale change to the UI would be a different story. I would try to use social processes to support such a change--have a few leaders try the new UI first, then teach others.
Somebody else’s question: Without solid continuous testing in place, CD is [a] fast track to continuous complaints from end users
Answer from Timothy Fitz: Not always, but usually. For the cases where it makes sense (small startup, or isolated segment that opts-in to alpha) you can find user segments who value features 100% over stability, and will gladly sign up for Continuous Deployment.
So what do I really think about Continuous Deployment
OK I can see how Continuous Deployment can work,
If: your architecture supports isolation, that it is horizontal and shallow, offering features that are clearly independent;
If: you don’t follow the all-or-none approach – that you recognize that some kinds of changes can be deployed continuously and some parts of the system are too important and require additional checks, tests, reviews, and more time;
If: you build up enough trust across the company;
If: your customers are willing to put up with more mistakes in return for faster delivery, if at least some of them are willing to help you do your testing for you;
If: you invest enough in tools and technology for automated layered testing and deployment and post-deployment checking and roll-back capabilities.
Continuous Deployment is still an immature approach and there are too many holes in it. And as Kent Beck has pointed out, there aren’t enough tools yet to support a lot of the ideas and requirements: you have to roll your own, which comes with its own costs and risks.
And finally, I have to question the fundamental importance of immediate feedback to a company. I can see that waiting a year, or even a month, for feedback can be too long. I fully understand and agree that sometimes changes need to be made quickly, that sometimes the windows of opportunity are small and we need to be ready immediately. And there’s first mover advantage, of course. But I have a hard time believing that any kind of changes need to be continuously made 50 times per day: that there are any changes that can be made that quickly that will have any real difference to customers or to the business. And I will go further and say that such rapid changes are not in the interests of customers, that they don’t need or even want this much change this fast. And that I don’t believe that it’s really about reducing waste, or maximizing velocity or increasing information value.
No, I suspect it is more about a need for immediate satisfaction – for programmers, and the people who drive them. Their desire to see what they’ve done get into production, and to see it right away, to get that little rush. The simple inability to delay gratification. And that’s not a good reason to adopt a model for change.