A typical system that we develop on and extend is a Complex System. We try to describe these systems using Flow Diagrams, Universal Modeling Language Diagrams and typical System Architecture Diagrams. In his talk Richard challenges this idea and explains that most systems are stochastic. A stochastic system is one where the behavior can be measured statistically and not predicted precisely.
Now a typical system has lots of moving parts, databases, message queues, network connections - alone one of these components may behave deterministically but interacting together can result in something that is best described stochastically.
When building these systems, Richard points out that we build these systems with the aim for them to be available, to achieve this once deployed and operating we:
- Monitor it
- Respond to it
- Adapt it
- Learn about it
By combining the above four activities we make our complex system resilient.
When we build complex systems we have a different set of goals, we design our systems to be reliable. To be reliable we:
- Encapsulate complexity by creating abstractions and only allowing a known set of inputs
- Implement defence in depth - creating multiple layers of security to provide fall backs in the case of a vulnerability or failure in one layer
- Create Redundancy - clusters, load balanced servers
- Embed assurances - formal reviews and audits
- Make people accountable for the system's
Now if our systems are completely deterministic, building for reliability makes sense because we still believe that we can predict precisely how it will operate. If we accept that our systems are stochastic, we accept that our complex systems will fail and that we should instead design for resilience.
To design to be resilient we can:
- Accept that systems continue to evolve and grow beyond delivery and need to be developed with processes in place to continue the evolution of the system over time
- Recognize the role of operations and maintenance people, build systems that expose controls and tools to these groups to support the system
- Envision operating failures and build systems that can recover
- Allow for simulation in the application so that the affect that operator changes will have can be tested
- Empower operator learning
- Make resilience engineering critical to the design of your systems
From this talk, I don't think I'll stop building systems that are reliable, reliability is still a characteristic I think is worth building for - however for a complex system to be available it is now clear that resilience needs to be designed for as well. Accepting that failures do happen and designing systems to be easily recoverable is also a very important characteristic. A characteristic that I have learned many times over by doing production support.