Release It! is one of the most useful books I've read. The advice and suggestions inside certainly change your perspective on how to write software. My key takeaway is that software should be cynical. Expect the worst, expect failures and put up boundaries. In the majority of cases these failures will be trigged by integration points with other systems, be it third parties or your own.
My rough notes and snippets will be spread across the following two posts. There is much more to the book than this, including various examples of real life systems failing and how they should have handled the problem in the first place.
- Part 1 - Shared Resources, Responses, SLA, Databases and Circuit Breakers
- Part 2 - Caches, Testing, HTML, Pre-Computation and Logging
- Shared Resources can jeopardize scalability.
- When a shared resource gets overloaded, it will become a bottleneck.
- If you provide the front end system, test what happens if the back end is slow/down. If you provide the back end, test what happens if the front end is under heavy load.
- Generating a slow response is worse than refusing to connect or timing out.
- Slow responses trigger cascading failures.
- Slow responses on the front end trigger more requests. Such as the user hitting refresh a few times, therefore generating more load ironically.
- You should error when a response exceeds the systems allowed time, rather than waiting.
- Most default timeouts of libraries and frameworks are far too generous - always configure manually.
- One of the worst places that scaling effects will bite you is with point to point communication. Favour other alternatives such as messaging to remove this problem.
- When calling third parties, services levels only decrease.
- Make sure even without a third party response your system can degrade gracefully.
- Be careful when crafting SLA's. Do not simply state 99.999%, it costs too much to hit this target and most systems don't need this sort of uptime.
- Reorient the discussion around SLA's to focus on features, not systems.
- You cannot offer a better SLA than the worst of any external dependencies you use.
- Your application probably trusts the database far too much.
- Design with scepticism and you will achieve resilience.
- What happens if the DB returns 5 million rows instead of 5 hundred? You could run out of memory trying to load all this. The only answers a query can return is 0, 1 or many. Don't rely on the database to follow this limit. Other systems or batch processes may not respect this rule and insert too much data.
- After a system is in production, fetch results can return huge result sets. Unlike developer testing where only a small subset of data is around.
- Limit your DB queries, e.g. SELECT * FROM table LIMIT 15 (the wildcard criteria would be substituted)
- Put limits into other application protocols such REST endpoints via paging or offsets.
- Now and forever networks will always be unreliable.
- The timeout pattern prevents calls to integration points from becoming blocked threads.
- Circuit Breakers area way of automatically degrading functionality when a system is under stress.
- Changes in a circuit breaker should always be logged and monitored.
- The frequency of state changes in a circuit breaker can help diagnose other problems with the system.
- When there is a problem with an integration point, stop calling it during a cool off period. The circuit breaker will enable this.
- Popping a circuit breaker always indicates a serious problem - log it.