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Release It - Highlights Part 2

This is the second part of my collection of notes and snippets from Release It!

Caches

  • Low memory conditions are a threat to both stability and capacity.
  • You need to ask whether the possible keys are infinite or finite and would the items ever need to change?
  • The simplest cache clearing mechanism is time based.
  • Improper use of caching is the major cause of memory leaks, which turn into horrors like daily server restarts.
  • Your system should run for at least the typical deployment cycle. If you deploy once every two weeks, it should be able to run for at least two weeks without restart.
  • Limit the amount of memory a cache can use.
  • Caches that do not limit memory will eventually eat all memory.

Testing

  • Every integration point should have a test harness that can be substituted.
  • Make your test harness act like a hacker - try large payloads, invalid characters, injection and so on.
  • Have your test harness log requests so you can see what has caused problems.
  • Run longevity tests - tests that put impulse and stress upon a system over long periods of time.
  • Someone saying "the odds of that happening is millions to one" is actually quite likely to happen. Given a average site, making thousands of requests a day this is an easy target to hit.
  • Sessions are the Achilles heel of every application server.
  • Most testing uses the app in the way it was expected to be use such as load testing a site using the correct workflow. What about load testing without using cookies? Would this spawn a new session each time?

HTML

  • Whitespace costs! In HTML (or the markup generated) remove all whitespace. It costs time to generate and money to send across the wire. You could argue this is for big traffic sites only, but this technique is very simple to apply as part of the build and speeds up client side rendering.
  • Omit needless characters in HTML such as comments. Use server side commenting instead, this will be removed when processed.

Pre-Computation

  • Precompute as much of the page as possible. Use "punch outs" for dynamic content. For example Slashdot generates its page once and serves to thousands of users. All users get the page equally as fast. Caching would mean handfuls of users would get a slow experience.
  • Precomputed content should be deployed as part of the build. For more frequent updates another strategy or "content deploys" would be required.

Logging

  • The human visual system is an excellent pattern matching machine. Make logs readable by using a custom format. Scanning logs is very easy then.
  • Two line log files are difficult. Harder to grep. Keep everything on one line.
  • Each week review the systems tickets. Try to identify and fix problems as you go. Try and predict future problems where possible based on this info.
  • Check the logs daily for stack traces that are suspicious. These could be common errors that are bugs/problems that need fixing.

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