[GitHub] davisp opened a new pull request #610: Optimize ddoc cache

[gi...@git.apache.org]

davisp opened a new pull request #610: Optimize ddoc cache
URL: https://github.com/apache/couchdb/pull/610
 
 
   ## Overview
   
   The previous version of ddoc_cache was written to rely on evicting
   entries after a maximum TTL. This leads to issues on clusters that have
   a large amount of load on databases with a large Q. What ends up
   happening is that when a design document is evicted we suddenly have a
   thundering herd scenario as every client attempts to reinsert it into
   the cache.
   
   This change instead relies on a monitor process for each cache entry
   that periodically attempts to refresh the cache. This way normal clients
   accessing a popular design document will never hit a point where it
   doesn't exist in cache. And we'll have at most one reader trying to
   write the value.
   
   ## Testing recommendations
   
   make eunit apps=ddoc_cache
   
   You'll find its got 100% test coverage if you enable cover.
   
   One note on the tests that's a bit different than other suites. I stumbled across a fun pattern of adding a function call to various locations in the code so that I can meck:wait on it. This is a compile time switch so its only affects the eunit runs. Production code has the calls compiled out and replaced by an atom which does nothing.
   
   ## Checklist
   
   - [x] Code is written and works correctly;
   - [x] Changes are covered by tests;
   
 
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