Github user dhutchis commented on the pull request:
https://github.com/apache/accumulo/pull/96#issuecomment-215535249
> An alternative solution to introducing SyncingTabletLocator could be to
change the implementation of clearLocators to the following. This clears all of
the caches while still maintaining the references to in uses locators.
>
> public static synchronized void clearLocators() {
> for(TabletLocator tl : locators.values()) {
> tl.invalidateCache();
> }
> }
> If worried about empty tablet location caches being left around in the
locators map, that could possible be changed to a weak hash map.
You could be on to something @keith-turner if we give it more thought. The
original reason why the `locators` cache is cleared every hour is to remove
TabletLocators that refer to tablets that don't exist or are rarely accessed,
ya?
A WeakHashMap wouldn't work because it uses weak references for the keys.
This would mean that the gc could reclaim keys as soon as the `getLocator`
method returns, which is inefficient for a cache and has correctness issues if
the static `invalidate` methods are called.
What do you think of switching locators to a [Guava
cache](https://github.com/google/guava/wiki/CachesExplained) with a 1 hour
timeout on entries since their last access?
I think this would let us remove the `synchronized` from `getLocator`
because Guava caches are thread-safe and designed for concurrency. It would
eliminate the need for any `clearLocators` call (and `SyncingTabletLocator`).
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