This method reduces the likelihood of a race condition where
you can add a (tracked) item to the cache, and the item isn't
the item you thought it was.
To support this, rather than adding another field/channel like
`getDroppedReq`, I added a `control` channel that can be used for these
miscellaneous interactions with the worker. The control can also be
used to take over for the `donec` channel
1 -
Previously, we determined if an item should be promoted in the main getter
thread. This required that we protect the item.promotions variable, as both
the getter and the worker were concurrently accessing it. This change pushes
the conditional promotion to the worker (from the getter's point of view, items
are always promoted). Since only the worker ever accesses .promotions, we no
longer must protect access to it.
2 -
The total size of the cache was being maintained by both the worker thread
and the calling code. This required that we protect access to cache.size. Now,
only the worker ever changes the size. While this simplifies much of the code,
it means that we can't easily replace an item (replacement either via Set or
Replace). A replcement now involves creating a new object and deleting the old
one (using the existing deletables and promotable infrastructure). The only
noticeable impact frmo this change is that, despite previous documentation,
Replace WILL cause the item to be promoted (but it still only does so if it
exists and it still doesn't extend the original TTL).
How do you decide you need to purge your cache? Relying on runtime.ReadMemStats
sucks for two reasons. First, it's a stop-the-world call, which is pretty bad
in general and down right stupid for a supposedly concurrent-focused package.
Secondly, it only tells you the total memory usage, but most time you really
want to limit the amount of memory the cache itself uses.
Since there's no great way to determine the size of an object, that means users
need to supply the size. One way is to make it so that any cached item satisfies
a simple interface which exposes a Size() method. With this, we can track how
much memory is set put and a delete releases. But it's hard for consumers to
know how much memory they're taking when storing complex object (the entire point
of an in-process cache is to avoid having to serialize the data). Since any Size()
is bound to be a rough guess, we can simplify the entire thing by evicting based
on # of items.
This works really bad when items vary greatly in size (an HTTP cache), but in
a lot of other cases it works great. Furthermore, even for an HTTP cache, given
enough values, it should average out in most cases.
Whatever. This improve performance and should improve the usability of the cache.
It is a pretty big breaking change though.
long-lived objects and won't clean them up. Oftentimes, the value returned
from a cache hit is short-lived. As a silly example:
func GetUser(http.responseWrite) {
user := cache.Get("user:1")
response.Write(serialize(user))
}
It's fine if the cache's GC cleans up "user:1" while the user variable has a reference to the
object..the cache's reference is removed and the real GC will clean it up
at some point after the user variable falls out of scope.
However, what if user is long-lived? Possibly stored as a reference to another
cached object? Normally (without this commit) the next time you call
cache.Get("user:1"), you'll get a miss and will need to refetch the object; even
though the original user object is still somewhere in memory - you just lost
your reference to it from the cache.
By enabling the Track() configuration flag, and calling TrackingGet() (instead
of Get), the cache will track that the object is in-use and won't GC it (even
if there's great memory pressure (what's the point? something else is holding on
to it anyways). Calling item.Release() will decrement the number of references.
When the count is 0, the item can be pruned from the cache.
The returned value is a TrackedItem which exposes:
- Value() interface{} (to get the actual cached value)
- Release() to release the item back in the cache
