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groupcache is a distributed caching and cache-filling library, intended as areplacement for a pool of memcached nodes in many cases.

For API docs and examples, seehttp://godoc.org/github.com/golang/groupcache

• shards by key to select which peer is responsible for that key

• does not require running a separate set of servers, thus massivelyreducing deployment/configuration pain. groupcache is a clientlibrary as well as a server. It connects to its own peers, forminga distributed cache.

• comes with a cache filling mechanism. Whereas memcached just says"Sorry, cache miss", often resulting in a thundering herd ofdatabase (or whatever) loads from an unbounded number of clients(which has resulted in several fun outages), groupcache coordinatescache fills such that only one load in one process of an entirereplicated set of processes populates the cache, then multiplexesthe loaded value to all callers.

• does not support versioned values. If key "foo" is value "bar",key "foo" must always be "bar". There are neither cache expirationtimes, nor explicit cache evictions. Thus there is also no CAS,nor Increment/Decrement. This also means that groupcache....

• ... supports automatic mirroring of super-hot items to multipleprocesses. This prevents memcached hot spotting where a machine'sCPU and/or NIC are overloaded by very popular keys/values.

• is currently only available for Go. It's very unlikely that I(bradfitz@) will port the code to any other language.

In a nutshell, a groupcache lookup ofGet("foo") looks like:

(On machine #5 of a set of N machines running the same code)

1. Is the value of "foo" in local memory because it's super hot? If so, use it.

2. Is the value of "foo" in local memory because peer #5 (the currentpeer) is the owner of it? If so, use it.

3. Amongst all the peers in my set of N, am I the owner of the key"foo"? (e.g. does it consistent hash to 5?) If so, load it. Ifother callers come in, via the same process or via RPC requestsfrom peers, they block waiting for the load to finish and get thesame answer. If not, RPC to the peer that's the owner and getthe answer. If the RPC fails, just load it locally (still withlocal dup suppression).

groupcache is in production use by dl.google.com (its original user),parts of Blogger, parts of Google Code, parts of Google Fiber, partsof Google production monitoring systems, etc.

Seehttp://talks.golang.org/2013/oscon-dl.slide

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