1# groupcache
2
3## Summary
4
5groupcache is a distributed caching and cache-filling library, intended as a
6replacement for a pool of memcached nodes in many cases.
7
8For API docs and examples, see http://godoc.org/github.com/golang/groupcache
9
10## Comparison to memcached
11
12### **Like memcached**, groupcache:
13
14 * shards by key to select which peer is responsible for that key
15
16### **Unlike memcached**, groupcache:
17
18 * does not require running a separate set of servers, thus massively
19 reducing deployment/configuration pain. groupcache is a client
20 library as well as a server. It connects to its own peers, forming
21 a distributed cache.
22
23 * comes with a cache filling mechanism. Whereas memcached just says
24 "Sorry, cache miss", often resulting in a thundering herd of
25 database (or whatever) loads from an unbounded number of clients
26 (which has resulted in several fun outages), groupcache coordinates
27 cache fills such that only one load in one process of an entire
28 replicated set of processes populates the cache, then multiplexes
29 the loaded value to all callers.
30
31 * does not support versioned values. If key "foo" is value "bar",
32 key "foo" must always be "bar". There are neither cache expiration
33 times, nor explicit cache evictions. Thus there is also no CAS,
34 nor Increment/Decrement. This also means that groupcache....
35
36 * ... supports automatic mirroring of super-hot items to multiple
37 processes. This prevents memcached hot spotting where a machine's
38 CPU and/or NIC are overloaded by very popular keys/values.
39
40 * is currently only available for Go. It's very unlikely that I
41 (bradfitz@) will port the code to any other language.
42
43## Loading process
44
45In a nutshell, a groupcache lookup of **Get("foo")** looks like:
46
47(On machine #5 of a set of N machines running the same code)
48
49 1. Is the value of "foo" in local memory because it's super hot? If so, use it.
50
51 2. Is the value of "foo" in local memory because peer #5 (the current
52 peer) is the owner of it? If so, use it.
53
54 3. Amongst all the peers in my set of N, am I the owner of the key
55 "foo"? (e.g. does it consistent hash to 5?) If so, load it. If
56 other callers come in, via the same process or via RPC requests
57 from peers, they block waiting for the load to finish and get the
58 same answer. If not, RPC to the peer that's the owner and get
59 the answer. If the RPC fails, just load it locally (still with
60 local dup suppression).
61
62## Users
63
64groupcache is in production use by dl.google.com (its original user),
65parts of Blogger, parts of Google Code, parts of Google Fiber, parts
66of Google production monitoring systems, etc.
67
68## Presentations
69
70See http://talks.golang.org/2013/oscon-dl.slide
71
72## Help
73
74Use the golang-nuts mailing list for any discussion or questions.
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