mirror of
https://github.com/mailgun/groupcache.git
synced 2024-11-19 05:13:57 +00:00
74 lines
2.7 KiB
Markdown
74 lines
2.7 KiB
Markdown
# groupcache
|
|
|
|
## Summary
|
|
|
|
groupcache is a caching and cache-filling library, intended as a
|
|
replacement for memcached in many cases.
|
|
|
|
For API docs and examples, see http://godoc.org/github.com/mailgun/groupcache
|
|
|
|
## Comparison to memcached
|
|
|
|
### **Like memcached**, groupcache:
|
|
|
|
* shards by key to select which peer is responsible for that key
|
|
|
|
### **Unlike memcached**, groupcache:
|
|
|
|
* does not require running a separate set of servers, thus massively
|
|
reducing deployment/configuration pain. groupcache is a client
|
|
library as well as a server. It connects to its own peers.
|
|
|
|
* comes with a cache filling mechanism. Whereas memcached just says
|
|
"Sorry, cache miss", often resulting in a thundering herd of
|
|
database (or whatever) loads from an unbounded number of clients
|
|
(which has resulted in several fun outages), groupcache coordinates
|
|
cache fills such that only one load in one process of an entire
|
|
replicated set of processes populates the cache, then multiplexes
|
|
the 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 expiration
|
|
times, 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 multiple
|
|
processes. This prevents memcached hot spotting where a machine's
|
|
CPU 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.
|
|
|
|
## Loading process
|
|
|
|
In a nutshell, a groupcache lookup of **Get("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 current
|
|
peer) 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. If
|
|
other callers come in, via the same process or via RPC requests
|
|
from peers, they block waiting for the load to finish and get the
|
|
same answer. If not, RPC to the peer that's the owner and get
|
|
the answer. If the RPC fails, just load it locally (still with
|
|
local dup suppression).
|
|
|
|
## Users
|
|
|
|
groupcache is in production use by dl.google.com (its original user),
|
|
parts of Blogger, parts of Google Code, parts of Google Fiber, parts
|
|
of Google production monitoring systems, etc.
|
|
|
|
## Presentations
|
|
|
|
See http://talks.golang.org/2013/oscon-dl.slide
|
|
|
|
## Help
|
|
|
|
Use the golang-nuts mailing list for any discussion or questions.
|