Groupcache with Evan Owen

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1. Content Processing and Delivery with Groupcache or, how we made media messaging fast. Evan Owen Director of Engineering @kainosnoema, cotap.me/evan
2. • WhatsApp for Work • Company directory without swapping numbers • Conversations synced across devices • Simple, powerful native apps • Integrated with Salesforce • Send files, photos, video Cotap: mobile messaging for business
3. Challenge: How can we deliver user media to hundreds of recipients and make it feel fast?
4. Goals for media messaging • Sending should feel as fast as transmitting the data • Latency between sending and receiving message as low as sending text • Delivery time should remain constant as number of recipients grows • One-to-many delivery with dozens of resolutions (phone, tablet, web, desktop)
5. Image processing patterns • Synchronous on upload - easy! • Asynchronous on upload - harder • On-demand - really hard at scale
6. groupcache • github.com/golang/groupcache • by Brad Fitzpatrick, Google (memcached, OpenID, etc) • Originally written for dl.google.com (GoSF, Sept. 2013) • See: talks.golang.org/2013/oscon-dl.slide
7. groupcache, cont. • memcached alternative (read-through only) • Go library, acts as server and client to peers • Distributed, coordinated cache fills (no thundering herds) • Replication of hot keys across cluster
8. Distributed read-through caching Node 2 Node 1 Node 3 Node 4 Node 5 group.Get(“key-1234”) Consistent hash ring determines “primary” node.
9. Concurrent requests for a given key Node 2 Node 1 Node 3 Node 4 Node 5
10. Replication of “hot” items Node 2 Node 1 Node 3 Node 4 Node 5
11. In-memory, immutable cache • Main and hot caches are LRU of configurable size • Data is cached and transmitted using Protocol Buffers • No `set` or `expire`: once filled, data is immutable • If remote node can’t be reached, key is filled locally
12. Cache peer configuration myself := "http://10.10.0.1" peers := groupcache.NewHTTPPool(me) peers.Set(myself, "http://10.10.0.2", "http://10.10.0.3")
13. Declare a group w/ getter function var getterFunc = func(ctx gc.Context, key string, dest gc.Sink) error { fileName, size := parseKey(key) dest.SetBytes(generateResized(fileName, size)) return nil } var group = gc.NewGroup("resized", 1024<<20, gc.GetterFunc(getterFunc))
14. Request keys var data []byte err := group.Get(nil, “original-sm.jpg”, gc.AllocatingByteSliceSink(&data)) // ... http.ServeContent(w, r, “original-sm.jpg”, modTime, bytes.NewReader(data)) • local memory cache • peer's memory cache • local computed value • peer’s computed value Filled from one of:
15. How we use groupcache for media • Content-addressable originals in “cold storage” • Small groupcache group in front of originals • Images resized in-process using OpenCV (w/ cgo) • Larger groupcache in front of resized images • Finally: CDN in front of outermost groupcache
16. Multi-level groupcache Originals GET /d41d8cd98f00b204e9800998ecf8427e-640x480.jpg CDN GET /d41d8cd98f00b204e9800998ecf8427e
17. Fast image messaging Originals CDN Sending: as fast as uploading, pre-warm asynchronously … … receiving: network latency (best-case) or a single processing operation
18. Cache item structure type Artifact struct { PbValue []byte `protobuf:"bytes,1,opt,name=value"` PbContentType *string `protobuf:"bytes,2,opt,name=content_type"` PbLastModified *string `protobuf:"bytes,3,opt,name=last_modified"` XXX_unrecognized []byte } func (a *Artifact) HTTPHeader() (h http.Header) { h = new(http.Header) h.Set("Content-Length", strconv.Itoa(len(a.PbValue)) h.Set("Content-Type", *a.PbContentType) h.Set("Etag", "\""+a.Md5String()+"\"") return }
19. Serve using http.ServeContent() a := cachegroup.Get(req.URL.Path) for k, v := range a.HTTPHeader() { res.Header().Set(k, v[0]) } contentSeeker := io.ReadSeeker(bytes.NewReader(a.Value())) http.ServeContent(res, req, req.URL.Path, a.ModTime(), contentSeeker)
20. Stats and metrics { "cache": { "bytes": 118650246, "evictions": 2772, "gets": 12211, "hits": 4395, "items": 528, "misses": 7816 }, "loads": 7499, "loads_deduped": 7496, "local_load_errors": 108, "local_loads": 3300, "peer_errors": 46, "peer_gets": 4875, "peer_loads": 4088 } var stats = group.Stats var cacheStats = group.CacheStats(gc.MainCache) • Exposed on localhost at /stats • Each node monitored with Sensu • Metrics sent to Graphite
21. groupcache “gotchas” • Inconsistent hash rings: always ensure local node is included in peer list, update peer lists as quickly as possible • Getter function inconsistency • Cache memory usage: leave enough headroom • Deadlocks: strict timeouts on cache getter functions • Cold cluster: use rolling restarts when upgrading
22. Thanks! Evan Owen Director of Engineering @kainosnoema, cotap.me/evan