A transparent cache for datastore, backed by memcache.

filter/dscache/doc.go

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Package dscache provides a transparent cache for RawDatastore which is
+// backed by Memcache.
+//
+// Inspiration
+//
+// Although this is not a port of any particular implementation, it takes
+// inspiration from these fine libraries:
+//   - https://cloud.google.com/appengine/docs/python/ndb/
+//   - https://github.com/qedus/nds
+//   - https://github.com/mjibson/goon
+//
+// Algorithm
+//
+// Memcache contains cache entries for single datastore entities. The memcache
+// key looks like
+//
+//   "gae:" | vers | ":" | shard | ":" | Base64_std_nopad(SHA1(rawdatastore.Key))
+//
+// Where:
+//   - vers is an ascii-hex-encoded number (currently 1).
+//   - shard is a zero-based ascii-hex-encoded number (depends on dscache.shards).
+//   - SHA1 has been chosen as unlikely (p == 1e-18) to collide, given dedicated
+//     memcache sizes of up to 170 Exabytes (assuming an average entry size of
+//     100KB including the memcache key). This is clearly overkill, but MD5
+//     could start showing collisions at this probability in as small as a 26GB
+//     cache (and also MD5 sucks).
+//
+// The memcache value is a compression byte, indicating the scheme (See
+// CompressionType), followed by the encoded (and possibly compressed) value.
+// Encoding is done with rawdatastore.PropertyMap.Write(). The memcache value
+// may also be the empty byte sequence, indicating that this entity is deleted.
+//
+// The memcache entry may also have a 'flags' value set to one of the following:
+//   - 0 "entity" (cached value)
+//   - 1 "lock"   (someone is mutating this entry)
+//
+// Algorithm - Put and Delete
+//
+// On a Put (or Delete), the memcache value written with a LockTimeSeconds
+// expiration (default 31 seconds), and a memcache flag value of 0x1 (indicating
+// that it's a put-locked key).
+//
+// The datastore operation will then occur. Assuming success, Put will then
+// delete all of the memcache locks (Not using CompareAndSwap).
+//
+// Algorithm - Get
+//
+// On a Get, "Add" a lock for it (which only does something if there's no entry
+// in memcache yet) with a nonce value. We immediately Get the memcache entries
+// back (for CAS purposes later).
+//
+// If it doesn't exist (unlikely since we just Add'd it) or if it's flag is
+// "lock" and the Value != the nonce we put there, go hit the datastore without
+// trying to update memcache.
+//
+// If its flag is "entity", decode the object and return it. If the Value is
+// the empty byte sequence, return ErrNoSuchEntity.
+//
+// If its flag is "lock" and the Value equals the nonce, go get it from the
+// datastore. If that's successful, then encode the value to bytes, and CAS
+// the object to memcache. The CAS will succeed if nothing else touched the
+// memcache in the meantime (like a Put, a memcache expiration/eviction, etc.).
+//
+// Algorithm - Transactions
+//
+// In a transaction, all Put memcache operations are held until the very end of
+// the transaction. Right before the transaction is committed, all accumulated
+// Put keys are locked. If the transaction is sucessfully committed (err ==
+// nil), then all the locks will be deleted.
+//
+// The assumption here is that get operations apply all outstanding
+// transactions before they return data (https://cloud.google.com/appengine/docs/go/datastore/#Go_Datastore_writes_and_data_visibility),
+// and so it is safe to purge all the locks if the transaction is known-good.
+//
+// If the transaction succeeds, but RunInTransaction returns an error (which can
+// happen), or if the transaction fails, then the lock entries time out
+// naturally. This will mean 31-ish seconds of direct datastore access, but it's
+// the more-correct thing to do.
+//
+// Cache control
+//
+// An entity may expose the following metadata (see
+// rawdatastore.PropertyLoadSaver.GetMeta) to control the behavior of its cache.
+//
+//   - "dscache.enable,<true|false>" - whether or not this entity should be
+//     cached at all. If ommitted, dscache defaults to true.
+//   - "dscache.expiration,#seconds" - the number of seconds of persistance to
+//     use when this item is cached. 0 is infinite. If omitted, defaults to 0.
+//
+// In addition, the application may set a function ShardsForKey(key) which
+// returns the number of shards to use for a given datastore key.
+//
+// Caveats
+//
+// A couple things to note that may differ from other appengine datastore
+// caching libraries (like goon, nds, or ndb).
+//
+//   - It does NOT provide in-memory ("per-request") caching.
+//   - It's tolerant of memcache failures (but will give potentially
+//     inconsistent results if memcache is non-operational). Using a transaction
+//     bypasses the cache logic, which will present a consistent view of the data.
+//   - Queries do not interact with the cache at all.
+//   - Negative lookups (e.g. ErrNoSuchEntity) are cached.
+//   - Allows sharding hot memcache entries as recommended by
+//     https://cloud.google.com/appengine/articles/best-practices-for-app-engine-memcache#distribute-load .
+package dscache