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net_instaweb::CacheBatcher Class Reference

#include "cache_batcher.h"

Inheritance diagram for net_instaweb::CacheBatcher:


struct  Options

Public Member Functions

 CacheBatcher (const Options &options, CacheInterface *cache, AbstractMutex *mutex, Statistics *statistics)
 Does not take ownership of the cache. Takes ownership of the mutex.
virtual void Get (const GoogleString &key, Callback *callback)
virtual void Put (const GoogleString &key, const SharedString &value)
virtual void Delete (const GoogleString &key)
virtual GoogleString Name () const
virtual bool IsBlocking () const
virtual bool IsHealthy () const
virtual void ShutDown ()
- Public Member Functions inherited from net_instaweb::CacheInterface
virtual void MultiGet (MultiGetRequest *request)
void PutSwappingString (const GoogleString &key, GoogleString *value)
virtual CacheInterfaceBackend ()
virtual bool MustEncodeKeyInValueOnPut () const
virtual void PutWithKeyInValue (const GoogleString &key, const SharedString &key_and_value)

Static Public Member Functions

static void InitStats (Statistics *statistics)
static GoogleString FormatName (StringPiece cache, int parallelism, int max)
- Static Public Member Functions inherited from net_instaweb::CacheInterface
static const char * KeyStateName (KeyState state)

Static Public Attributes

static const int kDefaultMaxParallelLookups = 1
static const size_t kDefaultMaxPendingGets = 1000


class CacheBatcherTestingPeer
 For testing use only (instrumentation, synchronization).

Additional Inherited Members

- Public Types inherited from net_instaweb::CacheInterface
enum  KeyState {
  kAvailable = 0, kNotFound = 1, kOverload = 2, kNetworkError = 3,
  kTimeout = 4
typedef std::vector< KeyCallbackMultiGetRequest
- Protected Member Functions inherited from net_instaweb::CacheInterface
void ValidateAndReportResult (const GoogleString &key, KeyState state, Callback *callback)
 Invokes callback->ValidateCandidate() and callback->Done() as appropriate.
void ReportMultiGetNotFound (MultiGetRequest *request)

Detailed Description

Batches up cache lookups to exploit implementations that have MultiGet support. A fixed limit of outstanding cache lookups are passed through as single-key Gets when received to avoid adding latency. Above that, the keys & callbacks are queued until one of the outstanding Gets completes. When that occurs, the queued requests are passed as a single MultiGet request.

There is also a maximum queue size. If Gets stream in faster than they are completed and the queue overflows, then we respond with a fast kNotFound.

Note that this class is designed for use with an asynchronous cache implementation. To use this with a blocking cache implementation, please wrap the blocking cache in an AsyncCache.

Member Function Documentation

virtual void net_instaweb::CacheBatcher::Get ( const GoogleString key,
Callback callback 

Initiates a cache fetch, calling callback->ValidateCandidate() and then callback->Done(state) when done.

Note: implementations should normally invoke the callback via ValidateAndReportResult, which will combine ValidateCandidate() and Done() together properly.

Implements net_instaweb::CacheInterface.

static void net_instaweb::CacheBatcher::InitStats ( Statistics statistics)

Startup-time (pre-construction) initialization of statistics variables so the correct-sized shared memory can be constructed in the root Apache process.

virtual bool net_instaweb::CacheBatcher::IsBlocking ( ) const

Note: CacheBatcher cannot do any batching if given a blocking cache, however it is still functional so pass on the bit.

Implements net_instaweb::CacheInterface.

virtual bool net_instaweb::CacheBatcher::IsHealthy ( ) const

IsHealthy() is a rough estimation of whether cache is available for any operations. If it's false, caller may reasonably expect that making calls right now is useless as they will fail or have high latency. If it's true, operations should succeed, but some still may fail occasionally. The primary goal is to avoid sending commands to 'unhealthy' caches, e.g. if cache is under heavy load, we do not want to send even more requests.

Memory and file-based caches can simply return 'true'. It should be safe to call this frequently – the implementation shouldn't do much more that check a bool flag under mutex.

Implements net_instaweb::CacheInterface.

virtual GoogleString net_instaweb::CacheBatcher::Name ( ) const

The name of this CacheInterface – used for logging and debugging.

It is strongly recommended that you provide a static GoogleString FormatName(...) method for use in formatting the Name() return, and in testing, e.g. in third_party/pagespeed/system/

Implements net_instaweb::CacheInterface.

virtual void net_instaweb::CacheBatcher::Put ( const GoogleString key,
const SharedString value 

Puts a value into the cache. The value that is passed in is not modified, but the SharedString is passed by non-const pointer because its reference count is bumped.

Implements net_instaweb::CacheInterface.

virtual void net_instaweb::CacheBatcher::ShutDown ( )

Stops all cache activity. Further Put/Delete calls will be dropped, and MultiGet/Get will call the callback with kNotFound immediately. Note there is no Enable(); once the cache is stopped it is stopped forever. This function is intended for use during process shutdown.

Implements net_instaweb::CacheInterface.

Member Data Documentation

const int net_instaweb::CacheBatcher::kDefaultMaxParallelLookups = 1

We are willing to only do a bounded number of parallel lookups. Note that this is independent of the number of keys in each lookup.

By setting the default at 1, we get maximum batching and minimize the number of parallel lookups we do. Note that independent of this count, there is already substantial lookup parallelism because each Apache process has its own batcher, and there can be multiple Apache servers talking to the same cache.

Further, the load-tests performed while developing this feature indicated that the best value was '1'.

const size_t net_instaweb::CacheBatcher::kDefaultMaxPendingGets = 1000

We batch up cache lookups until outstanding ones are complete. However, we bound the number of pending lookups in order to avoid exhausting memory. When the "queues" are saturated, we drop the requests, calling the callback immediately with kNotFound.

The documentation for this class was generated from the following file: