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Public Member Functions | Static Public Attributes
net_instaweb::AsyncCache Class Reference

#include "async_cache.h"

Inheritance diagram for net_instaweb::AsyncCache:
net_instaweb::CacheInterface

List of all members.

Public Member Functions

 AsyncCache (CacheInterface *cache, QueuedWorkerPool *pool)
virtual void Get (const GoogleString &key, Callback *callback)
virtual void Put (const GoogleString &key, SharedString *value)
virtual void Delete (const GoogleString &key)
virtual void MultiGet (MultiGetRequest *request)
virtual const char * Name () const
 The name of this CacheInterface -- used for logging and debugging.
virtual bool IsBlocking () const
virtual void ShutDown ()
void CancelPendingOperations ()
virtual bool IsHealthy () const
int32 outstanding_operations ()

Static Public Attributes

static const int64 kMaxQueueSize = 2000

Detailed Description

Employs a QueuedWorkerPool to turn a synchronous cache implementation into an asynchronous one. This makes sense to do only if the cache implemention is potentially slow, due to network latency or disk seek time.

This class also serves to serialize access to the passed-in cache, ensuring that it is accessed from only one thread at a time.


Constructor & Destructor Documentation

Takes ownership of the synchronous cache that is passed in. Does not take ownership of the pool, which might be shared with other users.

Note that in the future we may try to add multi-threaded access to the underlying cache (e.g. AprMemCache supports this), so we take the pool as the constructor arg.


Member Function Documentation

Cancels all pending cache operations. Puts and Deletes are dropped. Gets and MultiGets are retired by calling their callbacks with kNotFound.

virtual void net_instaweb::AsyncCache::Get ( const GoogleString key,
Callback callback 
) [virtual]

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.

virtual bool net_instaweb::AsyncCache::IsBlocking ( ) const [inline, virtual]

Returns true if this cache is guaranteed to call its callbacks before returning from Get and MultiGet.

Implements net_instaweb::CacheInterface.

virtual bool net_instaweb::AsyncCache::IsHealthy ( ) const [inline, virtual]

Returns true if the cache is in a healthy state. Memory and file-based caches can simply return 'true'. But for server-based caches, it is handy to be able to query to see whether it is in a good state. It should be safe to call this frequently -- the implementation shouldn't do much more than check a bool flag under mutex.

Implements net_instaweb::CacheInterface.

virtual void net_instaweb::AsyncCache::MultiGet ( MultiGetRequest *  request) [virtual]

Gets multiple keys, calling multiple callbacks. Default implementation simply loops over all the keys and calls Get.

MultiGetRequest, declared above, is a vector of structs of keys and callbacks.

Ownership of the request is transferred to this function.

Reimplemented from net_instaweb::CacheInterface.

virtual void net_instaweb::AsyncCache::Put ( const GoogleString key,
SharedString value 
) [virtual]

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::AsyncCache::ShutDown ( ) [virtual]

Prevent the AsyncCache from issuing any more Gets. Any subsequent Gets will have their callback invoked immediately with kNotFound. Outstanding Gets may be completed depending on timing.

This can be called during the process Shutdown flow to avoid introducing more work asynchronously that will have to be completed prior to Shutdown.

Implements net_instaweb::CacheInterface.


Member Data Documentation

const int64 net_instaweb::AsyncCache::kMaxQueueSize = 2000 [static]

The maximum number of operations that can be queued up while a server is slow. When this is reached, old Deletes/Puts get dropped, and old Gets are retired with a kNotFound.

This helps bound the amount of memory consumed by queued operations when the cache gets wedged. Note that when CacheBatcher is layered above AsyncCache, it will queue up its Gets at a level above this one, and ultimately send those using a MultiGet.

Todo:
TODO(jmarantz): Analyze whether we drop operations under load with a non-wedged cache. If it looks like we are dropping Puts the first time we encounter a page then I think we may need to bump this up.

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