src/pagespeed/kernel/cache/fallback_cache.cc

/*
 * Copyright 2012 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// Author: jmarantz@google.com (Joshua Marantz)

#include "pagespeed/kernel/cache/fallback_cache.h"

#include "base/logging.h"
#include "pagespeed/kernel/base/shared_string.h"

namespace {

// We decide in Put whether the value goes to large_object_cache_
// or small_object_cache_.  When a value goes to small_object_cache_,
// we suffix it with an 'S'.  When it goes to the large_object_cache_, we
// put into small_object_cache_ a single 'L', which will tell us
// to look in large_object_cache_.  Values stored in large_object_cache_
// do not have a suffix.
//
// We use suffixes so that we can strip the value from the
// SharedString via RemoveSuffix, which does not require mutating the
// base string data, which might, at least in the case of LRUCache, be
// accessed concurrently by multiple threads.
char kInSmallObjectCache = 'S';
char kInLargeObjectCache = 'L';

}  // namespace

namespace net_instaweb {

namespace {

// Forwards Get requests to the large_cache when the response matches
// kInLargeObjectCache.  For kInSmallObjectCache, unwraps the payload
// and delivers it to the callback.
class FallbackCallback : public CacheInterface::Callback {
 public:
  FallbackCallback(CacheInterface::Callback* callback,
                   CacheInterface* large_object_cache)
      : callback_(callback),
        large_object_cache_(large_object_cache),
        validate_candidate_called_(false) {
  }

  virtual ~FallbackCallback() {
  }

  virtual void Done(CacheInterface::KeyState state) {
    DCHECK(validate_candidate_called_);
    if (callback_ != NULL) {
      callback_->DelegatedDone(state);
    }
    delete this;
  }

  // This validation is called by the small-object cache.  We need to decode
  // the value and decide whether to unwrap the small value, or forward the
  // request to the large_object_cache_.
  virtual bool ValidateCandidate(const GoogleString& key,
                                 CacheInterface::KeyState state) {
    validate_candidate_called_ = true;
    size_t size = value()->size();
    const char* val = value()->data();
    if ((size == 1) && (val[0] == kInLargeObjectCache)) {
      // Delegate the fetch to large_object_cache_, passing the
      // original callback directly to the large_object_cache_.
      // We erase callback_ so we don't forward the Done report
      // from the small cache.
      Callback* callback = callback_;
      callback_ = NULL;
      large_object_cache_->Get(key, callback);
      return true;  // The forwarding-marker in the small object cache is OK.
    } else if ((size >= 1) && (val[size - 1] == kInSmallObjectCache)) {
      // Link values together, but strip the marker from the new view.
      *callback_->value() = *value();
      callback_->value()->RemoveSuffix(1);
      return callback_->DelegatedValidateCandidate(key, state);
    }
    // The value in the cache was missing or encoded incorrectly.
    callback_->DelegatedValidateCandidate(key, CacheInterface::kNotFound);
    return false;
  }

 private:
  CacheInterface::Callback* callback_;
  CacheInterface* large_object_cache_;
  bool validate_candidate_called_;

  DISALLOW_COPY_AND_ASSIGN(FallbackCallback);
};

}  // namespace

FallbackCache::FallbackCache(CacheInterface* small_object_cache,
                             CacheInterface* large_object_cache,
                             int threshold_bytes,
                             MessageHandler* handler)
    : small_object_cache_(small_object_cache),
      large_object_cache_(large_object_cache),
      threshold_bytes_(threshold_bytes),
      account_for_key_size_(true),
      message_handler_(handler) {
}

FallbackCache::~FallbackCache() {
}

GoogleString FallbackCache::FormatName(StringPiece small, StringPiece large) {
  return StrCat("Fallback(small=", small, ",large=", large, ")");
}

void FallbackCache::Get(const GoogleString& key, Callback* callback) {
  callback = new FallbackCallback(callback, large_object_cache_);
  small_object_cache_->Get(key, callback);
}

void FallbackCache::MultiGet(MultiGetRequest* request) {
  for (int i = 0, n = request->size(); i < n; ++i) {
    KeyCallback& key_callback = (*request)[i];
    key_callback.callback = new FallbackCallback(key_callback.callback,
                                                 large_object_cache_);
  }
  small_object_cache_->MultiGet(request);
}

void FallbackCache::Put(const GoogleString& key, SharedString* value) {
  int store_size = value->size();
  if (account_for_key_size_) {
    store_size += static_cast<int>(key.size());
  }
  store_size += 1;  // For kInSmallObjectCache marker.

  if (store_size > threshold_bytes_) {
    SharedString forwarding_value;
    forwarding_value.Assign(&kInLargeObjectCache, 1);
    small_object_cache_->Put(key, &forwarding_value);
    large_object_cache_->Put(key, value);
  } else {
    SharedString wrapped_value(*value);
    wrapped_value.Append(&kInSmallObjectCache, 1);
    small_object_cache_->Put(key, &wrapped_value);
  }
}

void FallbackCache::Delete(const GoogleString& key) {
  small_object_cache_->Delete(key);
  large_object_cache_->Delete(key);
}

}  // namespace net_instaweb