secondary_cache.h

// Copyright (c) 2021, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root directory).
#pragma once

#include <stdint.h>

#include <memory>
#include <string>

#include "rocksdb/cache.h"
#include "rocksdb/customizable.h"
#include "rocksdb/slice.h"
#include "rocksdb/statistics.h"
#include "rocksdb/status.h"

namespace ROCKSDB_NAMESPACE {

// A handle for lookup result. The handle may not be immediately ready or
// have a valid value. The caller must call isReady() to determine if its
// ready, and call Wait() in order to block until it becomes ready.
// The caller must call value() after it becomes ready to determine if the
// handle successfullly read the item.
class SecondaryCacheResultHandle {
 public:
  virtual ~SecondaryCacheResultHandle() = default;

  // Returns whether the handle is ready or not
  virtual bool IsReady() = 0;

  // Block until handle becomes ready
  virtual void Wait() = 0;

  // Return the value. If nullptr, it means the lookup was unsuccessful
  virtual void* Value() = 0;

  // Return the size of value
  virtual size_t Size() = 0;
};

// SecondaryCache
//
// Cache interface for caching blocks on a secondary tier (which can include
// non-volatile media, or alternate forms of caching such as compressed data)
//
// Exceptions MUST NOT propagate out of overridden functions into RocksDB,
// because RocksDB is not exception-safe. This could cause undefined behavior
// including data loss, unreported corruption, deadlocks, and more.
class SecondaryCache : public Customizable {
 public:
  ~SecondaryCache() override = default;

  static const char* Type() { return "SecondaryCache"; }
  static Status CreateFromString(const ConfigOptions& config_options,
                                 const std::string& id,
                                 std::shared_ptr<SecondaryCache>* result);

  // Insert the given value into this cache. Ownership of `value` is
  // transferred to the callee, who is reponsible for deleting the value
  // with helper->del_cb if del_cb is not nullptr. Unlike Cache::Insert(),
  // the callee is responsible for such cleanup even in case of non-OK
  // Status.
  // Typically, the value is not saved directly but the implementation
  // uses the SaveToCallback provided by helper to extract value's
  // persistable data (typically uncompressed block), which will be written
  // to this tier. The implementation may or may not write it to cache
  // depending on the admission control policy, even if the return status
  // is success (OK).
  //
  // If the implementation is asynchronous or otherwise uses `value` after
  // the call returns, then InsertSaved() must be overridden not to rely on
  // Insert(). For example, there could be a "holding area" in memory where
  // Lookup() might return the same parsed value back. But more typically, if
  // the implementation only uses `value` for getting persistable data during
  // the call, then the default implementation of `InsertSaved()` suffices.
  virtual Status Insert(const Slice& key, void* value,
                        const Cache::CacheItemHelper* helper) = 0;

  // Insert a value from its saved/persistable data (typically uncompressed
  // block), as if generated by SaveToCallback/SizeCallback. This can be used
  // in "warming up" the cache from some auxiliary source, and like Insert()
  // may or may not write it to cache depending on the admission control
  // policy, even if the return status is success.
  //
  // The default implementation assumes synchronous, non-escaping Insert(),
  // wherein `value` is not used after return of Insert(). See Insert().
  virtual Status InsertSaved(const Slice& key, const Slice& saved);

  // Lookup the data for the given key in this cache. The create_cb
  // will be used to create the object. The handle returned may not be
  // ready yet, unless wait=true, in which case Lookup() will block until
  // the handle is ready.
  //
  // advise_erase is a hint from the primary cache indicating that the handle
  // will be cached there, so the secondary cache is advised to drop it from
  // the cache as an optimization. To use this feature, SupportForceErase()
  // needs to return true.
  // This hint can also be safely ignored.
  //
  // is_in_sec_cache is to indicate whether the handle is possibly erased
  // from the secondary cache after the Lookup.
  virtual std::unique_ptr<SecondaryCacheResultHandle> Lookup(
      const Slice& key, const Cache::CreateCallback& create_cb, bool wait,
      bool advise_erase, bool& is_in_sec_cache) = 0;

  // Indicate whether a handle can be erased in this secondary cache.
  [[nodiscard]] virtual bool SupportForceErase() const = 0;

  // At the discretion of the implementation, erase the data associated
  // with key.
  virtual void Erase(const Slice& key) = 0;

  // Wait for a collection of handles to become ready.
  virtual void WaitAll(std::vector<SecondaryCacheResultHandle*> handles) = 0;

  // Set the maximum configured capacity of the cache.
  // When the new capacity is less than the old capacity and the existing usage
  // is greater than new capacity, the implementation will do its best job to
  // purge the released entries from the cache in order to lower the usage.
  //
  // The derived class can make this function no-op and return NotSupported().
  virtual Status SetCapacity(size_t /* capacity */) {
    return Status::NotSupported();
  }

  // The derived class can make this function no-op and return NotSupported().
  virtual Status GetCapacity(size_t& /* capacity */) {
    return Status::NotSupported();
  }
};

}  // namespace ROCKSDB_NAMESPACE