kv_store.h

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/** @file storage/temptable/include/temptable/kv_store.h
TempTable key-value store implementation. */
 
#ifndef TEMPTABLE_KV_STORE_H
#define TEMPTABLE_KV_STORE_H
 
#include <shared_mutex>
#include <string>
#include <unordered_map>
 
#include "storage/temptable/include/temptable/constants.h"
#include "storage/temptable/include/temptable/kv_store_logger.h"
 
namespace temptable {
 
/** Forward-declarations. */
class Table;
 
/** Key-value store, a convenience wrapper class which models a thread-safe
 * dictionary type.
 *
 * Thread-safety is accomplished by using a `Lock` which can be any of the usual
 * mutual exclusive algorithms from C++ thread-support library or any other
 * which satisfy C++ concurrency named requirements. E.g. mutex, timed_mutex,
 * recursive_mutex, recursive_timed_mutex, shared_mutex, shared_timed_mutex, ...
 * User code can opt-in for any of those. Also, whether the actual
 * implementation will use shared-locks or exclusive-locks (for read-only
 * operations) is handled automagically by this class.
 *
 * Furthermore, user code can similarly opt-in for different key-value
 * implementation but whose interface is compatible with the one of
 * std::unordered_map.
 * */
template <typename Lock,
          template <typename...> class KeyValueImpl = std::unordered_map>
class Key_value_store
    : public Key_value_store_logger<Key_value_store<Lock, KeyValueImpl>,
                                    DEBUG_BUILD> {
  /** Do not break encapsulation when using CRTP. */
  friend class Key_value_store_logger<Key_value_store<Lock, KeyValueImpl>,
                                      DEBUG_BUILD>;
 
  /** Help ADL to bring debugging/logging symbols into the scope. */
  using Key_value_store_logger<Key_value_store<Lock, KeyValueImpl>,
                               DEBUG_BUILD>::dbug_print;
  using Key_value_store_logger<Key_value_store<Lock, KeyValueImpl>,
                               DEBUG_BUILD>::log;
 
  /** Check whether we can use shared locks (which enable multiple concurrent
   * readers) or must we rather fallback to exclusive locks. Shared-locks will
   * be used only during read-only operations.
   * */
  using Exclusive_or_shared_lock =
      std::conditional_t<std::is_same<Lock, std::shared_timed_mutex>::value,
                         std::shared_lock<Lock>, std::lock_guard<Lock>>;
 
  /** Alias for our key-value store implementation. */
  using Key_value_store_impl = KeyValueImpl<std::string, Table>;
 
  /** Container holding (table-name, Table) tuples. */
  Key_value_store_impl m_kv_store;
 
  /** Lock type. */
  Lock m_lock;
 
 public:
  /** Inserts a new table into the container constructed in-place with the
   * given args if there is no table with the key in the container.
   *
   * [in] args Arguments to forward to the constructor of the table.
   * @return Returns a pair consisting of an iterator to the inserted table,
   * or the already-existing table if no insertion happened, and a bool
   * denoting whether the insertion took place (true if insertion happened,
   * false if it did not).
   * */
  template <class... Args>
  std::pair<typename Key_value_store_impl::iterator, bool> emplace(
      Args &&...args) {
    std::lock_guard<Lock> lock(m_lock);
    dbug_print();
    log(Key_value_store_stats::Event::EMPLACE);
    return m_kv_store.emplace(args...);
  }
 
  /** Searches for a table with given name (key).
   *
   * [in] key Name of a table to search for.
   * @return Pointer to table if found, nullptr otherwise.
   * */
  Table *find(const std::string &key) {
    Exclusive_or_shared_lock lock(m_lock);
    auto iter = m_kv_store.find(key);
    if (iter != m_kv_store.end()) {
      return &iter->second;
    } else {
      return nullptr;
    }
  }
 
  /** Removes the table (if one exists) with the given name (key).
   *
   * [in] key Name of a table to remove..
   * @return Number of elements removed.
   * */
  typename Key_value_store_impl::size_type erase(const std::string &key) {
    std::lock_guard<Lock> lock(m_lock);
    dbug_print();
    log(Key_value_store_stats::Event::ERASE);
    return m_kv_store.erase(key);
  }
};
 
}  // namespace temptable
 
#endif /* TEMPTABLE_KV_STORE_H */