YugabyteDB (2.13.0.0-b42, bfc6a6643e7399ac8a0e81d06a3ee6d6571b33ab)

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// under the License.

//

// The following only applies to changes made to this file as part of YugaByte development.

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// Portions Copyright (c) YugaByte, 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.

//

// This module is internal to the client and not a public API.

#ifndef YB_CLIENT_META_CACHE_H

#define YB_CLIENT_META_CACHE_H

#include <shared_mutex>

#include <map>

#include <string>

#include <memory>

#include <unordered_map>

#include <vector>

#include <boost/variant.hpp>

#include <gtest/gtest_prod.h>

#include "yb/client/client_fwd.h"

#include "yb/common/partition.h"

#include "yb/common/wire_protocol.h"

#include "yb/consensus/metadata.pb.h"

#include "yb/gutil/macros.h"

#include "yb/gutil/ref_counted.h"

#include "yb/gutil/thread_annotations.h"

#include "yb/master/master_client.fwd.h"

#include "yb/master/master_fwd.h"

#include "yb/rpc/rpc_fwd.h"

#include "yb/rpc/rpc.h"

#include "yb/tablet/metadata.pb.h"

#include "yb/tserver/tserver_fwd.h"

#include "yb/util/capabilities.h"

#include "yb/util/format.h"

#include "yb/util/locks.h"

#include "yb/util/lockfree.h"

#include "yb/util/metrics.h"

#include "yb/util/monotime.h"

#include "yb/util/semaphore.h"

#include "yb/util/status_fwd.h"

#include "yb/util/memory/arena.h"

#include "yb/util/net/net_util.h"

namespace yb {

class Histogram;

class YBPartialRow;

namespace client {

class ClientTest_TestMasterLookupPermits_Test;

class YBClient;

class YBTable;

namespace internal {

class LookupRpc;

class LookupByKeyRpc;

class LookupByIdRpc;

// The information cached about a given tablet server in the cluster.

//

// A RemoteTabletServer could be the local tablet server.

//

// This class is thread-safe.

class RemoteTabletServer {

 public:

  RemoteTabletServer(const std::string& uuid,

                     const std::shared_ptr<tserver::TabletServerServiceProxy>& proxy,

                     const tserver::LocalTabletServer* local_tserver = nullptr);

  explicit RemoteTabletServer(const master::TSInfoPB& pb);

  ~RemoteTabletServer();

  // Initialize the RPC proxy to this tablet server, if it is not already set up.

  // This will involve a DNS lookup if there is not already an active proxy.

  // If there is an active proxy, does nothing.

  CHECKED_STATUS InitProxy(YBClient* client);

  // Update information from the given pb.

  // Requires that 'pb''s UUID matches this server.

  void Update(const master::TSInfoPB& pb);

  // Is this tablet server local?

  bool IsLocal() const;

  const tserver::LocalTabletServer* local_tserver() const {

    return local_tserver_;

  }

  // Return the current proxy to this tablet server. Requires that InitProxy()

  // be called prior to this.

  std::shared_ptr<tserver::TabletServerServiceProxy> proxy() const;

  ::yb::HostPort ProxyEndpoint() const;

  std::string ToString() const;

  bool HasHostFrom(const std::unordered_set<std::string>& hosts) const;

  // Returns the remote server's uuid.

  const std::string& permanent_uuid() const;

  const CloudInfoPB& cloud_info() const;

  const google::protobuf::RepeatedPtrField<HostPortPB>& public_rpc_hostports() const;

  const google::protobuf::RepeatedPtrField<HostPortPB>& private_rpc_hostports() const;

  bool HasCapability(CapabilityId capability) const;

 private:

  mutable rw_spinlock mutex_;

  const std::string uuid_;

  google::protobuf::RepeatedPtrField<HostPortPB> public_rpc_hostports_;

  google::protobuf::RepeatedPtrField<HostPortPB> private_rpc_hostports_;

  yb::CloudInfoPB cloud_info_pb_;

  std::shared_ptr<tserver::TabletServerServiceProxy> proxy_;

  ::yb::HostPort proxy_endpoint_;

  const tserver::LocalTabletServer* const local_tserver_ = nullptr;

  scoped_refptr<Histogram> dns_resolve_histogram_;

  std::vector<CapabilityId> capabilities_;

  DISALLOW_COPY_AND_ASSIGN(RemoteTabletServer);

};

struct RemoteReplica {

  RemoteTabletServer* ts;

  PeerRole role;

  MonoTime last_failed_time = MonoTime::kUninitialized;

  // The state of this replica. Only updated after calling GetTabletStatus.

  tablet::RaftGroupStatePB state = tablet::RaftGroupStatePB::UNKNOWN;

  RemoteReplica(RemoteTabletServer* ts_, PeerRole role_)

      : ts(ts_), role(role_) {}

  void MarkFailed() {

    last_failed_time = MonoTime::Now();

  }

  void ClearFailed() {

    last_failed_time = MonoTime::kUninitialized;

  }

  bool Failed() const {

    return last_failed_time.Initialized();

  }

  std::string ToString() const;

};

typedef std::unordered_map<std::string, std::unique_ptr<RemoteTabletServer>> TabletServerMap;

YB_STRONGLY_TYPED_BOOL(UpdateLocalTsState);

YB_STRONGLY_TYPED_BOOL(IncludeFailedReplicas);

struct ReplicasCount {

  ReplicasCount(int expected_live_replicas, int expected_read_replicas) {

    SetExpectedReplicas(expected_live_replicas, expected_read_replicas);

  }

  int expected_live_replicas = 0;

  int expected_read_replicas = 0;

  // Number of live replicas in replicas_.

  int num_alive_live_replicas = 0;

  // Number of read replicas in replicas_.

  int num_alive_read_replicas = 0;

  bool IsReplicasCountConsistent() {

    return (expected_live_replicas + expected_read_replicas) ==

    (num_alive_live_replicas + num_alive_read_replicas);

  }

  // Set expected_live_replicas and expected_read_replicas.

  void SetExpectedReplicas(int live_replicas, int read_replicas) {

    expected_live_replicas = live_replicas;

    expected_read_replicas = read_replicas;

  }

  void SetAliveReplicas(int live_replicas, int read_replicas) {

    num_alive_live_replicas = live_replicas;

    num_alive_read_replicas = read_replicas;

  }

  std::string ToString();

};

// The client's view of a given tablet. This object manages lookups of

// the tablet's locations, status, etc.

//

// This class is thread-safe.

class RemoteTablet : public RefCountedThreadSafe<RemoteTablet> {

 public:

  RemoteTablet(std::string tablet_id,

               Partition partition,

               boost::optional<PartitionListVersion> partition_list_version,

               uint64 split_depth,

               const TabletId& split_parent_tablet_id);

  ~RemoteTablet();

  // Updates this tablet's replica locations.

  void Refresh(

      const TabletServerMap& tservers,

      const google::protobuf::RepeatedPtrField<master::TabletLocationsPB_ReplicaPB>& replicas);

  // Mark this tablet as stale, indicating that the cached tablet metadata is

  // out of date. Staleness is checked by the MetaCache when

  // LookupTabletByKey() is called to determine whether the fast (non-network)

  // path can be used or whether the metadata must be refreshed from the Master.

  void MarkStale();

  // Whether the tablet has been marked as stale.

  bool stale() const;

  // Mark this tablet as already split.

  void MarkAsSplit();

  bool is_split() const;

  // Returns table partition list version last known to the client for which this tablet was

  // serving partition_ key range.

  // This could be `none` for RemoteTablet instances requested by ID, because in that case we don't

  // get table partition list version from master.

  boost::optional<PartitionListVersion> partition_list_version() const {

    return partition_list_version_;

  }

  // Mark any replicas of this tablet hosted by 'ts' as failed. They will

  // not be returned in future cache lookups.

  //

  // The provided status is used for logging.

  // Returns true if 'ts' was found among this tablet's replicas, false if not.

  bool MarkReplicaFailed(RemoteTabletServer *ts, const Status& status);

  // Return the number of failed replicas for this tablet.

  int GetNumFailedReplicas() const;

  bool IsReplicasCountConsistent() const;

  std::string ReplicasCountToString() const;

  // Set expected_live_replicas and expected_read_replicas.

  void SetExpectedReplicas(int expected_live_replicas, int expected_read_replicas);

  void SetAliveReplicas(int alive_live_replicas, int alive_read_replicas);

  // Return the tablet server which is acting as the current LEADER for

  // this tablet, provided it hasn't failed.

  //

  // Returns NULL if there is currently no leader, or if the leader has

  // failed. Given that the replica list may change at any time,

  // callers should always check the result against NULL.

  RemoteTabletServer* LeaderTServer() const;

  // Writes this tablet's TSes (across all replicas) to 'servers' for all available replicas. If a

  // replica has failed recently, check if it is available now if it is local. For remote replica,

  // wait for some time (configurable) before retrying.

  void GetRemoteTabletServers(

      std::vector<RemoteTabletServer*>* servers,

      IncludeFailedReplicas include_failed_replicas = IncludeFailedReplicas::kFalse);

  std::vector<RemoteTabletServer*> GetRemoteTabletServers(

      IncludeFailedReplicas include_failed_replicas = IncludeFailedReplicas::kFalse) {

    std::vector<RemoteTabletServer*> result;

    GetRemoteTabletServers(&result, include_failed_replicas);

    return result;

  }

  // Return true if the tablet currently has a known LEADER replica

  // (i.e the next call to LeaderTServer() is likely to return non-NULL)

  bool HasLeader() const;

  const std::string& tablet_id() const { return tablet_id_; }

  const Partition& partition() const {

    return partition_;

  }

  // Mark the specified tablet server as the leader of the consensus configuration in the cache.

  // Returns whether server was found in replicas_.

  bool MarkTServerAsLeader(const RemoteTabletServer* server) WARN_UNUSED_RESULT;

  // Mark the specified tablet server as a follower in the cache.

  void MarkTServerAsFollower(const RemoteTabletServer* server);

  // Return stringified representation of the list of replicas for this tablet.

  std::string ReplicasAsString() const;

  std::string ToString() const;

  const std::string& LogPrefix() const { return log_prefix_; }

  MonoTime refresh_time() { return refresh_time_.load(std::memory_order_acquire); }

  // See TabletLocationsPB::split_depth.

  uint64 split_depth() const { return split_depth_; }

  const TabletId& split_parent_tablet_id() const { return split_parent_tablet_id_; }

  int64_t lookups_without_new_replicas() const { return lookups_without_new_replicas_; }

  // The last version of the table's partition list that we know the tablet was serving data with.

  PartitionListVersion GetLastKnownPartitionListVersion() const;

  void MakeLastKnownPartitionListVersionAtLeast(PartitionListVersion partition_list_version);

 private:

  // Same as ReplicasAsString(), except that the caller must hold mutex_.

  std::string ReplicasAsStringUnlocked() const;

  const std::string tablet_id_;

  const std::string log_prefix_;

  const Partition partition_;

  const boost::optional<PartitionListVersion> partition_list_version_;

  const uint64 split_depth_;

  const TabletId split_parent_tablet_id_;

  // All non-const members are protected by 'mutex_'.

  mutable rw_spinlock mutex_;

  bool stale_;

  bool is_split_ = false;

  std::vector<RemoteReplica> replicas_;

  PartitionListVersion last_known_partition_list_version_ = 0;

  std::atomic<ReplicasCount> replicas_count_{{0, 0}};

  // Last time this object was refreshed. Initialized to MonoTime::Min() so we don't have to be

  // checking whether it has been initialized everytime we use this value.

  std::atomic<MonoTime> refresh_time_{MonoTime::Min()};

  int64_t lookups_without_new_replicas_ = 0;

  DISALLOW_COPY_AND_ASSIGN(RemoteTablet);

};

class ToStringable {

 public:

  virtual std::string ToString() const = 0;

  virtual ~ToStringable() = default;

};

class RequestCleanup {

 public:

  virtual void CleanupRequest() = 0;

  virtual ~RequestCleanup() = default;

};

// We store partition_list_version in addition to start_key to be able to uniquely identify exact

// partition (as a result of split we can have new partition with same start key, but different

// version).

struct VersionedPartitionStartKey {

  PartitionKeyPtr key;

  PartitionListVersion partition_list_version;

  std::string ToString() const;

};

typedef PartitionKey PartitionGroupStartKey;

typedef PartitionKeyPtr PartitionGroupStartKeyPtr;

typedef VersionedPartitionStartKey VersionedPartitionGroupStartKey;

using LookupCallbackParam = boost::variant<RemoteTabletPtr, std::vector<RemoteTabletPtr>>;

using LookupCallback = boost::variant<LookupTabletCallback, LookupTabletRangeCallback>;

// Used to store callbacks for individual requests looking up tablet by partition key and those

// requests deadlines, so MetaCache can fire invoke those callbacks inside ProcessTabletLocations

// after receiving group of tablet locations from master.

struct LookupData : public MPSCQueueEntry<LookupData> {

  LookupData() {}

  LookupData(

      const LookupCallback& callback_, CoarseTimePoint deadline_,

      const PartitionKeyPtr& partition_start_)

      : callback(callback_), deadline(deadline_),

        partition_start(partition_start_) {

  }

  LookupCallback callback;

  CoarseTimePoint deadline;

  // Suitable only when lookup is performed for partition, nullptr otherwise.

  PartitionKeyPtr partition_start;

  std::string ToString() const {

    return Format("{ deadline: $1 partition_start: $2 }",

                  deadline, partition_start ? Slice(*partition_start).ToDebugHexString() : "");

  }

};

// Stores group of tablet lookups to be resolved by the same single RPC call.

// For this purpose, lookups by tablet ID are grouped by tablet ID and lookups by key

// are grouped by partitions group.

struct LookupDataGroup {

  MPSCQueue<LookupData> lookups;

  // 0 if the request is not yet sent

  std::atomic<int64_t> running_request_number{0};

  int64_t max_completed_request_number = 0;

  void Finished(int64_t request_no, const ToStringable& id, bool allow_absence = false);

  ~LookupDataGroup();

};

struct TableData {

  explicit TableData(const VersionedTablePartitionListPtr& partition_list_);

  VersionedTablePartitionListPtr partition_list;

  std::map<PartitionKey, RemoteTabletPtr> tablets_by_partition;

  std::unordered_map<PartitionGroupStartKey, LookupDataGroup> tablet_lookups_by_group;

  std::vector<RemoteTabletPtr> all_tablets;

  LookupDataGroup full_table_lookups;

  bool stale = false;

  // To resolve partition_key to tablet_id MetaCache uses client::FindPartitionStart with

  // TableData::partition_list and then translates partition_start to tablet_id based on

  // TableData::tablets_by_partition.

  //

  // We maintain the invariant that TableData::tablets_by_partition and

  // TableData::tablet_lookups_by_group always correspond to the version of

  // TableData::partition_list to avoid inconsistencies like the following:

  //

  // If TableData::tablets_by_partition was populated based on a newer version of Table partition

  // list than partition_list.version, for a key belonging to 2nd (with the higher key range)

  // post-split tablet we can get partition_start key of the parent partition as of the state before

  // the split and then translates it using newer TableData::tablets_by_partition into 1st

  // (with the lower key range) post-split tablet instead of 2nd post-split tablet. This way we will

  // miss the key, because it doesn't exist in 1st post-split tablet.

};

class LookupCallbackVisitor : public boost::static_visitor<> {

 public:

  explicit LookupCallbackVisitor(const LookupCallbackParam& param) : param_(param) {

  }

  explicit LookupCallbackVisitor(const Status& error_status) : error_status_(error_status) {

  }

  void operator()(const LookupTabletCallback& tablet_callback) const;

  void operator()(const LookupTabletRangeCallback& tablet_range_callback) const;

 private:

  const LookupCallbackParam param_;

  const boost::optional<Status> error_status_;

};

// Manager of RemoteTablets and RemoteTabletServers. The client consults

// this class to look up a given tablet or server.

//

// This class will also be responsible for cache eviction policies, etc.

class MetaCache : public RefCountedThreadSafe<MetaCache> {

 public:

  // The passed 'client' object must remain valid as long as MetaCache is alive.

  explicit MetaCache(YBClient* client);

  ~MetaCache();

  // Add a tablet server's proxy, and optionally the tserver itself it is local.

  void SetLocalTabletServer(const std::string& permanent_uuid,

                            const std::shared_ptr<tserver::TabletServerServiceProxy>& proxy,

                            const tserver::LocalTabletServer* local_tserver);

  // Look up which tablet hosts the given partition key for a table. When it is

  // available, the tablet is stored in 'remote_tablet' (if not NULL) and the

  // callback is fired. Only tablets with non-failed LEADERs are considered.

  //

  // NOTE: the callback may be called from an IO thread or inline with this

  // call if the cached data is already available.

  //

  // NOTE: the memory referenced by 'table' must remain valid until 'callback'

  // is invoked.

  void LookupTabletByKey(const std::shared_ptr<YBTable>& table,

                         const PartitionKey& partition_key,

                         CoarseTimePoint deadline,

                         LookupTabletCallback callback);

  std::future<Result<internal::RemoteTabletPtr>> LookupTabletByKeyFuture(

      const std::shared_ptr<YBTable>& table,

      const PartitionKey& partition_key,

      CoarseTimePoint deadline);

  // Lookup all tablets corresponding to a table.

  void LookupAllTablets(const std::shared_ptr<const YBTable>& table,

                        CoarseTimePoint deadline,

                        LookupTabletRangeCallback callback);

  // If table is specified and cache is not used or has no tablet leader also checks whether table

  // partitions are stale and returns ClientErrorCode::kTablePartitionListIsStale in that case.

  void LookupTabletById(const TabletId& tablet_id,

                        const std::shared_ptr<const YBTable>& table,

                        master::IncludeInactive include_inactive,

                        CoarseTimePoint deadline,

                        LookupTabletCallback callback,

                        UseCache use_cache);

  // Return the local tablet server if available.

  RemoteTabletServer* local_tserver() const {

    return local_tserver_;

  }

  // Mark any replicas of any tablets hosted by 'ts' as failed. They will

  // not be returned in future cache lookups.

  void MarkTSFailed(RemoteTabletServer* ts, const Status& status);

  // Acquire or release a permit to perform a (slow) master lookup.

  //

  // If acquisition fails, caller may still do the lookup, but is first

  // blocked for a short time to prevent lookup storms.

  bool AcquireMasterLookupPermit();

  void ReleaseMasterLookupPermit();

  // Called on the slow LookupTablet path when the master responds.

  // Populates the tablet caches.

  // If table_partition_list_version is specified, the function checks for it to match

  // TableData::partitions and returns Status with ClientErrorCode::kTablePartitionListIsStale if

  // versions do not match.

  // TableData::tablets_by_partition is only updated when these versions are defined and match.

  // Also notifies all callbacks that are waiting on received tablet ids.

  // REQUIRES locations to be in order of partitions and without overlaps.

  // There could be gaps due to post-tablets not yet being running, in this case, MetaCache will

  // just skip updating cache for these tablets until they become running.

  CHECKED_STATUS ProcessTabletLocations(

      const google::protobuf::RepeatedPtrField<master::TabletLocationsPB>& locations,

      boost::optional<PartitionListVersion> table_partition_list_version, LookupRpc* lookup_rpc);

  void InvalidateTableCache(const YBTable& table);

  const std::string& LogPrefix() const { return log_prefix_; }

 private:

  friend class LookupRpc;

  friend class LookupByKeyRpc;

  friend class LookupByIdRpc;

  friend class LookupFullTableRpc;

  FRIEND_TEST(client::ClientTest, TestMasterLookupPermits);

  // Lookup the given tablet by partition_start_key, only consulting local information.

  // Returns true and sets *remote_tablet if successful.

  RemoteTabletPtr LookupTabletByKeyFastPathUnlocked(

      const TableId& table_id,

      const VersionedPartitionStartKey& partition_key) REQUIRES_SHARED(mutex_);

  RemoteTabletPtr LookupTabletByIdFastPathUnlocked(const TabletId& tablet_id)

      REQUIRES_SHARED(mutex_);

  // Update our information about the given tablet server.

  //

  // This is called when we get some response from the master which contains

  // the latest host/port info for a server.

  void UpdateTabletServerUnlocked(const master::TSInfoPB& pb) REQUIRES(mutex_);

  // Notify appropriate callbacks that lookup of specified partition group of specified table

  // was failed because of specified status.

  void LookupByKeyFailed(

      const std::shared_ptr<const YBTable>& table,

      const VersionedPartitionGroupStartKey& partition_group_start,

      PartitionListVersion response_partition_list_version,

      int64_t request_no, const Status& status);

  void LookupByIdFailed(

      const TabletId& tablet_id,

      const std::shared_ptr<const YBTable>& table,

      master::IncludeInactive include_inactive,

      const boost::optional<PartitionListVersion>& response_partition_list_version,

      int64_t request_no,

      const Status& status);

  void LookupFullTableFailed(const std::shared_ptr<const YBTable>& table,

                             int64_t request_no, const Status& status);

  class CallbackNotifier;

  // Processes lookup failure.

  // status - failure status.

  // map - map that contains lookup data.

  // lock - lock of mutex_.

  // Returns deadline, if lookup should be restarted. CoarseTimePoint() if not.

  CoarseTimePoint LookupFailed(

      const Status& status, int64_t request_no, const ToStringable& lookup_id,

      LookupDataGroup* lookup_data_group,

      CallbackNotifier* notifier) REQUIRES(mutex_);

  RemoteTabletPtr FastLookupTabletByKeyUnlocked(

      const TableId& table_id,

      const VersionedPartitionStartKey& partition_start) REQUIRES_SHARED(mutex_);

  // Lookup from cache the set of tablets corresponding to a tiven table.

  // Returns empty vector if the cache is invalid or a tablet is stale,

  // otherwise returns a list of tablets.

  boost::optional<std::vector<RemoteTabletPtr>> FastLookupAllTabletsUnlocked(

      const std::shared_ptr<const YBTable>& table) REQUIRES_SHARED(mutex_);

  // If `tablet` is a result of splitting of pre-split tablet for which we already have

  // TabletRequests structure inside YBClient - updates TabletRequests.request_id_seq for the

  // `tablet` based on value for pre-split tablet.

  // This is required for correct tracking of duplicate requests to post-split tablets, if we

  // start from scratch - tserver will treat these requests as duplicates/incorrect, because

  // on tserver side related structure for tracking duplicate requests is also copied from

  // pre-split tablet to post-split tablets.

  void MaybeUpdateClientRequests(const RemoteTablet& tablet);

  std::unordered_map<TableId, TableData>::iterator InitTableDataUnlocked(

      const TableId& table_id, const VersionedTablePartitionListPtr& partitions)

      REQUIRES_SHARED(mutex_);

  template <class Lock>

  bool DoLookupTabletByKey(

      const std::shared_ptr<const YBTable>& table, const VersionedTablePartitionListPtr& partitions,

      const PartitionKeyPtr& partition_start, CoarseTimePoint deadline,

      LookupTabletCallback* callback, PartitionGroupStartKeyPtr* partition_group_start);

  template <class Lock>

  bool DoLookupTabletById(

      const TabletId& tablet_id,

      const std::shared_ptr<const YBTable>& table,

      master::IncludeInactive include_inactive,

      CoarseTimePoint deadline,

      UseCache use_cache,

      LookupTabletCallback* callback);

  template <class Lock>

  bool DoLookupAllTablets(const std::shared_ptr<const YBTable>& table,

                          CoarseTimePoint deadline,

                          LookupTabletRangeCallback* callback);

  YBClient* const client_;

  std::shared_timed_mutex mutex_;

  // Cache of Tablet Server locations: TS UUID -> RemoteTabletServer*.

  //

  // Given that the set of tablet servers is bounded by physical machines, we never

  // evict entries from this map until the MetaCache is destructed. So, no need to use

  // shared_ptr, etc.

  //

  // Protected by mutex_.

  TabletServerMap ts_cache_;

  // Local tablet server.

  RemoteTabletServer* local_tserver_ = nullptr;

  // Cache of tablets, keyed by table ID, then by start partition key.

  std::unordered_map<TableId, TableData> tables_ GUARDED_BY(mutex_);

  // Cache of tablets, keyed by tablet ID.

  std::unordered_map<TabletId, RemoteTabletPtr> tablets_by_id_ GUARDED_BY(mutex_);

  std::unordered_map<TabletId, LookupDataGroup> tablet_lookups_by_id_ GUARDED_BY(mutex_);

  // Prevents master lookup "storms" by delaying master lookups when all

  // permits have been acquired.

  Semaphore master_lookup_sem_;

  const std::string log_prefix_;

  DISALLOW_COPY_AND_ASSIGN(MetaCache);

};

int64_t TEST_GetLookupSerial();

} // namespace internal

} // namespace client

} // namespace yb

#endif /* YB_CLIENT_META_CACHE_H */