Iconic21Log/mgdb/include/mongocxx/read_preference.hpp

// Copyright 2014 MongoDB 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.

#pragma once

#include <chrono>
#include <cstdint>
#include <memory>
#include <string>

#include <bsoncxx/document/view_or_value.hpp>
#include <bsoncxx/stdx/optional.hpp>
#include <mongocxx/options/transaction.hpp>
#include <mongocxx/stdx.hpp>

#include <mongocxx/config/prelude.hpp>

namespace mongocxx {
MONGOCXX_INLINE_NAMESPACE_BEGIN

class client;
class collection;
class database;
class uri;

namespace events {
class topology_description;
}

///
/// Class representing a preference for how the driver routes read operations to members of a
/// replica set or to a sharded cluster.
///
/// By default read operations are directed to the primary member in a replica set. Reading from the
/// primary guarantees that read operations reflect the latest version of a document. However, by
/// distributing some or all reads to secondary members of the replica set, you can improve read
/// throughput or reduce latency for an application that does not require fully up-to-date data.
///
/// Read preference can be broadly specified by setting a mode. It is also possible to
/// set tags in the read preference for more granular control, and to target specific members of a
/// replica set via attributes other than their current state as a primary or secondary node.
/// Furthermore, it is also possible to set a staleness threshold, such that the read is limited to
/// targeting secondaries whose staleness is less than or equal to the given threshold.
///
/// Read preferences are ignored for direct connections to a single mongod instance. However,
/// in order to perform read operations on a direct connection to a secondary member of a replica
/// set, you must set a read preference that allows reading from secondaries.
///
/// @see https://docs.mongodb.com/master/core/read-preference/
///
class MONGOCXX_API read_preference {
   public:
    ///
    /// Determines which members in a replica set are acceptable to read from.
    ///
    /// @warning Read preference tags are not respected when the mode is set to primary.
    ///
    /// @warning All read preference modes except primary may return stale data because secondaries
    /// replicate operations from the primary with some delay. Ensure that your application
    /// can tolerate stale data if you choose to use a non-primary mode.
    ///
    /// @see https://docs.mongodb.com/master/core/read-preference/#read-preference-modes
    ///
    enum class read_mode : std::uint8_t {
        ///
        /// Only read from a primary node.
        ///
        k_primary,

        ///
        /// Prefer to read from a primary node.
        ///
        k_primary_preferred,

        ///
        /// Only read from secondary nodes.
        ///
        k_secondary,

        ///
        /// Prefer to read from secondary nodes.
        ///
        k_secondary_preferred,

        ///
        /// Read from the node with the lowest latency irrespective of state.
        ///
        k_nearest
    };

    ///
    /// Constructs a new read_preference with read_mode set to k_primary.
    ///
    read_preference();

    struct deprecated_tag {};

    ///
    /// Constructs a new read_preference.
    ///
    /// @param mode
    ///   Specifies the read_mode.
    ///
    /// @deprecated The constructor with no arguments and the method mode() should be used.
    ///
    MONGOCXX_DEPRECATED read_preference(read_mode mode);
    read_preference(read_mode mode, deprecated_tag);

    ///
    /// Constructs a new read_preference with tags.
    ///
    /// @param mode
    ///   A read_preference read_mode.
    /// @param tags
    ///   A document representing tags to use for the read_preference.
    ///
    /// @see https://docs.mongodb.com/master/core/read-preference/#tag-sets
    ///
    /// @deprecated The tags() method should be used instead.
    ///
    MONGOCXX_DEPRECATED read_preference(read_mode mode, bsoncxx::document::view_or_value tags);
    read_preference(read_mode mode, bsoncxx::document::view_or_value tags, deprecated_tag);

    ///
    /// Copy constructs a read_preference.
    ///
    read_preference(const read_preference&);

    ///
    /// Copy assigns a read_preference.
    ///
    read_preference& operator=(const read_preference&);

    ///
    /// Move constructs a read_preference.
    ///
    read_preference(read_preference&&) noexcept;

    ///
    /// Move assigns a read_preference.
    ///
    read_preference& operator=(read_preference&&) noexcept;

    ///
    /// Destroys a read_preference.
    ///
    ~read_preference();

    ///
    /// Sets a new mode for this read_preference.
    ///
    /// @param mode
    ///   The new read preference mode.
    ///
    /// @return
    ///   A reference to the object on which this member function is being called.  This facilitates
    ///   method chaining.
    ///
    read_preference& mode(read_mode mode);

    ///
    /// Returns the current read_mode for this read_preference.
    ///
    /// @return The current read_mode.
    ///
    read_mode mode() const;

    ///
    /// Sets or updates the tags for this read_preference.
    ///
    /// @param tags
    ///   Document representing the tags.
    ///
    /// @see https://docs.mongodb.com/master/core/read-preference/#tag-sets
    ///
    /// @return
    ///   A reference to the object on which this member function is being called.  This facilitates
    ///   method chaining.
    ///
    read_preference& tags(bsoncxx::document::view_or_value tags);

    ///
    /// Returns the current tags for this read_preference.
    ///
    /// @return The optionally set current tags.
    ///
    /// @see https://docs.mongodb.com/master/core/read-preference/#tag-sets
    ///
    stdx::optional<bsoncxx::document::view> tags() const;

    ///
    /// Sets the max staleness setting for this read_preference.  Secondary
    /// servers with an estimated lag greater than this value will be excluded
    /// from selection under modes that allow secondaries.
    ///
    /// Max staleness must be at least 90 seconds, and also at least
    /// the sum (in seconds) of the client's heartbeatFrequencyMS and the
    /// server's idle write period, which is 10 seconds.  For general use,
    /// 90 seconds is the effective minimum.  If less, an exception will be
    /// thrown when an operation is attempted.
    ///
    /// Max staleness may only be used with MongoDB version 3.4 or later.
    /// If used with an earlier version, an exception will be thrown when an
    /// operation is attempted.
    ///
    /// @note
    ///     The max-staleness feature is designed to prevent badly-lagging
    ///     servers from being selected.  The staleness estimate is
    ///     imprecise and shouldn't be used to try to select "up-to-date"
    ///     secondaries.
    ///
    /// @param max_staleness
    ///    The new max staleness setting.  It must be positive.
    ///
    /// @return
    ///   A reference to the object on which this member function is being called.  This facilitates
    ///   method chaining.
    ///
    /// @throws mongocxx::logic_error if the argument is invalid.
    ///
    read_preference& max_staleness(std::chrono::seconds max_staleness);

    ///
    /// Returns the current max staleness setting for this read_preference.
    ///
    /// @return The optionally current max staleness setting.
    ///
    stdx::optional<std::chrono::seconds> max_staleness() const;

    ///
    /// Sets the hedge document to be used for the read preference. Sharded clusters running MongoDB
    /// 4.4 or later can dispatch read operations in parallel, returning the result from the fastest
    /// host and cancelling the unfinished operations.
    ///
    /// This may be an empty document or a document of the form { enabled: <boolean> }.
    ///
    /// Hedged reads are automatically enabled in MongoDB 4.4+ when using a ``nearest`` read
    /// preference. To explicitly enable or disable hedging, the ``hedge`` document must be
    /// passed. An empty document uses server defaults to control hedging, but the ``enabled`` key
    /// may be set to ``true`` or ``false`` to explicitly enable or disable hedged reads.
    ///
    /// @param hedge
    ///   The hedge document to set. For example, the document { enabled: true }.
    ///
    /// @return A reference to the object on which this member function is being called. This
    /// facilitates method chaining.
    ///
    read_preference& hedge(bsoncxx::document::view_or_value hedge);

    ///
    /// Gets the current hedge document to be used for the read preference.
    ///
    /// @return A hedge document if one was set.
    ///
    const stdx::optional<bsoncxx::document::view> hedge() const;

   private:
    friend client;
    friend collection;
    friend database;
    friend options::transaction;
    friend events::topology_description;
    friend uri;

    ///
    /// @{
    ///
    /// Compares two read_preference objects for (in)-equality.
    ///
    /// @relates: read_preference
    ///
    friend MONGOCXX_API bool MONGOCXX_CALL operator==(const read_preference&,
                                                      const read_preference&);
    friend MONGOCXX_API bool MONGOCXX_CALL operator!=(const read_preference&,
                                                      const read_preference&);
    ///
    /// @}
    ///

    class MONGOCXX_PRIVATE impl;

    MONGOCXX_PRIVATE read_preference(std::unique_ptr<impl>&& implementation);

    std::unique_ptr<impl> _impl;
};

MONGOCXX_INLINE_NAMESPACE_END
}  // namespace mongocxx

#include <mongocxx/config/postlude.hpp>