# Copyright (C) 2020 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. module Mongo module ServerSelector class Base # Initialize the server selector. # # @example Initialize the selector. # Mongo::ServerSelector::Secondary.new(:tag_sets => [{'dc' => 'nyc'}]) # # @example Initialize the preference with no options. # Mongo::ServerSelector::Secondary.new # # @param [ Hash ] options The server preference options. # # @option options [ Integer ] :local_threshold The local threshold boundary for # nearest selection in seconds. # @option options [ Integer ] max_staleness The maximum replication lag, # in seconds, that a secondary can suffer and still be eligible for a read. # A value of -1 is treated identically to nil, which is to not # have a maximum staleness. # @option options [ Hash | nil ] hedge A Hash specifying whether to enable hedged # reads on the server. Hedged reads are not enabled by default. When # specifying this option, it must be in the format: { enabled: true }, # where the value of the :enabled key is a boolean value. # # @raise [ Error::InvalidServerPreference ] If tag sets are specified # but not allowed. # # @api private def initialize(options = nil) options = options ? options.dup : {} if options[:max_staleness] == -1 options.delete(:max_staleness) end @options = options @tag_sets = options[:tag_sets] || [] @max_staleness = options[:max_staleness] @hedge = options[:hedge] validate! end # @return [ Hash ] options The options. attr_reader :options # @return [ Array ] tag_sets The tag sets used to select servers. attr_reader :tag_sets # @return [ Integer ] max_staleness The maximum replication lag, in # seconds, that a secondary can suffer and still be eligible for a read. # # @since 2.4.0 attr_reader :max_staleness # @return [ Hash | nil ] hedge The document specifying whether to enable # hedged reads. attr_reader :hedge # Get the timeout for server selection. # # @example Get the server selection timeout, in seconds. # selector.server_selection_timeout # # @return [ Float ] The timeout. # # @since 2.0.0 # # @deprecated This setting is now taken from the cluster options when # a server is selected. Will be removed in version 3.0. def server_selection_timeout @server_selection_timeout ||= (options[:server_selection_timeout] || ServerSelector::SERVER_SELECTION_TIMEOUT) end # Get the local threshold boundary for nearest selection in seconds. # # @example Get the local threshold. # selector.local_threshold # # @return [ Float ] The local threshold. # # @since 2.0.0 # # @deprecated This setting is now taken from the cluster options when # a server is selected. Will be removed in version 3.0. def local_threshold @local_threshold ||= (options[:local_threshold] || ServerSelector::LOCAL_THRESHOLD) end # @api private def local_threshold_with_cluster(cluster) options[:local_threshold] || cluster.options[:local_threshold] || LOCAL_THRESHOLD end # Inspect the server selector. # # @example Inspect the server selector. # selector.inspect # # @return [ String ] The inspection. # # @since 2.2.0 def inspect "#<#{self.class.name}:0x#{object_id} tag_sets=#{tag_sets.inspect} max_staleness=#{max_staleness.inspect} hedge=#{hedge}>" end # Check equality of two server selectors. # # @example Check server selector equality. # preference == other # # @param [ Object ] other The other preference. # # @return [ true, false ] Whether the objects are equal. # # @since 2.0.0 def ==(other) name == other.name && hedge == other.hedge && max_staleness == other.max_staleness && tag_sets == other.tag_sets end # Select a server from the specified cluster, taking into account # mongos pinning for the specified session. # # If the session is given and has a pinned server, this server is the # only server considered for selection. If the server is of type mongos, # it is returned immediately; otherwise monitoring checks on this # server are initiated to update its status, and if the server becomes # a mongos within the server selection timeout, it is returned. # # If no session is given or the session does not have a pinned server, # normal server selection process is performed among all servers in the # specified cluster matching the preference of this server selector # object. Monitoring checks are initiated on servers in the cluster until # a suitable server is found, up to the server selection timeout. # # If a suitable server is not found within the server selection timeout, # this method raises Error::NoServerAvailable. # # @param [ Mongo::Cluster ] cluster The cluster from which to select # an eligible server. # @param [ true, false ] ping Whether to ping the server before selection. # Deprecated and ignored. # @param [ Session | nil ] session Optional session to take into account # for mongos pinning. Added in version 2.10.0. # # @return [ Mongo::Server ] A server matching the server preference. # # @raise [ Error::NoServerAvailable ] No server was found matching the # specified preference / pinning requirement in the server selection # timeout. # @raise [ Error::LintError ] An unexpected condition was detected, and # lint mode is enabled. # # @since 2.0.0 def select_server(cluster, ping = nil, session = nil) server_selection_timeout = cluster.options[:server_selection_timeout] || SERVER_SELECTION_TIMEOUT # Special handling for zero timeout: if we have to select a server, # and the timeout is zero, fail immediately (since server selection # will take some non-zero amount of time in any case). if server_selection_timeout == 0 msg = "Failing server selection due to zero timeout. " + " Requested #{name} in cluster: #{cluster.summary}" raise Error::NoServerAvailable.new(self, cluster, msg) end deadline = Time.now + server_selection_timeout if session && session.pinned_server if Mongo::Lint.enabled? unless cluster.sharded? raise Error::LintError, "Session has a pinned server in a non-sharded topology: #{topology}" end end if !session.in_transaction? session.unpin end if server = session.pinned_server # Here we assume that a mongos stays in the topology indefinitely. # This will no longer be the case once SRV polling is implemented. unless server.mongos? while (time_remaining = deadline - Time.now) > 0 wait_for_server_selection(cluster, time_remaining) end unless server.mongos? msg = "The session being used is pinned to the server which is not a mongos: #{server.summary} " + "(after #{server_selection_timeout} seconds)" raise Error::NoServerAvailable.new(self, cluster, msg) end end return server end end if cluster.replica_set? validate_max_staleness_value_early! end if cluster.addresses.empty? if Lint.enabled? unless cluster.servers.empty? raise Error::LintError, "Cluster has no addresses but has servers: #{cluster.servers.map(&:inspect).join(', ')}" end end msg = "Cluster has no addresses, and therefore will never have a server" raise Error::NoServerAvailable.new(self, cluster, msg) end =begin Add this check in version 3.0.0 unless cluster.connected? msg = 'Cluster is disconnected' raise Error::NoServerAvailable.new(self, cluster, msg) end =end loop do server = try_select_server(cluster) if server unless cluster.topology.compatible? raise Error::UnsupportedFeatures, cluster.topology.compatibility_error.to_s end if session && session.starting_transaction? && cluster.sharded? session.pin(server) end return server end cluster.scan!(false) time_remaining = deadline - Time.now if time_remaining > 0 wait_for_server_selection(cluster, time_remaining) # If we wait for server selection, perform another round of # attempting to locate a suitable server. Otherwise server selection # can raise NoServerAvailable message when the diagnostics # reports an available server of the requested type. else break end end msg = "No #{name} server is available in cluster: #{cluster.summary} " + "with timeout=#{server_selection_timeout}, " + "LT=#{local_threshold_with_cluster(cluster)}" msg += server_selection_diagnostic_message(cluster) raise Error::NoServerAvailable.new(self, cluster, msg) rescue Error::NoServerAvailable => e if session && session.in_transaction? && !session.committing_transaction? e.add_label('TransientTransactionError') end if session && session.committing_transaction? e.add_label('UnknownTransactionCommitResult') end raise e end # Tries to find a suitable server, returns the server if one is available # or nil if there isn't a suitable server. # # @return [ Server | nil ] A suitable server, if one exists. # # @api private def try_select_server(cluster) servers = suitable_servers(cluster) # This list of servers may be ordered in a specific way # by the selector (e.g. for secondary preferred, the first # server may be a secondary and the second server may be primary) # and we should take the first server here respecting the order server = servers.first if server if Lint.enabled? # It is possible for a server to have a nil average RTT here # because the ARTT comes from description which may be updated # by a background thread while server selection is running. # Currently lint mode is not a public feature, if/when this # changes (https://jira.mongodb.org/browse/RUBY-1576) the # requirement for ARTT to be not nil would need to be removed. if server.average_round_trip_time.nil? raise Error::LintError, "Server #{server.address} has nil average rtt" end end end server end # Returns servers of acceptable types from the cluster. # # Does not perform staleness validation, staleness filtering or # latency filtering. # # @param [ Cluster ] cluster The cluster. # # @return [ Array ] The candidate servers. # # @api private def candidates(cluster) servers = cluster.servers servers.each do |server| validate_max_staleness_support!(server) end if cluster.single? servers elsif cluster.sharded? servers elsif cluster.replica_set? select_in_replica_set(servers) else # Unknown cluster - no servers [] end end # Returns servers satisfying the server selector from the cluster. # # @param [ Cluster ] cluster The cluster. # # @return [ Array ] The suitable servers. # # @api private def suitable_servers(cluster) if cluster.single? candidates(cluster) elsif cluster.sharded? local_threshold = local_threshold_with_cluster(cluster) servers = candidates(cluster) near_servers(servers, local_threshold) elsif cluster.replica_set? validate_max_staleness_value!(cluster) candidates(cluster) else # Unknown cluster - no servers [] end end private # Convert this server preference definition into a format appropriate # for sending to a MongoDB server (i.e., as a command field). # # @return [ Hash ] The server preference formatted as a command field value. # # @since 2.0.0 def full_doc @full_doc ||= begin preference = { :mode => self.class.const_get(:SERVER_FORMATTED_NAME) } preference.update(tags: tag_sets) unless tag_sets.empty? preference.update(maxStalenessSeconds: max_staleness) if max_staleness preference.update(hedge: hedge) if hedge preference end end # Select the primary from a list of provided candidates. # # @param [ Array ] candidates List of candidate servers to select the # primary from. # # @return [ Array ] The primary. # # @since 2.0.0 def primary(candidates) candidates.select do |server| server.primary? end end # Select the secondaries from a list of provided candidates. # # @param [ Array ] candidates List of candidate servers to select the # secondaries from. # # @return [ Array ] The secondary servers. # # @since 2.0.0 def secondaries(candidates) matching_servers = candidates.select(&:secondary?) matching_servers = filter_stale_servers(matching_servers, primary(candidates).first) matching_servers = match_tag_sets(matching_servers) unless tag_sets.empty? # Per server selection spec the server selected MUST be a random # one matching staleness and latency requirements. # Selectors always pass the output of #secondaries to #nearest # which shuffles the server list, fulfilling this requirement. matching_servers end # Select the near servers from a list of provided candidates, taking the # local threshold into account. # # @param [ Array ] candidates List of candidate servers to select the # near servers from. # @param [ Integer ] local_threshold Local threshold. This parameter # will be required in driver version 3.0. # # @return [ Array ] The near servers. # # @since 2.0.0 def near_servers(candidates = [], local_threshold = nil) return candidates if candidates.empty? # Average RTT on any server may change at any time by the server # monitor's background thread. ARTT may also become nil if the # server is marked unknown. Take a snapshot of ARTTs for the duration # of this method. candidates = candidates.map do |server| {server: server, artt: server.average_round_trip_time} end.reject do |candidate| candidate[:artt].nil? end return candidates if candidates.empty? nearest_candidate = candidates.min_by do |candidate| candidate[:artt] end # Default for legacy signarure local_threshold ||= self.local_threshold threshold = nearest_candidate[:artt] + local_threshold candidates.select do |candidate| candidate[:artt] <= threshold end.map do |candidate| candidate[:server] end.shuffle! end # Select the servers matching the defined tag sets. # # @param [ Array ] candidates List of candidate servers from which those # matching the defined tag sets should be selected. # # @return [ Array ] The servers matching the defined tag sets. # # @since 2.0.0 def match_tag_sets(candidates) matches = [] tag_sets.find do |tag_set| matches = candidates.select { |server| server.matches_tag_set?(tag_set) } !matches.empty? end matches || [] end def filter_stale_servers(candidates, primary = nil) return candidates unless @max_staleness # last_scan is filled out by the Monitor, and can be nil if a server # had its description manually set rather than being normally updated # via the SDAM flow. We don't handle the possibility of a nil # last_scan here. if primary candidates.select do |server| validate_max_staleness_support!(server) staleness = (server.last_scan - server.last_write_date) - (primary.last_scan - primary.last_write_date) + server.cluster.heartbeat_interval staleness <= @max_staleness end else max_write_date = candidates.collect(&:last_write_date).max candidates.select do |server| validate_max_staleness_support!(server) staleness = max_write_date - server.last_write_date + server.cluster.heartbeat_interval staleness <= @max_staleness end end end def validate! if !@tag_sets.all? { |set| set.empty? } && !tags_allowed? raise Error::InvalidServerPreference.new(Error::InvalidServerPreference::NO_TAG_SUPPORT) elsif @max_staleness && !max_staleness_allowed? raise Error::InvalidServerPreference.new(Error::InvalidServerPreference::NO_MAX_STALENESS_SUPPORT) end if @hedge unless hedge_allowed? raise Error::InvalidServerPreference.new(Error::InvalidServerPreference::NO_HEDGE_SUPPORT) end unless @hedge.is_a?(Hash) && @hedge.key?(:enabled) && [true, false].include?(@hedge[:enabled]) raise Error::InvalidServerPreference.new( "`hedge` value (#{hedge}) is invalid - hedge must be a Hash in the " \ "format { enabled: true }" ) end end end def validate_max_staleness_support!(server) if @max_staleness && !server.features.max_staleness_enabled? raise Error::InvalidServerPreference.new(Error::InvalidServerPreference::NO_MAX_STALENESS_WITH_LEGACY_SERVER) end end def validate_max_staleness_value_early! if @max_staleness unless @max_staleness >= SMALLEST_MAX_STALENESS_SECONDS msg = "`max_staleness` value (#{@max_staleness}) is too small - it must be at least " + "`Mongo::ServerSelector::SMALLEST_MAX_STALENESS_SECONDS` (#{ServerSelector::SMALLEST_MAX_STALENESS_SECONDS})" raise Error::InvalidServerPreference.new(msg) end end end def validate_max_staleness_value!(cluster) if @max_staleness heartbeat_interval = cluster.heartbeat_interval unless @max_staleness >= [ SMALLEST_MAX_STALENESS_SECONDS, min_cluster_staleness = heartbeat_interval + Cluster::IDLE_WRITE_PERIOD_SECONDS, ].max msg = "`max_staleness` value (#{@max_staleness}) is too small - it must be at least " + "`Mongo::ServerSelector::SMALLEST_MAX_STALENESS_SECONDS` (#{ServerSelector::SMALLEST_MAX_STALENESS_SECONDS}) and (the cluster's heartbeat_frequency " + "setting + `Mongo::Cluster::IDLE_WRITE_PERIOD_SECONDS`) (#{min_cluster_staleness})" raise Error::InvalidServerPreference.new(msg) end end end # Waits for server state changes in the specified cluster. # # If the cluster has a server selection semaphore, waits on that # semaphore up to the specified remaining time. Any change in server # state resulting from SDAM will immediately wake up this method and # cause it to return. # # If the cluster des not have a server selection semaphore, waits # the smaller of 0.25 seconds and the specified remaining time. # This functionality is provided for backwards compatibilty only for # applications directly invoking the server selection process. # If lint mode is enabled and the cluster does not have a server # selection semaphore, Error::LintError will be raised. # # @param [ Cluster ] cluster The cluster to wait for. # @param [ Numeric ] time_remaining Maximum time to wait, in seconds. def wait_for_server_selection(cluster, time_remaining) if cluster.server_selection_semaphore # Since the semaphore may have been signaled between us checking # the servers list earlier and the wait call below, we should not # wait for the full remaining time - wait for up to 1 second, then # recheck the state. cluster.server_selection_semaphore.wait([time_remaining, 1].min) else if Lint.enabled? raise Error::LintError, 'Waiting for server selection without having a server selection semaphore' end sleep [time_remaining, 0.25].min end end # Creates a diagnostic message when server selection fails. # # The diagnostic message includes the following information, as applicable: # # - Servers having dead monitor threads # - Cluster is disconnected # # If none of the conditions for diagnostic messages apply, an empty string # is returned. # # @param [ Cluster ] cluster The cluster on which server selection was # performed. # # @return [ String ] The diagnostic message. def server_selection_diagnostic_message(cluster) msg = '' dead_monitors = [] cluster.servers_list.each do |server| thread = server.monitor.instance_variable_get('@thread') if thread.nil? || !thread.alive? dead_monitors << server end end if dead_monitors.any? msg += ". The following servers have dead monitor threads: #{dead_monitors.map(&:summary).join(', ')}" end unless cluster.connected? msg += ". The cluster is disconnected (client may have been closed)" end msg end end end end