lib/mongo/server_selector/base.rb in mongo-2.13.0.beta1 vs lib/mongo/server_selector/base.rb in mongo-2.13.0.rc1

@@ -14,12 +14,356 @@
 
 module Mongo
 
   module ServerSelector
 
-    # @api private
     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<Server> ] 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<Server> ] 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).
       #
@@ -32,9 +376,243 @@
           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