:tocdepth: 3 base/frameworks/cluster/main.zeek ================================= .. zeek:namespace:: Cluster A framework for establishing and controlling a cluster of Zeek instances. In order to use the cluster framework, a script named ``cluster-layout.zeek`` must exist somewhere in Zeek's script search path which has a cluster definition of the :zeek:id:`Cluster::nodes` variable. The ``CLUSTER_NODE`` environment variable or :zeek:id:`Cluster::node` must also be sent and the cluster framework loaded as a package like ``@load base/frameworks/cluster``. .. warning:: The file ``cluster-layout.zeek`` should only contain the definition of :zeek:id:`Cluster::nodes`. Specifically, avoid loading other Zeek scripts or using :zeek:see:`redef` for anything but :zeek:id:`Cluster::nodes`. Due to ``cluster-layout.zeek`` being loaded very early, it is easy to introduce circular loading issues. :Namespace: Cluster :Imports: :doc:`base/frameworks/broker `, :doc:`base/frameworks/control ` Summary ~~~~~~~ Redefinable Options ################### ==================================================================================================== =============================================================================== :zeek:id:`Cluster::default_backend`: :zeek:type:`Broker::BackendType` :zeek:attr:`&redef` The type of data store backend that will be used for all data stores if no other has already been specified by the user in :zeek:see:`Cluster::stores`. :zeek:id:`Cluster::default_master_node`: :zeek:type:`string` :zeek:attr:`&redef` Name of the node on which master data stores will be created if no other has already been specified by the user in :zeek:see:`Cluster::stores`. :zeek:id:`Cluster::default_persistent_backend`: :zeek:type:`Broker::BackendType` :zeek:attr:`&redef` The type of persistent data store backend that will be used for all data stores if no other has already been specified by the user in :zeek:see:`Cluster::stores`. :zeek:id:`Cluster::default_store_dir`: :zeek:type:`string` :zeek:attr:`&redef` Setting a default dir will, for persistent backends that have not been given an explicit file path via :zeek:see:`Cluster::stores`, automatically create a path within this dir that is based on the name of the data store. :zeek:id:`Cluster::enable_round_robin_logging`: :zeek:type:`bool` :zeek:attr:`&redef` Whether to distribute log messages among available logging nodes. :zeek:id:`Cluster::logger_topic`: :zeek:type:`string` :zeek:attr:`&redef` The topic name used for exchanging messages that are relevant to logger nodes in a cluster. :zeek:id:`Cluster::manager_is_logger`: :zeek:type:`bool` :zeek:attr:`&redef` Indicates whether or not the manager will act as the logger and receive logs. :zeek:id:`Cluster::manager_topic`: :zeek:type:`string` :zeek:attr:`&redef` The topic name used for exchanging messages that are relevant to manager nodes in a cluster. :zeek:id:`Cluster::node`: :zeek:type:`string` :zeek:attr:`&redef` This is usually supplied on the command line for each instance of the cluster that is started up. :zeek:id:`Cluster::node_topic_prefix`: :zeek:type:`string` :zeek:attr:`&redef` The topic prefix used for exchanging messages that are relevant to a named node in a cluster. :zeek:id:`Cluster::nodeid_topic_prefix`: :zeek:type:`string` :zeek:attr:`&redef` The topic prefix used for exchanging messages that are relevant to a unique node in a cluster. :zeek:id:`Cluster::nodes`: :zeek:type:`table` :zeek:attr:`&redef` The cluster layout definition. :zeek:id:`Cluster::proxy_topic`: :zeek:type:`string` :zeek:attr:`&redef` The topic name used for exchanging messages that are relevant to proxy nodes in a cluster. :zeek:id:`Cluster::retry_interval`: :zeek:type:`interval` :zeek:attr:`&redef` Interval for retrying failed connections between cluster nodes. :zeek:id:`Cluster::time_machine_topic`: :zeek:type:`string` :zeek:attr:`&redef` The topic name used for exchanging messages that are relevant to time machine nodes in a cluster. :zeek:id:`Cluster::worker_topic`: :zeek:type:`string` :zeek:attr:`&redef` The topic name used for exchanging messages that are relevant to worker nodes in a cluster. ==================================================================================================== =============================================================================== Constants ######### ====================================================== ================================================================== :zeek:id:`Cluster::broadcast_topics`: :zeek:type:`set` A set of topic names to be used for broadcasting messages that are relevant to all nodes in a cluster. ====================================================== ================================================================== State Variables ############### ================================================================================================ ====================================================================== :zeek:id:`Cluster::stores`: :zeek:type:`table` :zeek:attr:`&default` = *...* :zeek:attr:`&redef` A table of cluster-enabled data stores that have been created, indexed by their name. :zeek:id:`Cluster::worker_count`: :zeek:type:`count` :zeek:attr:`&deprecated` = *...* This gives the value for the number of workers currently connected to, and it's maintained internally by the cluster framework. ================================================================================================ ====================================================================== Types ##### ================================================================= ==================================================================== :zeek:type:`Cluster::Info`: :zeek:type:`record` :zeek:attr:`&log` The record type which contains the column fields of the cluster log. :zeek:type:`Cluster::NamedNode`: :zeek:type:`record` Record to represent a cluster node including its name. :zeek:type:`Cluster::Node`: :zeek:type:`record` Record type to indicate a node in a cluster. :zeek:type:`Cluster::NodeType`: :zeek:type:`enum` Types of nodes that are allowed to participate in the cluster configuration. :zeek:type:`Cluster::StoreInfo`: :zeek:type:`record` Information regarding a cluster-enabled data store. ================================================================= ==================================================================== Redefinitions ############# ======================================= ====================================== :zeek:type:`Log::ID`: :zeek:type:`enum` The cluster logging stream identifier. * :zeek:enum:`Cluster::LOG` ======================================= ====================================== Events ###### ================================================= ======================================================================= :zeek:id:`Cluster::hello`: :zeek:type:`event` When using broker-enabled cluster framework, nodes broadcast this event to exchange their user-defined name along with a string that uniquely identifies it for the duration of its lifetime. :zeek:id:`Cluster::node_down`: :zeek:type:`event` When using broker-enabled cluster framework, this event will be emitted locally whenever a connected cluster node becomes disconnected. :zeek:id:`Cluster::node_up`: :zeek:type:`event` When using broker-enabled cluster framework, this event will be emitted locally whenever a cluster node connects or reconnects. ================================================= ======================================================================= Hooks ##### ============================================================ ============================================= :zeek:id:`Cluster::log_policy`: :zeek:type:`Log::PolicyHook` A default logging policy hook for the stream. ============================================================ ============================================= Functions ######### ================================================================ ===================================================================== :zeek:id:`Cluster::create_store`: :zeek:type:`function` Sets up a cluster-enabled data store. :zeek:id:`Cluster::get_active_node_count`: :zeek:type:`function` Returns the number of nodes per type, the calling node is currently connected to. :zeek:id:`Cluster::get_node_count`: :zeek:type:`function` Returns the number of nodes defined in the cluster layout for a given node type. :zeek:id:`Cluster::is_enabled`: :zeek:type:`function` This function can be called at any time to determine if the cluster framework is being enabled for this run. :zeek:id:`Cluster::local_node_type`: :zeek:type:`function` This function can be called at any time to determine what type of cluster node the current Zeek instance is going to be acting as. :zeek:id:`Cluster::log`: :zeek:type:`function` Write a message to the cluster logging stream. :zeek:id:`Cluster::node_topic`: :zeek:type:`function` Retrieve the topic associated with a specific node in the cluster. :zeek:id:`Cluster::nodeid_topic`: :zeek:type:`function` Retrieve the topic associated with a specific node in the cluster. ================================================================ ===================================================================== Detailed Interface ~~~~~~~~~~~~~~~~~~ Redefinable Options ################### .. zeek:id:: Cluster::default_backend :source-code: base/frameworks/cluster/main.zeek 70 70 :Type: :zeek:type:`Broker::BackendType` :Attributes: :zeek:attr:`&redef` :Default: ``Broker::MEMORY`` The type of data store backend that will be used for all data stores if no other has already been specified by the user in :zeek:see:`Cluster::stores`. .. zeek:id:: Cluster::default_master_node :source-code: base/frameworks/cluster/main.zeek 66 66 :Type: :zeek:type:`string` :Attributes: :zeek:attr:`&redef` :Default: ``""`` Name of the node on which master data stores will be created if no other has already been specified by the user in :zeek:see:`Cluster::stores`. An empty value means "use whatever name corresponds to the manager node". .. zeek:id:: Cluster::default_persistent_backend :source-code: base/frameworks/cluster/main.zeek 76 76 :Type: :zeek:type:`Broker::BackendType` :Attributes: :zeek:attr:`&redef` :Default: ``Broker::SQLITE`` The type of persistent data store backend that will be used for all data stores if no other has already been specified by the user in :zeek:see:`Cluster::stores`. This will be used when script authors call :zeek:see:`Cluster::create_store` with the *persistent* argument set true. .. zeek:id:: Cluster::default_store_dir :source-code: base/frameworks/cluster/main.zeek 82 82 :Type: :zeek:type:`string` :Attributes: :zeek:attr:`&redef` :Default: ``""`` Setting a default dir will, for persistent backends that have not been given an explicit file path via :zeek:see:`Cluster::stores`, automatically create a path within this dir that is based on the name of the data store. .. zeek:id:: Cluster::enable_round_robin_logging :source-code: base/frameworks/cluster/main.zeek 25 25 :Type: :zeek:type:`bool` :Attributes: :zeek:attr:`&redef` :Default: ``T`` Whether to distribute log messages among available logging nodes. .. zeek:id:: Cluster::logger_topic :source-code: base/frameworks/cluster/main.zeek 29 29 :Type: :zeek:type:`string` :Attributes: :zeek:attr:`&redef` :Default: ``"zeek/cluster/logger"`` The topic name used for exchanging messages that are relevant to logger nodes in a cluster. Used with broker-enabled cluster communication. .. zeek:id:: Cluster::manager_is_logger :source-code: base/frameworks/cluster/main.zeek 241 241 :Type: :zeek:type:`bool` :Attributes: :zeek:attr:`&redef` :Default: ``T`` Indicates whether or not the manager will act as the logger and receive logs. This value should be set in the cluster-layout.zeek script (the value should be true only if no logger is specified in Cluster::nodes). Note that ZeekControl handles this automatically. .. zeek:id:: Cluster::manager_topic :source-code: base/frameworks/cluster/main.zeek 33 33 :Type: :zeek:type:`string` :Attributes: :zeek:attr:`&redef` :Default: ``"zeek/cluster/manager"`` The topic name used for exchanging messages that are relevant to manager nodes in a cluster. Used with broker-enabled cluster communication. .. zeek:id:: Cluster::node :source-code: base/frameworks/cluster/main.zeek 245 245 :Type: :zeek:type:`string` :Attributes: :zeek:attr:`&redef` :Default: ``""`` This is usually supplied on the command line for each instance of the cluster that is started up. .. zeek:id:: Cluster::node_topic_prefix :source-code: base/frameworks/cluster/main.zeek 56 56 :Type: :zeek:type:`string` :Attributes: :zeek:attr:`&redef` :Default: ``"zeek/cluster/node/"`` The topic prefix used for exchanging messages that are relevant to a named node in a cluster. Used with broker-enabled cluster communication. .. zeek:id:: Cluster::nodeid_topic_prefix :source-code: base/frameworks/cluster/main.zeek 60 60 :Type: :zeek:type:`string` :Attributes: :zeek:attr:`&redef` :Default: ``"zeek/cluster/nodeid/"`` The topic prefix used for exchanging messages that are relevant to a unique node in a cluster. Used with broker-enabled cluster communication. .. zeek:id:: Cluster::nodes :source-code: base/frameworks/cluster/main.zeek 226 226 :Type: :zeek:type:`table` [:zeek:type:`string`] of :zeek:type:`Cluster::Node` :Attributes: :zeek:attr:`&redef` :Default: ``{}`` The cluster layout definition. This should be placed into a filter named cluster-layout.zeek somewhere in the ZEEKPATH. It will be automatically loaded if the CLUSTER_NODE environment variable is set. Note that ZeekControl handles all of this automatically. The table is typically indexed by node names/labels (e.g. "manager" or "worker-1"). .. zeek:id:: Cluster::proxy_topic :source-code: base/frameworks/cluster/main.zeek 37 37 :Type: :zeek:type:`string` :Attributes: :zeek:attr:`&redef` :Default: ``"zeek/cluster/proxy"`` The topic name used for exchanging messages that are relevant to proxy nodes in a cluster. Used with broker-enabled cluster communication. .. zeek:id:: Cluster::retry_interval :source-code: base/frameworks/cluster/main.zeek 250 250 :Type: :zeek:type:`interval` :Attributes: :zeek:attr:`&redef` :Default: ``1.0 min`` Interval for retrying failed connections between cluster nodes. If set, the ZEEK_DEFAULT_CONNECT_RETRY (given in number of seconds) environment variable overrides this option. .. zeek:id:: Cluster::time_machine_topic :source-code: base/frameworks/cluster/main.zeek 45 45 :Type: :zeek:type:`string` :Attributes: :zeek:attr:`&redef` :Default: ``"zeek/cluster/time_machine"`` The topic name used for exchanging messages that are relevant to time machine nodes in a cluster. Used with broker-enabled cluster communication. .. zeek:id:: Cluster::worker_topic :source-code: base/frameworks/cluster/main.zeek 41 41 :Type: :zeek:type:`string` :Attributes: :zeek:attr:`&redef` :Default: ``"zeek/cluster/worker"`` The topic name used for exchanging messages that are relevant to worker nodes in a cluster. Used with broker-enabled cluster communication. Constants ######### .. zeek:id:: Cluster::broadcast_topics :source-code: base/frameworks/cluster/main.zeek 51 51 :Type: :zeek:type:`set` [:zeek:type:`string`] :Default: :: { "zeek/cluster/manager", "zeek/cluster/logger", "zeek/cluster/proxy", "zeek/cluster/worker", "zeek/cluster/time_machine" } A set of topic names to be used for broadcasting messages that are relevant to all nodes in a cluster. Currently, there is not a common topic to broadcast to, because enabling implicit Broker forwarding would cause a routing loop for this topic. State Variables ############### .. zeek:id:: Cluster::stores :source-code: base/frameworks/cluster/main.zeek 117 117 :Type: :zeek:type:`table` [:zeek:type:`string`] of :zeek:type:`Cluster::StoreInfo` :Attributes: :zeek:attr:`&default` = *[name=, store=, master_node=, master=F, backend=Broker::MEMORY, options=[sqlite=[path=, synchronous=, journal_mode=, failure_mode=Broker::SQLITE_FAILURE_MODE_FAIL, integrity_check=F]], clone_resync_interval=10.0 secs, clone_stale_interval=5.0 mins, clone_mutation_buffer_interval=2.0 mins]* :zeek:attr:`&redef` :Default: ``{}`` A table of cluster-enabled data stores that have been created, indexed by their name. This table will be populated automatically by :zeek:see:`Cluster::create_store`, but if you need to customize the options related to a particular data store, you may redef this table. Calls to :zeek:see:`Cluster::create_store` will first check the table for an entry of the same name and, if found, will use the predefined options there when setting up the store. .. zeek:id:: Cluster::worker_count :source-code: base/frameworks/cluster/main.zeek 218 218 :Type: :zeek:type:`count` :Attributes: :zeek:attr:`&deprecated` = *"Remove in v6.1. Active worker count can be obtained via get_active_node_count(Cluster::WORKER)"* :Default: ``0`` This gives the value for the number of workers currently connected to, and it's maintained internally by the cluster framework. It's primarily intended for use by managers to find out how many workers should be responding to requests. Types ##### .. zeek:type:: Cluster::Info :source-code: base/frameworks/cluster/main.zeek 138 145 :Type: :zeek:type:`record` ts: :zeek:type:`time` :zeek:attr:`&log` The time at which a cluster message was generated. node: :zeek:type:`string` :zeek:attr:`&log` The name of the node that is creating the log record. message: :zeek:type:`string` :zeek:attr:`&log` A message indicating information about the cluster's operation. :Attributes: :zeek:attr:`&log` The record type which contains the column fields of the cluster log. .. zeek:type:: Cluster::NamedNode :source-code: base/frameworks/cluster/main.zeek 195 198 :Type: :zeek:type:`record` name: :zeek:type:`string` node: :zeek:type:`Cluster::Node` Record to represent a cluster node including its name. .. zeek:type:: Cluster::Node :source-code: base/frameworks/cluster/main.zeek 172 192 :Type: :zeek:type:`record` node_type: :zeek:type:`Cluster::NodeType` Identifies the type of cluster node in this node's configuration. ip: :zeek:type:`addr` The IP address of the cluster node. zone_id: :zeek:type:`string` :zeek:attr:`&default` = ``""`` :zeek:attr:`&optional` If the *ip* field is a non-global IPv6 address, this field can specify a particular :rfc:`4007` ``zone_id``. p: :zeek:type:`port` :zeek:attr:`&default` = ``0/unknown`` :zeek:attr:`&optional` The port that this node will listen on for peer connections. A value of ``0/unknown`` means the node is not pre-configured to listen. interface: :zeek:type:`string` :zeek:attr:`&optional` Identifier for the interface a worker is sniffing. manager: :zeek:type:`string` :zeek:attr:`&optional` Name of the manager node this node uses. For workers and proxies. time_machine: :zeek:type:`string` :zeek:attr:`&optional` Name of a time machine node with which this node connects. id: :zeek:type:`string` :zeek:attr:`&optional` A unique identifier assigned to the node by the broker framework. This field is only set while a node is connected. lb_filter: :zeek:type:`string` :zeek:attr:`&optional` (present if :doc:`/scripts/policy/misc/load-balancing.zeek` is loaded) A BPF filter for load balancing traffic sniffed on a single interface across a number of processes. In normal uses, this will be assigned dynamically by the manager and installed by the workers. Record type to indicate a node in a cluster. .. zeek:type:: Cluster::NodeType :source-code: base/frameworks/cluster/main.zeek 149 170 :Type: :zeek:type:`enum` .. zeek:enum:: Cluster::NONE Cluster::NodeType A dummy node type indicating the local node is not operating within a cluster. .. zeek:enum:: Cluster::CONTROL Cluster::NodeType A node type which is allowed to view/manipulate the configuration of other nodes in the cluster. .. zeek:enum:: Cluster::LOGGER Cluster::NodeType A node type responsible for log management. .. zeek:enum:: Cluster::MANAGER Cluster::NodeType A node type responsible for policy management. .. zeek:enum:: Cluster::PROXY Cluster::NodeType A node type for relaying worker node communication and synchronizing worker node state. .. zeek:enum:: Cluster::WORKER Cluster::NodeType The node type doing all the actual traffic analysis. .. zeek:enum:: Cluster::TIME_MACHINE Cluster::NodeType A node acting as a traffic recorder using the `Time Machine `_ software. Types of nodes that are allowed to participate in the cluster configuration. .. zeek:type:: Cluster::StoreInfo :source-code: base/frameworks/cluster/main.zeek 85 108 :Type: :zeek:type:`record` name: :zeek:type:`string` :zeek:attr:`&optional` The name of the data store. store: :zeek:type:`opaque` of Broker::Store :zeek:attr:`&optional` The store handle. master_node: :zeek:type:`string` :zeek:attr:`&default` = :zeek:see:`Cluster::default_master_node` :zeek:attr:`&optional` The name of the cluster node on which the master version of the data store resides. master: :zeek:type:`bool` :zeek:attr:`&default` = ``F`` :zeek:attr:`&optional` Whether the data store is the master version or a clone. backend: :zeek:type:`Broker::BackendType` :zeek:attr:`&default` = :zeek:see:`Cluster::default_backend` :zeek:attr:`&optional` The type of backend used for storing data. options: :zeek:type:`Broker::BackendOptions` :zeek:attr:`&default` = *[sqlite=[path=, synchronous=, journal_mode=, failure_mode=Broker::SQLITE_FAILURE_MODE_FAIL, integrity_check=F]]* :zeek:attr:`&optional` Parameters used for configuring the backend. clone_resync_interval: :zeek:type:`interval` :zeek:attr:`&default` = :zeek:see:`Broker::default_clone_resync_interval` :zeek:attr:`&optional` A resync/reconnect interval to pass through to :zeek:see:`Broker::create_clone`. clone_stale_interval: :zeek:type:`interval` :zeek:attr:`&default` = :zeek:see:`Broker::default_clone_stale_interval` :zeek:attr:`&optional` A staleness duration to pass through to :zeek:see:`Broker::create_clone`. clone_mutation_buffer_interval: :zeek:type:`interval` :zeek:attr:`&default` = :zeek:see:`Broker::default_clone_mutation_buffer_interval` :zeek:attr:`&optional` A mutation buffer interval to pass through to :zeek:see:`Broker::create_clone`. Information regarding a cluster-enabled data store. Events ###### .. zeek:id:: Cluster::hello :source-code: base/frameworks/cluster/main.zeek 350 379 :Type: :zeek:type:`event` (name: :zeek:type:`string`, id: :zeek:type:`string`) When using broker-enabled cluster framework, nodes broadcast this event to exchange their user-defined name along with a string that uniquely identifies it for the duration of its lifetime. This string may change if the node dies and has to reconnect later. .. zeek:id:: Cluster::node_down :source-code: base/frameworks/cluster/main.zeek 264 264 :Type: :zeek:type:`event` (name: :zeek:type:`string`, id: :zeek:type:`string`) When using broker-enabled cluster framework, this event will be emitted locally whenever a connected cluster node becomes disconnected. .. zeek:id:: Cluster::node_up :source-code: base/frameworks/cluster/main.zeek 260 260 :Type: :zeek:type:`event` (name: :zeek:type:`string`, id: :zeek:type:`string`) When using broker-enabled cluster framework, this event will be emitted locally whenever a cluster node connects or reconnects. Hooks ##### .. zeek:id:: Cluster::log_policy :source-code: base/frameworks/cluster/main.zeek 135 135 :Type: :zeek:type:`Log::PolicyHook` A default logging policy hook for the stream. Functions ######### .. zeek:id:: Cluster::create_store :source-code: base/frameworks/cluster/main.zeek 424 499 :Type: :zeek:type:`function` (name: :zeek:type:`string`, persistent: :zeek:type:`bool` :zeek:attr:`&default` = ``F`` :zeek:attr:`&optional`) : :zeek:type:`Cluster::StoreInfo` Sets up a cluster-enabled data store. They will also still properly function for uses that are not operating a cluster. :param name: the name of the data store to create. :param persistent: whether the data store must be persistent. :returns: the store's information. For master stores, the store will be ready to use immediately. For clones, the store field will not be set until the node containing the master store has connected. .. zeek:id:: Cluster::get_active_node_count :source-code: base/frameworks/cluster/main.zeek 319 322 :Type: :zeek:type:`function` (node_type: :zeek:type:`Cluster::NodeType`) : :zeek:type:`count` Returns the number of nodes per type, the calling node is currently connected to. This is primarily intended for use by the manager to find out how many nodes should be responding to requests. .. zeek:id:: Cluster::get_node_count :source-code: base/frameworks/cluster/main.zeek 306 318 :Type: :zeek:type:`function` (node_type: :zeek:type:`Cluster::NodeType`) : :zeek:type:`count` Returns the number of nodes defined in the cluster layout for a given node type. .. zeek:id:: Cluster::is_enabled :source-code: base/frameworks/cluster/main.zeek 324 327 :Type: :zeek:type:`function` () : :zeek:type:`bool` This function can be called at any time to determine if the cluster framework is being enabled for this run. :returns: True if :zeek:id:`Cluster::node` has been set. .. zeek:id:: Cluster::local_node_type :source-code: base/frameworks/cluster/main.zeek 329 338 :Type: :zeek:type:`function` () : :zeek:type:`Cluster::NodeType` This function can be called at any time to determine what type of cluster node the current Zeek instance is going to be acting as. If :zeek:id:`Cluster::is_enabled` returns false, then :zeek:enum:`Cluster::NONE` is returned. :returns: The :zeek:type:`Cluster::NodeType` the calling node acts as. .. zeek:id:: Cluster::log :source-code: base/frameworks/cluster/main.zeek 501 504 :Type: :zeek:type:`function` (msg: :zeek:type:`string`) : :zeek:type:`void` Write a message to the cluster logging stream. .. zeek:id:: Cluster::node_topic :source-code: base/frameworks/cluster/main.zeek 340 343 :Type: :zeek:type:`function` (name: :zeek:type:`string`) : :zeek:type:`string` Retrieve the topic associated with a specific node in the cluster. :param name: the name of the cluster node (e.g. "manager"). :returns: a topic string that may used to send a message exclusively to a given cluster node. .. zeek:id:: Cluster::nodeid_topic :source-code: base/frameworks/cluster/main.zeek 345 348 :Type: :zeek:type:`function` (id: :zeek:type:`string`) : :zeek:type:`string` Retrieve the topic associated with a specific node in the cluster. :param id: the id of the cluster node (from :zeek:see:`Broker::EndpointInfo` or :zeek:see:`Broker::node_id`. :returns: a topic string that may used to send a message exclusively to a given cluster node.