# join_node Policies¶

[flow_graph.join_node_policies]

join_node supports three buffering policies at its input ports: reserving, queueing, and key_matching.

// Defined in header <tbb/flow_graph.h>

namespace tbb {
namespace flow {

struct reserving;
struct queueing;
template<typename K, class KHash=tbb_hash_compare<K> > struct key_matching;
using tag_matching = key_matching<tag_value>;

} // namespace flow
} // namespace tbb

• queueing - As each input port is put to, the incoming message is added to an unbounded first-in first-out queue in the port. When there is at least one message at each input port, the join_node broadcasts a tuple containing the head of each queue to all successors. If at least one successor accepts the tuple, the head of each input port’s queue is removed; otherwise, the messages remain in their respective input port queues.

• reserving - As each input port is put to, the join_node marks that an input may be available at that port and returns false. When all ports have been marked as possibly available, the join_node tries to reserve a message at each port from their known predecessors. If it is unable to reserve a message at a port, it unmarks that port, and releases all previously acquired reservations. If it is able to reserve a message at all ports, it broadcasts a tuple containing these messages to all successors. If at least one successor accepts the tuple, the reservations are consumed; otherwise, they are released.

• key_matching<typename K, class KHash=tbb_hash_compare<K>> - As each input port is put to, a user-provided function object is applied to the message to obtain its key. The message is then added to a hash table of the input port. When there is a message at each input port for a given key, the join_node removes all matching messages from the input ports, constructs a tuple containing the matching messages and attempts to broadcast it to all successors. If no successor accepts the tuple, it is saved and will be forwarded on a subsequent try_get.

• tag_matching - A specialization of key_matching that accepts keys of type tag_value.