[PATCH v3 2/6] [RAW] swim: introduce failure detection component

[Vladislav Shpilevoy <v.shpilevoy@tarantool.org>]

Failure detection components allows to find which members are
already dead.

Part of #3234
---
 src/lib/swim/swim.c           | 439 +++++++++++++++++++++++++++++++++-
 src/lib/swim/swim_io.c        |  14 ++
 src/lib/swim/swim_io.h        |  16 ++
 src/lib/swim/swim_transport.h |   4 +-
 4 files changed, 463 insertions(+), 10 deletions(-)

diff --git a/src/lib/swim/swim.c b/src/lib/swim/swim.c
index 08c377374..c7bc11bca 100644
--- a/src/lib/swim/swim.c
+++ b/src/lib/swim/swim.c
@@ -89,6 +89,22 @@
 enum {
 	/** How often to send membership messages and pings. */
 	HEARTBEAT_RATE_DEFAULT = 1,
+	/**
+	 * If a ping was sent, it is considered to be lost after
+	 * this time without an ack.
+	 */
+	ACK_TIMEOUT = 1,
+	/**
+	 * If a member has not been responding to pings this
+	 * number of times, it is considered to be dead.
+	 */
+	NO_ACKS_TO_DEAD = 3,
+	/**
+	 * If a not pinned member confirmed to be dead, it is
+	 * removed from the membership after at least this number
+	 * of failed pings.
+	 */
+	NO_ACKS_TO_GC = NO_ACKS_TO_DEAD + 2,
 };
 
 /**
@@ -109,11 +125,17 @@ enum swim_member_status {
 	 * members table.
 	 */
 	MEMBER_ALIVE = 0,
+	/**
+	 * The member is considered to be dead. It will disappear
+	 * from the membership, if it is not pinned.
+	 */
+	MEMBER_DEAD,
 	swim_member_status_MAX,
 };
 
 static const char *swim_member_status_strs[] = {
 	"alive",
+	"dead",
 };
 
 /**
@@ -138,6 +160,30 @@ struct swim_member {
 	 * Position in a queue of members in the current round.
 	 */
 	struct rlist in_queue_round;
+	/**
+	 *
+	 *               Failure detection component
+	 */
+	/**
+	 * True, if the member is configured explicitly and can
+	 * not disappear from the membership.
+	 */
+	bool is_pinned;
+	/** Growing number to refute old messages. */
+	uint64_t incarnation;
+	/**
+	 * How many pings did not receive an ack in a row. After
+	 * a threshold the instance is marked as dead. After more
+	 * it is removed from the table (if not pinned).
+	 */
+	int failed_pings;
+	/** When the latest ping was sent to this member. */
+	double ping_ts;
+	/** Ready at hand regular ACK task. */
+	struct swim_task ack_task;
+	struct swim_task ping_task;
+	/** Position in a queue of members waiting for an ack. */
+	struct rlist in_queue_wait_ack;
 };
 
 /**
@@ -189,6 +235,19 @@ struct swim {
 	 * anti-entropy message.
 	 */
 	struct swim_member **shuffled_members;
+	/**
+	 *
+	 *               Failure detection component
+	 */
+	/**
+	 * Members waiting for an ACK. On too long absence of ACK
+	 * a member is considered to be dead and is removed. The
+	 * list is sorted by time in ascending order (tail is
+	 * newer, head is older).
+	 */
+	struct rlist queue_wait_ack;
+	/** Generator of ack checking events. */
+	struct ev_periodic wait_ack_tick;
 };
 
 static inline uint64_t
@@ -204,8 +263,84 @@ sockaddr_in_hash(const struct sockaddr_in *a)
  */
 enum swim_component_type {
 	SWIM_ANTI_ENTROPY = 0,
+	SWIM_FAILURE_DETECTION,
 };
 
+/** {{{                Failure detection component              */
+
+/** Possible failure detection keys. */
+enum swim_fd_key {
+	/** Type of the failure detection message: ping or ack. */
+	SWIM_FD_MSG_TYPE,
+	/**
+	 * Incarnation of the sender. To make the member alive if
+	 * it was considered to be dead, but ping/ack with greater
+	 * incarnation was received from it.
+	 */
+	SWIM_FD_INCARNATION,
+};
+
+/**
+ * Failure detection message now has only two types: ping or ack.
+ * Indirect ping/ack are todo.
+ */
+enum swim_fd_msg_type {
+	SWIM_FD_MSG_PING,
+	SWIM_FD_MSG_ACK,
+	swim_fd_msg_type_MAX,
+};
+
+static const char *swim_fd_msg_type_strs[] = {
+	"ping",
+	"ack",
+};
+
+/** SWIM failure detection MsgPack header template. */
+struct PACKED swim_fd_header_bin {
+	/** mp_encode_uint(SWIM_FAILURE_DETECTION) */
+	uint8_t k_header;
+	/** mp_encode_map(2) */
+	uint8_t m_header;
+
+	/** mp_encode_uint(SWIM_FD_MSG_TYPE) */
+	uint8_t k_type;
+	/** mp_encode_uint(enum swim_fd_msg_type) */
+	uint8_t v_type;
+
+	/** mp_encode_uint(SWIM_FD_INCARNATION) */
+	uint8_t k_incarnation;
+	/** mp_encode_uint(64bit incarnation) */
+	uint8_t m_incarnation;
+	uint64_t v_incarnation;
+};
+
+static inline void
+swim_fd_header_bin_create(struct swim_fd_header_bin *header,
+			  enum swim_fd_msg_type type, uint64_t incarnation)
+{
+	header->k_header = SWIM_FAILURE_DETECTION;
+	header->m_header = 0x82;
+
+	header->k_type = SWIM_FD_MSG_TYPE;
+	header->v_type = type;
+
+	header->k_incarnation = SWIM_FD_INCARNATION;
+	header->m_incarnation = 0xcf;
+	header->v_incarnation = mp_bswap_u64(incarnation);
+}
+
+static void
+swim_member_schedule_ack_wait(struct swim *swim, struct swim_member *member)
+{
+	if (rlist_empty(&member->in_queue_wait_ack)) {
+		member->ping_ts = fiber_time();
+		rlist_add_tail_entry(&swim->queue_wait_ack, member,
+				     in_queue_wait_ack);
+	}
+}
+
+/** }}}               Failure detection component               */
+
 /** {{{                  Anti-entropy component                 */
 
 /**
@@ -220,6 +355,7 @@ enum swim_member_key {
 	 */
 	SWIM_MEMBER_ADDRESS,
 	SWIM_MEMBER_PORT,
+	SWIM_MEMBER_INCARNATION,
 	swim_member_key_MAX,
 };
 
@@ -243,7 +379,7 @@ swim_anti_entropy_header_bin_create(struct swim_anti_entropy_header_bin *header,
 
 /** SWIM member MsgPack template. */
 struct PACKED swim_member_bin {
-	/** mp_encode_map(3) */
+	/** mp_encode_map(4) */
 	uint8_t m_header;
 
 	/** mp_encode_uint(SWIM_MEMBER_STATUS) */
@@ -262,6 +398,12 @@ struct PACKED swim_member_bin {
 	/** mp_encode_uint(addr.sin_port) */
 	uint8_t m_port;
 	uint16_t v_port;
+
+	/** mp_encode_uint(SWIM_MEMBER_INCARNATION) */
+	uint8_t k_incarnation;
+	/** mp_encode_uint(64bit incarnation) */
+	uint8_t m_incarnation;
+	uint64_t v_incarnation;
 };
 
 static inline void
@@ -271,17 +413,20 @@ swim_member_bin_reset(struct swim_member_bin *header,
 	header->v_status = member->status;
 	header->v_addr = mp_bswap_u32(member->addr.sin_addr.s_addr);
 	header->v_port = mp_bswap_u16(member->addr.sin_port);
+	header->v_incarnation = mp_bswap_u64(member->incarnation);
 }
 
 static inline void
 swim_member_bin_create(struct swim_member_bin *header)
 {
-	header->m_header = 0x83;
+	header->m_header = 0x84;
 	header->k_status = SWIM_MEMBER_STATUS;
 	header->k_addr = SWIM_MEMBER_ADDRESS;
 	header->m_addr = 0xce;
 	header->k_port = SWIM_MEMBER_PORT;
 	header->m_port = 0xcd;
+	header->k_incarnation = SWIM_MEMBER_INCARNATION;
+	header->m_incarnation = 0xcf;
 }
 
 /** }}}                  Anti-entropy component                 */
@@ -289,17 +434,51 @@ swim_member_bin_create(struct swim_member_bin *header)
 /**
  * SWIM message structure:
  * {
+ *     SWIM_FAILURE_DETECTION: {
+ *         SWIM_FD_MSG_TYPE: uint, enum swim_fd_msg_type,
+ *         SWIM_FD_INCARNATION: uint
+ *     },
+ *
+ *                 OR/AND
+ *
  *     SWIM_ANTI_ENTROPY: [
  *         {
  *             SWIM_MEMBER_STATUS: uint, enum member_status,
  *             SWIM_MEMBER_ADDRESS: uint, ip,
- *             SWIM_MEMBER_PORT: uint, port
+ *             SWIM_MEMBER_PORT: uint, port,
+ *             SWIM_MEMBER_INCARNATION: uint
  *         },
  *         ...
  *     ],
  * }
  */
 
+/**
+ * Update status and incarnation of the member if needed. Statuses
+ * are compared as a compound key: {incarnation, status}. So @a
+ * new_status can override an old one only if its incarnation is
+ * greater, or the same, but its status is "bigger". Statuses are
+ * compared by their identifier, so "alive" < "dead". This
+ * protects from the case when a member is detected as dead on one
+ * instance, but overriden by another instance with the same
+ * incarnation "alive" message.
+ */
+static inline void
+swim_member_update_status(struct swim *swim, struct swim_member *member,
+			  enum swim_member_status new_status,
+			  uint64_t incarnation)
+{
+	(void) swim;
+	assert(member != swim->self);
+	if (member->incarnation == incarnation) {
+		if (member->status < new_status)
+			member->status = new_status;
+	} else if (member->incarnation < incarnation) {
+		member->status = new_status;
+		member->incarnation = incarnation;
+	}
+}
+
 /**
  * Remove the member from all queues, hashes, destroy it and free
  * the memory.
@@ -313,6 +492,11 @@ swim_member_delete(struct swim *swim, struct swim_member *member)
 	mh_i64ptr_del(swim->members, rc, NULL);
 	rlist_del_entry(member, in_queue_round);
 
+	/* Failure detection component. */
+	rlist_del_entry(member, in_queue_wait_ack);
+	swim_task_destroy(&member->ack_task);
+	swim_task_destroy(&member->ping_task);
+
 	free(member);
 }
 
@@ -322,7 +506,7 @@ swim_member_delete(struct swim *swim, struct swim_member *member)
  */
 static struct swim_member *
 swim_member_new(struct swim *swim, const struct sockaddr_in *addr,
-		enum swim_member_status status)
+		enum swim_member_status status, uint64_t incarnation)
 {
 	struct swim_member *member =
 		(struct swim_member *) calloc(1, sizeof(*member));
@@ -343,6 +527,12 @@ swim_member_new(struct swim *swim, const struct sockaddr_in *addr,
 	}
 	rlist_add_entry(&swim->queue_round, member, in_queue_round);
 
+	/* Failure detection component. */
+	member->incarnation = incarnation;
+	rlist_create(&member->in_queue_wait_ack);
+	swim_task_create(&member->ack_task, &swim->scheduler, swim_task_reset);
+	swim_task_create(&member->ping_task, &swim->scheduler, swim_task_reset);
+
 	return member;
 }
 
@@ -442,6 +632,28 @@ swim_encode_anti_entropy(struct swim *swim, struct swim_msg *msg)
 	return 1;
 }
 
+/**
+ * Encode failure detection component.
+ * @retval -1 Error.
+ * @retval 1 Success, something is encoded.
+ */
+static int
+swim_encode_failure_detection(struct swim *swim, struct swim_msg *msg,
+			      enum swim_fd_msg_type type)
+{
+	struct swim_fd_header_bin fd_header_bin;
+	int size = sizeof(fd_header_bin);
+	struct swim_packet *packet = swim_msg_reserve(msg, size);
+	if (packet == NULL)
+		return -1;
+	char *pos = swim_packet_alloc(packet, size);
+	swim_fd_header_bin_create(&fd_header_bin, type,
+				  swim->self->incarnation);
+	memcpy(pos, &fd_header_bin, size);
+	swim_packet_flush(packet);
+	return 1;
+}
+
 /** Encode SWIM components into a sequence of UDP packets. */
 static int
 swim_encode_round_msg(struct swim *swim, struct swim_msg *msg)
@@ -453,6 +665,11 @@ swim_encode_round_msg(struct swim *swim, struct swim_msg *msg)
 	char *header = swim_packet_alloc(packet, 1);
 	int rc, map_size = 0;
 
+	rc = swim_encode_failure_detection(swim, msg, SWIM_FD_MSG_PING);
+	if (rc < 0)
+		goto error;
+	map_size += rc;
+
 	rc = swim_encode_anti_entropy(swim, msg);
 	if (rc < 0)
 		goto error;
@@ -490,10 +707,11 @@ swim_round_step_begin(struct ev_loop *loop, struct ev_periodic *p, int events)
 		return;
 	}
 	struct swim_member *m =
-		rlist_shift_entry(&swim->queue_round, struct swim_member,
+		rlist_first_entry(&swim->queue_round, struct swim_member,
 				  in_queue_round);
 	swim_task_schedule(&swim->round_step_task,
 			   swim->transport->send_round_msg, &m->addr);
+	swim_member_schedule_ack_wait(swim, m);
 	ev_periodic_stop(loop, p);
 }
 
@@ -501,16 +719,84 @@ static void
 swim_round_step_complete(struct swim_task *task)
 {
 	struct swim *swim = container_of(task, struct swim, round_step_task);
+	rlist_shift_entry(&swim->queue_round, struct swim_member,
+			  in_queue_round);
 	swim_msg_reset(&task->msg);
 	ev_periodic_start(loop(), &swim->round_tick);
 }
 
+/** Send a failure detection message. */
+static void
+swim_schedule_fd_request(struct swim *swim, struct swim_task *task,
+			 struct swim_member *m, enum swim_fd_msg_type type,
+			 swim_transport_send_f send)
+{
+	struct swim_msg *msg = &task->msg;
+	int rc = swim_encode_failure_detection(swim, msg, type);
+	if (rc < 0) {
+		diag_log();
+		swim_task_delete(task);
+		return;
+	}
+	assert(rc > 0);
+	say_verbose("SWIM: send %s to %s", swim_fd_msg_type_strs[type],
+		    sio_strfaddr((struct sockaddr *) &m->addr,
+				 sizeof(m->addr)));
+	swim_task_schedule(task, send, &m->addr);
+}
+
+static inline void
+swim_schedule_ack(struct swim *swim, struct swim_member *member)
+{
+	swim_schedule_fd_request(swim, &member->ack_task, member,
+				 SWIM_FD_MSG_ACK, swim->transport->send_ack);
+}
+
+static inline void
+swim_schedule_ping(struct swim *swim, struct swim_member *member)
+{
+	swim_schedule_fd_request(swim, &member->ping_task, member,
+				 SWIM_FD_MSG_PING, swim->transport->send_ping);
+	swim_member_schedule_ack_wait(swim, member);
+}
+
+/**
+ * Check for failed pings. A ping is failed if an ack was not
+ * received during ACK_TIMEOUT. A failed ping is resent here.
+ */
+static void
+swim_check_acks(struct ev_loop *loop, struct ev_periodic *p, int events)
+{
+	assert((events & EV_PERIODIC) != 0);
+	(void) loop;
+	(void) events;
+	struct swim *swim = (struct swim *) p->data;
+	struct swim_member *m, *tmp;
+	double current_time = fiber_time();
+	rlist_foreach_entry_safe(m, &swim->queue_wait_ack, in_queue_wait_ack,
+				 tmp) {
+		if (current_time - m->ping_ts < ACK_TIMEOUT)
+			break;
+		++m->failed_pings;
+		if (m->failed_pings >= NO_ACKS_TO_GC) {
+			if (!m->is_pinned)
+				swim_member_delete(swim, m);
+			continue;
+		}
+		if (m->failed_pings >= NO_ACKS_TO_DEAD)
+			m->status = MEMBER_DEAD;
+		swim_schedule_ping(swim, m);
+		rlist_del_entry(m, in_queue_wait_ack);
+	}
+}
+
 /**
  * SWIM member attributes from anti-entropy and dissemination
  * messages.
  */
 struct swim_member_def {
 	struct sockaddr_in addr;
+	uint64_t incarnation;
 	enum swim_member_status status;
 };
 
@@ -530,9 +816,35 @@ swim_process_member_update(struct swim *swim, struct swim_member_def *def)
 	 * members table.
 	 */
 	if (member == NULL) {
-		member = swim_member_new(swim, &def->addr, def->status);
+		member = swim_member_new(swim, &def->addr, def->status,
+					 def->incarnation);
 		if (member == NULL)
 			diag_log();
+		return;
+	}
+	struct swim_member *self = swim->self;
+	if (member != self) {
+		swim_member_update_status(swim, member, def->status,
+					  def->incarnation);
+		return;
+	}
+	/*
+	 * It is possible that other instances know a bigger
+	 * incarnation of this instance - such thing happens when
+	 * the instance restarts and loses its local incarnation
+	 * number. It will be restored by receiving dissemination
+	 * messages about self.
+	 */
+	if (self->incarnation < def->incarnation)
+		self->incarnation = def->incarnation;
+	if (def->status != MEMBER_ALIVE &&
+	    def->incarnation == self->incarnation) {
+		/*
+		 * In the cluster a gossip exists that this
+		 * instance is not alive. Refute this information
+		 * with a bigger incarnation.
+		 */
+		self->incarnation++;
 	}
 }
 
@@ -576,6 +888,15 @@ swim_process_member_key(enum swim_member_key key, const char **pos,
 		}
 		def->addr.sin_port = port;
 		break;
+	case SWIM_MEMBER_INCARNATION:
+		if (mp_typeof(**pos) != MP_UINT ||
+		    mp_check_uint(*pos, end) > 0) {
+			say_error("%s member incarnation should be uint",
+				  msg_pref);
+			return -1;
+		}
+		def->incarnation = mp_decode_uint(pos);
+		break;
 	default:
 		unreachable();
 	}
@@ -627,12 +948,95 @@ swim_process_anti_entropy(struct swim *swim, const char **pos, const char *end)
 	return 0;
 }
 
+/**
+ * Decode a failure detection message. Schedule pings, process
+ * acks.
+ */
+static int
+swim_process_failure_detection(struct swim *swim, const char **pos,
+			       const char *end, const struct sockaddr_in *src)
+{
+	const char *msg_pref = "Invalid SWIM failure detection message:";
+	if (mp_typeof(**pos) != MP_MAP || mp_check_map(*pos, end) > 0) {
+		say_error("%s root should be a map", msg_pref);
+		return -1;
+	}
+	uint64_t size = mp_decode_map(pos);
+	if (size != 2) {
+		say_error("%s root map should have two keys - message type "\
+			  "and incarnation", msg_pref);
+		return -1;
+	}
+	enum swim_fd_msg_type type = swim_fd_msg_type_MAX;
+	uint64_t incarnation = 0;
+	for (int i = 0; i < (int) size; ++i) {
+		if (mp_typeof(**pos) != MP_UINT ||
+		    mp_check_uint(*pos, end) > 0) {
+			say_error("%s a key should be uint", msg_pref);
+			return -1;
+		}
+		uint64_t key = mp_decode_uint(pos);
+		switch(key) {
+		case SWIM_FD_MSG_TYPE:
+			if (mp_typeof(**pos) != MP_UINT ||
+			    mp_check_uint(*pos, end) > 0) {
+				say_error("%s message type should be uint",
+					  msg_pref);
+				return -1;
+			}
+			key = mp_decode_uint(pos);
+			if (key >= swim_fd_msg_type_MAX) {
+				say_error("%s unknown message type", msg_pref);
+				return -1;
+			}
+			type = key;
+			break;
+		case SWIM_FD_INCARNATION:
+			if (mp_typeof(**pos) != MP_UINT ||
+			    mp_check_uint(*pos, end) > 0) {
+				say_error("%s incarnation should be uint",
+					  msg_pref);
+				return -1;
+			}
+			incarnation = mp_decode_uint(pos);
+			break;
+		default:
+			say_error("%s unknown key", msg_pref);
+			return -1;
+		}
+	}
+	if (type == swim_fd_msg_type_MAX) {
+		say_error("%s message type should be specified", msg_pref);
+		return -1;
+	}
+	struct swim_member *sender = swim_find_member(swim, src);
+	if (sender == NULL) {
+		sender = swim_member_new(swim, src, MEMBER_ALIVE, incarnation);
+		if (sender == NULL) {
+			diag_log();
+			return 0;
+		}
+	} else {
+		swim_member_update_status(swim, sender, MEMBER_ALIVE,
+					  incarnation);
+	}
+	if (type == SWIM_FD_MSG_PING) {
+		swim_schedule_ack(swim, sender);
+	} else {
+		assert(type == SWIM_FD_MSG_ACK);
+		if (incarnation >= sender->incarnation) {
+			sender->failed_pings = 0;
+			rlist_del_entry(sender, in_queue_wait_ack);
+		}
+	}
+	return 0;
+}
+
 /** Receive and process a new message. */
 static void
 swim_on_input(struct swim_scheduler *scheduler,
 	      const struct swim_packet *packet, const struct sockaddr_in *src)
 {
-	(void) src;
 	const char *msg_pref = "Invalid SWIM message:";
 	struct swim *swim = container_of(scheduler, struct swim, scheduler);
 	const char *pos = packet->body;
@@ -655,6 +1059,12 @@ swim_on_input(struct swim_scheduler *scheduler,
 			if (swim_process_anti_entropy(swim, &pos, end) != 0)
 				return;
 			break;
+		case SWIM_FAILURE_DETECTION:
+			say_verbose("SWIM: process failure detection");
+			if (swim_process_failure_detection(swim, &pos, end,
+							   src) != 0)
+				return;
+			break;
 		default:
 			say_error("%s unknown component type component is "\
 				  "supported", msg_pref);
@@ -726,6 +1136,12 @@ swim_new(void)
 			 swim_round_step_complete);
 	swim->transport = &swim_udp_transport;
 	swim_scheduler_create(&swim->scheduler, swim_on_input, swim->transport);
+
+	/* Failure detection component. */
+	rlist_create(&swim->queue_wait_ack);
+	ev_init(&swim->wait_ack_tick, swim_check_acks);
+	ev_periodic_set(&swim->wait_ack_tick, 0, ACK_TIMEOUT, NULL);
+	swim->wait_ack_tick.data = (void *) swim;
 	return swim;
 }
 
@@ -738,7 +1154,7 @@ swim_cfg(struct swim *swim, const char *uri, double heartbeat_rate,
 		return -1;
 	struct swim_member *new_self = NULL;
 	if (swim_find_member(swim, &addr) == NULL) {
-		new_self = swim_member_new(swim, &addr, MEMBER_ALIVE);
+		new_self = swim_member_new(swim, &addr, MEMBER_ALIVE, 0);
 		if (new_self == NULL)
 			return -1;
 	}
@@ -747,6 +1163,7 @@ swim_cfg(struct swim *swim, const char *uri, double heartbeat_rate,
 		return -1;
 	}
 	ev_periodic_start(loop(), &swim->round_tick);
+	ev_periodic_start(loop(), &swim->wait_ack_tick);
 
 	if (swim->round_tick.interval != heartbeat_rate && heartbeat_rate > 0)
 		ev_periodic_set(&swim->round_tick, 0, heartbeat_rate, NULL);
@@ -767,9 +1184,10 @@ swim_add_member(struct swim *swim, const char *uri)
 		return -1;
 	struct swim_member *member = swim_find_member(swim, &addr);
 	if (member == NULL) {
-		member = swim_member_new(swim, &addr, MEMBER_ALIVE);
+		member = swim_member_new(swim, &addr, MEMBER_ALIVE, 0);
 		if (member == NULL)
 			return -1;
+		member->is_pinned = true;
 	}
 	return 0;
 }
@@ -800,6 +1218,8 @@ swim_info(struct swim *swim, struct info_handler *info)
 					      sizeof(member->addr)));
 		info_append_str(info, "status",
 				swim_member_status_strs[member->status]);
+		info_append_int(info, "incarnation",
+				(int64_t) member->incarnation);
 		info_table_end(info);
 	}
 	info_end(info);
@@ -810,6 +1230,7 @@ swim_delete(struct swim *swim)
 {
 	swim_scheduler_destroy(&swim->scheduler);
 	ev_periodic_stop(loop(), &swim->round_tick);
+	ev_periodic_stop(loop(), &swim->wait_ack_tick);
 	swim_task_destroy(&swim->round_step_task);
 	mh_int_t node = mh_first(swim->members);
 	while (node != mh_end(swim->members)) {
diff --git a/src/lib/swim/swim_io.c b/src/lib/swim/swim_io.c
index 8a1eca819..00a16a2bb 100644
--- a/src/lib/swim/swim_io.c
+++ b/src/lib/swim/swim_io.c
@@ -23,6 +23,8 @@ swim_udp_recv_msg(int fd, void *buffer, size_t size, struct sockaddr *addr,
 
 struct swim_transport swim_udp_transport = {
 	/* .send_round_msg = */ swim_udp_send_msg,
+	/* .send_ping = */ swim_udp_send_msg,
+	/* .send_ack = */ swim_udp_send_msg,
 	/* .recv_msg = */ swim_udp_recv_msg,
 };
 
@@ -39,6 +41,18 @@ swim_packet_new(struct swim_msg *msg)
 	return res;
 }
 
+struct swim_task *
+swim_task_new(struct swim_scheduler *scheduler, swim_task_f complete)
+{
+	struct swim_task *task = (struct swim_task *) malloc(sizeof(*task));
+	if (task == NULL) {
+		diag_set(OutOfMemory, sizeof(*task), "malloc", "task");
+		return NULL;
+	}
+	swim_task_create(task, scheduler, complete);
+	return task;
+}
+
 void
 swim_task_schedule(struct swim_task *task, swim_transport_send_f send,
 		   const struct sockaddr_in *dst)
diff --git a/src/lib/swim/swim_io.h b/src/lib/swim/swim_io.h
index fc44fd0a7..f08bd1ef3 100644
--- a/src/lib/swim/swim_io.h
+++ b/src/lib/swim/swim_io.h
@@ -279,12 +279,21 @@ swim_task_create(struct swim_task *task, struct swim_scheduler *scheduler,
 	task->scheduler = scheduler;
 }
 
+struct swim_task *
+swim_task_new(struct swim_scheduler *scheduler, swim_task_f complete);
+
 static inline bool
 swim_task_is_active(struct swim_task *task)
 {
 	return ! rlist_empty(&task->in_queue_output);
 }
 
+static inline void
+swim_task_reset(struct swim_task *task)
+{
+	swim_msg_reset(&task->msg);
+}
+
 static inline void
 swim_task_destroy(struct swim_task *task)
 {
@@ -292,6 +301,13 @@ swim_task_destroy(struct swim_task *task)
 	swim_msg_destroy(&task->msg);
 }
 
+static inline void
+swim_task_delete(struct swim_task *task)
+{
+	swim_task_destroy(task);
+	free(task);
+}
+
 #if defined(__cplusplus)
 }
 #endif
diff --git a/src/lib/swim/swim_transport.h b/src/lib/swim/swim_transport.h
index d629526ac..0bf4aa186 100644
--- a/src/lib/swim/swim_transport.h
+++ b/src/lib/swim/swim_transport.h
@@ -52,7 +52,9 @@ struct swim_transport {
 	 * are like sendto().
 	 */
 	swim_transport_send_f send_round_msg;
-
+	/** Send failure detection message. */
+	swim_transport_send_f send_ping;
+	swim_transport_send_f send_ack;
 	/**
 	 * Receive a message. Not necessary round or failure
 	 * detection. Before message is received, its type is
-- 
2.17.2 (Apple Git-113)