applier.cc

/*
 * Copyright 2010-2016, Tarantool AUTHORS, please see AUTHORS file.
 *
 * Redistribution and use in source and binary forms, with or
 * without modification, are permitted provided that the following
 * conditions are met:
 *
 * 1. Redistributions of source code must retain the above
 *    copyright notice, this list of conditions and the
 *    following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above
 *    copyright notice, this list of conditions and the following
 *    disclaimer in the documentation and/or other materials
 *    provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY <COPYRIGHT HOLDER> ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
 * <COPYRIGHT HOLDER> OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
 * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 * THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */
#include "applier.h"

#include <msgpuck.h>

#include "xlog.h"
#include "fiber.h"
#include "fiber_cond.h"
#include "coio.h"
#include "coio_buf.h"
#include "wal.h"
#include "xrow.h"
#include "replication.h"
#include "iproto_constants.h"
#include "version.h"
#include "trigger.h"
#include "xrow_io.h"
#include "error.h"
#include "session.h"
#include "cfg.h"
#include "schema.h"
#include "txn.h"
#include "box.h"
#include "scoped_guard.h"

STRS(applier_state, applier_STATE);

static inline void
applier_set_state(struct applier *applier, enum applier_state state)
{
	applier->state = state;
	say_debug("=> %s", applier_state_strs[state] +
		  strlen("APPLIER_"));
	trigger_run_xc(&applier->on_state, applier);
}

/**
 * Write a nice error message to log file on SocketError or ClientError
 * in applier_f().
 */
static inline void
applier_log_error(struct applier *applier, struct error *e)
{
	uint32_t errcode = box_error_code(e);
	if (applier->last_logged_errcode == errcode)
		return;
	switch (applier->state) {
	case APPLIER_CONNECT:
		say_info("can't connect to master");
		break;
	case APPLIER_CONNECTED:
	case APPLIER_READY:
		say_info("can't join/subscribe");
		break;
	case APPLIER_AUTH:
		say_info("failed to authenticate");
		break;
	case APPLIER_SYNC:
	case APPLIER_FOLLOW:
	case APPLIER_INITIAL_JOIN:
	case APPLIER_FINAL_JOIN:
		say_info("can't read row");
		break;
	default:
		break;
	}
	error_log(e);
	switch (errcode) {
	case ER_LOADING:
	case ER_CFG:
	case ER_ACCESS_DENIED:
	case ER_NO_SUCH_USER:
	case ER_SYSTEM:
	case ER_UNKNOWN_REPLICA:
	case ER_PASSWORD_MISMATCH:
		say_info("will retry every %.2lf second",
			 replication_reconnect_interval());
		break;
	default:
		break;
	}
	applier->last_logged_errcode = errcode;
}

/**
 * A helper function which switches the applier to FOLLOW state
 * if it has synchronized with its master.
 */
static inline void
applier_check_sync(struct applier *applier)
{
	/*
	 * Stay 'orphan' until appliers catch up with
	 * the remote vclock at the time of SUBSCRIBE
	 * and the lag is less than configured.
	 */
	if (applier->state == APPLIER_SYNC &&
	    applier->lag <= replication_sync_lag &&
	    vclock_compare(&applier->remote_vclock_at_subscribe,
			   &replicaset.vclock) <= 0) {
		/* Applier is synced, switch to "follow". */
		applier_set_state(applier, APPLIER_FOLLOW);
	}
}

/*
 * Fiber function to write vclock to replication master.
 * To track connection status, replica answers master
 * with encoded vclock. In addition to DML requests,
 * master also sends heartbeat messages every
 * replication_timeout seconds (introduced in 1.7.7).
 * On such requests replica also responds with vclock.
 */
static int
applier_writer_f(va_list ap)
{
	struct applier *applier = va_arg(ap, struct applier *);
	struct ev_io io;
	coio_create(&io, applier->io.fd);

	while (!fiber_is_cancelled()) {
		/*
		 * Tarantool >= 1.7.7 sends periodic heartbeat
		 * messages so we don't need to send ACKs every
		 * replication_timeout seconds any more.
		 */
		if (applier->version_id >= version_id(1, 7, 7))
			fiber_cond_wait_timeout(&applier->writer_cond,
						TIMEOUT_INFINITY);
		else
			fiber_cond_wait_timeout(&applier->writer_cond,
						replication_timeout);
		/*
		 * A writer fiber is going to be awaken after a commit or
		 * a heartbeat message. So this is an appropriate place to
		 * update an applier status because the applier state could
		 * yield and doesn't fit into a commit trigger.
		 */
		applier_check_sync(applier);

		/* Send ACKs only when in FOLLOW mode ,*/
		if (applier->state != APPLIER_SYNC &&
		    applier->state != APPLIER_FOLLOW)
			continue;
		try {
			struct xrow_header xrow;
			xrow_encode_vclock(&xrow, &replicaset.vclock);
			coio_write_xrow(&io, &xrow);
		} catch (SocketError *e) {
			/*
			 * There is no point trying to send ACKs if
			 * the master closed its end - we would only
			 * spam the log - so exit immediately.
			 */
			if (e->get_errno() == EPIPE)
				break;
			/*
			 * Do not exit, if there is a network error,
			 * the reader fiber will reconnect for us
			 * and signal our cond afterwards.
			 */
			e->log();
		} catch (Exception *e) {
			/*
			 * Out of memory encoding the message, ignore
			 * and try again after an interval.
			 */
			e->log();
		}
		fiber_gc();
	}
	return 0;
}

static int
apply_initial_join_row(struct xrow_header *row)
{
	struct txn *txn = txn_begin();
	if (txn == NULL)
		return -1;
	struct request request;
	xrow_decode_dml(row, &request, dml_request_key_map(row->type));
	struct space *space = space_cache_find(request.space_id);
	if (space == NULL)
		goto rollback;
	/* no access checks here - applier always works with admin privs */
	if (space_apply_initial_join_row(space, &request))
		goto rollback;
	int rc;
	rc = txn_commit(txn);
	if (rc < 0)
		return -1;
	fiber_gc();
	return rc;
rollback:
	txn_rollback(txn);
	fiber_gc();
	return -1;
}

/**
 * Process a no-op request.
 *
 * A no-op request does not affect any space, but it
 * promotes vclock and is written to WAL.
 */
static int
process_nop(struct request *request)
{
	assert(request->type == IPROTO_NOP);
	struct txn *txn = in_txn();
	if (txn_begin_stmt(txn, NULL) != 0)
		return -1;
	return txn_commit_stmt(txn, request);
}

static int
apply_row(struct xrow_header *row)
{
	struct request request;
	if (xrow_decode_dml(row, &request, dml_request_key_map(row->type)) != 0)
		return -1;
	if (request.type == IPROTO_NOP)
		return process_nop(&request);
	struct space *space = space_cache_find(request.space_id);
	if (space == NULL)
		return -1;
	if (box_process_rw(&request, space, NULL) != 0) {
		say_error("error applying row: %s", request_str(&request));
		return -1;
	}
	return 0;
}

static int
apply_final_join_row(struct xrow_header *row)
{
	struct txn *txn = txn_begin();
	if (txn == NULL)
		return -1;
	if (apply_row(row) != 0) {
		txn_rollback(txn);
		fiber_gc();
		return -1;
	}
	if (txn_commit(txn) != 0)
		return -1;
	fiber_gc();
	return 0;
}

/**
 * Connect to a remote host and authenticate the client.
 */
void
applier_connect(struct applier *applier)
{
	struct ev_io *coio = &applier->io;
	struct ibuf *ibuf = &applier->ibuf;
	if (coio->fd >= 0)
		return;
	char greetingbuf[IPROTO_GREETING_SIZE];
	struct xrow_header row;

	struct uri *uri = &applier->uri;
	/*
	 * coio_connect() stores resolved address to \a &applier->addr
	 * on success. &applier->addr_len is a value-result argument which
	 * must be initialized to the size of associated buffer (addrstorage)
	 * before calling coio_connect(). Since coio_connect() performs
	 * DNS resolution under the hood it is theoretically possible that
	 * applier->addr_len will be different even for same uri.
	 */
	applier->addr_len = sizeof(applier->addrstorage);
	applier_set_state(applier, APPLIER_CONNECT);
	coio_connect(coio, uri, &applier->addr, &applier->addr_len);
	assert(coio->fd >= 0);
	coio_readn(coio, greetingbuf, IPROTO_GREETING_SIZE);
	applier->last_row_time = ev_monotonic_now(loop());

	/* Decode instance version and name from greeting */
	struct greeting greeting;
	if (greeting_decode(greetingbuf, &greeting) != 0)
		tnt_raise(LoggedError, ER_PROTOCOL, "Invalid greeting");

	if (strcmp(greeting.protocol, "Binary") != 0) {
		tnt_raise(LoggedError, ER_PROTOCOL,
			  "Unsupported protocol for replication");
	}

	if (applier->version_id != greeting.version_id) {
		say_info("remote master %s at %s running Tarantool %u.%u.%u",
			 tt_uuid_str(&greeting.uuid),
			 sio_strfaddr(&applier->addr, applier->addr_len),
			 version_id_major(greeting.version_id),
			 version_id_minor(greeting.version_id),
			 version_id_patch(greeting.version_id));
	}

	/* Save the remote instance version and UUID on connect. */
	applier->uuid = greeting.uuid;
	applier->version_id = greeting.version_id;

	/* Don't display previous error messages in box.info.replication */
	diag_clear(&fiber()->diag);

	/*
	 * Send an IPROTO_VOTE request to fetch the master's ballot
	 * before proceeding to "join". It will be used for leader
	 * election on bootstrap.
	 */
	xrow_encode_vote(&row);
	coio_write_xrow(coio, &row);
	coio_read_xrow(coio, ibuf, &row);
	if (row.type == IPROTO_OK) {
		xrow_decode_ballot_xc(&row, &applier->ballot);
	} else try {
		xrow_decode_error_xc(&row);
	} catch (ClientError *e) {
		if (e->errcode() != ER_UNKNOWN_REQUEST_TYPE)
			e->raise();
		/*
		 * Master isn't aware of IPROTO_VOTE request.
		 * It's OK - we can proceed without it.
		 */
	}

	applier_set_state(applier, APPLIER_CONNECTED);

	/* Detect connection to itself */
	if (tt_uuid_is_equal(&applier->uuid, &INSTANCE_UUID))
		tnt_raise(ClientError, ER_CONNECTION_TO_SELF);

	/* Perform authentication if user provided at least login */
	if (!uri->login)
		goto done;

	/* Authenticate */
	applier_set_state(applier, APPLIER_AUTH);
	xrow_encode_auth_xc(&row, greeting.salt, greeting.salt_len, uri->login,
			    uri->login_len,
			    uri->password != NULL ? uri->password : "",
			    uri->password_len);
	coio_write_xrow(coio, &row);
	coio_read_xrow(coio, ibuf, &row);
	applier->last_row_time = ev_monotonic_now(loop());
	if (row.type != IPROTO_OK)
		xrow_decode_error_xc(&row); /* auth failed */

	/* auth succeeded */
	say_info("authenticated");
done:
	applier_set_state(applier, APPLIER_READY);
}

/**
 * Execute and process JOIN request (bootstrap the instance).
 */
static void
applier_join(struct applier *applier)
{
	/* Send JOIN request */
	struct ev_io *coio = &applier->io;
	struct ibuf *ibuf = &applier->ibuf;
	struct xrow_header row;
	xrow_encode_join_xc(&row, &INSTANCE_UUID);
	coio_write_xrow(coio, &row);

	/**
	 * Tarantool < 1.7.0: if JOIN is successful, there is no "OK"
	 * response, but a stream of rows from checkpoint.
	 */
	if (applier->version_id >= version_id(1, 7, 0)) {
		/* Decode JOIN response */
		coio_read_xrow(coio, ibuf, &row);
		if (iproto_type_is_error(row.type)) {
			xrow_decode_error_xc(&row); /* re-throw error */
		} else if (row.type != IPROTO_OK) {
			tnt_raise(ClientError, ER_UNKNOWN_REQUEST_TYPE,
				  (uint32_t) row.type);
		}
		/*
		 * Start vclock. The vclock of the checkpoint
		 * the master is sending to the replica.
		 * Used to initialize the replica's initial
		 * vclock in bootstrap_from_master()
		 */
		xrow_decode_vclock_xc(&row, &replicaset.vclock);
	}

	applier_set_state(applier, APPLIER_INITIAL_JOIN);

	/*
	 * Receive initial data.
	 */
	uint64_t row_count = 0;
	while (true) {
		coio_read_xrow(coio, ibuf, &row);
		applier->last_row_time = ev_monotonic_now(loop());
		if (iproto_type_is_dml(row.type)) {
			if (apply_initial_join_row(&row) != 0)
				diag_raise();
			if (++row_count % 100000 == 0)
				say_info("%.1fM rows received", row_count / 1e6);
		} else if (row.type == IPROTO_OK) {
			if (applier->version_id < version_id(1, 7, 0)) {
				/*
				 * This is the start vclock if the
				 * server is 1.6. Since we have
				 * not initialized replication
				 * vclock yet, do it now. In 1.7+
				 * this vclock is not used.
				 */
				xrow_decode_vclock_xc(&row, &replicaset.vclock);
			}
			break; /* end of stream */
		} else if (iproto_type_is_error(row.type)) {
			xrow_decode_error_xc(&row);  /* rethrow error */
		} else {
			tnt_raise(ClientError, ER_UNKNOWN_REQUEST_TYPE,
				  (uint32_t) row.type);
		}
	}
	say_info("initial data received");

	applier_set_state(applier, APPLIER_FINAL_JOIN);

	/*
	 * Tarantool < 1.7.0: there is no "final join" stage.
	 * Proceed to "subscribe" and do not finish bootstrap
	 * until replica id is received.
	 */
	if (applier->version_id < version_id(1, 7, 0))
		return;

	/*
	 * Receive final data.
	 */
	while (true) {
		coio_read_xrow(coio, ibuf, &row);
		applier->last_row_time = ev_monotonic_now(loop());
		if (iproto_type_is_dml(row.type)) {
			vclock_follow_xrow(&replicaset.vclock, &row);
			if (apply_final_join_row(&row) != 0)
				diag_raise();
			if (++row_count % 100000 == 0)
				say_info("%.1fM rows received", row_count / 1e6);
		} else if (row.type == IPROTO_OK) {
			/*
			 * Current vclock. This is not used now,
			 * ignore.
			 */
			break; /* end of stream */
		} else if (iproto_type_is_error(row.type)) {
			xrow_decode_error_xc(&row);  /* rethrow error */
		} else {
			tnt_raise(ClientError, ER_UNKNOWN_REQUEST_TYPE,
				  (uint32_t) row.type);
		}
	}
	say_info("final data received");

	applier_set_state(applier, APPLIER_JOINED);
	applier_set_state(applier, APPLIER_READY);
}

/**
 * A helper struct to link xrow objects in a list.
 */
struct applier_tx_row {
	/* Next transaction row. */
	struct stailq_entry next;
	/* xrow_header struct for the current transaction row. */
	struct xrow_header row;
};

static struct applier_tx_row *
applier_read_tx_row(struct applier *applier)
{
	struct ev_io *coio = &applier->io;
	struct ibuf *ibuf = &applier->ibuf;

	struct applier_tx_row *tx_row = (struct applier_tx_row *)
		region_alloc(&fiber()->gc, sizeof(struct applier_tx_row));

	if (tx_row == NULL)
		tnt_raise(OutOfMemory, sizeof(struct applier_tx_row),
			  "region", "struct applier_tx_row");

	struct xrow_header *row = &tx_row->row;

	double timeout = replication_disconnect_timeout();
	/*
	 * Tarantool < 1.7.7 does not send periodic heartbeat
	 * messages so we can't assume that if we haven't heard
	 * from the master for quite a while the connection is
	 * broken - the master might just be idle.
	 */
	if (applier->version_id < version_id(1, 7, 7))
		coio_read_xrow(coio, ibuf, row);
	else
		coio_read_xrow_timeout_xc(coio, ibuf, row, timeout);

	applier->lag = ev_now(loop()) - row->tm;
	applier->last_row_time = ev_monotonic_now(loop());
	return tx_row;
}

/**
 * Read one transaction from network using applier's input buffer.
 * Transaction rows are placed onto fiber gc region.
 * We could not use applier input buffer to store rows because
 * rpos is adjusted as xrow is decoded and the corresponding
 * network input space is reused for the next xrow.
 */
static void
applier_read_tx(struct applier *applier, struct stailq *rows)
{
	int64_t tsn = 0;

	stailq_create(rows);
	do {
		struct applier_tx_row *tx_row = applier_read_tx_row(applier);
		struct xrow_header *row = &tx_row->row;

		if (iproto_type_is_error(row->type))
			xrow_decode_error_xc(row);

		/* Replication request. */
		if (row->replica_id == REPLICA_ID_NIL ||
		    row->replica_id >= VCLOCK_MAX) {
			/*
			 * A safety net, this can only occur
			 * if we're fed a strangely broken xlog.
			 */
			tnt_raise(ClientError, ER_UNKNOWN_REPLICA,
				  int2str(row->replica_id),
				  tt_uuid_str(&REPLICASET_UUID));
		}
		if (tsn == 0) {
			/*
			 * Transaction id must be derived from the log sequence
			 * number of the first row in the transaction.
			 */
			tsn = row->tsn;
			if (row->lsn != tsn)
				tnt_raise(ClientError, ER_PROTOCOL,
					  "Transaction id must be equal to "
					  "LSN of the first row in the "
					  "transaction.");
		}
		if (tsn != row->tsn)
			tnt_raise(ClientError, ER_UNSUPPORTED,
				  "replication",
				  "interleaving transactions");

		assert(row->bodycnt <= 1);
		if (row->bodycnt == 1 && !row->is_commit) {
			/*
			 * Save row body to gc region.
			 * Not done for single-statement
			 * transactions knowing that the input
			 * buffer will not be used while the
			 * transaction is applied.
			 */
			void *new_base = region_alloc(&fiber()->gc,
						      row->body->iov_len);
			if (new_base == NULL)
				tnt_raise(OutOfMemory, row->body->iov_len,
					  "region", "xrow body");
			memcpy(new_base, row->body->iov_base,
			       row->body->iov_len);
			/* Adjust row body pointers. */
			row->body->iov_base = new_base;
		}
		stailq_add_tail(rows, &tx_row->next);

	} while (!stailq_last_entry(rows, struct applier_tx_row,
				    next)->row.is_commit);
}

static void
applier_txn_rollback_cb(struct trigger *trigger, void *event)
{
	(void) trigger;
	/* Setup shared applier diagnostic area. */
	diag_set(ClientError, ER_WAL_IO);
	diag_move(&fiber()->diag, &replicaset.applier.diag);
	/* Broadcast the rollback event across all appliers. */
	trigger_run(&replicaset.applier.on_rollback, event);
	/* Rollback applier vclock to the committed one. */
	vclock_copy(&replicaset.applier.vclock, &replicaset.vclock);
}

static void
applier_txn_commit_cb(struct trigger *trigger, void *event)
{
	(void) trigger;
	/* Broadcast the commit event across all appliers. */
	trigger_run(&replicaset.applier.on_commit, event);
}

/**
 * Apply all rows in the rows queue as a single transaction.
 *
 * Return 0 for success or -1 in case of an error.
 */
static int
applier_apply_tx(struct stailq *rows)
{
	struct xrow_header *first_row = &stailq_first_entry(rows,
					struct applier_tx_row, next)->row;
	struct replica *replica = replica_by_id(first_row->replica_id);
	/*
	 * In a full mesh topology, the same set of changes
	 * may arrive via two concurrently running appliers.
	 * Hence we need a latch to strictly order all changes
	 * that belong to the same server id.
	 */
	struct latch *latch = (replica ? &replica->order_latch :
			       &replicaset.applier.order_latch);
	latch_lock(latch);
	if (vclock_get(&replicaset.applier.vclock,
		       first_row->replica_id) >= first_row->lsn) {
		latch_unlock(latch);
		return 0;
	}

	/**
	 * Explicitly begin the transaction so that we can
	 * control fiber->gc life cycle and, in case of apply
	 * conflict safely access failed xrow object and allocate
	 * IPROTO_NOP on gc.
	 */
	struct txn *txn = txn_begin();
	struct applier_tx_row *item;
	if (txn == NULL) {
		latch_unlock(latch);
		return -1;
	}
	stailq_foreach_entry(item, rows, next) {
		struct xrow_header *row = &item->row;
		int res = apply_row(row);
		if (res != 0) {
			struct error *e = diag_last_error(diag_get());
			/*
			 * In case of ER_TUPLE_FOUND error and enabled
			 * replication_skip_conflict configuration
			 * option, skip applying the foreign row and
			 * replace it with NOP in the local write ahead
			 * log.
			 */
			if (e->type == &type_ClientError &&
			    box_error_code(e) == ER_TUPLE_FOUND &&
			    replication_skip_conflict) {
				diag_clear(diag_get());
				row->type = IPROTO_NOP;
				row->bodycnt = 0;
				res = apply_row(row);
			}
		}
		if (res != 0)
			goto rollback;
	}
	/*
	 * We are going to commit so it's a high time to check if
	 * the current transaction has non-local effects.
	 */
	if (txn_is_distributed(txn)) {
		/*
		 * A transaction mixes remote and local rows.
		 * Local rows must be replicated back, which
		 * doesn't make sense since the master likely has
		 * new changes which local rows may overwrite.
		 * Raise an error.
		 */
		diag_set(ClientError, ER_UNSUPPORTED,
			 "Replication", "distributed transactions");
		goto rollback;
	}

	/* We are ready to submit txn to wal. */
	struct trigger *on_rollback, *on_commit;
	on_rollback = (struct trigger *)region_alloc(&txn->region,
						     sizeof(struct trigger));
	on_commit = (struct trigger *)region_alloc(&txn->region,
						   sizeof(struct trigger));
	if (on_rollback == NULL || on_commit == NULL)
		goto rollback;

	trigger_create(on_rollback, applier_txn_rollback_cb, NULL, NULL);
	txn_on_rollback(txn, on_rollback);

	trigger_create(on_commit, applier_txn_commit_cb, NULL, NULL);
	txn_on_commit(txn, on_commit);

	if (txn_write(txn) < 0)
		goto fail;

	/* Transaction was sent to journal so promote vclock. */
	vclock_follow(&replicaset.applier.vclock,
		      first_row->replica_id, first_row->lsn);
	latch_unlock(latch);
	return 0;
rollback:
	txn_rollback(txn);
fail:
	latch_unlock(latch);
	fiber_gc();
	return -1;
}

/*
 * A trigger to update an applier state after a replication commit.
 */
static void
applier_on_commit(struct trigger *trigger, void *event)
{
	(void) event;
	struct applier *applier = (struct applier *)trigger->data;
	fiber_cond_signal(&applier->writer_cond);
}

/*
 * A trigger to update an applier state after a replication rollback.
 */
static void
applier_on_rollback(struct trigger *trigger, void *event)
{
	(void) event;
	struct applier *applier = (struct applier *)trigger->data;
	/* Setup a shared error. */
	if (!diag_is_empty(&replicaset.applier.diag)) {
		diag_add_error(&applier->diag,
			       diag_last_error(&replicaset.applier.diag));
	}
	/* Stop the applier fiber. */
	fiber_cancel(applier->reader);
}

/**
 * Execute and process SUBSCRIBE request (follow updates from a master).
 */
static void
applier_subscribe(struct applier *applier)
{
	/* Send SUBSCRIBE request */
	struct ev_io *coio = &applier->io;
	struct ibuf *ibuf = &applier->ibuf;
	struct xrow_header row;
	struct tt_uuid cluster_id = uuid_nil;

	struct vclock vclock;
	vclock_create(&vclock);
	vclock_copy(&vclock, &replicaset.vclock);
	/*
	 * Stop accepting local rows coming from a remote
	 * instance as soon as local WAL starts accepting writes.
	 */
	uint32_t id_filter = box_is_orphan() ? 0 : 1 << instance_id;
	xrow_encode_subscribe_xc(&row, &REPLICASET_UUID, &INSTANCE_UUID,
				 &vclock, id_filter);
	coio_write_xrow(coio, &row);

	/* Read SUBSCRIBE response */
	if (applier->version_id >= version_id(1, 6, 7)) {
		coio_read_xrow(coio, ibuf, &row);
		if (iproto_type_is_error(row.type)) {
			xrow_decode_error_xc(&row);  /* error */
		} else if (row.type != IPROTO_OK) {
			tnt_raise(ClientError, ER_PROTOCOL,
				  "Invalid response to SUBSCRIBE");
		}
		/*
		 * In case of successful subscribe, the server
		 * responds with its current vclock.
		 *
		 * Tarantool > 2.1.1 also sends its cluster id to
		 * the replica, and replica has to check whether
		 * its and master's cluster ids match.
		 */
		vclock_create(&applier->remote_vclock_at_subscribe);
		xrow_decode_subscribe_response_xc(&row, &cluster_id,
					&applier->remote_vclock_at_subscribe);
		/*
		 * If master didn't send us its cluster id
		 * assume that it has done all the checks.
		 * In this case cluster_id will remain zero.
		 */
		if (!tt_uuid_is_nil(&cluster_id) &&
		    !tt_uuid_is_equal(&cluster_id, &REPLICASET_UUID)) {
			tnt_raise(ClientError, ER_REPLICASET_UUID_MISMATCH,
				  tt_uuid_str(&cluster_id),
				  tt_uuid_str(&REPLICASET_UUID));
		}

		say_info("subscribed");
		say_info("remote vclock %s local vclock %s",
			 vclock_to_string(&applier->remote_vclock_at_subscribe),
			 vclock_to_string(&vclock));
	}
	/*
	 * Tarantool < 1.6.7:
	 * If there is an error in subscribe, it's sent directly
	 * in response to subscribe.  If subscribe is successful,
	 * there is no "OK" response, but a stream of rows from
	 * the binary log.
	 */

	if (applier->state == APPLIER_READY) {
		/*
		 * Tarantool < 1.7.7 does not send periodic heartbeat
		 * messages so we cannot enable applier synchronization
		 * for it without risking getting stuck in the 'orphan'
		 * mode until a DML operation happens on the master.
		 */
		if (applier->version_id >= version_id(1, 7, 7))
			applier_set_state(applier, APPLIER_SYNC);
		else
			applier_set_state(applier, APPLIER_FOLLOW);
	} else {
		/*
		 * Tarantool < 1.7.0 sends replica id during
		 * "subscribe" stage. We can't finish bootstrap
		 * until it is received.
		 */
		assert(applier->state == APPLIER_FINAL_JOIN);
		assert(applier->version_id < version_id(1, 7, 0));
	}

	/* Re-enable warnings after successful execution of SUBSCRIBE */
	applier->last_logged_errcode = 0;
	if (applier->version_id >= version_id(1, 7, 4)) {
		/* Enable replication ACKs for newer servers */
		assert(applier->writer == NULL);

		char name[FIBER_NAME_MAX];
		int pos = snprintf(name, sizeof(name), "applierw/");
		uri_format(name + pos, sizeof(name) - pos, &applier->uri, false);

		applier->writer = fiber_new_xc(name, applier_writer_f);
		fiber_set_joinable(applier->writer, true);
		fiber_start(applier->writer, applier);
	}

	applier->lag = TIMEOUT_INFINITY;

	/* Register triggers to handle replication commits and rollbacks. */
	struct trigger on_commit;
	trigger_create(&on_commit, applier_on_commit, applier, NULL);
	trigger_add(&replicaset.applier.on_commit, &on_commit);

	struct trigger on_rollback;
	trigger_create(&on_rollback, applier_on_rollback, applier, NULL);
	trigger_add(&replicaset.applier.on_rollback, &on_rollback);

	auto trigger_guard = make_scoped_guard([&] {
		trigger_clear(&on_commit);
		trigger_clear(&on_rollback);
	});

	/*
	 * Process a stream of rows from the binary log.
	 */
	while (true) {
		if (applier->state == APPLIER_FINAL_JOIN &&
		    instance_id != REPLICA_ID_NIL) {
			say_info("final data received");
			applier_set_state(applier, APPLIER_JOINED);
			applier_set_state(applier, APPLIER_READY);
			applier_set_state(applier, APPLIER_FOLLOW);
		}

		struct stailq rows;
		applier_read_tx(applier, &rows);

		applier->last_row_time = ev_monotonic_now(loop());
		/*
		 * In case of an heartbeat message wake a writer up
		 * and check applier state.
		 */
		if (stailq_first_entry(&rows, struct applier_tx_row,
				       next)->row.lsn == 0)
			fiber_cond_signal(&applier->writer_cond);
		else if (applier_apply_tx(&rows) != 0)
			diag_raise();

		if (ibuf_used(ibuf) == 0)
			ibuf_reset(ibuf);
		fiber_gc();
	}
}

static inline void
applier_disconnect(struct applier *applier, enum applier_state state)
{
	applier_set_state(applier, state);
	if (applier->writer != NULL) {
		fiber_cancel(applier->writer);
		fiber_join(applier->writer);
		applier->writer = NULL;
	}

	coio_close(loop(), &applier->io);
	/* Clear all unparsed input. */
	ibuf_reinit(&applier->ibuf);
	fiber_gc();
}

static int
applier_f(va_list ap)
{
	struct applier *applier = va_arg(ap, struct applier *);
	/*
	 * Set correct session type for use in on_replace()
	 * triggers.
	 */
	struct session *session = session_create_on_demand();
	if (session == NULL)
		return -1;
	session_set_type(session, SESSION_TYPE_APPLIER);

	/* Re-connect loop */
	while (!fiber_is_cancelled()) {
		try {
			applier_connect(applier);
			if (tt_uuid_is_nil(&REPLICASET_UUID)) {
				/*
				 * Execute JOIN if this is a bootstrap.
				 * The join will pause the applier
				 * until WAL is created.
				 */
				applier_join(applier);
			}
			applier_subscribe(applier);
			/*
			 * subscribe() has an infinite loop which
			 * is stoppable only with fiber_cancel().
			 */
			unreachable();
			return 0;
		} catch (ClientError *e) {
			if (e->errcode() == ER_CONNECTION_TO_SELF &&
			    tt_uuid_is_equal(&applier->uuid, &INSTANCE_UUID)) {
				/* Connection to itself, stop applier */
				applier_disconnect(applier, APPLIER_OFF);
				return 0;
			} else if (e->errcode() == ER_LOADING) {
				/* Autobootstrap */
				applier_log_error(applier, e);
				applier_disconnect(applier, APPLIER_LOADING);
				goto reconnect;
			} else if (e->errcode() == ER_CFG ||
				   e->errcode() == ER_ACCESS_DENIED ||
				   e->errcode() == ER_NO_SUCH_USER ||
				   e->errcode() == ER_PASSWORD_MISMATCH) {
				/* Invalid configuration */
				applier_log_error(applier, e);
				applier_disconnect(applier, APPLIER_LOADING);
				goto reconnect;
			} else if (e->errcode() == ER_SYSTEM) {
				/* System error from master instance. */
				applier_log_error(applier, e);
				applier_disconnect(applier, APPLIER_DISCONNECTED);
				goto reconnect;
			} else {
				/* Unrecoverable errors */
				applier_log_error(applier, e);
				applier_disconnect(applier, APPLIER_STOPPED);
				return -1;
			}
		} catch (FiberIsCancelled *e) {
			if (!diag_is_empty(&applier->diag)) {
				diag_move(&applier->diag, &fiber()->diag);
				applier_disconnect(applier, APPLIER_STOPPED);
				break;
			}
			applier_disconnect(applier, APPLIER_OFF);
			break;
		} catch (SocketError *e) {
			applier_log_error(applier, e);
			applier_disconnect(applier, APPLIER_DISCONNECTED);
			goto reconnect;
		} catch (SystemError *e) {
			applier_log_error(applier, e);
			applier_disconnect(applier, APPLIER_DISCONNECTED);
			goto reconnect;
		} catch (Exception *e) {
			applier_log_error(applier, e);
			applier_disconnect(applier, APPLIER_STOPPED);
			return -1;
		}
		/* Put fiber_sleep() out of catch block.
		 *
		 * This is done to avoid the case when two or more
		 * fibers yield inside their try/catch blocks and
		 * throw an exception. Seems like the exception unwinder
		 * uses global state inside the catch block.
		 *
		 * This could lead to incorrect exception processing
		 * and crash the program.
		 *
		 * See: https://github.com/tarantool/tarantool/issues/136
		*/
reconnect:
		fiber_sleep(replication_reconnect_interval());
	}
	return 0;
}

void
applier_start(struct applier *applier)
{
	char name[FIBER_NAME_MAX];
	assert(applier->reader == NULL);

	int pos = snprintf(name, sizeof(name), "applier/");
	uri_format(name + pos, sizeof(name) - pos, &applier->uri, false);

	struct fiber *f = fiber_new_xc(name, applier_f);
	/**
	 * So that we can safely grab the status of the
	 * fiber any time we want.
	 */
	fiber_set_joinable(f, true);
	applier->reader = f;
	fiber_start(f, applier);
}

void
applier_stop(struct applier *applier)
{
	struct fiber *f = applier->reader;
	if (f == NULL)
		return;
	fiber_cancel(f);
	fiber_join(f);
	applier_set_state(applier, APPLIER_OFF);
	applier->reader = NULL;
}

struct applier *
applier_new(const char *uri)
{
	struct applier *applier = (struct applier *)
		calloc(1, sizeof(struct applier));
	if (applier == NULL) {
		diag_set(OutOfMemory, sizeof(*applier), "malloc",
			 "struct applier");
		return NULL;
	}
	coio_create(&applier->io, -1);
	ibuf_create(&applier->ibuf, &cord()->slabc, 1024);

	/* uri_parse() sets pointers to applier->source buffer */
	snprintf(applier->source, sizeof(applier->source), "%s", uri);
	int rc = uri_parse(&applier->uri, applier->source);
	/* URI checked by box_check_replication() */
	assert(rc == 0 && applier->uri.service != NULL);
	(void) rc;

	applier->last_row_time = ev_monotonic_now(loop());
	rlist_create(&applier->on_state);
	fiber_cond_create(&applier->resume_cond);
	fiber_cond_create(&applier->writer_cond);
	diag_create(&applier->diag);

	return applier;
}

void
applier_delete(struct applier *applier)
{
	assert(applier->reader == NULL && applier->writer == NULL);
	ibuf_destroy(&applier->ibuf);
	assert(applier->io.fd == -1);
	trigger_destroy(&applier->on_state);
	diag_destroy(&applier->diag);
	free(applier);
}

void
applier_resume(struct applier *applier)
{
	assert(!fiber_is_dead(applier->reader));
	applier->is_paused = false;
	fiber_cond_signal(&applier->resume_cond);
}

void
applier_pause(struct applier *applier)
{
	/* Sleep until applier_resume() wake us up */
	assert(fiber() == applier->reader);
	assert(!applier->is_paused);
	applier->is_paused = true;
	while (applier->is_paused && !fiber_is_cancelled())
		fiber_cond_wait(&applier->resume_cond);
}

struct applier_on_state {
	struct trigger base;
	struct applier *applier;
	enum applier_state desired_state;
	struct fiber_cond wakeup;
};

static void
applier_on_state_f(struct trigger *trigger, void *event)
{
	(void) event;
	struct applier_on_state *on_state =
		container_of(trigger, struct applier_on_state, base);

	struct applier *applier = on_state->applier;

	if (applier->state != APPLIER_OFF &&
	    applier->state != APPLIER_STOPPED &&
	    applier->state != on_state->desired_state)
		return;

	/* Wake up waiter */
	fiber_cond_signal(&on_state->wakeup);

	applier_pause(applier);
}

static inline void
applier_add_on_state(struct applier *applier,
		     struct applier_on_state *trigger,
		     enum applier_state desired_state)
{
	trigger_create(&trigger->base, applier_on_state_f, NULL, NULL);
	trigger->applier = applier;
	fiber_cond_create(&trigger->wakeup);
	trigger->desired_state = desired_state;
	trigger_add(&applier->on_state, &trigger->base);
}

static inline void
applier_clear_on_state(struct applier_on_state *trigger)
{
	fiber_cond_destroy(&trigger->wakeup);
	trigger_clear(&trigger->base);
}

static inline int
applier_wait_for_state(struct applier_on_state *trigger, double timeout)
{
	struct applier *applier = trigger->applier;
	double deadline = ev_monotonic_now(loop()) + timeout;
	while (applier->state != APPLIER_OFF &&
	       applier->state != APPLIER_STOPPED &&
	       applier->state != trigger->desired_state) {
		if (fiber_cond_wait_deadline(&trigger->wakeup, deadline) != 0)
			return -1; /* ER_TIMEOUT */
	}
	if (applier->state != trigger->desired_state) {
		assert(applier->state == APPLIER_OFF ||
		       applier->state == APPLIER_STOPPED);
		/* Re-throw the original error */
		assert(!diag_is_empty(&applier->reader->diag));
		diag_move(&applier->reader->diag, &fiber()->diag);
		return -1;
	}
	return 0;
}

void
applier_resume_to_state(struct applier *applier, enum applier_state state,
			double timeout)
{
	struct applier_on_state trigger;
	applier_add_on_state(applier, &trigger, state);
	applier_resume(applier);
	int rc = applier_wait_for_state(&trigger, timeout);
	applier_clear_on_state(&trigger);
	if (rc != 0)
		diag_raise();
	assert(applier->state == state);
}