Syntax:
START REPLICA [thread_types] [until_option] [connection_options] [channel_option]

thread_types:
    [thread_type [, thread_type] ... ]

thread_type:
    IO_THREAD | SQL_THREAD

until_option:
    UNTIL {   {SQL_BEFORE_GTIDS | SQL_AFTER_GTIDS} = gtid_set
          |   MASTER_LOG_FILE = 'log_name', MASTER_LOG_POS = log_pos
          |   SOURCE_LOG_FILE = 'log_name', SOURCE_LOG_POS = log_pos
          |   RELAY_LOG_FILE = 'log_name', RELAY_LOG_POS = log_pos
          |   SQL_AFTER_MTS_GAPS  }

connection_options:
    [USER='user_name'] [PASSWORD='user_pass'] [DEFAULT_AUTH='plugin_name'] [PLUGIN_DIR='plugin_dir']


channel_option:
    FOR CHANNEL channel

gtid_set:
    uuid_set [, uuid_set] ...
    | ''

uuid_set:
    uuid:interval[:interval]...

uuid:
    hhhhhhhh-hhhh-hhhh-hhhh-hhhhhhhhhhhh

h:
    [0-9,A-F]

interval:
    n[-n]

    (n >= 1)

START REPLICA starts the replication threads, either together or
separately. From MySQL 8.0.22, use START REPLICA in place of START
SLAVE, which is deprecated from that release. In releases before MySQL
8.0.22, use START SLAVE.

START REPLICA requires the REPLICATION_SLAVE_ADMIN privilege (or the
deprecated SUPER privilege). START REPLICA causes an implicit commit of
an ongoing transaction. See
https://dev.mysql.com/doc/refman/8.0/en/implicit-commit.html.

For the thread type options, you can specify IO_THREAD, SQL_THREAD,
both of these, or neither of them. Only the threads that are started
are affected by the statement.

o START REPLICA with no thread type options starts all of the
  replication threads, and so does START REPLICA with both of the
  thread type options.

o IO_THREAD starts the replication receiver thread, which reads events
  from the source server and stores them in the relay log.

o SQL_THREAD starts the replication applier thread, which reads events
  from the relay log and executes them. A multithreaded replica (with
  replica_parallel_workers or slave_parallel_workers > 0) applies
  transactions using a coordinator thread and multiple applier threads,
  and SQL_THREAD starts all of these.

*Important*:

START REPLICA sends an acknowledgment to the user after all the
replication threads have started. However, the replication receiver
thread might not yet have connected to the source successfully, or an
applier thread might stop when applying an event right after starting.
START REPLICA does not continue to monitor the threads after they are
started, so it does not warn you if they subsequently stop or cannot
connect. You must check the replica's error log for error messages
generated by the replication threads, or check that they are running
satisfactorily with SHOW REPLICA STATUS. A successful START REPLICA
statement causes SHOW REPLICA STATUS to show Replica_SQL_Running=Yes,
but it might or might not show Replica_IO_Running=Yes, because
Replica_IO_Running=Yes is only shown if the receiver thread is both
running and connected. For more information, see
https://dev.mysql.com/doc/refman/8.0/en/replication-administration-stat
us.html.

The optional FOR CHANNEL channel clause enables you to name which
replication channel the statement applies to. Providing a FOR CHANNEL
channel clause applies the START REPLICA statement to a specific
replication channel. If no clause is named and no extra channels exist,
the statement applies to the default channel. If a START REPLICA
statement does not have a channel defined when using multiple channels,
this statement starts the specified threads for all channels. See
https://dev.mysql.com/doc/refman/8.0/en/replication-channels.html for
more information.

The replication channels for Group Replication
(group_replication_applier and group_replication_recovery) are managed
automatically by the server instance. START REPLICA cannot be used at
all with the group_replication_recovery channel, and should only be
used with the group_replication_applier channel when Group Replication
is not running. The group_replication_applier channel only has an
applier thread and has no receiver thread, so it can be started if
required by using the SQL_THREAD option without the IO_THREAD option.

START REPLICA supports pluggable user-password authentication (see
https://dev.mysql.com/doc/refman/8.0/en/pluggable-authentication.html)
with the USER, PASSWORD, DEFAULT_AUTH and PLUGIN_DIR options, as
described in the following list. When you use these options, you must
start the receiver thread (IO_THREAD option) or all the replication
threads - you cannot start the replication applier thread (SQL_THREAD
option) alone.

o USER

  The user name for the account. You must set this if PASSWORD is used.
  The option cannot be set to an empty or null string.

o PASSWORD

  The password for the named user account.

o DEFAULT_AUTH

  The name of the authentication plugin. The default is MySQL native
  authentication.

o PLUGIN_DIR

  The location of the authentication plugin.

*Important*:

The password that you set using START REPLICA is masked when it is
written to MySQL Server's logs, Performance Schema tables, and SHOW
PROCESSLIST statements. However, it is sent in plain text over the
connection to the replica server instance. To protect the password in
transit, use SSL/TLS encryption, an SSH tunnel, or another method of
protecting the connection from unauthorized viewing, for the connection
between the replica server instance and the client that you use to
issue START REPLICA.

The UNTIL clause makes the replica start replication, then process
transactions up to the point that you specify in the UNTIL clause, then
stop again. The UNTIL clause can be used to make a replica proceed
until just before the point where you want to skip a transaction that
is unwanted, and then skip the transaction as described in
https://dev.mysql.com/doc/refman/8.0/en/replication-administration-skip
.html. To identify a transaction, you can use mysqlbinlog with the
source's binary log or the replica's relay log, or use a SHOW BINLOG
EVENTS statement.

You can also use the UNTIL clause for debugging replication by
processing transactions one at a time or in sections. If you are using
the UNTIL clause to do this, start the replica with the
--skip-slave-start option, or from MySQL 8.0.24, the skip_slave_start
system variable, to prevent the SQL thread from running when the
replica server starts. Remove the option or system variable setting
after the procedure is complete, so that it is not forgotten in the
event of an unexpected server restart.

The SHOW REPLICA STATUS statement includes output fields that display
the current values of the UNTIL condition. The UNTIL condition lasts
for as long as the affected threads are still running, and is removed
when they stop.

The UNTIL clause operates on the replication applier thread (SQL_THREAD
option). You can use the SQL_THREAD option or let the replica default
to starting both threads. If you use the IO_THREAD option alone, the
UNTIL clause is ignored because the applier thread is not started.

The point that you specify in the UNTIL clause can be any one (and only
one) of the following options:

o SOURCE_LOG_FILE and SOURCE_LOG_POS (from MySQL 8.0.23), or
  MASTER_LOG_FILE and MASTER_LOG_POS (to MySQL 8.0.22)

  These options make the replication applier process transactions up to
  a position in its relay log, identified by the file name and file
  position of the corresponding point in the binary log on the source
  server. The applier thread finds the nearest transaction boundary at
  or after the specified position, finishes applying the transaction,
  and stops there. For compressed transaction payloads, specify the end
  position of the compressed Transaction_payload_event.

  These options can still be used when the GTID_ONLY option was set on
  the CHANGE REPLICATION SOURCE TO statement to stop the replication
  channel from persisting file names and file positions in the
  replication metadata repositories. The file names and file positions
  are tracked in memory.

o RELAY_LOG_FILE and RELAY_LOG_POS

  These options make the replication applier process transactions up to
  a position in the replica's relay log, identified by the relay log
  file name and a position in that file. The applier thread finds the
  nearest transaction boundary at or after the specified position,
  finishes applying the transaction, and stops there. For compressed
  transaction payloads, specify the end position of the compressed
  Transaction_payload_event.

  These options can still be used when the GTID_ONLY option was set on
  the CHANGE REPLICATION SOURCE TO statement to stop the replication
  channel from persisting file names and file positions in the
  replication metadata repositories. The file names and file positions
  are tracked in memory.

o SQL_BEFORE_GTIDS

  This option makes the replication applier start processing
  transactions and stop when it encounters any transaction that is in
  the specified GTID set. The encountered transaction from the GTID set
  is not applied, and nor are any of the other transactions in the GTID
  set. The option takes a GTID set containing one or more global
  transaction identifiers as an argument (see
  https://dev.mysql.com/doc/refman/8.0/en/replication-gtids-concepts.ht
  ml#replication-gtids-concepts-gtid-sets). Transactions in a GTID set
  do not necessarily appear in the replication stream in the order of
  their GTIDs, so the transaction before which the applier stops is not
  necessarily the earliest.

o SQL_AFTER_GTIDS

  This option makes the replication applier start processing
  transactions and stop when it has processed all of the transactions
  in a specified GTID set. The option takes a GTID set containing one
  or more global transaction identifiers as an argument (see
  https://dev.mysql.com/doc/refman/8.0/en/replication-gtids-concepts.ht
  ml#replication-gtids-concepts-gtid-sets).

  With SQL_AFTER_GTIDS, the replication threads stop when they have
  processed the transactions in the GTID set and encounter the first
  transaction whose GTID is not part of the GTID set. For example,
  executing START REPLICA UNTIL SQL_AFTER_GTIDS =
  3E11FA47-71CA-11E1-9E33-C80AA9429562:11-56 causes the replica to
  obtain all transactions just mentioned from the source, including all
  of the transactions having the sequence numbers 11 through 56, and
  then to stop without processing any additional transactions.

o SQL_AFTER_MTS_GAPS

  For a multithreaded replica only (with replica_parallel_workers or
  slave_parallel_workers > 0), this option makes the replica process
  transactions up to the point where there are no more gaps in the
  sequence of transactions executed from the relay log. When using a
  multithreaded replica, there is a chance of gaps occurring in the
  following situations:

  o The coordinator thread is stopped.

  o An error occurs in the applier threads.

  o mysqld shuts down unexpectedly.

  When a replication channel has gaps, the replica's database is in a
  state that might never have existed on the source. The replica tracks
  the gaps internally and disallows CHANGE REPLICATION SOURCE TO
  statements that would remove the gap information if they executed.

  Before MySQL 8.0.26, issuing START REPLICA on a multithreaded replica
  with gaps in the sequence of transactions executed from the relay log
  generates a warning. To correct this situation, the solution is to
  use START REPLICA UNTIL SQL_AFTER_MTS_GAPS. See
  https://dev.mysql.com/doc/refman/8.0/en/replication-features-transact
  ion-inconsistencies.html for more information.

  From MySQL 8.0.26, the process of checking for gaps in the sequence
  of transactions is skipped entirely when GTID-based replication and
  GTID auto-positioning (SOURCE_AUTO_POSITION=1) are in use for the
  channel, because gaps in transactions can be resolved using GTID
  auto-positioning. In that situation, START REPLICA UNTIL
  SQL_AFTER_MTS_GAPS just stops the applier thread when it finds the
  first transaction to execute, and does not attempt to check for gaps
  in the sequence of transactions. You can also continue to use CHANGE
  REPLICATION SOURCE TO statements as normal, and relay log recovery is
  possible for the channel.

  From MySQL 8.0.27, all replicas are multithreaded by default. When
  replica_preserve_commit_order=ON or slave_preserve_commit_order=ON is
  set for the replica, which is also the default setting from MySQL
  8.0.27, gaps should not occur except in the specific situations
  listed in the description for replica_preserve_commit_order and
  slave_preserve_commit_order. If replica_preserve_commit_order=OFF or
  slave_preserve_commit_order=OFF is set for the replica, which is the
  default before MySQL 8.0.27, the commit order of transactions is not
  preserved, so the chance of gaps occurring is much larger.

  If GTIDs are not in use and you need to change a failed multithreaded
  replica to single-threaded mode, you can issue the following series
  of statements, in the order shown:

START SLAVE UNTIL SQL_AFTER_MTS_GAPS;
START SLAVE SQL_THREAD;

Or from MySQL 8.0.26:
START REPLICA UNTIL SQL_AFTER_MTS_GAPS;
START REPLICA SQL_THREAD;

URL: https://dev.mysql.com/doc/refman/8.0/en/start-replica.html

