> For the complete documentation index, see [llms.txt](https://mariadb.com/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mariadb.com/docs/server/reference/clientserver-protocol/replication-protocol/com_register_slave.md).

# COM\_REGISTER\_SLAVE

This command is used by a replica to register its details, such as server ID, hostname, and port, with the primary server.

This command is sent by the replica server to start MariaDB replication, and should be sent before requesting binlog events with [COM\_BINLOG\_DUMP](/docs/server/reference/clientserver-protocol/replication-protocol/com_binlog_dump.md).

The payload is:

* [uint<1>](/docs/server/reference/clientserver-protocol/protocol-data-types.md#fixed-length-bytes) command (`COM_REGISTER_SLAVE` = `0x15`).
* [uint<4>](/docs/server/reference/clientserver-protocol/protocol-data-types.md#fixed-length-bytes) Replica server ID.
* [uint<1>](/docs/server/reference/clientserver-protocol/protocol-data-types.md#fixed-length-bytes) Replica hostname length.
* [string\<n>](/docs/server/reference/clientserver-protocol/protocol-data-types.md#fixed-length-bytes) Hostname.
* [uint<1>](/docs/server/reference/clientserver-protocol/protocol-data-types.md#fixed-length-bytes) Replica username length.
* [string\<n>](/docs/server/reference/clientserver-protocol/protocol-data-types.md#fixed-length-bytes) Username.
* [uint<1>](/docs/server/reference/clientserver-protocol/protocol-data-types.md#fixed-length-bytes) Replica password length.
* [string\<n>](/docs/server/reference/clientserver-protocol/protocol-data-types.md#fixed-length-bytes) Replica password.
* [uint<2>](/docs/server/reference/clientserver-protocol/protocol-data-types.md#fixed-length-bytes) Replica connection port.
* [uint<4>](/docs/server/reference/clientserver-protocol/protocol-data-types.md#fixed-length-bytes) Replication rank.
* [uint<4>](/docs/server/reference/clientserver-protocol/protocol-data-types.md#fixed-length-bytes) Master server ID.

**Note**:

* Replica hostname, replica user, replica password, and replica port are usually not set. Some replica [replication parameters](/docs/server/ha-and-performance/standard-replication/replication-and-binary-log-system-variables.md) can be used for such settings (`report_host`, `report_port`, and so forth).
* Replication rank is not set.
* Master server ID is not set either.

## Example of COM\_REGISTER\_SLA

The replica server is configured with:

* server-id=10101;
* report-host=slave\_n\_1;
* report-port=23241.

```
1b 00 00 00 15 75 27 00    00 09 73 6c 61 76 65 5f    .....u'...slave_
 6e 5f 31 00 00 c9 5a 00    00 00 00 00 00 00 00       n_1...Z.......
```

We can see from the example:

* server\_id \[4] = 75 27 00 00 => 10101.
* hostname\_len \[1] = 09.
* hostname\[n] = slave\_n\_1 (9 bytes).
* username len \[1] = 0 (not set).
* password len \[1] = 0 (not set).
* slave port \[2] = c9 5a => 23241.
* rank \[4] = 0.
* master server id = 0.

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