MySQL Replication: "Can't connect to MySQL server" (110) - Troubleshooting Case

Nana Zedginidze

— 1 min read
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A production replica suddenly stopped receiving updates.

SHOW REPLICA STATUS showed:

Replica_IO_Running: No
Replica_SQL_Running: Yes
Last_IO_Error: Can't connect to MySQL server ... (110)

The first step was verifying the source.

I checked that binary logging was enabled, GTID mode was active, and the source was generating binlogs normally. Everything looked healthy.

The next step was testing connectivity directly from the replica.

Both a TCP connection (nc) and a direct MySQL client connection to the source timed out, confirming that the issue was outside MySQL.

After network connectivity was restored, replication still didn't resume automatically because the I/O thread had already stopped after exhausting its reconnect attempts.

Starting the I/O thread manually was enough:

START REPLICA IO_THREAD;

A few minutes later:

Replica_IO_Running: Yes
Replica_SQL_Running: Yes
Seconds_Behind_Source: 0

Takeaway

When Replica_IO_Running = No, don't immediately focus on GTIDs, binlogs, or replication configuration.

First, verify that the replica can actually reach the source.

Sometimes the issue is simply the network.

#MySQL #Replication #DBA #AWS #Troubleshooting

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