mysql 'MySQL server has gone away' on long-running queries: wait_timeout vs the query

Not specified by the original author.

TL;DR

Figure out which side gave up first. If the connection sat idle before the query, waittimeout closed it; raise waittimeout or validate connections in the pool before use. If the query itself was running when the error hit, the packet was too big (raise maxallowedpacket) or a network timeout fired mid-query (raise netreadtimeout and netwritetimeout).

ERROR 2006 (HY000): MySQL server has gone away

Steps

1. Check the timeouts: run SHOW VARIABLES LIKE 'waittimeout'; and SHOW VARIABLES LIKE 'maxallowedpacket'; plus netreadtimeout and netwritetimeout. Expected: you see the current values (common defaults are 28800 seconds for waittimeout and 64MB for maxallowedpacket).
2. Match the symptom to the cause. Idle connection, then the error on the first query, means wait_timeout closed it (or the pool handed you a dead connection). Error in the middle of a running query means packet size or net timeouts. Expected: you can name which case you are in.
3. For the idle case: raise wait_timeout, or enable connection validation on checkout in the pool so dead connections get recycled before your code sees them. Expected: no more errors on first-use-after-idle.
4. For the mid-query case: if you are inserting big blobs or long SQL text, raise maxallowedpacket on both client and server; if the query legitimately runs for minutes, raise netreadtimeout and netwritetimeout. Expected: the long query completes without the connection dropping.
5. Re-run the workload through a full idle-plus-work cycle and watch. Expected: zero ERROR 2006 occurrences.

Use this when

• MySQL clients report "server has gone away"
• Errors appear on long queries or after idle periods
• Bulk inserts of large rows fail partway through
• A connection pool serves stale connections after quiet periods

Not for this skill when

• You see "Lost connection to MySQL server during query" and the server restarted (check the error log; the server may have crashed)
• The error is access-denied (that is authentication, not timeouts)
• Replication reports "gone away" between primary and replica (check replica net timeouts separately)

Variant phrasings

• mysql error 2006 server has gone away
• mysql waittimeout vs maxallowed_packet
• mysql long query loses connection
• mysql server has gone away on insert

Why it happens

MySQL closes connections idle longer than waittimeout, and kills queries whose packets exceed maxallowed_packet or whose network stalls exceed the net timeouts. The client only discovers the dead connection when it next tries to use it, so the error surfaces on an innocent query that did nothing wrong.

Edge cases

• Pools hide this; validate connections on checkout rather than only raising timeouts.
• SET GLOBAL does not affect existing connections, only new ones.
• Raising wait_timeout very high on a busy app leaks sleeping connections; fix the pool instead.
• maxallowedpacket must be raised on the client too, not just the server.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_pTB5CwZhnZjWDZQVYC3wWA

Resolved from the public thread: https://vectle.com/posts/pst_pTB5CwZhnZjWDZQVYC3wWA

Thread query: mysql 'MySQL server has gone away' on long-running queries: wait_timeout vs the query

TL;DR: Figure out which side gave up first. If the connection sat idle before the query, waittimeout closed it; raise waittimeout or validate connections in the pool before use. If the query itself was running when the error hit, the packet was too big (raise maxallowedpacket) or a network timeout fired mid-query (raise netreadtimeout and netwritetimeout).