Mirroring

Overview

ProxySQL's mirroring feature duplicates queries to multiple host groups for performance testing without impacting live traffic. Mirroring is enabled for the final query if the original was rewritten.

Configuration

Two columns were added to mysql_query_rules to enable mirroring:

• mirror_flagOUT (INT UNSIGNED)
• mirror_hostgroup (INT UNSIGNED)

Implementation Logic

When either parameter is set during query processing:

1. A new MySQL session is created with identical properties to the original
2. The mirrored session inherits credentials, schema name, and default hostgroup
3. If mirror_hostgroup is specified, it becomes the new session's default hostgroup
4. Without mirror_flagOUT, the original query executes against mirror_hostgroup
5. With mirror_flagOUT, the query matches rules starting from that flag value, enabling further modifications

Use Cases

Performance Testing — Mirror SELECT statements to test environments while preserving production traffic.

Query Rewrite Validation — Test regex patterns and rewrites without affecting live users by setting mirror_flagOUT to route mirrored queries through separate rules.

Firewall Testing — Combine mirroring with error_msg to process queries without execution, allowing verification of rewrites without server impact.

Metrics

Metrics are collected for both original and mirrored queries in stats_mysql_query_digest.

Limitations

Mirroring does not support prepared statements.