Mirror rules are a feature for testing how queries will perform when executed against a different keyspace. It is intended to reduce the uncertainty involved when migrating application queries from one keyspace to another via MoveTables.

Viewing Mirror Rules #

The mirror rules are global and can be viewed using the GetMirrorRules client command.

Updating Mirror Rules #

Mirror rules are managed by the MoveTables MirrorTraffic client command.

Syntax #

Mirror rules are managed using the JSON format. Here's an example, showing the mirror rules that would be produced using MoveTables MirrorTraffic against the local examples, where the queries to customer and corder tables are mirrored from the the commerce keyspace to the customer keyspace:

$ vtctldclient --server=localhost:15999 GetMirrorRules
{
  "rules": [
    {
      "from_table": "commerce.corder",
      "to_table": "customer.corder",
      "percent": 1.0
    },
    {
      "from_table": "commerce.corder@replica",
      "to_table": "customer.corder",
      "percent": 1.0
    },
    {
      "from_table": "commerce.corder@rdonly",
      "to_table": "customer.corder",
      "percent": 1.0
    },
    {
      "from_table": "commerce.customer",
      "to_table": "customer.customer",
      "percent": 1.0
    },
    {
      "from_table": "commerce.customer@replica",
      "to_table": "customer.customer",
      "percent": 1.0
    },
    {
      "from_table": "commerce.customer@rdonly",
      "to_table": "customer.customer",
      "percent": 1.0
    }
  ]
}

When Mirror Rules Are Applied #

Mirror rules are evaluated after routing rules. So, if there are routing rules in place redirecting traffic from commerce tables to customer tables, then the mirror rules in the example above would not be applied.

Evaluating the Impact of Mirror Rules #

At the VTGate-level, the impact of mirrored queries can be seen in the query log (MirrorSourceExecuteTime, MirrorTargetExecuteTime, and MirrorTargetError), and in metrics reporting the performance of mirrored queries. Check VTTablet-level metrics to observe the performance of mirrored queries.