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. For advanced use cases, you can manage mirror rules using the ApplyMirrorRules 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 moment, there are no VTGate-level metrics reporting the performance of mirrored queries. Check VTTablet-level metrics to observe the performance of mirrored queries.

Additional Details #

For most cases, you should use MoveTables MirrorTraffic to manage mirror rules. Here are some details to keep in mind if you will be creating and managing your own custom mirror rules:

• from_table may optionally specify a @<tablet-type>; to_table may not.
• from_table and to_table must both be fully qualified.
• For a given from_table value, there can be at most one mirror rule.
• A keyspace that is named in the from_table of one rule may not be named in the to_table of that rule or any other rule.
• percent may be a value between 0 and 100, inclusive.
• Setting percent to 0 removes that mirror rule.