DNS Cache

Overview

Since the release 2.4.5 of ProxySQL, DNS caching is available.

Historically, whenever client connection took place, a Domain Name System (DNS) lookup for each client connection was performed. DNS lookup or DNS quering is a blocking process and impacts performance if DNS server is slow or misconfigured. By caching the results, the server avoids doing a DNS lookup for each client connection for a given host.

Feature

ProxySQL DNS caching allows storing DNS records locally and re-using them in future, eliminating the need for new DNS queries.

Every DNS record has a time-to-live (TTL). TTL specifies the expiry time of the record and is managed using mysql-monitor_local_dns_cache_ttl variable.

DNS records are checked after every interval and expired records are refreshed by issuing a DNS query. The interval value is specified in mysql-monitor_local_dns_cache_refresh_interval variable.

Other than that, cached records are checked when mysql_servers and proxysql_servers tables are updated.

If the domain contains multiple mapped IPs, the load is balanced among them in a round-robin fashion on socket connection.

Configuration

By default, DNS caching is enabled with these values:

mysql> SELECT * FROM global_variables WHERE Variable_Name LIKE 'mysql-monitor_local_dns_%';
+------------------------------------------------+----------------+
| Variable_Name                                  | Variable_Value |
+------------------------------------------------+----------------+
| mysql-monitor_local_dns_cache_ttl              | 300000         |
| mysql-monitor_local_dns_cache_refresh_interval | 60000          |
| mysql-monitor_local_dns_resolver_queue_maxsize | 128            |
+------------------------------------------------+----------------+
3 rows in set (0.01 sec)

They represent:

mysql-monitor_local_dns_cache_ttl : 300000 (ms) – TTL time-to-live per DNS record.
mysql-monitor_local_dns_cache_refresh_interval : 60000 (ms) – The interval at which the Monitor module of the proxy will check ‘mysql_servers’ and ‘proxysql_servers’ tables for new records and send DNS queries for expired DNS records.
mysql-monitor_local_dns_resolver_queue_maxsize : 128 – The variable controls how many DNS query requests are queued before starting new threads, up to 32 threads.

Setting value of mysql-monitor_local_dns_cache_ttl or mysql-monitor_local_dns_cache_refresh_interval to '0' disables DNS caching feature.

Metrics

Metrics related to DNS Caching are available through the stats table stats_mysql_global:

mysql> SELECT * FROM stats_mysql_global WHERE Variable_Name LIKE 'MySQL_Monitor_dns_cache_%';
+----------------------------------------+----------------+
| Variable_Name                          | Variable_Value |
+----------------------------------------+----------------+
| MySQL_Monitor_dns_cache_queried        | 65             |
| MySQL_Monitor_dns_cache_lookup_success | 60             |
| MySQL_Monitor_dns_cache_record_updated | 30             |
+----------------------------------------+----------------+
3 rows in set (0.01 sec)