HAProxy determines if a server is available for request routing by performing so called health checks. By default, a health check consists of trying to establish a TCP connection to the server.
This is usually not enough for a robust setup with a database backend since the database server itself could be in a non-operational state while still being able to respond to connection requests.
Severalnines provides an HTTP request based health check which determines whether a MySQL Server is available or not by carefully examining its internal state which depends on the clustering solution used.
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