Error Handling

Introduction

In an application connected to MariaDB/MySQL database server, an error can occur either on server or client side. Almost all the APIs, available through Go's database/sql package, return error. So, its critical for applications to check for its nil-ness on return.

Unknown driver

if db, err := sql.Open("foo", dsn); err != nil {
        fmt.Println(err)
}
// Output:
//sql: unknown driver "foo" (forgotten import?)

Client error: connection failure

if err := db.Ping(); err != nil {
        fmt.Println(err)
}
// Output:
// mysql: [9002] Can't connect to the server (dial tcp 127.0.0.1:16001: getsockopt: connection refused)

Server error: test.t1 already exists

if _, err := db.Exec("CREATE TABLE test.t1(i INT)"); err != nil {
        fmt.Println(err)
}
// Output:
// mysqld: [1050] (42S01) Table 't1' already exists

As seen above, the error messages are prefixed appropriately and they give an idea about the source of their origin.

Vaquita also provides APIs to access error code, SQLSTATE, message and time of error, individually. It can be done using type assertion.

if _, err = db.Exec("CREATE TABLE test.t1(i INT)"); err != nil {
        /* check whether its a mysql.Error */
        if e, ok := err.(*mysql.Error); ok {
                fmt.Println("Code     : ", e.Code())
                fmt.Println("SQLSTATE : ", e.SqlState())
                fmt.Println("Message  : ", e.Message())
                fmt.Println("When     : ", e.When())
        } else {
                fmt.Println(err)
        }
}
// Output:
// Code     :  1050
// SQLSTATE :  42S01
// Message  :  Table 't1' already exists
// When     :  2015-10-16 16:59:05.438575626 -0400 EDT

List of errors

Code	Constant	Message
0	ErrWarning	Execution of last statement resulted in warning(s)
9000	ErrUnknown	Unknown error
9001	ErrConnection	Can't connect to the server (%s)
9002	ErrRead	Can't read data from connection (%s)
9003	ErrWrite	Can't write data to connection (%s)
9004	ErrSSLSupport	Server does not support SSL connection
9005	ErrSSLConnection	Can't establish SSL connection with the server (%s)
9006	ErrCompressionSupport	Server does not support packet compression
9007	ErrCompression	Compression error (%s)
9008	ErrInvalidType	Invalid type (%s)
9009	ErrInvalidDSN	Can't parse data source name (%s)
9010	ErrInvalidProperty	Invalid value for property '%s' (%s)
9011	ErrScheme	Unsupported scheme '%s'
9012	ErrCursor	Cursor is closed
9013	ErrFile	File operation failed (%s)
9014	ErrInvalidPacket	Invalid/unexpected packet received

How to handle warnings?

Go's sql interface does not provide any explicit mechanism to handle warnings. In Vaquita, by default, warnings are not reported. However, this behavior can be changed by setting ReportWarnings connection property. When set, if the execution of a statement results in warning(s), the execution function will return an error with code 0.

dsn = "mysql://root@localhost:16001/test?ReportWarnings=true"
..skip..
// test.t1 already exists
if _, err = db.Exec("CREATE TABLE IF NOT EXISTS test.t1(i INT)"); err != nil {
            fmt.Println(err)
}
// Output:
// mysql: [0] last command resulted in warning(s)

The warnings can further be retrieved using SHOW WARNINGS.