5.12.2.2 Building Custom Listeners

To build custom listeners that work with the MySQL Connector/NET Trace Source, it is necessary to understand the key methods used, and the event data formats used.

The main method involved in passing trace messages is the TraceSource.TraceEvent method. This has the prototype:

public void TraceEvent(
    TraceEventType eventType,
    int id,
    string format,
    params Object[] args
)

This trace source method will process the list of attached listeners and call the listener's TraceListener.TraceEvent method. The prototype for the TraceListener.TraceEvent method is as follows:

public virtual void TraceEvent(
    TraceEventCache eventCache,
    string source,
    TraceEventType eventType,
    int id,
    string format,
    params Object[] args
)

The first three parameters are used in the standard as defined by Microsoft. The last three parameters contain MySQL-specific trace information. Each of these parameters is now discussed in more detail.

int id

This is a MySQL-specific identifier. It identifies the MySQL event type that has occurred, resulting in a trace message being generated. This value is defined by the MySqlTraceEventType public enum contained in the Connector/NET code:

public enum MySqlTraceEventType : int
{
    ConnectionOpened = 1,
    ConnectionClosed,
    QueryOpened,
    ResultOpened,
    ResultClosed,
    QueryClosed,
    StatementPrepared,
    StatementExecuted,
    StatementClosed,
    NonQuery,
    UsageAdvisorWarning,
    Warning,
    Error
}

The MySQL event type also determines the contents passed using the parameter params Object[] args. The nature of the args parameters are described in further detail in the following material.

string format

This is the format string that contains zero or more format items, which correspond to objects in the args array. This would be used by a listener such as ConsoleTraceListener to write a message to the output device.

params Object[] args

This is a list of objects that depends on the MySQL event type, id. However, the first parameter passed using this list is always the driver id. The driver id is a unique number that is incremented each time the connector is opened. This enables groups of queries on the same connection to be identified. The parameters that follow driver id depend on the MySQL event id, and are as follows:

MySQL-specific event type	Arguments (params Object[] args)
ConnectionOpened	Connection string
ConnectionClosed	No additional parameters
QueryOpened	mysql server thread id, query text
ResultOpened	field count, affected rows (-1 if select), inserted id (-1 if select)
ResultClosed	total rows read, rows skipped, size of result set in bytes
QueryClosed	No additional parameters
StatementPrepared	prepared sql, statement id
StatementExecuted	statement id, mysql server thread id
StatementClosed	statement id
NonQuery	Varies
UsageAdvisorWarning	usage advisor flag. NoIndex = 1, BadIndex = 2, SkippedRows = 3, SkippedColumns = 4, FieldConversion = 5.
Warning	level, code, message
Error	error number, error message

This information allows you to create custom trace listeners that can actively monitor the MySQL-specific events.