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Debugging MySQL Stored Procedures


The Proposed Approach

The current approach uses several of MySQL's special features to create a logging scheme that is both robust and practically free. We create two tables: a temporary log table using the Memory engine and a permanent log table using the MyISAM engine. MySQL supports several storage engines that act as handlers for different table types. MySQL storage engines include both those that handle transaction-safe tables and those that handle non-transaction-safe tables. The memory engine performs all operations in memory and never writes to disk. As such it is very fast, though transient. The MyISAM engine is a non-transactional engine, transactions can include both transactional and non-transactional tables (the non-transactional table simply ignore the transactional commands).

Log messages are inserted into the tmplog table, which is a practically free operation since that table lives in memory. The extremely low cost of this operation means that a developer can be very verbose in their use of logging. Rather than agonize over whether to log something or not, you can simply log at will.

In the (hopefully) usual case where nothing goes wrong, the Stored Procedures does not have to do anything. Temporary tables only exist for the duration of a connection. In the typical J2EE usage pattern, an external request arrives at the system, a connection is retrieved from the connection pool, is used and then returned to the pool. When the connection is returned to the pool the temporary table is effectively dropped -- the code does not have to do anything to remove the log messages that where inserted during the successful invocation of the Stored Procedures. In this way the system suffers little to no performance cost for a successful operation.

When the Stored Procedure detects a failure condition it does an INSERT SELECT from the temporary memory table into the permanent MyISAM table. Thus it preserves all log messages that were written to the memory table into the MyISAM table. In this way the system records all of the information it needs but only in the cases where it needs it.

Log4J users can imagine being able to run your production system at DEBUG level for all the failure cases but only pay the overhead of running at WARN level for all of the successful cases -- without having to know which cases were which ahead of time.

An important thing to note is the choice of the MyISAM engine for the permanent log table. Remember that rows are typically only written to that table when things are going badly. That would usually result in the current transaction being rolled back -- except that we really want the inserts into the log table to succeed! The MyISAM engine is not a transactional engine. This means that even if a transaction is rolled back the inserts into the log table are preserved -- which is exactly the desired behavior.

The Code

There are four SPs defined in the debugLogging.sql package. Two SPs help setup the tables to be used, one performs temporary logging and the last copies messages to the permanent table.

This first procedure creates both the temporary and permanant tables. Note the use of the engine specifier to distinguish the two tables. The temporary log contains a single interesting column which was called msg. The permanent table adds an automatic timestamp and a thingID. The assumption is that the logs are written during an operation that creates, destroys, or modifies some object, and that that object has a unique identifier. In the video-on-demand space that I work in, this can be the identifier of a movie being streamed to a customer's set-top box.

Create Procedure setupLogging() BEGIN create temporary table if not exists tmplog (msg varchar(512)) engine = memory; create table if not exists log (ts timestamp default current_timestamp, thingID bigint, msg varchar(512)) engine = myisam; END;

This next procedure creates just the temporary log table. We've seen cases in the field where the temporary table does not exist at the time that we are needing to insert into it. In that case we have a procedure to recreate the table.


Create Procedure setupTmpLog()
 BEGIN
     create temporary table if not exists tmplog (msg varchar(512)) engine = memory;
  END;

The next procedure is the one that is called the most and performs the actual logging. A message parameter is written to the temporary table. There is a continue handler that creates the temporary table if it does not already exist.


Create Procedure doLog(in logMsg varchar(512))
BEGIN
  Declare continue handler for 1146 -- Table not found
  BEGIN
     call setupTmpLog();
     insert into tmplog values('resetup tmp table');
     insert into tmplog values(logMsg);
  END;

  insert into tmplog values(logMsg);
END;

The last procedure is the one to call when an error is detected within the user's SP and all of the logging needed to be saved for later analysis. This procedure takes a final message, presumably one saying what the final error condition is. It then inserts all of the rows in the temporary table into the permanent table, setting the timestamp along the way. To handle the situation where the user of the logging system chooses to continue after an error, and then runs into another error we make sure to delete all rows in the temporary table after inserting them into the permenant table.


Create Procedure saveAndLog(in thingId int, in lastMsg varchar(512))
BEGIN
   call dolog(lastMsg);
   insert into log(thingId, msg) (select thingId, msg from tmplog);
   truncate table tmplog;
END;

Sample Use

The following code sample shows a possible usage of the logging procedures. The example is based on the notion of a Stored Procedure that would be called from within another stored procedure. The mission of the inner procedure is to parse a comma separated list of values and insert each value into a cache table for use in a query by the calling procedure. It illustrates the value of being able to log not just the final results but the steps followed along the way since it might be very valuable to know which iteration of the loop contained the failure.



Create Procedure parseAndStoreList(in thingId int,
                                                             in i_list varchar (128),
                                                             out returnCode smallInt)
 BEGIN
    DECLARE v_loopIndex default 0;
    DECLARE Exit Handler for SQLEXCEPTION
    BEGIN
         call saveAndLog(thingId, 'got exception parsing list');    -- save the logs if things go badly
         set returnCode = -1;
    END;

    call dolog(concat_ws('got list:', i_list));   -- say we got to the start
    pase_loop: LOOP
         set v_loopIndex = v_loopIndex + 1;
         call dolog(concat_wc(',', 'at loop iteration ',  v_loopIndex);  -- say we got to nth iteration
         -- actually do the parsing, or whatever
    END LOOOP parse_loop;
    set returnCode = 0;
 END;

Conclusion

The ability to do low- or no-cost logging with Stored Procedures has proven to be an extremely useful technique. It lets you instrument your code with extensive information, which is available at production runtime, but only when needed.


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