Yet another Database Blog

MySQL stored procedures with Ruby

Ruby's getting an incredible amount of attention recently, largely as the result of Ruby on Rails.  I've played a little with Ruby on Rails and it certainly is the easiest way I've seen so far to develop  web interfaces to a back-end database.

At the same time,  I've been shifting from perl to Java as my language of choice for any serious database utility development.  But I still feel the need for something dynamic and hyper-productive when I'm writing something one-off or for my own use.  I've been playing with Python, but if Ruby has the upper ground as a web platform then maybe I should try Ruby. 

So seeing as how I've just finished the MySQL stored procedure book, first thing is to see if I can use Ruby for MySQL stored procedures.

Database - and MySQL - support for Ruby is kind of all over the place.  There's a DBI option (similar to perl) which provides a consistent interface and there's also native drivers.  For MySQL there are pure-ruby native drivers and drivers written in C.  Since the DBI is based on the native driver, I thought I'd try the native driver first.  The pure-ruby driver gave me some problems so I started with the C driver on Linux (RHAS4). 

Retrieving multiple result sets

The main trick with stored procedures is that they might return multiple result sets. OUT or INOUT parameters can be an issue too, but you can always work around that using session variables. 

If you try to call a stored procedure that returns a result set, you'll at first get a "procedure foo() can't return a result set in the given context error".  This is because the CLIENT_MULTI_RESULTS flag is not set by default when the connection is created.  Luckily we can set that in our own code:

dbh=Mysql.init
dbh.real_connect("127.0.0.1", "root", "secret", "prod",3306,nil,Mysql::CLIENT_MULTI_RESULTS)

The "query" method returns a result set as soon as it is called, but I found it easier to retrieve each result set manually, so i set the query_with_result attribute to false:

dbh.query_with_result=false

The next_result and more_results methods are implemented in the Ruby MySql driver, but there's some weird things about the more_results C API call that causes problems in python and PHP.  In Ruby, the more_results call returns true whether or not there is an additional result.   The only reliable way I found to determine if there is another result set is to try and grab the results and bail out if an exception fires (the exception doesn't generate an error code, btw);
      
    dbh.query("CALL foo()")
    begin
      rs=dbh.use_result
    rescue Mysql::Error => e 
      no_more_results=true
    end
.
We can then call more_results at the end of each rowset loop.  So here's a method that dumps all the result sets from a stored procedure call as XML using this approach (I'm know the Ruby is probably crap, it's like my 3rd Ruby program):

def procXML(dbh,sql)
  connect(dbh)
  no_more_results=false
  dbh.query(sql)
  printf("<?xml version='1.0'?>\n");
  printf("<proc sql=\"%s\">\n",sql)
  result_no=0
  until no_more_results
    begin
      rs=dbh.use_result
    rescue Mysql::Error => e 
      no_more_results=true
    end 
     if no_more_results==false
      result_no+=1
      colcount=rs.fetch_fields.size
      rowno=0
      printf("\t<resultset id=%d columns=%s>\n",result_no,colcount)
      rs.each do |row|
        rowno+=1
        printf "\t\t<row no=%d>\n",rowno
        rs.fetch_fields.each_with_index do |col,i|
          printf("\t\t\t<colvalue column=\"%s\">%s</colvalue>\n",col.name,row[i])
        end
        printf("\t\t</row>\n")
      end
      printf("\t</resultset>\n");
      rs.free
      dbh.next_result
    end
  end
  printf("</proc>\n")
end

No C programming required!

Whew!  No need to hack into the C code.  So you can use MySQL stored procedures in Ruby with the existing native C driver. The problem is that the C driver is not yet available as a binary on Windows yet and trying to compile it turns out to be beyond my old brain (and yes, I used minGW and all the other "right" things).   Hopefully a copy of the MySQL binary driver it will be available in the one-click installer Ruby installer eventually.

The above code doesn't work using the pure-Ruby driver on windows by the way -  there's an "out of sequence" error when trying to execute the stored proc.  I might hack around on that later (at the moment I'm 35,000 ft with 15 minutes of battery left on the way to the MySQL UC).  For now if you want to use MySQL stored procedures in a ruby program on windows I can't help.

Note that ruby seems to hit a bug that causes MySQL to go away if there are two calls to the same stored proc in the same session and the stored proc is created using server-side prepared statements.  Fixed soon hopefully, but for now if you get a "MySQL server has gone away error" you might be hitting the same problem.   Wez posted on this problem here.

I suppose the end of this investigation will probably be to see if there's any way to use stored procedure calls to maintain a Rails AcitveRecord object.  Not that I think you'd necessarily want to, but it would probably be a good learning exercise.

Plain Old SQL Statements in Java

Lot's of Java developers want to avoid writing SQL or even avoid directly accessing with a relational data store.  I, on the other hand, want to use SQL in my Java programs (which are mostly database utilities) but I want it to be as easy to use SQL in Java as it is in Python, Perl or PHP. 

Recently, I decided to use Java rather than Perl as my language of choice for database utilities.  Perl is quick to write, but tends to be easier to write than to read.  I played with Python for a while, but in the end decided that by using Java I would have an easier time distributing by stuff and it could more easily be re-used inside of my company, which has Java developers but not many Perl people.  So developed a set of JDBC wrappers that would let me use SQL easily within my utilities.   

My Design :

• Require no configuration files (no XML mappings for instance).  All the data access logic should be right there in the code.  Although I did end up allowing SQL statement to be held in an XML file just to avoid the whole messy string handling involved in really long SQL statements.   
• Easy processing of result sets, leveraging Java collections (which were not around when JDBC was first speced).
• Make it easiest to follow best practices.  For instance, make it very easy to use bind variables, re-use cursors, etc.
• Allow interoperability with the underlying JDBC objects so that I could use these classes without worrying that I would run into a brick wall.
• Work well with dynamic SQL.
• Where appropriate, avoid some of the tedium involved with DML and DDL.
• Cache prepared statements so that you don't have to worry about which prepared statements to keep open and when to close.
• Be RDBMS neutral. 

This was a bit of a learning experience - had not programmed in Java for quite a while and I tried to follow best Java practice such as creating Junit tests, JavaDoc and ant builds.  If anyone's interested,  here's the latest versions:

• jar file
• javadoc
• Short page of examples

I don't really expect anyone else to use this, posting it was I guess one of the ways to enforce a bit of discipline on myself as regards quality. However, it will be embedded in most of my database utilities so I guess it will see some use in our internal benchmarking routines and the like.

MySQL 5.1 events

I finally got around to working with the 5.1 scheduler.  I wanted to have a simple but non-trivial example, and when I saw Brian Akers post on the new processlist table, I thought of a useful little application:  I would submit an event that would summarize the users and their statuses at regular intervals so I could track user trends.

First off,  I needed to enable the scheduler by adding the following line to my configuration file:

event_scheduler=1

Now the scheduler is ready for action.  So I created a table to hold my process list details:

CREATE TABLE processhistory (h_timestamp DATETIME,
                             processcount INTEGER,
                             activecount INTEGER,
                             lockedcount INTEGER)$$

And then the DML to create an event:

CREATE EVENT evt_process_history
     ON SCHEDULE EVERY 2 MINUTE
     DO
BEGIN
    INSERT INTO processhistory (h_timestamp,processcount,
            activecount,lockedcount)
     SELECT NOW() AS h_timestamp,COUNT(*) AS processcount,
            SUM(active) AS activecount  ,
            SUM(locked) AS lockedcount
       FROM (SELECT CASE command WHEN 'Sleep' THEN 0 ELSE 1
                     END AS active ,
                    CASE state WHEN 'Locked' THEN 1 ELSE 0
                     END AS locked
               FROM information_schema.`PROCESSLIST` P) Q;
END$$

Every two minutes, the event summarizes the status of the sessions currently connected and stores them to the table.  I could use various tools to analze this data, but for convenience I used Excel with the ODBC driver to create a chart of activity:

Cool! Now I can keep track of active sessions over time, which could be useful. On the test database, there is a little ruby program that locks up a table needed by my java TP simulation, so we see those spikes of lock activity. I'm hoping that MySQL expose the SHOW STATUS command as a table as well, since we can't get at the contents of SHOW STATUS from within the stored program language.