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+# you will find instructions how to write extensions here
+
+Self-written code is addressed by using a mode which starts with my-
+--mode=my-thing-does ?
+
+check_mysql_health will then look for a package named MyThing.
+
+So you first have to write a Module which describes MyThing. Such a thing iinherits from DBD::MySQL::Server and needs two methods: init and nagios.
+
+Start with a file called CheckMySQLHealthExt1.pm and skeleton code:
+
+###################################################
+package MyThing;
+
+our @ISA = qw(DBD::MySQL::Server);
+
+sub init {
+  my $self = shift;
+  my %params = @_;
+}
+
+sub nagios {
+  my $self = shift;
+  my %params = @_;
+}
+###################################################
+
+When you call check_mysql_health with --mode=my-thing-does, it will
+- create a DBD::MySQL::Server object
+  $obj = DBD::MySQL::Server->new()
+- connect to the database
+  $obj->connect()
+- re-bless the object
+  bless $obj, "MyThing"
+- call $obj->init()
+- if that was ok, call $obj->nagios()
+
+
+So you need to write code which 
+- initializes the parameters you want to check
+- calculates the nagios result from these parameters
+
+For your convenience there are some predefined methods and variables:
+
+Variable $self
+ $self is a hash-based object of type My::Thing
+ You can pass metrics from the init() method to the nagios() method by
+ adding attributes to the hash.
+ One important predefined attribute is $self->{handle} which points to
+ a database Connection object. You surely will use this.
+
+Variable %params
+ $params{mode} contains the string you passed to the 
+ --mode command line parameter, only with the "-" replaced by "::".
+ In the above example it will be "my::thing::does". 
+ Because you can have only one init() method for your MyThing object but 
+ more than one related modes (my-thing-does, my-thing-length, my-thing-rate)
+ you use $params{mode} for branching in your code. (see the example file)
+
+Method add_nagios
+ $self->add_nagios(1, "i warn you");
+ This method can be called several times. The messages will be concatenated.
+ The first parameter is one of 0..3 and sets the nagios level. The worst level
+ among several calls to add_nagios will determine the plugin's exit code.
+
+Method add_nagios_[ok|warning|critical|unknown] 
+ $self->add_nagios_critical("i warned you!!! now it's too late");
+ $self->add_nagios_ok("everything is ok. i am the exit message");
+ These methods are wrappers for add_nagios which make your code more readable.
+
+Method add_perfdata
+ $self->add_perfdata("metric1=0 metric2=100");
+ $self->add_perfdata("metric3=0);
+ $self->add_perfdata(sprintf "metric_xy=%d", $self->{xy});
+ You can call add_perfdata as often as you like. 
+ The strings will be concatenated.
+
+Method valdiff
+ $self->valdiff(\%params, qw(metric1 metric2));
+ Use this if you want to know how a metric has changed since the last run
+ of check_mysql_health.
+ Provided you have attributes metric1 and metric2 this method will create
+ new attributes for your $self object.
+ $self->{delta_metric1} which is the difference between the value of
+ $self->{metric1} during the current run and $self->{metric1} during the
+ last run.
+ $self->{delta_timestamp} which is the number of seconds which passed
+ since the last run of check_mysql_health.
+ If you have ever-growing values, you can simply calculate the rate:
+ $self->{metric1_per_second} = $self->{delta_metric1} / $self->{delta_timestamp}
+ The valdiff method will automatically save the current value to a state file
+ and read the past value from this file.
+ If you used the --name parameter which appears as $params{name} in your code
+ then you probably need to separate the saved values from each other. Otherwise
+ name1 would read the same saved value as name2. They would overwrite the 
+ saved values. Use $params{differentiator} to use different state files.
+ $params{differenciator} = lc $self->{name};
+ $self->valdiff(\%params, qw(gets misses));
+
+Method fetchrow_array
+ my($column1, $column2) = $self->{handle}->fetchrow_array(q{
+     SELECT col1, col2 FROM table1 where col1 = 'val1'
+ });
+ $self->{connected_users} = $self->{handle}->fetchrow_array(q{
+     SELECT COUNT(*) FROM v$session WHERE type = 'USER'
+ });
+ This method is used like the Perl DBI method fetchrow_array.
+
+
+Method fetchall_array
+ my @results = $self->{handle}->fetchall_array(q{
+   SELECT col1, col2, col3 FROM table1
+ });
+ foreach (@results) {
+   my($column1, $column2, $column3) = @{$_};
+   ...
+ }
+ This method is used like the Perl DBI method fetchall_array.
+
+
+
+
+Now you have written your first extension to check_mysql_health. Maybe you
+just modified the example file contrib/CheckMySQLHealthExt1.pm
+There are two methods how to import your own code into check_mysql_health:
+
+- the static method
+with ./configure --with-mymodules-dir=<dir> parameter you build a plugin which
+contains both my code and your code in a single file. When you call "make"
+every file in <dir> which matches CheckMySQLHealthExt*.pm is appended
+to the final plugin check_mysql_health.
+
+- the dynamic method
+with ./configure --with-mymodules-dyn-dir=<dir> you build a plugin which will
+search at runtime the <dir> for files matching CheckMySQLHealthExt*.pm and
+import them. This way you can have different extensions on different hosts.
+You also can modify your extension without having to rebuild the plugin.
+On the other hand you have to distribute your extensions along with the plugin.
+