First and foremost, the most common error message is a variant of the following:

$ sudo gem install ibm_db
Building native extensions.  This could take a while...
ERROR:  Error installing ibm_db:
	ERROR: Failed to build gem native extension.

/usr/bin/ruby1.8 extconf.rb install ibm_db
checking for SQLConnect() in -ldb2... no
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for more
details.  You may need configuration options.

Provided configuration options:
	--with-opt-dir
	--without-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
  ...
  ...

The gem fails to install because the SQLConnect() function provided by DB2’s development headers (which are required to build the gem) cannot be found/loaded.

This brings us to our quick checklist:

1. Have you sourced the profile? This is done by running:

. /home/db2inst1/sqllib/db2profile

You may need to replace the dot, with source depending on your shell.

2. Have you specified where DB2’s directory and library are to be found? You can do so by running:

export IBM_DB_DIR=/home/db2inst1/sqllib
export IBM_DB_LIB=/home/db2inst1/sqllib/lib32

3. Did you install the required development headers during the installation? These are available by selecting “Base application development tools” under the section “Application Development Tools” in the DB2 Setup Wizard (having selected a “custom” installation beforehand).

You can double check this by running:

ls /home/db2inst1/sqllib/include

This should show you a series of .h files, including sqlcli1.h. If you don’t see them, then you need to repeat the setup procedure so as to add the development tools. Likewise, you should also see a series of .so files, including
libdb2.so when you run: ls /home/db2inst1/sqllib/lib32.

4. Are you trying to perform the installation when logged in as the db2inst1 user? If you’re doing this, it’s best not to. It doesn’t matter if db2inst1 is a suoder or not. Don’t run sudo gem install ibm_db as the instance user.

5. If nothing else works, try to install as the root user (on certain systems sudo won’t cut it). The following complete procedure works in 99% of cases:

sudo -s
. /home/db2inst1/sqllib/db2profile
export IBM_DB_DIR=/home/db2inst1/sqllib
export IBM_DB_LIB=/home/db2inst1/sqllib/lib32
gem install ibm_db

If this checklist doesn’t help you, then you can reach for the RubyForge forum, where the DB2 on Rails community and the IBM team will be happy to help you.

$ . /home/db2inst1/db2profile
$ export IBM_DB_DIR=/opt/ibm/db2/V9.5
$ export IBM_DB_LIB=/opt/ibm/db2/V9.5/lib32
$ sudo gem install ibm_db

Adjust these commands for your paths, shell and architecture (e.g. 32 vs. 64 bits).

Please note that at this stage we don’t recommend that you download the outdated Starter Toolkit for DB2 on Rails. Instead, please grab and install DB2 Express-C, install Ruby through the One-Click Installer, install Rails through gems (gem install rails) and then proceed to install the ibm_db gem as explained above. When you install DB2, ensure to select the “Base application development tools” as illustrated here.

conn = IBM_DB::connect("mydb","myuser","my_wrong_pass")
#=> false

false. That doesn't really help us too much. We know that something went wrong, but we don't exactly know what. Luckily for us, the IBM_DB driver gives us all the tools that we need to properly troubleshoot problems. The example above can be rewritten in this way:

if conn = IBM_DB::connect("mydb","myuser","my_wrong_pass")
  # do something interesting
else
  # conn is false
  raise IBM_DB::conn_errormsg
end
#=> RuntimeError: [IBM][CLI Driver] SQL30082N Security processing failed with reason
"24" ("USERNAME AND/OR PASSWORD INVALID").  SQLSTATE=08001 SQLCODE=-30082

Okay, that should have us covered when it comes to failed attempts to connect to the database, but what about failed queries? Analogously, the IBM_DB driver provides us with the IBM_DB::stmt_errormsg method:

if stmt = IBM_DB::exec(conn, "SELECT * FROM WRONG_TABLE)
  # process the results
else
  # stmt is false
  raise IBM_DB::stmt_errormsg
end
#=> RuntimeError: => [IBM][CLI Driver][DB2/NT] SQL0204N " DB2ADMIN.WRONG_TABLE" is
an undefined name.  SQLSTATE=42704 SQLCODE=-204
    

DB2 error messages are usually easy to understand and with a bit of help from the DB2 Information Center you should be able to get out of trouble most of the time.

DB2 CLI Tracing

Despite the helpful error messages, there are situations in which troubleshooting can be hard because everything seems to be in the right place but your application is still acting up for some reason. The ultimate tool when it comes to troubleshooting for DB2 and Ruby/Rails application problems is to enable DB2 CLI (Call Level Interface) tracing. The CLI trace captures information about applications that access the DB2 CLI driver. Tracing gives you the ability to analyze low level calls to the C driver API with details on the input and output, to and from the database. The resulting trace is not particularly easy to understand for a DB2 novice, but it can offer a microscopic view which is invaluable for understanding problems that are hidden by the abstractions of higher level APIs, and allow you to see to a certain degree what's happening under the hood.

The two free PDFs, Call Level Interface Guide and Reference, Volume 1 and Volume 2 are the best references if you need to look up calls in your CLI trace files. Instructions on how to enable CLI level tracing for DB2 Express-C LUW (Linux/Unix/Windows) can be found here.

rake db:test:purge

Running rake db:test:purge generates the following error:

  rake aborted!
  Task not supported by 'ibm_db'

db:test:purge is a rake task defined in the Rails gem within the file rails-1.2.3/lib/tasks/databases.rake. This file takes care of, amongst other things, defining the specific behavior for each database adapter known to rails, when the user requests a database purge. It is essentially a big case statement with specific ruby code which drops all the user objects from the supported database.

ibm_db is currently shipped independently from Rails, therefore it is missing from that case statement, and as a result the task will appear to not be supported by ibm_db.

If you are using the ibm_db adapter for DB2, you don’t want to miss the opportunity to use rake db:test:purge and take advantage of other tasks that rely on this in order to work. What you can do is, manually insert a snippet of code for the ibm_db case in databases.rake.

An easy and straightforward implementation of this would be dropping the database and recreating it from scratch. I’m not allowed into the other adapters implementations, but I assume this could be what some of the other adapters may be doing. I believe this is not the right thing to do with DB2 though. In fact, this would have two main drawbacks. Firstly, creating a database every time, implies that the task becomes quite slow, because the creation of a database in DB2 is a “magical process” that can take up to a minute (for good reason, and that minute can save you lots of money in the years to come as you use the database). Secondly, the database that you have created in the first place may have many options and parameters configured, and collecting them all and reapplying them may not be the easiest or the smartest thing to do.

A different approach would be to handle this the right way, by dropping all the user schemas and the objects contained within the database. The code needs to be placed within the case statement we mentioned above:

    desc "Empty the test database"
    task :purge => :environment do
      abcs = ActiveRecord::Base.configurations
      case abcs["test"]["adapter"]
        #...
      end
   end

With the current (Rails 1.2.3) databases.rake you can practically just copy and paste the following code at line 145 in the file:

when "ibm_db"
  ActiveRecord::Base.establish_connection(:test)
  conn = ActiveRecord::Base.connection.connection

  begin
    # Required for the stored procedure ADMIN_DROP_SCHEMA
    ActiveRecord::Base.connection.execute("CREATE TABLESPACE SYSTOOLSPACE")
    systool_existing = false
  rescue
    # The SYSTOOLSPAGE already exists
    systool_existing = true
  end

  # Collects all the user defined schemas
  user_schemas_sql = "SELECT SCHEMANAME FROM SYSCAT.SCHEMATA WHERE DEFINER <> 'SYSIBM' AND 
  SCHEMANAME NOT IN ('NULLID', 'ERRORSCHEMA', 'SYSTOOLS')"
  schemas = ActiveRecord::Base.connection.select_all(user_schemas_sql)

  unless schemas.empty?
    errortabschema = 'ERRORSCHEMA'
    errortab = 'ERRORTABLE'

    # Drop each schema and all its objects
    schemas.each do |schema|
      schema_name = schema["schemaname"].strip.upcase
      sql = "CALL SYSPROC.ADMIN_DROP_SCHEMA('#{schema_name}', NULL, ?, ?)"
      stmt = IBM_DB::prepare(conn, sql)
      IBM_DB::bind_param(stmt, 1, "errortabschema", IBM_DB::SQL_PARAM_INPUT)
      IBM_DB::bind_param(stmt, 2, "errortab", IBM_DB::SQL_PARAM_INPUT)
      IBM_DB::execute(stmt)
    end

    # If the tablespace SYSTOOLSPACE didn't exist initially, it gets dropped  
    ActiveRecord::Base.connection.execute("DROP TABLESPACE SYSTOOLSPACE") unless systool_existing

    # Drops the remaining schema "ERRORSCHEMA"
    ActiveRecord::Base.connection.execute("DROP SCHEMA ERRORSCHEMA RESTRICT")
  end

This can also be used do define a method on its own (e.g. purge_database(:mydb)), should you require that functionality somewhere in your code. In that case, just make sure to modify the first lines in order to use the already established connection or define your own.

rake db:test:clone

Now that rake db:test:purge is working, you will be able to successfully run rake db:test:clone. Out of the box, there are two limitations though. Rails doesn’t acknowledge tablespaces and foreign keys (the lack of the first is understandable as it is strongly related to DB2, but the lack of the second is much less justifiable).

This doesn’t affect many developers but if it does affect you, it is both annoying and problematic. Suppose in fact, that you have created the database objects in your development db through migrations. You may have specified a certain tablespace name by passing ptions => “IN mytablespace” to the create_table method. Running rake db:test:clone will generate the table in the test database within the default tablespace (USERSPACE1) rather than in the one that you’ve specified for the development database. Not only this, but if you manually defined foreign keys by executing sql statements in the migrations or directly, these will not appear in the cloned test database.

This is not an issue specific to DB2, it is just the way it works in Rails at the moment. In fact there are third party plugins that attempt to introduce these and other features that are shortcoming of Rails’ core (e.g. Foreign Key Schema Dumper Plugin for MySQL and PostgreSQL).

In order to address those two concerns when using DB2, you don’t have to operate on the definition of the task directly. In fact, the db:test:clone loads the dumped schema in the test environment. The culprit is therefore ActiveRecord SchemaDumper which doesn’t know anything about DB2 Tablespaces and about foreign keys. Changes to this will also affect the db:schema:dump task, which will in turn produce more correct and “database aware” db\schema.rb files.

The file schema_dumper.rb within the ActiveRecord gem (activerecord-1.15.3\lib\active_record\schema_dumper.rb) can be directly modified for your specific needs. At line 21 the dump method becomes:

    def dump(stream)
      header(stream)
      tables(stream)
      # Foreign keys are added for DB2 only
      if @connection.adapter_name == "IBM_DB"
        foreign_keys(stream)
      end
      trailer(stream)
      stream
    end

At line 89 within the table method, we need to specify code to handle the possibility of a non-default tablespace:

# Options to retrieve the right tablespace are enabled for DB2 only
if @connection.adapter_name == "IBM_DB"
  tbspace_sql = "select TBSPACE from syscat.tables where tabname='#{table.upcase}'"
  table_space = @connection.select_one(tbspace_sql)["tbspace"]
  if table_space != "USERSPACE1"
    # A different tablespace was defined       
    tbl.print %Q(,  ptions => "IN #{table_space}")
  end
end

At this point, just after the index method, we need to define the method foreign_keys:

def foreign_keys(stream)
  references = @connection.select_all("SELECT * FROM SYSCAT.REFERENCES")
  for reference in references
    constraint = reference["constname"]
    schema = reference["tabschema"]
    table = reference["tabname"]
    cols = reference["fk_colnames"]
    ref_schema = reference["reftabschema"]
    ref_table = reference["reftabname"]
    ref_cols = reference["pk_colnames"]
    if reference["updaterule"] == "R"
      update_action = "RESTRICT"
    else
      update_action = "NO ACTION"
    end

    delete_action = case reference["deleterule"]
                          when "A"
                            "NO ACTION"
                          when "R"
                            "RESTRICT"
                          when "C"
                            "CASCADE"
                          when "N"
                            "SET NULL"
                          end

    foreign_key_sql = "  execute("ALTER TABLE #{schema}.#{table} add 
CONSTRAINT #{constraint} FOREIGN KEY (#{cols.strip})n  
REFERENCES #{ref_schema}.#{ref_table} (#{ref_cols.strip}) 
ON UPDATE #{update_action} ON DELETE #{delete_action}n  
ENFORCED ENABLE QUERY OPTIMIZATION")n  "
    stream.print foreign_key_sql
    stream.puts
  end
end

As you can imagine, it is possible to prevent the need to modify the file directly, by simply extending the SchemaDumper class in, for example, a plugin. You would have to overwrite the original methods within the SchemaDumper class:

module ActiveRecord
  class SchemaDumper

    # ...

    def dump(stream)
      #...
    end

    private

    # ...

    def table(table, stream)
      # ...
    end

    # ...

    def foreign_keys(stream)
    # ...
    end

  end
end

It would be beneficial to aggregate several improvements in a “DB2 PowerPack” plugin of some sort, and it’s very likely that we will eventually work on publishing something like this.

Please visit and rate the first part of the article here.