database schema evolution versus schema versioning

Why we have choosen to use dbdeploy for our database evolution over flyway and some other candidates.
 
Flyway as most other tools, assumes that you have a schema version, that is strictly monotonically increasing. This means, you can only evolve from a lower version to a higher version. As we are not yet doing continuous delivery, we still have a staging environment, where we test a release candidate. And from time to time it happens, that this RC needs a database change. And that is where trouble starts.
Let us assume, that the latest staging version uses db version 22.13, the trunk is currently on 23.5. If you now need a db change in staging, it will increas the schema version to 22.14. But your dev databases are allready at 23.5. So flyway will not allow to add the missing script to advance from 22.13 to 22.14 on your dev databases, as these are allready on 23.5. The only way to add the required change would be to recreate the database from scratch, which gets a little bit complicated and time consuming, if you are working an a more than 10 years old application, as we are.

The main reason I can come up with, for this behaviour is, that this guarantees a consistent order of changes. And thus solves eventually occuring dependencies between database changes. For example db version 23.5 may change a table that was introduces with 22.12. Thus 23.5 will fail if 22.12 is not yet applied.

However, with 100 people working on one big web application -which needs changing, but this is another story- most changes will not depend on each other as they affect different functional parts of the application. And often changes are developed in parallel which makes sctrictly monotonically increasing nmbering in svn difficult.

To allow an efficient development, the dbdeploy way of doing things looked more appropriate for us. Dbdeploy also uses a unique numbering scheme for naming the changes, but it does not enforce the order as strictly als flyway. If you allready applied 100, 110 and 120, you can still create a 115, and get it deployed. Dbdeploy basically removes the allready applied scripts from the set of available scripts and applies the remaining scripts in the order given by their numbers. Dependencies between scripts are at your risk.  

So the basic difference is versioning a schema versus evolving a schema by applying (hopefully small and independent) changes. The only thing we missed in dbdeploy was the capability to warn about changed scripts allready applied to the database. Thus we added a sha256 checksum to the dbdeploy changelog table, and added checksum comparison for scripts allready applied. If a allready applied script was changed, we will set up the database to a previous production version by importing an anonymized dump and apply the missing features. As this is currently our normal way of database deployment, we know how to do that. But my strong hope is that we will only have to do that in one out of a hundred cases, as this takes 15 minutes. Applying the missing changes takes less than a minute as far as we have experienced up to now.

Oracle database shutdown and startup via jdbc

While working on the automation of our database setup procedures, we learned that you can shut down and start up an oracle db instance remotely via jdbc. It took some trials and finally reading this blog post to understand how to achive that.

From the oracle java api documentation it is not obvious, that you have to call several methods to achieve the same effect of a shutdown transactional from within sqlplus. You actually should issue four commands as shown in the following code example

   import static oracle.jdbc.OracleConnection.DatabaseShutdownMode.TRANSACTIONAL;

   ...

   OracleConnection connection = connectionHolder.getConnection();
    try {
      connection.shutdown(TRANSACTIONAL);

      Statement stmt = connection.createStatement();
      stmt.execute("alter database close normal");
      stmt.execute("alter database dismount");
      stmt.close();
      connection.shutdown(OracleConnection.DatabaseShutdownMode.FINAL);
    } finally {
      connection.close();
    }

For startup  it is :

    connectionHolder.getPrelimAuthConnection().startup(NO_RESTRICTION);

    Statement statement = connectionHolder.getConnection().createStatement();
    statement.execute("alter database mount");
    statement.close();
 

Both calls require a connection with sysdba or sysoper role. Startup requires a preliminary auth connection. To get one use

  private OracleConnection getPrelimAuthConnection() 

      throws SQLException 

  {
    Properties props = new Properties();
    props.put(USER_KEY, username);
    props.put(PASSWORD_KEY, password);
    props.put(ROLE_KEY, "sysdba");
    props.put(PRELIM_AUTH_KEY, "true");

    OracleConnection newConnection = 

       (OracleConnection) DriverManager.getConnection(connectionString, props);
    return newConnection;
  }

How to get things done aka agile change

In an IT organisation with 100 or more employees and a 10 year old million lines of code base, things tend to become slow. Some reasons for that are complexity, fear of breaking something, and lack of knowledge, especially about the legacy stuff. The older problems are, the more hesitation is felt.

After a couple of infrastructure improvement projects I participated in, there is the one most important lesson I learned. The one thing that helps most, is to stop hesitating and "just" start doing something.

• Find the right people, 
• understand the problems, 
• pick the most important problem, 
• collect solution ideas to reduce this problem a bit 
• and decide on the idea to start with. 

An important criteria to select the first / next idea to work on should be the time required to evaluate the idea. You should get feedback after one or two days of working on it (aka time boxing).  This makes simple solution ideas more likely to be choosen, which is good. Suprisingly often the first idea I can come up with, is much more complex than the idea finally followed after discussing the ideas with a small team. The implementation of the idea is not necessarily finished after one or two days. But if you do not see sufficient progress, you should rethink problem and solution with what was learned during these days. Often new ideas are borne in this time. Just do not ride that horse until it is dead.

As an example we were looking for a solution to get our database deployment in our dev environment faster. Someone came up with the idea to use an empty database instead of an anonymized production dump. This ideas was around for at least five years, but nobody really tried and and many problems were expected to happen. Having decided to try that idea, we managed to have a minimal dump available and our application starting and running with it within less than a day. So we knew it would work. We still spend two weeks to automate the creation of this dump and to build some quality assurance around the automation of minimal dump creation and database deployment, but we knew that it would work and save us around 10 minutes of deployment time.