Programming Space

One of the issues I had to deal with in building my mobile app was the evolving database schema as more features were added and the app grew.  I needed a way to update the database scheme without blowing away the old one and forcing the app to redownload all the data.  While I could have done that without any loss of data (there is no user supplied data to be lost), the main data set is over 8 MB in size and that isn’t nice to users’ wireless plans.  What I needed was a way to incrementally change the schema as the app was updated.

First Attempt

My first attempts just used if() statements.  i.e. if the current app version was one value and the new app version was a different value, apply this update.  This worked well enough for my development versions where I was strictly controlling the upgrade versions.  But I quickly realized that if I were to try to do that with versions of the app out in the wild, where I had no control over when it was updated, I’d quickly have a mess of conditionals to deal with.  So I scrapped that idea and went looking for something new.

Final Solution

I don’t think this solution is new to me as I’m sure I saw it somewhere to give me the idea but I couldn’t tell you where I saw it off the top of my head.  The answer was a switch() statement, with each case() block having a bit of the upgrade path from old to new schemas.  In order for this to work, you need to track a version number for the database schema that increases each time you make a change.  Simply start at one and every time you change the schema increment the value.  This value, which I called current_version, is simply a coded variable.

Additionally, the app needs a second value that it stores as part of the application data which contains the database version that is currently installed (installed_version).  Then, each time the application is run, you check these values.  There are three possibilities^[1]:

1. There is no stored value for installed_version. This should only be true the very first time the application is run.  In this case, you simply need to create the entire database schema as it now stands and store the current_version as the installed_version.
2. The current_version equals the installed_version.  This is the simplest case of all.  Since the database schema is all up-to- date you don’t have to do anything.  Continue on to the main part of the application.
3. The current_version is greater than the installed_version.  In this case there is work to do and we now need to do the update.

To make the update we simply enter a switch() statement based on the installed_version.  Each case() block should have arguments that start at 1 and increase by one each time.  The first case() block (i.e. case 1:) would hold the commands that were necessary to upgrade from version 1 to version 2.  The second would hold the commands modify the schema from version 2 to version 3, and so forth.

None of the case statements should have the break command associated with them so that after execution they would fall through to the next one and cascade all the way to the end.  That way if a user skips one or more app updates that include database upgrades, each one will be applied in turn and none will be skipped.

Finally, the default() block at the end of the switch statement should contain the code up update the value of the stored installed_version so that it is also up-to-date for the next time the application is run.

Here is a bit of sample code that shows how this works.  It is lifted in modified (with additional comments added) from my application which is written in using Titanium Studio’s Mobile SDK.  It should be noted that the c object is a configuration object that holds a lot of information such as the database name, the table names, and column names among other things.

// Check that we have a stored value for the installed database version
if(!Ti.App.Properties.hasProperty('installed_db_version')){
  // if not, the database has never been created
  createDB();
} else {
  var installed_version = Ti.App.Properties.getInt('installed_db_version');
  if (c.current_version > installed_version){
    Ti.API.info("Performing an upgrade from database version "+old_version+" to version "+c.version);
    var db = Ti.Database.open(c.name);
    // We need to do an upgrade to the new version based on the old version.
    // We'll do this incrementally adding in all the changes since the installed
    // version was created.
    switch(installed_version) {
      case 1:
        db.execute("REPLACE INTO daily_data (source,start,duration,flux_1000_300000,error_1000_300000,ul_1000_300000," +
        "flux_300_1000,error_300_1000,ul_300_1000,flux_100_300000,error_100_300000,ul_100_300000,test_statistic)" +
        "VALUES (1,56000,86400,10,1,0,9,2,0,1,0.5,0,32)");
        db.execute("REPLACE INTO daily_data (source,start,duration,flux_1000_300000,error_1000_300000,ul_1000_300000," +
        "flux_300_1000,error_300_1000,ul_300_1000,flux_100_300000,error_100_300000,ul_100_300000,test_statistic)" +
        "VALUES (1,57000,86400,12,2,0,10,1.5,0,2,0.5,1,32)");
      case 2:
        db.execute("DELETE FROM " + c.T_DAILY);
        db.execute("DELETE FROM " + c.T_WEEKLY);
        Ti.App.Properties.setString('last_lc_data_update','0');
      case 3:
        db.execute(DATABASE_CREATE_GCN);
        Ti.App.Properties.setString('last_GCNNotice_date', '0');
      case 4:
      ...
      ...
      default:
        Ti.App.Properties.setInt('installed_db_version',c.current_version);
        break;
    }
  db.close();
  }
}

In version 1, I was using some dummy data that I was setting explicitly.  In version 2, I had updated the code to get the data from a web service so I needed to remove all the old data and add a stored variable to track the date of the last data point.  In version 3, I added a table to the database and an internal stored variable to track the date that data was last updated.

At the beginning of the code block, I check to see if the database is installed at all.  If not I call a createDB() function.  This function simply contains the code that creates the current version of the database schema.  Another option would be to just set the installed_version to 0 and have a case 0: block that creates the original database schema that all the modifications are applied to but I believe it is faster and cleaner to just create the current schema rather than run through all the old ones applying one updated at a time.  While probably not an issue if you only have a few changes, as the number of modifications grow, so does the time involved in the creation of the new database.

And there you have it.  A simple straightforward way to make sure that your database updates go smoothly across updates of your application code.

One final note is that you don’t have to call this each time the app runs, but only the first time the database is used.  Now I suspect in most cases, the database is used every time the application is run but if there are sections of your app that don’t use the database, this check can be delayed until first use of the database.  This allows the app to load up a bit faster, although the difference may not be noticeable.

[1] There is a fourth possibility and that is that current_version is less than installed_version.  But if you’re properly incrementing current_version then this should never happen unless you roll back a distributed version of your application.  If you do that, all bets are off and you’ll need to find another way to handle the update.  Of course this is only a worry if the older code can’t work with the newer schema.

Once you have an idea for an application, it really helps to get a prototype up and running to understand better how the application will function and even if it is possible.  Building the mobile data application for the Fermi Science Support Center was no different.

This is the third of several articles about the journey of developing an application for the Fermi Gamma-ray Space Telescope mission.  Earlier articles can be found here:

• My First Mobile App
• Designing the Mobile App

When I started this project my mobile app development experience comprised a sum total of about two weeks of playing around with the Android SDK.  I had it up and installed, had read a few tutorials, and started in on a small app that was to be a character generator for an old role-playing game to have something (semi-)practical to work on.  But beyond that I was a complete novice.

However, that was infinitely more experience than I had with iOS development and that fact, combined with the fact that I owned an Android smartphone (a Droid X) and the only Apple devices in my house were my daughter’s 3rd gen iPod Nano and a work MacBook that I used occasionally to test out builds of our science software, I decided to try the prototype as a native Android app written in Java.

Breaking New Ground

This was a completely uphill journey, although not necessarily a difficult or unpleasant one.  First was the fact that I was working in Java.  I’m primarily a C++/Perl programmer although I’m familiar with many other languages.  I have a passing knowledge of Java but have never really used it for any real project.  So working in Java required more than a bit of looking up syntax and functions.

Doing event driven, GUI programming wasn’t completely new to me.  On of my side projects that I work on off-and-on is a computer version of the spaceship combat game from the old Star Frontiers role-playing game.  This game is GUI based and so the concepts of responding asynchronously to input events isn’t new.  In fact to a lesser scale, the main data server that I built for the mission does the same thing but the events are coming from sockets instead of a user interface.

And of course the entire Android development process was new.  Screen layouts, the SDK, etc was virgin territory.  I spent a lot of time looking at the developer documentation to figure things out and learn about the various classes, what they did and how they interacted.

What to Build in the Prototype

The next question was what the prototype should contain.  The goal of the prototype was two-fold.  First, I wanted it to demo the basic functionality of the application so I could show it to the project scientist to get approval to do the full application.  And second, it was a way for me to explore a bit of how to do thing in the mobile space and how the whole mobile development paradigm worked.

In the end I decided to implement three major components:

1. Plotting the light curves – This was to be the major functionality of the application to begin with so it made sense to try this out from the get go.  It didn’t have to pull data from the web or perform updates, but it did need to plot regular data points with errors as well as upper limits and give source and data range selection options.
2. Show the Fermi Sky Blog – I mainly did this one because it was easy as it was effectively just providing a link to the web page of the blog.  This provided some experience in launching other tasks (Activities in Android parlance) that were external to the application I was writing.  However, while everything I learned here could have been learned from implementing the ATels (#3 below) it did provided a little more functionality to have in the demo.
3. Building and displaying the list of Fermi related Astronomer’s Telegrams – The main draw to implementing this one was that it provided some experience with reading an RSS feed and parsing the related XML.  Both of which I’d never done before.  It also provided some experience with the process of using the network interface to go out and get data.

Putting It All Together

It took me about a week and a half to get the main application screen and get the basic plotting implemented.  I know because a week and a half after I got back from the AAS meeting I was on the plane headed out to NASA Goddard.  The trip was to help train a new staff member on our data server but I would also be pitching the mobile application to the project scientist.

One thing I was reminded of during that time is how effective and efficient you can be when you are really excited about what you are working on.  I was really excited about this project and wanted to pursue it and so I found myself very focused and working hard to get this done.  It’s amazing what you can accomplish when you don’t let yourself get distracted.

While at Goddard, I had a chance to show the prototype as it existed to the scientist who I had worked on the design with as well as the project scientist.  Both of them liked what I had so far and the project scientist gave the green light for me to work on this officially and implement the full version.

I still had work to do on the prototype to fulfill it’s role as an exploratory tool for some of the techniques in building the app but it had fulfilled at least one of its main purposes as a demo tool.  In the end, the functionality of the prototype was expanded some what to include other things like notifications, automatic data updates, and pulling live data from our servers.  Especially in the latter case, it served as a test client to back end services that I needed to develop for the final applications but that that is the topic for another post.  But all along the way it was a tool to help me learn what was possible, what wasn’t, and how to implement those things.  And in the end, that was really its primary purpose which it fulfilled admirably.