Tuesday, April 22, 2008

Database versioning part 2

In part 1, I discussed database versioning with MS SQL Server. SQL Server has robust tools available that simplifies the process of differencing and creating upgrade scripts. DBISAM doesn't have these tools, but using datamodules and table components makes versioning close to painless.

Situation 2
Delphi app
DBISAM database
1 developer

Rental Property Manager uses a DBISAM database (DBISAM v3 in RPM 1, DBISAM v4 in RPM 2). In the 4 years since it was released, it has been through about 20 changes.

Unlike sql server, there is no database comparison utility to generate upgrade scripts. That means things have to be done a bit differently.

I still use the same general process as for sql server:
  1. Record the database version number in the database
  2. Define the expected db version in the application
  3. Keep a master db for comparison
  4. Only make one set of changes at a time
  5. Automate differencing as much as possible
  6. Unit test, test and test again. The tests should fail the moment a table is modified.
Because dbisam doesn't support views, the version number is stored (along with a bunch of other info) in an ini file in the database directory.

I have a datamodule, TdmodCheckDatabase. This has a TdbisamTable component for every table in the database. The table component contains all fields in the table and is updated whenever the table is changed.

To make database changes, the following process was used:

  1. Increase the version number in the application
  2. Make and test DB changes.
  3. Update the affected tables in TdmodCheckDatabase
  4. If necessary (rarely) add further upgrade queries to TdmodCheckDatabase. E.g. to set the values of new fields, or to add new data rows.
  5. Generate a CreateDatabase unit script using the supplied database tools.
  6. Update unit tests to suit the new db
When the application is run, it goes through the following process
  1. If no database is found, then run CreateDatabase unit and then do step 3
  2. Get the current version number from the database ini file
  3. If it is less than the expected version number then
    Run CreateDatabase (to create any new tables)
    Check every table component in TdmodCheckDatabase
    Apply any table changes
    run any manual upgrade scripts
  4. Update the version number in the database ini file.
In code this is:


class procedure TdmodCheckDatabase.UpgradeDatabase(databasePath: string; currentVersion, newVersion: integer);
var
module: TdmodCheckDatabase;
f: integer;
begin
module:= TdmodCheckDatabase.create(nil);
try
module.OpenDatabase( databasePath );

for f:= 0 to module.ComponentCount -1 do
begin
if module.Components[f] is TDBISAMTable then
begin
try
// if we need to upgrade table to dbisam 4
if currentVersion <= DB_VERSION_FOR_DBISAM4 then
TDBISAMTable(module.Components[f]).UpgradeTable;

module.UpgradeTable(TDBISAMTable(module.Components[f]));
except
// logging and error stuff removed
end;
end;
end;

for f:= currentVersion + 1 to newVersion do
module.RunUpgradeScripts(f);

module.sqlMakeIndexes.ExecSQL;
// have to create additional indexes manually
finally
module.DBISAMDatabase1.Close;
module.free;
end;
end;

procedure TdmodCheckDatabase.UpgradeTable(table: TDBISAMTable);
var
fieldIndex: integer;
needsRestructure: boolean;
canonical: TField;
begin
needsRestructure:= false;

table.FieldDefs.Update;

// add any new fields to the FieldDefs
if table.FieldDefs.Count < table.FieldCount then
begin
for fieldIndex := table.FieldDefs.Count to table.Fields.Count -1 do
begin
table.FieldDefs.Add(fieldIndex + 1, table.Fields[fieldIndex].FieldName, table.Fields[fieldIndex].DataType, table.Fields[fieldIndex].Size, table.Fields[fieldIndex].Required);
end;
needsRestructure:= true;
end;

// make sure we have correct size for string fields
for fieldIndex := 0 to table.FieldDefs.Count -1 do
begin
if (table.FieldDefs[fieldIndex].DataType = ftString) then
begin
canonical:= table.FindField(table.FieldDefs[fieldIndex].Name);
if assigned(canonical) and (table.FieldDefs[fieldIndex].Size <> canonical.Size) then
begin
// field size has changed
needsRestructure:= true;
table.FieldDefs[fieldIndex].Size:= canonical.Size;
end;
end;
end;

if needsRestructure then
table.AlterTable(); // upgrades table using the new FieldDef values
end;

procedure TdmodCheckDatabase.RunUpgradeScripts(newVersion: integer);
begin
case newVersion of
3: sqlVersion3.ExecSQL;
9: sqlVersion9.ExecSQL;
11: begin // change to DBISAM 4
sqlVersion11a.ExecSQL;
sqlVersion11b.ExecSQL;
sqlVersion11c.ExecSQL;
sqlVersion11d.ExecSQL;
sqlVersion11e.ExecSQL;
end;
19: sqlVersion19.ExecSQL;
20: sqlVersion20.ExecSQL;
end;
end;

Unit tests included:
  • Make sure the current version is correct
  • Make sure that every table and every field exists
  • Create a new blank database (for a number of different versions) and work though the upgrade process to make sure the final database is correct.
  • Restore an existing older database with data and upgrade to the latest version
With this process, altering the database structure is trivial for most changes. Adding fields and table usually requires no more work than updating the table components and generating a new creation script .

The current implementation does have a couple of restrictions in that it won't remove tables or fields. However if that is required, it won't take long to add.

4 comments:

Anonymous said...

You can try to use this component, altougth you pay. It compares two databases and syncronize a reference database (you can send with the update) with the work database, and automatically without sql updates.

Anonymous said...

http://www.clevercomponents.com/products/dbcvcl/dbcvcl.asp

Thomas Mueller said...

Does anybody know a tool for comparing two MS Access databases? It would be perfect if it could also extract the database structure to some kind of text format and vice versa, so I could actually add that text file to the SCM rather than that un-diff-able .mdb file.

Anonymous said...

CONTEXT DATABASE DESIGNER
http://www.contextsoft.com/products/dbdesign/

Simply rocks for this sort of thing and more.