Question: What is the most elegant way to reuse code generated by SubSonic for tables that share the same schema, in different databases.

  • Ideally, I would have a shared schema definition, generated once, and seamlessly integrated into the code generated for each separate provider.
  • Creating a separate DataProvider for a subset of tables reduces the amount of code that is generated, but is not very convenient to use if you do not use the same namespace for all your projects.
  • Creating a separate DataProvider does not solve the problem of database selection at runtime.

Multiple Databases, Same SchemaLavaBlast's integrated solution for franchise management solution operates on a centralized database and a data warehouse which collects data from all our points of sale. Recently, we decided we wanted to create some management pages for our various e-commerce websites in our centralized portal. Because our recently developed e-commerce backend is the same as our point of sale (reuse++), we automatically obtained features like centralized product line and pricing management for our store fleet (featureSynergy++). However, we wanted to be able to process website users and orders from this same central portal, not on each individual site.

My first question was how do we get the union of the data from the same table in multiple databases? One solution would be to merge these into the data warehouse, but we didn't want to go through complex infrastructure to bring the data into the warehouse and push the changes back out when necessary. I suppose having everything in the same database in the first place would be a solution, but it is not how we architecture our systems. SQL Server Replication might be useful, but it is not bidirectional with SQL Server Express. I can easily write a view that would be a UNION query that would merge the data from the set of databases, but that would be a maintenance problem. For each table, I would have to hardcode the list of databases.

I wrote a quick stored procedure that builds the UNION query from a table of Website to DatabaseName mappings, given a few parameters. It is inefficient and is not strongly-typed (hence it feels dirty) but given the volume of data on these sites, it is good enough for now without being a maintenance pain. Passing in a in a few parameters to the stored procedure, we can filter the rows before the union, we can improve performance. I am curious to know if there are more elegant solutions to this problem.

Anyhow, with this first problem solved, we could bind our GridView to a DataTable produced by the execution of a StoredProcedure and see the merged results. However, because we have a standard infrastructure that makes good use of SubSonic magic for filtering, paging, and sorting, this was not enough. Our infrastructure only works on views or tables in our central database, not on arbitrary results returned by stored procedures. Therefore, SubSonic did not generate any code for the merged tables, in the central database. Still, thanks to the SubSonic Provider model, we managed to load a collection based on the type defined in one DataProvider (point of sale) using data provided by the stored procedure, in another DataProvider (central server). Below, an example without any filtering, sorting or paging.

SubSonic.StoredProcedure sp = SPs.WebsiteUnionOfTables(POSBOLib.Generated.ShoppingCart.ViewWebUser.Schema.TableName, "*", string.Empty, string.Empty);
POSBOLib.Generated.ShoppingCart.ViewWebUserCollection coll = new POSBOLib.Generated.ShoppingCart.ViewWebUserCollection();
coll.LoadAndCloseReader(sp.GetReader());

With a bit more work on the stored procedure, we can make it efficient, but we don't want to use T-SQL all that much, to make everything easier to maintain. (We could use CLR stored procedures, but that's another story).

My second question was how am I going to update this data? When manipulating the data, I know from which database it comes from thanks to an additional column appended by my stored procedure, but I cannot create an updatable SubSonic object with this, and I don't feel like writing SQL anymore, now that we use SubSonic. However, the DataProvider name is a hardcoded string in the auto-generated code… and changing the templates to pass in extra parameters looks like too much work in addition to breaking the simplicity of the framework.

Having played with the DataProvider model, one idea that came to me was to switch the provider context dynamically at runtime. The framework doesn't support this, so I had to hack it in and make sure all my data access was contained in critical sections (using the lock keyword) which begin with an invocation of the following method.

Another option, which just came to me now, would be to obtain the SQL generated by SubSonic during an operation and perform string replacements to forward the requests to the appropriate database. This too is too much of a hack, however, since it depends on the implementation details and the DBMS.

In conclusion, I did manage to build a working prototype using locks and the above code, but I feel the code is too dirty and I am open to suggestions from SubSonic experts (I'm looking at you Rob Conery and Eric Kemp). If there is a clean way to do it, I would love to contribute it to the SubSonic project!

Read Part 2.