XQuery is a powerful language. Add MarkLogic-specific extension functions and constructs and there is very little you can’t do all in XQuery.
But, of course, sometimes you do find things you can’t accomplish–like generating a GUID value. Or let’s say you have a lot of business logic already implemented in a programming language like Java or C#. Why recreate all that work in XQuery?
Most XQuery developers working in MarkLogic know about MLJAM, a code library that lets you instantiate and use Java classes from XQuery. It’s a great way to access functionality that XQuery either can’t support or would take too long to re-implement.
But what about those of us working in a Microsoft environment? In my old organization, I had many .NET assemblies already written to handle complex business processes (especially getting information out of and back into SQL Server databases and pre-MarkLogic Content Management Systems). It would be great if I could access all that functionality.
What I really need is a .NET version of MLJAM—an “MLNET”, as it were. The problem is that MLJAM is built on top of an open source project named “BeanShell,” which provides a scripting environment for evaluating Java. The .NET framework does not have a comparable scripting engine—it requires compiled code, which XQuery can’t provide.
After thinking about the problem, however, I realized that .NET has a set of classes that are perfectly capable of dynamically compiling source code. Indeed, after a little more research, I also learned that there is a .NET scripting language called “PowerShell,” as well as a set of classes that can provide a run-time environment for PowerShell scripts.
This past weekend, I decided to see if I could build a simple version of MLNET that would take arbitrary C# code, compile it, and run commands. I was pleasantly surprised to find that it was not nearly as difficult as I thought.
My simple version of MLNET consists of a set of XQuery functions that send C# code to a web service. The service uses PowerShell to compile and execute the code, then sends back the results. As this was only a test, my version of MLNET is limited in scope. It doesn’t have any real security or error handling; and while it can evaluate any C# code, it can only return strings as results.
For those interested in the technical details, I’ve provided source code and set-up instructions with this post.
But what can MLNET do? Let’s take that hypothetical case of needing to generate a GUID. The .NET framework has a dedicated class that generates these identifiers, which I can now invoke like this:
I can also call existing custom classes with MLNET. For example, let’s say that I have a Data Access Layer class already defined that saves a new username and password to a database. The class is named “Acme.DAL.UserManager” and is compiled inside an assembly named AcmeDAL.dll. I can invoke it like this:
Note that in this example, I pass the username and password values in to the net:execute() function. In other words, I don’t need to know any values ahead of time–the XQuery can supply the necessary information when the code is run.
And of course, I can also use classes that I declare directly in XQuery. For example, I can write and instantiate my own class to calculate an MD5 hash:
MLNet fills an important gap that was preventing me from developing all my applications in pure XQuery. Or at least, it will once I get the code in better shape!
Unfortunately, the current version of MLNET is just a proof-of-concept. It can evaluate the code shown in these examples, but it is not efficient and doesn’t provide a rich set of features, like support for passing in and returning arrays of values, or handling any data type other than a string.
Nevertheless, I don’t think it would be very difficult to write a true, production-ready version of MLNET. Indeed, I encourage anyone who’s interested to download and improve the attached code. I know I was planning to keep working on the project.
(Note: I’m sharing the code through Google Docs, so the link will take you to a page displaying everything inside the zip. Select File > Download to get the zip file itself.)