Wednesday, 24 September 2014

Working with Entity Framework Code First and SQL Bulk Copy

There's a few of these that I haven't written, but it seems that you could keep a blog going pretty well with just "Working with Entity Framework and ..." posts. That's not because Entity Framework is particularly bad, I really quite like it myself, but because if you're writing an application of any considerable size or complexity you will at some point reach the limitations of what is possible "out of the box". I'm also aware of the number of "ORMs are evil" posts circulating currently, and possibly you're someone who thinks the same, but for what I'm working on now they make perfect sense, and I'm all for using the right tool for the job.

So what's the problem this time?

General purpose ORMs are great when you're working with transactions, it's what they're meant for.  A user interacts with the system and you need to persist the data from their most recent transaction.  The problem this time is that as the system grows you suddenly find yourself with a couple of use cases where you need to persist a lot of data.  In the example application I've put together for this post I'm using an XML data source with 10,000 records which need to be persisted.

The problem here is that when running with this size data set (with auto-tracking changes disabled) is that it is taking around 40 seconds to run.  10,000 records in 40 seconds is certainly more that I can process manually but for a modern computer it's not so great.  The problem is that as you're adding more records to the context it's getting bloated, it has to start tracking more and more entities, then each entity is persisted individually.  That last point is important because each insert in Entity Framework inserts the new record and then pulls back out the newly created ID code and updates the entity in memory with the new ID code, which is not a trivial amount of work.

So what are the solutions?

Disable features: The first thing to check is that you are telling the context to not auto-track changes, it's a small thing but you're giving the context less work to do, performance isn't about making your code faster, it's about making it do less.

If you were to run this again you would find that you've taken a few seconds of the total run time, which is better but it's still no where near fast enough.

Disabling auto-detect changes

Commit early, commit often: A fairly obvious option but rather than waiting until we've attached all of the entities to the context before saving, save more frequently (e.g. every 100 entities).  Again this is reducing the amount of work the context is having to perform when it figures out which entities it needs to persist and makes a more significant impact in our figures, but it's still got a way to go.

You might also remember that I mentioned about the context getting bloated, well we can do something about that as well by re-creating the context after each time we save the changes.  This stops the context from getting too bloated and again reduces the amount of effort needed to work out which entities need to be persisted.  We've added in some work now for context initialisation but this is typically cheaper.  This does take a bit more effort to maintain and ensure that we're not breaking the system by doing anything stupid, but it again takes a bit more of a chunk out of the run time.

Committing frequently

Get to the DbSet differently: The typical route to adding an entity is to add it to the contexts DbSet collection.  Bizarrely this collection doesn't have an AddRange method, but there is a way to get at one by asking the context for the set directly.  By adding the entities using the AddRange method we can skip all of the tedious foreach looping and adding the entities one at a time.  So we can now make a simple call to AddRange followed by a call to SaveChanges, this is much more performant than the previous solutions, getting down to some slightly more reasonable numbers.

Using AddRange

But what about SQL Bulk Copy?

So the title of the post is about bulk copying, and having read through the above you're probably wondering why I didn't just jump to this as a solution.  Well, Entity Framework has no out of the box support for SQL Bulk Copy because it's database agnostic.  But I've done this before when working with SQL FILESTREAM so why can't I do it again?

Being a lazy developer the first thing I did was look for an existing solution and one turned up in the shape of EntityFramework.BulkInsert.  It seems pretty popular online for this kind of a problem, is available through NuGet and is pretty easy to use.  After adding the package and creating a method to try it out I ran the sample application and waited for it to finish.  It took me a while before I realised that it already had!  For 10,000 records it ran in under 1 second.

Using EntityFramework.BulkInsert

So surely EntityFramework.BulkInsert is the answer then?  Well if you want to stop reading here and go and download it then please do, it's a great little package.  Naturally there are a few things that you need to take into consideration.  First of all bulk copying doesn't bring the ID codes back, so if you need the values you will have to think of a way around this (think SQL 2012 sequences and sp_sequence_get_range).  Next you have to think about how bulk copying works and make sure you get the bulk copy options correct.  By default it won't check any constraints you have in place and it might not observe NULL values, instead putting in default values for the column type.  It also works within its own transaction (unless you provide a TransactionScope), but if you can work around these then you have a great little solution in your hands.

SQL Bulk Copy

I'm a lazy developer but I'm also a fascinated one, I wanted to know if I could still write the code using the System.Data.SqlClient.SqlBulkCopy class instead of relying on 3rd party packages or falling back to ADO.NET (which is an option, but not one I'm going to cover).

I already know that I can get the connection information from the context, and I've previously shown how to get the mapped table name for a given entity, so surely this is possible.  But I am going to be a little bit lazy and not implement an IDataReader for my collection, instead I'm going to load the entities into a DataTable and use that (note, this option really isn't going to scale well).

This is actually a fairly easy solution to implement with probably the most complicated piece being a fairly simple extension method which pulls out the entity properties and their types, then using this to create a DataTable and copy the data using reflection (again, this isn't going to scale well).  Once you have that you just need to write the data to the database for your chosen batch size.

Using System.Data.SqlClient.SqlBulkCopy

This solution isn't quite as fast as the EntityFramework.BulkInsert component, mostly for the reasons I mention, but it can still persist 100,000 records in about 1 second.

I've created a project which is available on GitHub under an MIT license for you to grab and look at.  I've done this because the code isn't really that difficult to follow and is pretty similar to my previous post on SQL FILESTREAM and me talking through lines of code is boring.  Also available is a LinqPad file which I used to create the input data files, just change the number of entities and run it.  But for convenience I've added a 1,000 and 10,000 entity files into the project anyway.

Tuesday, 8 July 2014

Where does your FILESTREAM data live?

This is hopefully just a short one but follows up on a previous post about using FILESTREAM with Entity Framework.

After implementing the solution almost as posted we ran into some problems on the environment it was being deployed to (for reference, do NOT disable netBIOS!).  Whilst investigating these issues we needed to figure out where on the server the FILESTREAM data was being stored.  Looking around the internet I found many posts about getting the path name from a SELECT statement but that's useless if you want to know "my data is at C:\<path>" because you want to check disk space, permissions etc...  Not having the DBA who installed available wasn't useful either and there doesn't appear to be a way to get this information back from SQL Server Management Studio.

But, there is a way to find it from the system tables.  So I put together a SQL statement which pulls the information out.  It worked for me but you may want to tweak it to give you the information you want, to filter stuff out and so on.

Thursday, 30 January 2014

Working with Entity Framework Code First and SQL FILESTREAM

Whilst looking around at these two technologies I couldn't find any information which was particularly useful about how to get the solution working.  So I wanted to put this up so that should anyone else want to try something similar then there's some (hopefully) useful information out there.

What am I trying to do?

The system that I am involved with at the moment has the need to store files with some of it's entities.  We've started out by using Entity Framework 5 Code First (although in this post I'll be working with Entity Framework 6) to create the data model and database mappings which is working nicely and is proving itself a very useful framework.

When looking at saving file data along with an entity there are a few choices:
  1. Store the file contents directly in the database
  2. Save the file contents to the file system
  3. Use a database table with FILESTREAM enabled
  4. Use a FILETABLE
The first option is fine for small files, but I don't really want to load all of the file data each time I query the table and the files might get pretty big.

The next option is a reasonable option and with recent versions of Windows Server we have a transactional file system.  So far so good, but it would be nice to not have to think about two persistence mechanisms.

FILESTREAMs were introduced in SQL Server 2008 and allow you to store unstructured data on the file system, so it feels like we're using the right tools for the job but they're all nicely in the same package.  The problem here is that Entity Framework doesn't support FILESTREAM.

Lastly there's FILETABLE which was introduced with SQL Server 2012.  This is like FILESTREAM, but rather than defining it at a column level you get a table created for you which provides information from the file system.  It is a really nice system, but it didn't quite fit with how we're structuring data and it's also not supported by Entity Framework.

So the option that I would ideally like to work with here is the FILESTREAM option as it gives me all of the database goodness but with the performance of the file system.  But there is just that minor sticking point of it not being supported by Entity Framework.  After a fair amount of playing around with the technology and research into the problem I figured that I could probably make it work by falling back to basic ADO.NET for handling the FILESTREAM part of the requests.  Whilst this was an option I didn't really want to start having different technology choices for doing database work, so the goal was to see how much I could get away with in Entity Framework.

Setting up the test solution

The database server

With SQL Server the default install options will not give you a FILESTREAM enabled server but you can enable it.  I'm not going to go into how with this post as Microsoft have some pretty good documentation available on how to do this.

This also means that we can't let Entity Framework create the database for us, so you will need to create an empty, FILESTREAM enabled database and point to that.

The outline of the project

I created the solution in Visual Studio 2013, and delving into the most creative parts of my mind I came up with a solution that has hotels, with rooms and multiple pictures of each room (okay, not my most creative moment, but it gets the job done).

So in this solution I have my data model which is pretty simple.  I have some locations, at each location there are a number of hotels, each hotel has rooms and each room has photos.

The Location, Hotel, Room and Photot entities
Solution Data Model
Of all of these the import one is Photo.  This entity has some basic properties, Title and Description, which describe the photo, then there's the navigation properties for getting back to the room and then lastly there's the Data property which is intended to hold the content of the file.  Normally Entity Framework would see this property and it's type (a byte array) and map it to an appropriately named column of type VARBINARY(max).  Whilst we could still let it do this, it would somewhat defeat the purpose of the exercise as we'd be storing the contents of the file directly in the database, so we need to add some configuration to tell Entity Framework to ignore this property when mapping.

Photo configuration information
Photo entity configuration
I'm using the Fluent API here, but you should be able to do this using Data Annotations as well.

At this point if we were to deploy the database we would get a table with no data information and a blank property in our entity.  What we need to do next before any of this is useful is to somehow get a FILESTREAM column into the Photo table.  The solution to this is to use Entity Framework migrations, the basics of which I'll not cover here and leave it as an exercise to the reader.

Migrations provides us with a migration class for each migration added to uplift and roll-back the changes to the database.  The useful method for us in this class is the Sql method which allows us to execute SQL commands; using this we can add our ROWGUID column and our FILESTREAM column with all the constraints we need (and of course the appropriate commands to remove it all again as well for the Down method).

Migrations code
Migrations SQL commands
Now if we run the Update-Database command from the Package Manager Console we get a table with all the right columns of the right types for being able to use FILESTREAM.

So that's half the battle won, the next challenge is being able to read to and write from the table.

Storing and retrieving file data

So how do we query data in a FILESTREAM column?  Well this is the bit where we fall back to the System.Data.SqlTypes namespace, specifically the SqlFileStream class.  We use this class to read the contents of the file back from the server as a stream, but this only works in the context of a SQL transaction.

So the first thing we need to do is get the file path and the SQL transaction information, we can then pass this to the SqlFileStream constructor to get our stream, after which it's just a case of reading from the byte array in our entity and writing to the SqlFileStream stream.  To get this information we need to run a custom SQL statement.  We could do this using a SqlCommand object, but I still want to stick to Entity Framework a bit more, fortunately there's the DbContext.Database.SqlQuery<TElement> class which we can use to run raw SQL statements, it also handles parameters so we can parametrize the query (great for guarding against SQL injection attacks) and it an enumerable collection mapped to TElement (which does not have to be a part of our data model).
Raw Data Query
Raw Data Query
The FileStreamRowData class here is a custom class with a string property for the path, and a byte array for the transaction context.

Running all of this inside of a transaction scope will get information required (the call to "First" will enumerate the collection) to pass to the SqlFileStream constructor, we can then use this to write data to the stream.
Writing to the FILESTREAM
Writing to the FILESTREAM
The same applies when writing to the database as well, but with the source and destination reversed.  Also when writing to the database you would need to save the entity first.  Wrapping up the Entity Framework bit in the same transaction scope means that even if you call "SaveChanges" on your context, if the transaction does not successfully complete then the changes are stilled rolled back.

So does it work?

Well, yes it does, and it works pretty nicely as well.  It's maybe not the final solution that I'll use as I'm still investigating a couple of other options, but it's certainly not a solution that I would be upset at using, and by hiding the complexity in the data services the client need never know how the file information is being held in the database or using which technologies.

How do I play with it?

You could probably work most of what you need out from this post, but for convenience sake I've also put up the whole solution onto GitHub, so feel free to head over and take a look.  If you come up with any suggestions or improvements then feel free to contribute.