I have multiple processes accessing the same database table. The table holds "TakenBy" column that is supposed to hold the ID of the taker process.
Entity Framework is my data access layer.
My question would be how can I use my DataContext object so I can retrieve rows from the above table, and have the "TakenBy" column updated at the same time.
This would allow me to overcome race-condition with the other processes, who also try to get the same records.
EF will not handle that for you. You must either use stored procedure or you must perform update once you load the record through your application and handle concurrency (either by optimistic way which means to use times tamp or row version column or by pessimistic way which means manual SQL query).
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It is my first time using EF Core and DDD concepts. Our database is Microsoft SQL Server. We use optimistic concurrency based on the RowVersion for user requests. This handles concurrent read and writes by users.
With the DDD paradigma user changes are not written directly to the database nor is the logic handled in database with a stored procedure. It is a three step process:
get aggregate from repository that pulls it from the database
update aggregate through domain commands that implement business logic
save aggregate back to repository that writes it to the database
The separation of read and write in the application logic can lead again to race conditions between parallel commands.
Since the time between read and write in the backend is normally fairly short, those race conditions can be handled with optimistic and also pessimistic locking.
To my understanding optimistic concurrency using RowVersion is sufficient for lost update problem, but not for write skew as is shown in Martin Kleppmann's book "Designing Data-Intensive Applications". This would require locking the read records.
To prevent write skew a common solution is to lock the records in step 1 with FOR UPDATE or in SQL Server with the hints UPDLOCK and HOLDLOCK.
EF Core does neither support FOR UPDATE nor SQL Server's WITH.
If I'm not able to lock records with EF Core does it mean there is no way to prevent write skew except using Raw SQL or Stored Procedures?
If I use RowVersion, I first check the RowVersion after getting the aggregate from the database. If it doesn't match I can fail fast. If it matches it is checked through EF Core in step 3 when updating the database. Is this pattern sufficient to eliminate all race conditions except write skew?
Since the write skew race condition occurs when read and write is on different records, it seems that there can always be a transaction added maybe later during development that makes a decision on a read. In a complex system I would not feel safe if it is not just simple CRUD access. Is there another solution when using EF Core to prevent write skew without locking records for update?
If you tell EF Core about the RowVersion attribute, it will use it in any update statement. BUT you have to be careful to preserve the RowVersion value from your data retrieval. The usual work pattern would retrieve the data, the user potentially edits the data, and then the user saves the data. When the user saves the data, you would normally have EF retrieve the entity, update the entity with the user's changes, and save the updates. EF uses the RowVersion in a Where clause to ensure nothing has changed since you read the data. This is the tricky part- you want to make sure the RowVersion is still the same as your initial data retrieval, not the second retrieval used to update the entity before saving.
I am working on my first project using an ORM (currently using Entiry Framework, although that's not set in stone) and am unsure what is the best practice when I need to add or subtract a given amount from a database field, when I am not interested in the new value and I know the field in question is frequently updated, so concurrency conflicts are a concern.
For example, in a retail system where I am recording a sale, as well as creating records for the sale and each of the line items, I need to update the quantity on hand of the items sold. It seems unnecessary to query the database for the existing quantity on hand, just so that I can populate the entity model before saving the updated quantity - and in the time taken for that round-trip, there is a chance that the same item will have been sold through another checkout or the website, so I either have a conflict or (if using a transaction) the other sale is blocked until I complete my update.
In SQL I would simply write
UPDATE Item SET Quantity=Quantity-1 WHERE ...
It seems the best option in this case is to fall back to ADO.NET + stored procedure for this one update, but is there a better way within Entity Framework?
You're right. ORMs are specialized in tracking changes to each individual entity, and applying those changes to the DB individually. Some ORMs support sending thechanges in btaches, but, even so, to modify all the records in a table implies reading them all, modifyng each one, and sending the changes back to the DB as individual UPDATEs.
And that's a big no-no! as you have corectly thought. It implies loading all the rows into memory, modifying all of them, track their changes, and send them back to the DB as indivudal updates, which is way more expensive that running a single UPDATE on the DB.
As to the final question, to run a SQL command you don't need to use traditional ADO.NET. You can run SQL queries directly from an EF DbContext using ExecuteSqlCommand like this:
MyDbContext.Database.ExecuteSqlCommand('Your SQL here!!');
I recommend you to look at the MSDN docs for Database class, to learn all the things that can be done, for example managing transactions, executing commands that return no data (as the previous example) or executing queries that return data, and even mapping them to entities (classes) in your model: SqlQuery().
So you can run SQL commands and queries without using a different technology.
I am looking into adding history tables to my database. The easiest way is to intercept all Insert, Update and Delete calls that EF Makes and add in a merge that will also insert a history row into a history table.
Right now all my Entities just let EF figure out how to do the inserts, updates and deletes.
If I go and add in stored procedures (instead of the EF Generated stuff) will EF still function the same on the business tier?
Or does it change how I have to work with my entities? If so, how?
Everything works the same, it is transparent.
Stored procedures need to return the rows affected, in order for EF to know that the update succeeded or not. Additionally, if you do an update and need to map any property back to your entity (e.g. timestamps) you must select them in the sproc and then map them back in the EF designer (since you can only have one output parameter, and that should be the rows affected).
You might consider using triggers on the DB to solve your issue, though?
Doing this in stored procedures means that you will write all inserts, updates and deletes yourselves. It is like throwing 30% of feature set (and 50% productivity) away. Create audit records in your application and save them together with main records through EF.
I have multiple string values, I want to insert in an sql server db table, But i want to check values one by one if it already exist in the db I will update, if not I will insert it.
I am using Entity Framework 4.1, and I hope I can do that with best performance, means less calls to db as I can.
I saw this question before, but they are using linq to sql not entity framework.
One way you could do it is to batch up the queries for existence ... for example, using the .Contains method (like this), you can query for some or all of the items which may or may not exist at once. Then once you have the data locally, you can quickly check if it's there before inserting
We have an application that creates new tables at runtime, but always with the same table schema. The only thing that varies from one of these tables to the next is the table name. Is it possible to access these tables using Entity Framework, specifying which table to access by name?
Entity Framework is not designed for DDL, it's an ORM tool for data access. You would want to use a simple ADO.NET query to create/drop the table.
Creating and dropping tables for every user session will make your log file grow very big very fast. I would consider carefully the reasons you think this is necessary. If the data is temporary, why not save the Session ID in each row and truncate the table on a daily basis?
UPDATE:
No, not really. The Entity Data Model is not dynamic, it's a static XML document that describes the structure of the database. If you want to interact with a table with a dynamic name, you're going to have to stick to "classic" ADO.NET.
With Linq to SQL I guess it would be possible with a stored procedure taking the table Name as a parameter.
A nice post about SP in L2SQL: http://weblogs.asp.net/scottgu/archive/2007/08/16/linq-to-sql-part-6-retrieving-data-using-stored-procedures.aspx
I don't know if that feature exists in EF.