Entity Framework One Database Multiple DbContexts Is this a bad idea

Question

My impression to date has been that a DbContext is meant to represent your database  and thus  if your application uses one database  you d want only one DbContext   However  some colleagues want to break functional areas out into separate DbContext classes   I believe this comes from a good place -- a desire to keep the code cleaner -- but it seems volatile  My gut s telling me it s a bad idea  but unfortunately  my gut feeling is not a sufficient condition for a design decision   So I m looking for   A  concrete examples of why this might be a bad idea   B  assurances that this will all work out just fine

User · Answer

I wrote this answer about four years ago and my opinion hasn't changed. But since then there have been significant developments on the micro-services front. I added micro-services specific notes at the end...

I'll weigh in against the idea, with real-world experience to back up my vote.

I was brought on to a large application that had five contexts for a single database. In the end, we ended up removing all of the contexts except for one - reverting back to a single context.

At first the idea of multiple contexts seems like a good idea. We can separate our data access into domains and provide several clean lightweight contexts. Sounds like DDD, right? This would simplify our data access. Another argument is for performance in that we only access the context that we need.

But in practice, as our application grew, many of our tables shared relationships across our various contexts. For example, queries to table A in context 1 also required joining table B in context 2.

This left us with a couple poor choices. We could duplicate the tables in the various contexts. We tried this. This created several mapping problems including an EF constraint that requires each entity to have a unique name. So we ended up with entities named Person1 and Person2 in the different contexts. One could argue this was poor design on our part, but despite our best efforts, this is how our application actually grew in the real world.

We also tried querying both contexts to get the data we needed. For example, our business logic would query half of what it needed from context 1 and the other half from context 2. This had some major issues. Instead of performing one query against a single context, we had to perform multiple queries across different contexts. This has a real performance penalty.

In the end, the good news is that it was easy to strip out the multiple contexts. The context is intended to be a lightweight object. So I don't think performance is a good argument for multiple contexts. In almost all cases, I believe a single context is simpler, less complex, and will likely perform better, and you won't have to implement a bunch of work-arounds to get it to work.

I thought of one situation where multiple contexts could be useful. A separate context could be used to fix a physical issue with the database in which it actually contains more than one domain. Ideally, a context would be one-to-one to a domain, which would be one-to-one to a database. In other words, if a set of tables are in no way related to the other tables in a given database, they should probably be pulled out into a separate database. I realize this isn't always practical. But if a set of tables are so different that you would feel comfortable separating them into a separate database (but you choose not to) then I could see the case for using a separate context, but only because there are actually two separate domains.

Regarding micro-services, one single context still makes sense. However, for micro-services, each service would have its own context which includes only the database tables relevant to that service. In other words, if service x accesses tables 1 and 2, and service y accesses tables 3 and 4, each service would have its own unique context which includes tables specific to that service.

I'm interested in your thoughts.

User · Answer

Simple example to achieve the below       ApplicationDbContext forumDB   new ApplicationDbContext        MonitorDbContext monitor   new MonitorDbContext      Just scope the properties in the main context   used to create and maintain the DB  Note  Just use protected   Entity is not exposed here   public class ApplicationDbContext   IdentityDbContext lt ApplicationUser gt        public ApplicationDbContext             base  QAForum   throwIfV1Schema  false                   protected DbSet lt Diagnostic gt  Diagnostics   get  set        public DbSet lt Forum gt  Forums   get  set        public DbSet lt Post gt  Posts   get  set        public DbSet lt Thread gt  Threads   get  set        public static ApplicationDbContext Create                 return new ApplicationDbContext               protected override void OnModelCreating DbModelBuilder modelBuilder                base OnModelCreating modelBuilder             MonitorContext  Expose separate Entity here  public class MonitorDbContext  DbContext       public MonitorDbContext             base  QAForum                    public DbSet lt Diagnostic gt  Diagnostics   get  set           add more here     Diagnostics Model   public class Diagnostic        Key      public Guid DiagnosticID   get  set        public string ApplicationName   get  set        public DateTime DiagnosticTime   get  set        public string Data   get  set        If you like you could mark all entities as protected inside the main ApplicationDbContext  then create additional contexts as needed for each separation of schemas   They all use the same connection string  however they use separate connections  so do not cross transactions and be aware of locking issues  Generally your designing separation so this shouldn t happen anyway

User · Answer

Another bit of  wisdom   I have a database facing both  the internet and an internal app  I have a context for each face  That helps me to keep a disciplined  secured segregation

User · Answer

In code first  you can have multiple DBContext and just one database  You just have to specify the connection string in the constructor   public class MovieDBContext   DbContext       public MovieDBContext             base  DefaultConnection                    public DbSet lt Movie gt  Movies   get  set

User · Answer

Huh  spent quite a lot of time on a problem with separate DB contexts for each DB schema  hope that it will help somebody else     I recently started working on a project that had one database with 3 schemas  DB first approach   one of them for user management  There was a DB context scaffolded from each separate schema  Of course  users were related to other schemas as well  eg  schema KB had a table Topic  which had  created by    last modified by  etc  FK to identity schema  table appuser   These objects were loaded separately in C   firstly  topic was loaded from 1 context  then users were loaded via user IDs from the other db context - not nice  have to fix this   similar to using multiple dbcontexts in the same database with EF 6   First  I tried to add missing FK instructions from identity schema to KB schema  to EF modelBuilder in KB DB context  The same as if there was only 1 context  but I separated it to 2   modelBuilder Entity lt Topic gt  entity   gt      entity HasOne d   gt  d Creator       WithMany p   gt  p TopicCreator       HasForeignKey d   gt  d CreatorId       HasConstraintName  fk topic app users      It didnt work  because kb db context didn t have any information about the user object  postgres returned error relation  AppUsers  does not exist  Select statement didnt have proper information about schema  field names etc   I almost gave up  but then i noticed a switch  -d  when running dotnet ef dbcontext scaffold  Its short for -data-annotations - Use attributes to configure the model  where possible   If omitted  only the fluent API is used  With this switch specified  object properties were defined not in db context OnModelCreating    but rather on the object itself  with attributes   This way  EF got sufficient information for generating a proper SQL statement with proper field names and schemas   TL DR  separate DB contexts don t handle relations  FKs  between them well  each context only has information about its own entities  When specifying  -data-annotations  switch on dotnet ef dbcontext scaffold  these informations arent stored in each separate context  but on DB objects themselves

User · Answer

I want to share a case  where I think the possibility of having multiple DBContexts in the same database makes good sense    I have a solution with two database  One is for domain data except user information  The other is solely for user information  This division is primarily driven by the EU General Data Protection Regulation  By having two databases  I can freely move the domain data around  e g  from Azure to my development environment  as long as the user data stays in one secure place    Now for the user database I have implemented two schemas through EF  One is the default one provided by the AspNet Identity framework  The other is our own implementing anything else user related  I prefer this solution over extending the ApsNet schema  because I can easily handle future changes to AspNet Identity and at the same time the separation makes it clear to the programmers  that  our own user information  goes in the specific user schema we have defined

User · Answer

Reminder  If you do combine multiple contexts make sure you cut n paste all the functionality in your various RealContexts OnModelCreating   into your single CombinedContext OnModelCreating     I just wasted time hunting down why my cascade delete relationships weren t being preserved only to discover that I hadn t ported the modelBuilder Entity lt T gt       WillCascadeOnDelete    code from my real context into my combined context

User · Answer

Distinguishing contexts by setting the default schema  In EF6 you can have multiple contexts  just specify the name for the default database schema in the OnModelCreating method of you DbContext derived class  where the Fluent-API configuration is   This will work in EF6   public partial class CustomerModel   DbContext          protected override void OnModelCreating DbModelBuilder modelBuilder                modelBuilder HasDefaultSchema  Customer                Fluent API configuration              This example will use  Customer  as prefix for your database tables  instead of  dbo    More importantly it will also prefix the   MigrationHistory table s   e g  Customer   MigrationHistory  So you can have more than one   MigrationHistory table in a single database  one for each context  So the changes you make for one context will not mess with the other   When adding the migration  specify the fully qualified name of your configuration class  derived from DbMigrationsConfiguration  as parameter in the add-migration command   add-migration NAME OF MIGRATION -ConfigurationTypeName FULLY QUALIFIED NAME OF CONFIGURATION CLASS     A short word on the context key  According to this MSDN article  Chapter - Multiple Models Targeting the Same Database  EF 6 would probably handle the situation even if only one MigrationHistory table existed  because in the table there is a ContextKey column to distinguish the migrations   However I prefer having more than one MigrationHistory table by specifying the default schema like explained above     Using separate migration folders  In such a scenario you might also want to work with different  Migration  folders in you project  You can set up your DbMigrationsConfiguration derived class accordingly using the MigrationsDirectory property   internal sealed class ConfigurationA   DbMigrationsConfiguration lt ModelA gt        public ConfigurationA                 AutomaticMigrationsEnabled   false          MigrationsDirectory     Migrations ModelA            internal sealed class ConfigurationB   DbMigrationsConfiguration lt ModelB gt        public ConfigurationB                 AutomaticMigrationsEnabled   false          MigrationsDirectory     Migrations ModelB               Summary  All in all  you can say that everything is cleanly separated  Contexts  Migration folders in the project and tables in the database   I would choose such a solution  if there are groups of entities which are part of a bigger topic  but are not related  via foreign keys  to one another   If the groups of entities do not have anything to do which each other  I would created a separate database for each of them and also access them in different projects  probably with one single context in each project

User · Answer

My gut told me the same thing when I came across this design  I am working on a code base where there are three dbContexts to one database   2 out of the 3 dbcontexts are dependent on information from 1 dbcontext because it serves up the administrative data  This design has placed constraints on how you can query your data   I ran into this problem where you cannot join across dbcontexts   Instead what you are required to do is query the two separate dbcontexts then do a join in memory or iterate through both to get the combination of the two as a result set  The problem with that is instead of querying for a specific result set you are now loading all your records into memory and then doing a join against the two result sets in memory  It can really slow things down   I would ask the question  just because you can  should you    See this article for the problem I came across related to this design  The specified LINQ expression contains references to queries that are associated with different contexts

User · Answer

You can have multiple contexts for single database  It can be useful for example if your database contains multiple database schemas and you want to handle each of them as separate self contained area    The problem is when you want to use code first to create your database - only single context in your application can do that  The trick for this is usually one additional context containing all your entities which is used only for database creation  Your real application contexts containing only subsets of your entities must have database initializer set to null   There are other issues you will see when using multiple context types - for example shared entity types and their passing from one context to another  etc  Generally it is possible  it can make your design much cleaner and separate different functional areas but it has its costs in additional complexity

User · Answer

Inspired by   JulieLerman  s DDD MSDN Mag Article 2013  1       public class ShippingContext   BaseContext lt ShippingContext gt       public DbSet lt Shipment gt  Shipments   get  set       public DbSet lt Shipper gt  Shippers   get  set       public DbSet lt OrderShippingDetail gt  Order   get  set      Orders table    public DbSet lt ItemToBeShipped gt  ItemsToBeShipped   get  set       protected override void OnModelCreating DbModelBuilder modelBuilder              modelBuilder Ignore lt LineItem gt            modelBuilder Ignore lt Order gt            modelBuilder Configurations Add new ShippingAddressMap                public class BaseContext lt TContext gt    DbContext where TContext   DbContext     static BaseContext           Database SetInitializer lt TContext gt  null         protected BaseContext     base  DPSalesDatabase                   If you   re doing new development and you want to let Code First create or migrate your database based on your classes  you   ll need to create an    uber-model    using a DbContext that includes all of the classes and relationships needed to build a complete model that represents the database  However  this context must not inherit from BaseContext   JL

[entity-framework] Entity Framework: One Database, Multiple DbContexts. Is this a bad idea?

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