Is AddDbContextPool supposed to release contexts from the pool?

[Steve887]

Question

I have a large context model, so I'm experimenting with context pooling.

My current setup is:

service.AddDbContextPool<AppDbContext>((provider, optionsBuilder) =>
{
    AppDbContextOptionsFactory.Get(optionsBuilder);
});

When I run my app, everything seems to work ok, except the active_dbcontexts seems to indicate the pooled dbcontexts are created but never released. Is this the expected behaviour for this?

From debugging with EFCore symbols, it appears that AddDbContextPool uses a ScopedDbContextFactory that does not allow the lease to be released because IsStandalone is set to false.

EF Core version

9.0.0

Database provider

SqlServer

Target framework

.Net 9

Operating system

Windows 10

IDE

vs 2022

[roji]

@Steve887 the point of context pooling is to keep unused contexts in the pool, so they can be reused later. There's no mechanism for pruning unused contexts after a while, so if you e.g. have a load spike which allocates 100 contexts, these will afterwards sit idle in the pool without getting removed. This is intentional - unlike e.g. physical database connections, which can consume considerable resources on the databases side and therefore need to be pruned, contexts just consume a bit of memory - nothing more - and keeping them around has very little impact.

Having said that, IIRC the intention behind the active_dbcontexts telemetry was to track non-idle instances, i.e, how many are at this moment in-use, out of the pool; you're saying that this isn't the behavior you're seeing?

[Steve887]

@roji that makes sense from a design viewpoint, I suppose trading a small amount of memory for better performance makes sense.

I was expecting the metric to go down once the request was finished, however. If I run a test with 10 simultaneous requests, it goes up to 10 and stays there forever. They are reused, so running the request again doesn't increase it further. I just wanted to make sure that's expected or if there's an issue with my setup.

[roji]

@cincuranet assigning to you as you worked in the metrics area - I think active_dbcontext is indeed supposed to represent active contexts in use by the program, and should not take into account idle contexts stored in the pool. Let's discuss if anything's unclear.