Running simultaneous queries on different DbContext instances leads to `NpgsqlOperationInProgressException` about efcore.pg

[roji]

@jhinder yes, as long as different DbContext instances are being used, you shouldn't get this exception. However, I can't really tell you what you're doing specifically in your code that triggers this - definitely not from the two snippets posted above (and no stack trace); it's always a good idea to post a fully runnable, minimal project or code sample, rather than a partial set of snippets - there's very frequently something hiding in unposted code that explains the problem. I'd take a very careful look at your DbContext usages - of course as a function of the exception stack trace - and try to see where concurrent usage may be happening.

[jhinder]

OK, so I've continued debugging this a bit and I guess I solved it.

Previously, we would export a NpgsqlConnection (initialized with our connection string) instance with a service type of DbConnection and a lifetime of Singleton. The optionsAction of AddDbContext would retrieve that instance from the supplied IServiceProvider and pass it as the first argument to UseNpgsql.

I've now simplified that a bit so that the optionsAction now only retrieves the connection string (via a simple custom type called ConnectionStringProvider whose only method returns the connection string, and that type is exported into the DI as a singleton because it needs IConfiguration) and passes that to UseNpgsql.

I guess the former scenario (multiple scoped services operate on a singleton DbConnection) is unsupported because Npgsql does something in the way of connection management?

Anyway, I can now throw a hundred parallel queries against the database with the code above and I can run the infinite request loop as well, both seem to work just fine.

[roji]

I guess the former scenario (multiple scoped services operate on a singleton DbConnection) is unsupported because Npgsql does something in the way of connection management?

Well, a DbConnection represents an actual single physical connection to the database - it's very much not usable concurrently from multiple threads (since the same TCP connection can't be used simultaneously...). I'm not sure what the idea was behind having a single DbConnection previously, but it's definitely not a typical or recommended way to do things, and would indeed lead to NpgsqlOperationInProgressException very quickly.

BTW as an alternative to passing in the connection string, you may want to look at NpgsqlDataSource (see the Npgsql docs).

I'll go ahead and close this as everything seems clear and there doesn't seem to be an issue with Npgsql.