Explain what is meant by a database that stores all data in system RAM.

An in-memory database, which stores all data in RAM, is a database management system that saves data exclusively in RAM rather than on disk. This provides very high speed of data access and processing, as reading and writing to memory is significantly faster than to a hard disk.

Such databases are often used where performance and low latency are critical, for example, in caches, real-time systems, or analytics.

An example of an in-memory database in C# could be using collections or specialized solutions, such as Redis (which can be used as an in-memory storage). Built-in DBMSs like SQLite usually work with disk, but there are also in-memory modes.

Example of creating an in-memory database using Entity Framework Core:

var options = new DbContextOptionsBuilder<MyDbContext>()
    .UseInMemoryDatabase(databaseName: "TestDb")
    .Options;

using var context = new MyDbContext(options);
// Now all data will be stored in memory, not on disk