Ensure SQLite in-memory database is persisted for the whole application lifetime.

appsettings.json

{
  "ConnectionStrings": {
    "umbracoDbDSN": "Data Source=Umbraco;Mode=Memory;Cache=Shared;Foreign Keys=True;Pooling=True",
    "umbracoDbDSN_ProviderName": "Microsoft.Data.Sqlite"
  }
}

SQLiteMemoryComposer.cs

using Microsoft.Data.Sqlite;
using Umbraco.Cms.Core;
using Umbraco.Cms.Core.Composing;

/// <summary>
/// Ensure SQLite in-memory database is persisted for the whole application lifetime.
/// </summary>
internal sealed class SQLiteMemoryComposer : IComposer
{
    public void Compose(IUmbracoBuilder builder)
    {
        string? connectionString = builder.Config.GetUmbracoConnectionString(out var providerName);
        if (!string.IsNullOrEmpty(connectionString) &&
            Constants.ProviderNames.SQLLite.InvariantEquals(providerName))
        {
            var connectionStringBuilder = new SqliteConnectionStringBuilder(connectionString);
            if (connectionStringBuilder.Mode == SqliteOpenMode.Memory)
            {
                var connection = new SqliteConnection(connectionString);
                connection.Open();

                builder.Services.AddHostedService(_ => new SQLiteMemoryHostedService(connection));
            }
        }
    }

    /// <summary>
    /// Ensure connection is kept open (by keeping a reference) and gets gracefully closed/disposed when application stops.
    /// </summary>
    private sealed class SQLiteMemoryHostedService : IHostedService, IAsyncDisposable
    {
        private readonly SqliteConnection _connection;

        public SQLiteMemoryHostedService(SqliteConnection connection) => _connection = connection;

        public Task StartAsync(CancellationToken cancellationToken) => Task.CompletedTask;

        public async Task StopAsync(CancellationToken cancellationToken) => await _connection.CloseAsync();

        public async ValueTask DisposeAsync() => await _connection.DisposeAsync();
    }
}