Write to two databases, each with its own service

Runs the sample. Generated for this page; it reads the connection string from the environment.

using ArxSamples.MSSQL;
using ArxSamples.MSSQL.Operations.Create.AdvancedCases;

// The connection string comes from the environment, never from the code (see "Configure the connection").
var connectionString = Environment.GetEnvironmentVariable("ARX_SAMPLES_SQLSERVER")
    ?? throw new InvalidOperationException("Set the ARX_SAMPLES_SQLSERVER environment variable to your SQL Server connection string.");

await new Advanced4_TwoDatabases().RunAsync(new SampleContext(connectionString));

The sample, as it is in the repository.

using ARX.SDK;
using ARX.SDK.SQL.Services;
using ARX.SDK.Services;
using Microsoft.Extensions.DependencyInjection;
using static ArxSamples.MSSQL.SampleOutput;

namespace ArxSamples.MSSQL.Operations.Create.AdvancedCases;

/// <summary>
/// Two databases in one application, in three scenarios: one service per database, an order placed in the CRM (the
/// source of truth) and recorded in a reporting database (an event log), and a retry of the copy. One <see cref="ArxMsSqlDbService"/> per database, each with its own connection string and schema catalog,
/// registered as keyed services and injected by key.
/// </summary>
/// <remarks>
/// Each database has its own transactions: ARX can't make a write to two databases atomic (there's no distributed
/// transaction). Write the source of truth first and commit it, then write the copy. Make the copy safe to repeat (here
/// the event is unique per order number), so a failed copy can simply be retried.
/// </remarks>
public sealed class Advanced4_TwoDatabases : ISample
{
    private const string REPORTING_DATABASE = "arx_sdk_reporting";

    public string Name => "create-advanced-4-two-databases";

    public string Title => "Write to two databases, each with its own service";

    public async Task RunAsync(SampleContext dbContext)
    {
        // Setup only: create the reporting database and its table (plain SQL, not ARX).
        await dbContext.Direct.EnsureDatabaseExistsAsync(REPORTING_DATABASE);
        var reportingDirect = dbContext.Direct.ForDatabase(REPORTING_DATABASE);
        await reportingDirect.ExecuteAsync(
            "DROP TABLE IF EXISTS order_event; "
            + "CREATE TABLE order_event (id bigint IDENTITY(1, 1) PRIMARY KEY, order_number varchar(20) NOT NULL UNIQUE, account_name nvarchar(200) NOT NULL, "
            + "total decimal(14, 2) NOT NULL, recorded_at datetimeoffset NOT NULL DEFAULT SYSDATETIMEOFFSET())");

        await using var provider = Scenario1_RegisterOneServicePerDatabase(dbContext);
        using var scope = provider.CreateScope();
        var desk = scope.ServiceProvider.GetRequiredService<OrderDesk>();

        await Scenario2_PlaceAnOrderThenRecordItAsync(desk, reportingDirect);

        await Scenario3_ARetryOfTheCopyIsHarmlessAsync(desk, reportingDirect);
    }

    /// <summary>1. Register one service per database, by key (in ASP.NET Core: builder.Services).</summary>
    private static ServiceProvider Scenario1_RegisterOneServicePerDatabase(SampleContext dbContext)
    {
        Step("1. Register one service per database, by key (in ASP.NET Core: builder.Services)");

        var reportingConnectionString = dbContext.Direct.ConnectionStringFor(REPORTING_DATABASE);
        var services = new ServiceCollection();

        // Each database has its own schema catalog: only the tables that database has.
        services.AddKeyedSingleton("crm", (_, _) => new ArxMsSqlDbService(dbContext.ConnectionString, SampleCatalog.Build()));
        services.AddKeyedSingleton("reporting", (_, _) => new ArxMsSqlDbService(reportingConnectionString, ArxSchemaCatalog.Create()
            .Table("order_event", ArxIdColumn.Generated<long>("id"))
            .Build()));
        services.AddScoped<OrderDesk>();

        /* ===========================================================
         * [DIRECT QUERY CHECK]
         * -----------------------------------------------------------
         * SELECT name
         * FROM sys.databases
         * WHERE name IN ('arx_samples', 'arx_sdk_reporting');
         * -- Expect: 2 rows: the CRM database and the reporting database.
         * ===========================================================
         */

        return services.BuildServiceProvider();
    }

    /// <summary>2. The source of truth first, in one CRM transaction; then the copy in reporting.</summary>
    private static async Task Scenario2_PlaceAnOrderThenRecordItAsync(OrderDesk desk, MsSqlDirectHelper reportingDirect)
    {
        Step("2. Place an order in the CRM, then record it in reporting");

        var orderId = await desk.PlaceOrderAsync("Fourth Coffee", "SO-6001", 149.70m);
        Show($"CRM: order {orderId} committed; reporting: {await reportingDirect.CountAsync("order_event", "order_number = @p0", "SO-6001")} event");

        /* ===========================================================
         * [DIRECT QUERY CHECK]
         * -----------------------------------------------------------
         * SELECT o.order_number, o.total, a.name AS account
         * FROM sales_order o
         * JOIN account a ON a.id = o.account_id
         * WHERE o.order_number = 'SO-6001';
         *
         * -- In the arx_sdk_reporting database:
         * SELECT order_number, account_name, total, recorded_at
         * FROM arx_sdk_reporting.dbo.order_event
         * WHERE order_number = 'SO-6001';
         * -- Expect: the order in the CRM (Fourth Coffee, 149.70), and its event in reporting.
         * ===========================================================
         */
    }

    /// <summary>3. Recording the same event again (a retry after a failure) is harmless: the order number is unique there.</summary>
    private static async Task Scenario3_ARetryOfTheCopyIsHarmlessAsync(OrderDesk desk, MsSqlDirectHelper reportingDirect)
    {
        Step("3. Recording the same event again (a retry after a failure) is harmless");

        var recordedAgain = await desk.RecordEventAsync("Fourth Coffee", "SO-6001", 149.70m);
        Show($"second attempt recorded a new row: {recordedAgain}; events for SO-6001: {await reportingDirect.CountAsync("order_event", "order_number = @p0", "SO-6001")}");

        /* ===========================================================
         * [DIRECT QUERY CHECK]
         * -----------------------------------------------------------
         * -- In the arx_sdk_reporting database:
         * SELECT count(*)
         * FROM arx_sdk_reporting.dbo.order_event
         * WHERE order_number = 'SO-6001';
         * -- Expect: 1: the retry didn't record a second event.
         * ===========================================================
         */
    }

    /// <summary>An application service that writes to both databases. Each service comes in by its key.</summary>
    private sealed class OrderDesk
    {
        private readonly ArxMsSqlDbService _crm;
        private readonly ArxMsSqlDbService _reporting;

        public OrderDesk([FromKeyedServices("crm")] ArxMsSqlDbService crm, [FromKeyedServices("reporting")] ArxMsSqlDbService reporting)
        {
            _crm = crm;
            _reporting = reporting;
        }

        /// <summary>The source of truth first, in one CRM transaction; then the copy in reporting.</summary>
        public async Task<ArxRecordId> PlaceOrderAsync(string accountName, string orderNumber, decimal total)
        {
            ArxRecordId orderId = null!;

            await _crm.RunBatchAsync(async batch =>
            {
                var account = new ArxEntity("account");
                account["name"] = accountName;
                await batch.CreateAsync(account);

                var order = new ArxEntity("sales_order");
                order["order_number"] = orderNumber;
                order["account_id"] = new ArxEntityReference("account", account.RecordId);
                order["ordered_on"] = DateTime.UtcNow;
                order["total"] = total;
                orderId = await batch.CreateAsync(order);
            });

            // Committed in the CRM. If this next write fails, the order still stands: retry the event later.
            await RecordEventAsync(accountName, orderNumber, total);
            return orderId;
        }

        /// <summary>Records the order in reporting. Returns false when it was already recorded (safe to call again).</summary>
        public async Task<bool> RecordEventAsync(string accountName, string orderNumber, decimal total)
        {
            var orderEvent = new ArxEntity("order_event");
            orderEvent["order_number"] = orderNumber;
            orderEvent["account_name"] = accountName;
            orderEvent["total"] = total;

            try
            {
                await _reporting.CreateAsync(orderEvent);
                return true;
            }
            catch (ArxDatabaseException error) when (error.Code == ArxErrorCode.UniqueViolation)
            {
                return false;
            }
        }
    }
}

What every sample is given: the connection string (and the ISample interface the samples implement).

namespace ArxSamples.MSSQL;

/// <summary>One documentation sample: a scenario a real application has, shown end to end.</summary>
public interface ISample
{
    /// <summary>The name to run it alone: <c>dotnet run -- &lt;name&gt;</c>.</summary>
    string Name { get; }

    /// <summary>One line on what it shows.</summary>
    string Title { get; }

    Task RunAsync(SampleContext dbContext);
}

/// <summary>
/// What a sample is given. Every sample gets the connection string; the application's service and catalog
/// (SampleContext.Application.cs) and the plain SqlClient read-back helper (SampleContext.Direct.cs) live in their own
/// files, so a sample needs only the files it uses.
/// </summary>
public sealed partial class SampleContext
{
    public SampleContext(string connectionString)
    {
        ConnectionString = connectionString;
    }

    public string ConnectionString { get; }
}

How the samples print: Step for each scenario, Show for what it did, ShowError for an error caught on purpose.

using ARX.SDK;

namespace ArxSamples.MSSQL;

/// <summary>How the samples print: a numbered step for each scenario, then what it did.</summary>
public static class SampleOutput
{
    /// <summary>The start of a scenario: <c>Step("1. By the id Create returned")</c>.</summary>
    public static void Step(string text) => Console.WriteLine($"  > {text}");

    /// <summary>What the scenario did.</summary>
    public static void Show(string text) => Console.WriteLine($"      {text}");

    /// <summary>An ARX error the scenario caught on purpose.</summary>
    public static void ShowError(ArxException exception)
        => Console.WriteLine($"      caught {exception.GetType().Name}: {exception.Message}");
}

The application's schema catalog: every table's id column and keys, and its selection sets.

using ARX.SDK.Services;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace ArxSamples.MSSQL;

/// <summary>An account's industry. Stored in account.industry (smallint) as the enum's number.</summary>
public enum Industry : short
{
    Retail = 1,
    Manufacturing = 2,
    Healthcare = 3,
    Finance = 4,
    Technology = 5
}

/// <summary>
/// The application's schema catalog: every table's id column and keys, and every picklist (selection set),
/// defined once.
/// </summary>
/// <remarks>
/// <para>
/// <b>Ids.</b> Every table the application writes is declared with its one id column: its name, its .NET type, and
/// whether the database generates it (<c>ArxIdColumn.Generated</c>: identity, sequence, default) or the caller supplies
/// it (<c>ArxIdColumn.Supplied</c>: a natural id, a client-generated Guid). A table with no id column, such as a log or a
/// table identified by several columns together, is declared <c>ArxIdColumn.NoIdColumn</c>. ARX never reads ids from
/// the database: the developer knows the schema, and an undeclared table is refused (ARX1016).
/// </para>
/// <para>
/// <b>Keys.</b> Optional business keys: one or more columns whose values identify a record, such as an account number,
/// or a region and a SKU together. Create doesn't use them (the database's unique constraints do the enforcing); Update
/// and Delete will be able to find rows by them. <c>ValidateSchemaAsync</c> checks ids and keys against the database
/// when you ask it to.
/// </para>
/// <para>
/// <b>Selection sets.</b> /// A selection set (picklist, option set) is a column that only accepts a fixed list of codes, each with a display
/// text: an account's status (ACTIVE, ON_HOLD, CLOSED), a contact's preferred channel (E, P, S). The catalog tells ARX,
/// for each set:
/// </para>
/// <list type="bullet">
/// <item><b>Name</b>: what an <see cref="ArxSelectionSetValue"/> refers to, for example <c>new ArxSelectionSetValue("AccountStatus", "ON_HOLD")</c>.</item>
/// <item><b>Kind</b>: what the codes are, matching the column: Integer, String, Character, Boolean, Decimal or Guid.
/// (SQL Server has no enum types: a fixed list of text values is a String set, and a CHECK can guard the column.)</item>
/// <item><b>Multiplicity</b>: Single (one code), or several codes stored as bit flags (Flags), a delimited string
/// (Delimited) or a JSON array (Json). SQL Server has no array columns.</item>
/// <item><b>Options</b>: each code with its display text and order. A retired option (<c>IsActive: false</c>) stays
/// readable in old rows, but can't be written any more.</item>
/// </list>
/// <para>
/// Build the catalog once at startup and give it to the service: <c>new ArxMsSqlDbService(connectionString, SampleCatalog.Build())</c>.
/// ARX checks every <see cref="ArxSelectionSetValue"/> against it in memory, before any SQL is sent, and stores the code
/// itself in the column: no lookup table, no extra query. A plain value (a string or number that isn't an
/// ArxSelectionSetValue) isn't checked.
/// </para>
/// <para>
/// The catalog is plain code, so the options can come from wherever you keep them: written out here, taken from a .NET
/// enum (<c>AddEnum</c>), loaded from JSON (<see cref="SetsJson"/> and <see cref="FromJson"/> below), or loaded at
/// startup from your own option table and passed to <c>new ArxSelectionSetDefinition(name, kind, options)</c>. ARX
/// never reads option tables or files itself.
/// </para>
/// </remarks>
public static class SampleCatalog
{
    /// <summary>
    /// Selection sets kept as JSON, as an application might keep them in a file, a settings store or a database column.
    /// Here the JSON is in the class itself; in an application, read it from wherever you keep it
    /// (<c>File.ReadAllText("selection-sets.json")</c>, configuration, an HTTP call) and pass it to <see cref="FromJson"/>.
    /// "multiplicity" and "active" are optional (Single and true).
    /// </summary>
    public const string SetsJson = """
        [
          {
            "name": "LeadSource",
            "kind": "String",
            "options": [
              { "value": "WEB",       "text": "Website",    "order": 0 },
              { "value": "REFERRAL",  "text": "Referral",   "order": 1 },
              { "value": "EVENT",     "text": "Trade show", "order": 2 },
              { "value": "COLD_CALL", "text": "Cold call",  "order": 3, "active": false }
            ]
          },
          {
            "name": "ContactRating",
            "kind": "Integer",
            "multiplicity": "Single",
            "options": [
              { "value": 1, "text": "Poor" },
              { "value": 2, "text": "Fair" },
              { "value": 3, "text": "Good" },
              { "value": 4, "text": "Very good" },
              { "value": 5, "text": "Excellent" }
            ]
          }
        ]
        """;

    public static ArxSchemaCatalog Build()
    {
        var builder = ArxSchemaCatalog.Create()

            // ---------- tables: id column and keys ----------
            // Generated bigint ids (IDENTITY): the database fills them in. The keys are the unique columns that also
            // identify a record.
            .Table("account", ArxIdColumn.Generated<long>("id")).Key("account", "account_number")
            .Table("contact", ArxIdColumn.Generated<long>("id")).Key("contact", "email")

            // Retrieve: a contact's account_id reads back as a reference to its account, named by the account's
            // name; a contact is named by its first and last names.
            .Lookup("contact", "account_id", "account")
            .DisplayName("account", "name")
            .DisplayName("contact", new[] { "first_name", "last_name" }, " ")

            .Table("product_category", ArxIdColumn.Generated<long>("id")).Key("product_category", "name")
            .Table("product", ArxIdColumn.Generated<long>("id")).Key("product", "sku")
            .Table("sales_order", ArxIdColumn.Generated<long>("id")).Key("sales_order", "order_number")
            .Table("order_line", ArxIdColumn.Generated<long>("id"))
            .Table("data_type_showcase", ArxIdColumn.Generated<long>("id"))

            // A natural id: the caller gives the ISO country code.
            .Table("country", ArxIdColumn.Supplied<string>("code"))

            // A generated uuid id (DEFAULT gen_random_uuid()).
            .Table("device", ArxIdColumn.Generated<Guid>("id")).Key("device", "serial_number")

            // No id column: a price is identified by its region and SKU together, a compound key.
            .Table("price_list", ArxIdColumn.NoIdColumn).Key("price_list", "region", "sku")

            // No primary key at all: the same action can be logged twice.
            .Table("activity_log", ArxIdColumn.NoIdColumn)

            // A partitioned table without an id: a visit is a fact, never updated (Retrieve Advanced8).
            .Table("web_visit", ArxIdColumn.NoIdColumn)

            // ---------- selection sets ----------

            // From a .NET enum: the set is named after the enum ("Industry"), the codes are its numbers (Retail = 1, …),
            // the display texts are its member names. Column: account.industry smallint.
            .AddEnum<Industry>()

            // Written out: String codes with display texts, single choice. Column: account.status varchar(10).
            .Add("AccountStatus", ArxSelectionSetKind.String, ("ACTIVE", "Active"), ("ON_HOLD", "On hold"), ("CLOSED", "Closed"))

            // Multiple choice, stored as delimited text ("ENTERPRISE,PUBLIC"). RESELLER is retired: rows that have it still
            // read back, but new writes can't use it. Column: account.segments nvarchar(200).
            .Add(new ArxSelectionSetDefinition("CustomerSegment", ArxSelectionSetKind.String, new[]
            {
                new ArxSelectionSetOption("SMB", "Small business", 0),
                new ArxSelectionSetOption("ENTERPRISE", "Enterprise", 1),
                new ArxSelectionSetOption("PUBLIC", "Public sector", 2),
                new ArxSelectionSetOption("RESELLER", "Reseller", 3, IsActive: false)
            }, ArxSelectionSetMultiplicity.Delimited))

            // Single-character codes. Column: contact.preferred_channel char(1).
            .Add("ContactChannel", ArxSelectionSetKind.Character, ('E', "Email"), ('P', "Phone"), ('S', "Text message"))

            // A fixed list of text values the column's CHECK also guards (SQL Server has no enum types).
            // Column: data_type_showcase.mood nvarchar(10).
            .Add(new ArxSelectionSetDefinition("Mood", ArxSelectionSetKind.String, new[]
            {
                new ArxSelectionSetOption("happy", "Happy", 0),
                new ArxSelectionSetOption("neutral", "Neutral", 1),
                new ArxSelectionSetOption("sad", "Sad", 2)
            }));

        // Loaded from JSON: LeadSource (contact.lead_source text) and ContactRating (contact.rating smallint).
        foreach (var set in FromJson(SetsJson))
        {
            builder.Add(set);
        }

        return builder.Build();
    }

    /// <summary>Reads selection set definitions from JSON (the shape of <see cref="SetsJson"/>).</summary>
    public static IEnumerable<ArxSelectionSetDefinition> FromJson(string json)
    {
        var options = new JsonSerializerOptions
        {
            PropertyNameCaseInsensitive = true,
            Converters = { new JsonStringEnumConverter() }   // "String", "Integer", "Array", … as names
        };

        var sets = JsonSerializer.Deserialize<List<JsonSet>>(json, options)
            ?? throw new InvalidOperationException("The selection set JSON is empty.");

        foreach (var set in sets)
        {
            yield return new ArxSelectionSetDefinition(
                set.Name,
                set.Kind,
                set.Options.Select(x => new ArxSelectionSetOption(Code(x.Value, set.Kind), x.Text, x.Order, x.Active)),
                set.Multiplicity,
                set.DatabaseTypeName);
        }
    }

    /// <summary>A JSON value as a code of the set's kind: JSON has no char, Guid or 64-bit integer of its own.</summary>
    private static object Code(JsonElement value, ArxSelectionSetKind kind) => kind switch
    {
        ArxSelectionSetKind.Integer => value.GetInt64(),
        ArxSelectionSetKind.Decimal => value.GetDecimal(),
        ArxSelectionSetKind.Boolean => value.GetBoolean(),
        ArxSelectionSetKind.Guid => value.GetGuid(),
        ArxSelectionSetKind.Character => value.GetString()![0],
        _ => value.GetString()!   // String and DatabaseEnum
    };

    private sealed record JsonSet(
        string Name,
        ArxSelectionSetKind Kind,
        List<JsonOption> Options,
        ArxSelectionSetMultiplicity Multiplicity = ArxSelectionSetMultiplicity.Single,
        string? DatabaseTypeName = null);

    private sealed record JsonOption(JsonElement Value, string Text, int Order = 0, bool Active = true);
}

The plain Microsoft.Data.SqlClient read-back helper, for the samples that show what SQL Server stored.

namespace ArxSamples.MSSQL;

/// <summary>plain SqlClient outside the SDK, for the samples that show what SQL Server stored.</summary>
public sealed partial class SampleContext
{
    private MsSqlDirectHelper? _direct;

    /// <summary>plain SqlClient outside the SDK, used only to reset the tables and to show what was stored.</summary>
    public MsSqlDirectHelper Direct => _direct ??= new MsSqlDirectHelper(ConnectionString);
}

Plain Microsoft.Data.SqlClient, not part of ARX: reads rows back so a sample can show what SQL Server stored.

using Microsoft.Data.SqlClient;
using System.Text.RegularExpressions;

namespace ArxSamples.MSSQL;

/// <summary>
/// Plain Microsoft.Data.SqlClient, outside ARX.SDK: resets the sample tables, reads rows back so a sample can show what
/// SQL Server actually stored (printing is SampleOutput.cs). It isn't part of the SDK and isn't how an application uses
/// ARX; it only lets the samples verify and show their results.
/// </summary>
public sealed class MsSqlDirectHelper
{
    private readonly string _connectionString;

    public MsSqlDirectHelper(string connectionString)
    {
        _connectionString = connectionString;
    }

    /// <summary>The same helper pointed at another database on the same server.</summary>
    public MsSqlDirectHelper ForDatabase(string database) => new MsSqlDirectHelper(ConnectionStringFor(database));

    /// <summary>The connection string of <see cref="ForDatabase"/>'s database, for a second ARX service.</summary>
    public string ConnectionStringFor(string database)
        => new SqlConnectionStringBuilder(_connectionString) { InitialCatalog = database }.ConnectionString;

    /// <summary>Creates <paramref name="database"/> on the server if it doesn't exist yet (connecting to master).</summary>
    public async Task EnsureDatabaseExistsAsync(string database)
    {
        await using var connection = new SqlConnection(ConnectionStringFor("master"));
        await connection.OpenAsync();

        // CREATE DATABASE takes no parameters; the name is the sample's own constant, quoted.
        await using var create = new SqlCommand($"IF DB_ID(@name) IS NULL CREATE DATABASE [{database.Replace("]", "]]")}]; ALTER DATABASE [{database.Replace("]", "]]")}] SET ALLOW_SNAPSHOT_ISOLATION ON", connection);
        create.Parameters.AddWithValue("@name", database);
        await create.ExecuteNonQueryAsync();
    }

    /// <summary>Drops and recreates the sample tables (Schema/sample-schema.sql), batch by batch (split at GO lines).</summary>
    public async Task ResetSchemaAsync()
    {
        await EnsureDatabaseExistsAsync(new SqlConnectionStringBuilder(_connectionString).InitialCatalog);

        // The read-backs read committed rows without waiting for locks (snapshot isolation, allowed on this database).
        await ExecuteAsync("ALTER DATABASE CURRENT SET ALLOW_SNAPSHOT_ISOLATION ON");
        var script = await File.ReadAllTextAsync(Path.Combine(AppContext.BaseDirectory, "Schema", "sample-schema.sql"));

        foreach (var batch in Regex.Split(script, @"^\s*GO\s*$", RegexOptions.Multiline | RegexOptions.IgnoreCase).Where(x => !string.IsNullOrWhiteSpace(x)))
        {
            await ExecuteAsync(batch);
        }
    }

    public async Task ExecuteAsync(string sql)
    {
        await using var connection = await OpenConnectionAsync();
        await using var command = new SqlCommand(sql, connection);
        await command.ExecuteNonQueryAsync();
    }

    public async Task<SqlConnection> OpenConnectionAsync()
    {
        var connection = new SqlConnection(_connectionString);
        await connection.OpenAsync();
        return connection;
    }

    /// <summary>
    /// Counts committed rows, optionally with a WHERE clause using @p0, @p1, … for <paramref name="parameters"/>. It reads
    /// under SNAPSHOT isolation, so it shows what is committed without waiting for another transaction's locks (a
    /// READ COMMITTED read would wait for them), and sets READ COMMITTED back on the pooled connection afterwards.
    /// </summary>
    public async Task<long> CountAsync(string table, string where = "1 = 1", params object[] parameters)
    {
        await using var connection = await OpenConnectionAsync();
        await using var command = new SqlCommand(
            $"SET TRANSACTION ISOLATION LEVEL SNAPSHOT; SELECT COUNT_BIG(*) FROM {table} WHERE {where}; SET TRANSACTION ISOLATION LEVEL READ COMMITTED;", connection);

        for (var i = 0; i < parameters.Length; i++)
        {
            command.Parameters.AddWithValue("@p" + i, parameters[i] ?? DBNull.Value);
        }

        return (long)(await command.ExecuteScalarAsync())!;
    }

    /// <summary>Reads one row by id as column → value (null for SQL NULL).</summary>
    public async Task<Dictionary<string, object?>> RowAsync(string table, long id)
    {
        await using var connection = await OpenConnectionAsync();
        await using var command = new SqlCommand($"SELECT * FROM {table} WHERE id = @p0", connection);
        command.Parameters.AddWithValue("@p0", id);
        await using var reader = await command.ExecuteReaderAsync();

        if (!await reader.ReadAsync())
        {
            throw new InvalidOperationException($"No {table} row with id {id}.");
        }

        var row = new Dictionary<string, object?>(StringComparer.OrdinalIgnoreCase);

        for (var i = 0; i < reader.FieldCount; i++)
        {
            row[reader.GetName(i)] = reader.IsDBNull(i) ? null : reader.GetValue(i);
        }

        return row;
    }

    /// <summary>
    /// Reads columns of one row as SQL Server prints them as text (<c>CONVERT(nvarchar(max), column, 126)</c>: ISO 8601
    /// for dates, with a datetimeoffset's own offset), which shows the stored value exactly, whatever its type.
    /// </summary>
    public async Task<Dictionary<string, string?>> TextRowAsync(string table, long id, params string[] columns)
    {
        await using var connection = await OpenConnectionAsync();
        var select = string.Join(", ", columns.Select(x => $"CONVERT(nvarchar(max), [{x}], 126)"));
        await using var command = new SqlCommand($"SELECT {select} FROM {table} WHERE id = @p0", connection);
        command.Parameters.AddWithValue("@p0", id);
        await using var reader = await command.ExecuteReaderAsync();

        if (!await reader.ReadAsync())
        {
            throw new InvalidOperationException($"No {table} row with id {id}.");
        }

        var row = new Dictionary<string, string?>(StringComparer.OrdinalIgnoreCase);

        for (var i = 0; i < columns.Length; i++)
        {
            row[columns[i]] = reader.IsDBNull(i) ? null : reader.GetString(i);
        }

        return row;
    }
}

  > 1. Register one service per database, by key (in ASP.NET Core: builder.Services)
  > 2. Place an order in the CRM, then record it in reporting
      CRM: order 1 committed; reporting: 1 event
  > 3. Recording the same event again (a retry after a failure) is harmless
      second attempt recorded a new row: False; events for SO-6001: 1