Custom Host Setup

MigrationHost.RunAsync provides a simplified entry point that handles everything. If you need more control, use the overload that accepts IConfiguration and ILoggerFactory.

Default setup (one line)

using System.Reflection;
using Cosmigrator;

await MigrationHost.RunAsync(args, Assembly.GetExecutingAssembly());

This internally:

1. Creates a .NET Generic Host with Host.CreateDefaultBuilder
2. Configures Serilog with console output
3. Loads appsettings.json, environment variables, and CLI args
4. Creates a CosmosClient and runs migrations

Custom setup

Use the RunAsync(IConfiguration, ILoggerFactory, string[], Assembly) overload when you need:

• Custom Serilog sinks (Seq, Application Insights, file)
• Environment-specific configuration
• Integration with an existing host pipeline

using System.Reflection;
using Cosmigrator;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Serilog;

Log.Logger = new LoggerConfiguration()
    .WriteTo.Console()
    .WriteTo.Seq("http://localhost:5341")
    .CreateLogger();

var host = Host.CreateDefaultBuilder(args)
    .ConfigureAppConfiguration((context, config) =>
    {
        config
            .SetBasePath(AppContext.BaseDirectory)
            .AddJsonFile("appsettings.json", optional: false)
            .AddJsonFile($"appsettings.{context.HostingEnvironment.EnvironmentName}.json", optional: true)
            .AddEnvironmentVariables()
            .AddCommandLine(args);
    })
    .UseSerilog((ctx, cfg) => cfg.ReadFrom.Configuration(ctx.Configuration))
    .Build();

var loggerFactory = host.Services.GetRequiredService<ILoggerFactory>();
var configuration = host.Services.GetRequiredService<IConfiguration>();

await MigrationHost.RunAsync(
    configuration,
    loggerFactory,
    args,
    Assembly.GetExecutingAssembly());

What MigrationHost.RunAsync reads from configuration

Key	Required	Description
CosmosDb:ConnectionString	Yes	Full Cosmos DB connection string
CosmosDb:DatabaseName	Yes	Target database name

CosmosClient configuration

Both overloads create a CosmosClient with these settings:

new CosmosClient(connectionString, new CosmosClientOptions
{
    UseSystemTextJsonSerializerWithOptions = new JsonSerializerOptions
    {
        PropertyNamingPolicy = JsonNamingPolicy.CamelCase
    },
    AllowBulkExecution = true,
    MaxRetryAttemptsOnRateLimitedRequests = 10,
    MaxRetryWaitTimeOnRateLimitedRequests = TimeSpan.FromSeconds(60)
});

Key settings:

• System.Text.Json with camelCase naming
• Bulk execution enabled for BulkOperationHelper
• 10 retry attempts on rate-limited requests with up to 60s wait