Delta

Delta is an approach to implementing a 304 Not Modified leveraging SqlServer change tracking

The approach uses a last updated timestamp from the database to generate an ETag. All dynamic requests then have that ETag checked/applied.

This approach works well when the frequency of updates is relatively low. In this scenario, the majority of requests will leverage the result in a 304 Not Modified being returned and the browser loading the content its cache.

Effectively consumers will always receive the most current data, while the load on the server remains low.

See Milestones for release notes.

Assumptions

Frequency of updates to data is relatively low compared to reads
Using either SQL Server Change Tracking and/or SQL Server Row Versioning

304 Not Modified Flow

graph TD
    Request
    CalculateEtag[Calculate current ETag<br/>based on timestamp<br/>from web assembly and SQL]
    IfNoneMatch{Has<br/>If-None-Match<br/>header?}
    EtagMatch{Current<br/>Etag matches<br/>If-None-Match?}
    AddETag[Add current ETag<br/>to Response headers]
    304[Respond with<br/>304 Not-Modified]
    Request --> CalculateEtag
    CalculateEtag --> IfNoneMatch
    IfNoneMatch -->|Yes| EtagMatch
    IfNoneMatch -->|No| AddETag
    EtagMatch -->|No| AddETag
    EtagMatch -->|Yes| 304

ETag calculation logic

The ETag is calculated from a combination several parts

AssemblyWriteTime

The last write time of the web entry point assembly

var webAssemblyLocation = Assembly.GetEntryAssembly()!.Location;
AssemblyWriteTime = File.GetLastWriteTime(webAssemblyLocation).Ticks.ToString();

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SQL timestamp

A combination of change_tracking_current_version (if tracking is enabled) and @@DBTS (row version timestamp)

declare @changeTracking bigint = change_tracking_current_version();
declare @timeStamp bigint = convert(bigint, @@dbts);

if (@changeTracking is null)
  select cast(@timeStamp as varchar)
else
  select cast(@timeStamp as varchar) + '-' + cast(@changeTracking as varchar)

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Suffix

An optional string suffix that is dynamically calculated at runtime based on the current HttpContext.

var app = builder.Build();
app.UseDelta(suffix: httpContext => "MySuffix");

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Combining the above

internal static string BuildEtag(string timeStamp, string? suffix)
{
    if (suffix == null)
    {
        return $"\"{AssemblyWriteTime}-{timeStamp}\"";
    }

    return $"\"{AssemblyWriteTime}-{timeStamp}-{suffix}\"";
}

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NuGet

Delta is shipped as two nugets:

Delta: Delivers functionality using SqlConnection and SqlTransaction.
Delta.EF: Delivers functionality using SQL Server EF Database Provider.

Only one of the above should be used.

Usage

DB Schema

Ensure SQL Server Change Tracking and/or SQL Server Row Versioning is enabled for all relevant tables.

Example SQL schema:

-- Tables

CREATE TABLE [dbo].[Companies](
	[Id] [uniqueidentifier] NOT NULL,
	[RowVersion] [timestamp] NOT NULL,
	[Content] [nvarchar](max) NULL,
 CONSTRAINT [PK_Companies] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

CREATE TABLE [dbo].[Employees](
	[Id] [uniqueidentifier] NOT NULL,
	[RowVersion] [timestamp] NOT NULL,
	[CompanyId] [uniqueidentifier] NOT NULL,
	[Content] [nvarchar](max) NULL,
	[Age] [int] NOT NULL,
 CONSTRAINT [PK_Employees] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

CREATE NONCLUSTERED INDEX [IX_Employees_CompanyId] ON [dbo].[Employees]
(
	[CompanyId] ASC
) ON [PRIMARY]

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Add to WebApplicationBuilder

var builder = WebApplication.CreateBuilder();
builder.Services.AddScoped(_ => new SqlConnection(connectionString));
var app = builder.Build();
app.UseDelta();

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Add to a Route Group

To add to a specific Route Group:

app.MapGroup("/group")
    .UseDelta()
    .MapGet("/", () => "Hello Group!");

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ShouldExecute

Optionally control what requests Delta is executed on.

var app = builder.Build();
app.UseDelta(
    shouldExecute: httpContext =>
    {
        var path = httpContext.Request.Path.ToString();
        return path.Contains("match");
    });

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Custom Connection discovery

By default, Delta uses HttpContext.RequestServices to discover the SqlConnection and SqlTransaction:

static Connection DiscoverConnection(HttpContext httpContext)
{
    var provider = httpContext.RequestServices;
    var connection = provider.GetRequiredService<SqlConnection>();
    var transaction = provider.GetService<SqlTransaction>();
    return new(connection, transaction);
}

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To use custom connection discovery:

var application = webApplicationBuilder.Build();
application.UseDelta(
    getConnection: httpContext => httpContext.RequestServices.GetRequiredService<SqlConnection>());

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To use custom connection and transaction discovery:

var webApplication = webApplicationBuilder.Build();
webApplication.UseDelta(
    getConnection: httpContext =>
    {
        var provider = httpContext.RequestServices;
        var sqlConnection = provider.GetRequiredService<SqlConnection>();
        var sqlTransaction = provider.GetService<SqlTransaction>();
        return new(sqlConnection, sqlTransaction);
    });

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EF Usage

DbContext using RowVersion

Enable row versioning in Entity Framework

public class SampleDbContext(DbContextOptions options) :
    DbContext(options)
{
    public DbSet<Employee> Employees { get; set; } = null!;
    public DbSet<Company> Companies { get; set; } = null!;

    protected override void OnModelCreating(ModelBuilder builder)
    {
        var company = builder.Entity<Company>();
        company.HasKey(_ => _.Id);
        company
            .HasMany(_ => _.Employees)
            .WithOne(_ => _.Company)
            .IsRequired();
        company
            .Property(_ => _.RowVersion)
            .IsRowVersion()
            .HasConversion<byte[]>();

        var employee = builder.Entity<Employee>();
        employee.HasKey(_ => _.Id);
        employee
            .Property(_ => _.RowVersion)
            .IsRowVersion()
            .HasConversion<byte[]>();
    }
}

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Add to WebApplicationBuilder

var builder = WebApplication.CreateBuilder();
builder.Services.AddSqlServer<SampleDbContext>(database.ConnectionString);
var app = builder.Build();
app.UseDelta<SampleDbContext>();

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Add to a Route Group

To add to a specific Route Group:

app.MapGroup("/group")
    .UseDelta<SampleDbContext>()
    .MapGet("/", () => "Hello Group!");

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ShouldExecute

Optionally control what requests Delta is executed on.

var app = builder.Build();
app.UseDelta<SampleDbContext>(
    shouldExecute: httpContext =>
    {
        var path = httpContext.Request.Path.ToString();
        return path.Contains("match");
    });

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UseResponseDiagnostics

Response diagnostics is an opt-in feature that includes extra log information in the response headers.

Enable by setting UseResponseDiagnostics to true at startup:

DeltaExtensions.UseResponseDiagnostics = true;

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Response diagnostics headers are prefixed with Delta-.

Example Response header when the Request has not If-None-Match header.

Helpers

Utility methods for working with databases using the Delta conventions.

GetLastTimeStamp

For a `SqlConnection`:

var timeStamp = await sqlConnection.GetLastTimeStamp();

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For a `DbContext`:

var timeStamp = await dbContext.GetLastTimeStamp();

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GetDatabasesWithTracking

Get a list of all databases with change tracking enabled.

var trackedDatabases = await sqlConnection.GetTrackedDatabases();
foreach (var db in trackedDatabases)
{
    Trace.WriteLine(db);
}

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Uses the following SQL:

select d.name
from sys.databases as d inner join
  sys.change_tracking_databases as t on
  t.database_id = d.database_id

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GetTrackedTables

Get a list of all tracked tables in database.

var trackedTables = await sqlConnection.GetTrackedTables();
foreach (var db in trackedTables)
{
    Trace.WriteLine(db);
}

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Uses the following SQL:

select t.Name
from sys.tables as t left join
  sys.change_tracking_tables as c on t.[object_id] = c.[object_id]
where c.[object_id] is not null

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IsTrackingEnabled

Determine if change tracking is enabled for a database.

var isTrackingEnabled = await sqlConnection.IsTrackingEnabled();

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Uses the following SQL:

select count(d.name)
from sys.databases as d inner join
  sys.change_tracking_databases as t on
  t.database_id = d.database_id
where d.name = '{database}'

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EnableTracking

Enable change tracking for a database.

await sqlConnection.EnableTracking();

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Uses the following SQL:

alter database {database}
set change_tracking = on
(
  change_retention = {retentionDays} days,
  auto_cleanup = on
)

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DisableTracking

Disable change tracking for a database and all tables within that database.

await sqlConnection.DisableTracking();

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Uses the following SQL:

For disabling tracking on a database:

alter database [{database}] set change_tracking = off;

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For disabling tracking on tables:

alter table [{table}] disable change_tracking;

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SetTrackedTables

Enables change tracking for all tables listed, and disables change tracking for all tables not listed.

await sqlConnection.SetTrackedTables(["Companies"]);

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Uses the following SQL:

For enabling tracking on a database:

alter database {database}
set change_tracking = on
(
  change_retention = {retentionDays} days,
  auto_cleanup = on
)

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For enabling tracking on tables:

alter table [{table}] enable change_tracking

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For disabling tracking on tables:

alter table [{table}] disable change_tracking;

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Icon

Estuary designed by Daan from The Noun Project.

License

DEVBOX10/SimonCropp-Delta

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