Add base for apollo federation connector

src/All.slnx

@@ -214,9 +214,11 @@
     <Project Path="HotChocolate/Fusion/src/Fusion.Aspire/HotChocolate.Fusion.Aspire.csproj" />
     <Project Path="HotChocolate/Fusion/src/Fusion.AspNetCore/HotChocolate.Fusion.AspNetCore.csproj" />
     <Project Path="HotChocolate/Fusion/src/Fusion.Composition/HotChocolate.Fusion.Composition.csproj" />
+    <Project Path="HotChocolate/Fusion/src/Fusion.Composition.ApolloFederation/HotChocolate.Fusion.Composition.ApolloFederation.csproj" />
     <Project Path="HotChocolate/Fusion/src/Fusion.Caching/HotChocolate.Fusion.Caching.csproj" />
     <Project Path="HotChocolate/Fusion/src/Fusion.Diagnostics/HotChocolate.Fusion.Diagnostics.csproj" />
     <Project Path="HotChocolate/Fusion/src/Fusion.Execution.Types/HotChocolate.Fusion.Execution.Types.csproj" />
+    <Project Path="HotChocolate/Fusion/src/Fusion.Connectors.ApolloFederation/HotChocolate.Fusion.Connectors.ApolloFederation.csproj" />
     <Project Path="HotChocolate/Fusion/src/Fusion.Connectors.InMemory/HotChocolate.Fusion.Connectors.InMemory.csproj" />
     <Project Path="HotChocolate/Fusion/src/Fusion.Execution/HotChocolate.Fusion.Execution.csproj" />
     <Project Path="HotChocolate/Fusion/src/Fusion.Language/HotChocolate.Fusion.Language.csproj" />
@@ -227,6 +229,9 @@
   <Folder Name="/HotChocolate/Fusion/test/">
     <Project Path="HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/HotChocolate.Fusion.AspNetCore.Tests.csproj" />
     <Project Path="HotChocolate/Fusion/test/Fusion.Composition.Tests/HotChocolate.Fusion.Composition.Tests.csproj" />
+    <Project Path="HotChocolate/Fusion/test/Fusion.Composition.ApolloFederation.Tests/HotChocolate.Fusion.Composition.ApolloFederation.Tests.csproj" />
+    <Project Path="HotChocolate/Fusion/test/Fusion.Connectors.ApolloFederation.Compliance.Tests/HotChocolate.Fusion.Connectors.ApolloFederation.Compliance.Tests.csproj" />
+    <Project Path="HotChocolate/Fusion/test/Fusion.Connectors.ApolloFederation.Tests/HotChocolate.Fusion.Connectors.ApolloFederation.Tests.csproj" />
     <Project Path="HotChocolate/Fusion/test/Fusion.Connectors.InMemory.Tests/HotChocolate.Fusion.Connectors.InMemory.Tests.csproj" />
     <Project Path="HotChocolate/Fusion/test/Fusion.Caching.Tests/HotChocolate.Fusion.Caching.Tests.csproj" />
     <Project Path="HotChocolate/Fusion/test/Fusion.Diagnostics.Tests/HotChocolate.Fusion.Diagnostics.Tests.csproj" />

src/HotChocolate/Fusion/HotChocolate.Fusion.slnx

@@ -6,6 +6,8 @@
     <Project Path="src/Fusion.Aspire/HotChocolate.Fusion.Aspire.csproj" />
     <Project Path="src/Fusion.AspNetCore/HotChocolate.Fusion.AspNetCore.csproj" />
     <Project Path="src/Fusion.Composition/HotChocolate.Fusion.Composition.csproj" />
+    <Project Path="src/Fusion.Composition.ApolloFederation/HotChocolate.Fusion.Composition.ApolloFederation.csproj" />
+    <Project Path="src/Fusion.Connectors.ApolloFederation/HotChocolate.Fusion.Connectors.ApolloFederation.csproj" />
     <Project Path="src/Fusion.Caching/HotChocolate.Fusion.Caching.csproj" />
     <Project Path="src/Fusion.Diagnostics/HotChocolate.Fusion.Diagnostics.csproj" />
     <Project Path="src/Fusion.Execution.Types/HotChocolate.Fusion.Execution.Types.csproj" />
@@ -19,6 +21,9 @@
   <Folder Name="/test/">
     <Project Path="test/Fusion.AspNetCore.Tests/HotChocolate.Fusion.AspNetCore.Tests.csproj" />
     <Project Path="test/Fusion.Composition.Tests/HotChocolate.Fusion.Composition.Tests.csproj" />
+    <Project Path="test/Fusion.Composition.ApolloFederation.Tests/HotChocolate.Fusion.Composition.ApolloFederation.Tests.csproj" />
+    <Project Path="test/Fusion.Connectors.ApolloFederation.Compliance.Tests/HotChocolate.Fusion.Connectors.ApolloFederation.Compliance.Tests.csproj" />
+    <Project Path="test/Fusion.Connectors.ApolloFederation.Tests/HotChocolate.Fusion.Connectors.ApolloFederation.Tests.csproj" />
     <Project Path="test/Fusion.Caching.Tests/HotChocolate.Fusion.Caching.Tests.csproj" />
     <Project Path="test/Fusion.Diagnostics.Tests/HotChocolate.Fusion.Diagnostics.Tests.csproj" />
     <Project Path="test/Fusion.EventSources.Tests/HotChocolate.Fusion.EventSources.Tests.csproj" />

src/HotChocolate/Fusion/src/Fusion.Composition.ApolloFederation/EntityKeyInfo.cs

@@ -0,0 +1,18 @@
+namespace HotChocolate.Fusion.ApolloFederation;
+
+/// <summary>
+/// Represents a single <c>@key</c> directive on a federation entity type.
+/// </summary>
+internal sealed class EntityKeyInfo
+{
+    /// <summary>
+    /// Gets the raw field selection string from <c>@key(fields: "...")</c>.
+    /// </summary>
+    public required string Fields { get; init; }
+
+    /// <summary>
+    /// Gets a value indicating whether the key is resolvable.
+    /// Defaults to <c>true</c> when the <c>resolvable</c> argument is omitted.
+    /// </summary>
+    public bool Resolvable { get; init; } = true;
+}

src/HotChocolate/Fusion/src/Fusion.Composition.ApolloFederation/FederationDirectiveNames.cs

@@ -0,0 +1,19 @@
+namespace HotChocolate.Fusion.ApolloFederation;
+
+internal static class FederationDirectiveNames
+{
+    public const string Key = "key";
+    public const string Requires = "requires";
+    public const string Provides = "provides";
+    public const string External = "external";
+    public const string Link = "link";
+    public const string Shareable = "shareable";
+    public const string Inaccessible = "inaccessible";
+    public const string Override = "override";
+    public const string Tag = "tag";
+    public const string InterfaceObject = "interfaceObject";
+    public const string ComposeDirective = "composeDirective";
+    public const string Authenticated = "authenticated";
+    public const string RequiresScopes = "requiresScopes";
+    public const string Policy = "policy";
+}

src/HotChocolate/Fusion/src/Fusion.Composition.ApolloFederation/FederationFieldNames.cs

@@ -0,0 +1,7 @@
+namespace HotChocolate.Fusion.ApolloFederation;
+
+internal static class FederationFieldNames
+{
+    public const string Entities = "_entities";
+    public const string Service = "_service";
+}

src/HotChocolate/Fusion/src/Fusion.Composition.ApolloFederation/FederationSchemaAnalyzer.cs

@@ -0,0 +1,105 @@
+using HotChocolate.Fusion.Errors;
+using HotChocolate.Language;
+using HotChocolate.Types.Mutable;
+using static HotChocolate.Fusion.ApolloFederation.Properties.FederationResources;
+
+namespace HotChocolate.Fusion.ApolloFederation;
+
+/// <summary>
+/// Validates a <see cref="MutableSchemaDefinition"/> for Apollo Federation v2 compatibility
+/// and detects unsupported directives.
+/// </summary>
+internal static class FederationSchemaAnalyzer
+{
+    private const string FederationUrlPrefix = "specs.apollo.dev/federation";
+
+    private static readonly HashSet<string> s_unsupportedDirectives =
+    [
+        FederationDirectiveNames.ComposeDirective,
+        FederationDirectiveNames.Authenticated,
+        FederationDirectiveNames.RequiresScopes,
+        FederationDirectiveNames.Policy,
+        FederationDirectiveNames.InterfaceObject
+    ];
+
+    /// <summary>
+    /// Validates the given federation schema and returns any composition errors.
+    /// </summary>
+    /// <param name="schema">
+    /// The mutable schema definition to validate.
+    /// </param>
+    /// <returns>
+    /// A list of <see cref="CompositionError"/> instances. An empty list indicates success.
+    /// </returns>
+    public static List<CompositionError> Validate(MutableSchemaDefinition schema)
+    {
+        var errors = new List<CompositionError>();
+
+        ValidateFederationVersion(schema, errors);
+        ValidateUnsupportedDirectives(schema, errors);
+
+        return errors;
+    }
+
+    private static void ValidateFederationVersion(
+        MutableSchemaDefinition schema,
+        List<CompositionError> errors)
+    {
+        var federationVersion = FindFederationVersion(schema);
+
+        if (federationVersion is null)
+        {
+            errors.Add(new CompositionError(FederationSchemaAnalyzer_FederationV1NotSupported));
+        }
+    }
+
+    private static string? FindFederationVersion(MutableSchemaDefinition schema)
+    {
+        foreach (var directive in schema.Directives)
+        {
+            if (!directive.Name.Equals(FederationDirectiveNames.Link, StringComparison.Ordinal))
+            {
+                continue;
+            }
+
+            if (!directive.Arguments.TryGetValue("url", out var urlValue)
+                || urlValue is not StringValueNode urlString)
+            {
+                continue;
+            }
+
+            var url = urlString.Value;
+
+            if (!url.Contains(FederationUrlPrefix, StringComparison.Ordinal))
+            {
+                continue;
+            }
+
+            // Extract version from URL like
+            // "https://specs.apollo.dev/federation/v2.5"
+            var lastSlash = url.LastIndexOf('/');
+
+            if (lastSlash >= 0 && lastSlash < url.Length - 1)
+            {
+                return url[(lastSlash + 1)..];
+            }
+        }
+
+        return null;
+    }
+
+    private static void ValidateUnsupportedDirectives(
+        MutableSchemaDefinition schema,
+        List<CompositionError> errors)
+    {
+        foreach (var name in s_unsupportedDirectives)
+        {
+            if (schema.DirectiveDefinitions.ContainsName(name))
+            {
+                errors.Add(new CompositionError(string.Format(
+                    FederationSchemaAnalyzer_DirectiveNotSupported,
+                    name)));
+            }
+        }
+    }
+}

src/HotChocolate/Fusion/src/Fusion.Composition.ApolloFederation/FederationSchemaTransformer.cs

@@ -0,0 +1,58 @@
+using System.Collections.Immutable;
+using HotChocolate.Fusion.Errors;
+using HotChocolate.Fusion.Results;
+using HotChocolate.Language;
+using HotChocolate.Types.Mutable;
+using HotChocolate.Types.Mutable.Serialization;
+using static HotChocolate.Fusion.ApolloFederation.Properties.FederationResources;
+
+namespace HotChocolate.Fusion.ApolloFederation;
+
+/// <summary>
+/// Transforms an Apollo Federation v2 subgraph SDL into a Composite Schema Spec
+/// source schema SDL suitable for the HotChocolate Fusion composition pipeline.
+/// </summary>
+public static class FederationSchemaTransformer
+{
+    /// <summary>
+    /// Transforms the given Apollo Federation v2 subgraph SDL.
+    /// </summary>
+    /// <param name="federationSdl">
+    /// The Apollo Federation v2 subgraph SDL to transform.
+    /// </param>
+    /// <returns>
+    /// A <see cref="CompositionResult{TValue}"/> containing the transformed SDL string
+    /// on success, or composition errors on failure.
+    /// </returns>
+    public static CompositionResult<string> Transform(string federationSdl)
+    {
+        ArgumentException.ThrowIfNullOrEmpty(federationSdl);
+
+        MutableSchemaDefinition schema;
+
+        try
+        {
+            schema = SchemaParser.Parse(federationSdl);
+        }
+        catch (SyntaxException ex)
+        {
+            return new CompositionError(
+                string.Format(FederationSchemaTransformer_ParseFailed, ex.Message));
+        }
+
+        var errors = FederationSchemaAnalyzer.Validate(schema);
+
+        if (errors.Count > 0)
+        {
+            return errors.ToImmutableArray();
+        }
+
+        RemoveFederationInfrastructure.Apply(schema);
+        GenerateLookupFields.Apply(schema);
+        RewriteKeyDirectives.Apply(schema);
+        TransformRequiresToRequire.Apply(schema);
+        RemoveExternalFields.Apply(schema);
+
+        return SchemaFormatter.FormatAsString(schema);
+    }
+}

src/HotChocolate/Fusion/src/Fusion.Composition.ApolloFederation/FederationTypeNames.cs

@@ -0,0 +1,10 @@
+namespace HotChocolate.Fusion.ApolloFederation;
+
+internal static class FederationTypeNames
+{
+    public const string Any = "_Any";
+    public const string Entity = "_Entity";
+    public const string Service = "_Service";
+    public const string FieldSet = "FieldSet";
+    public const string LegacyFieldSet = "_FieldSet";
+}