Add Support for Hot Reload of GraphQL Schemas (#2510)

## Why make this change?

- Closes #2431 
- This change is necessary to enable dynamic updates to GraphQL schemas
without requiring a full application restart.
- It enhances the developer experience by allowing schema changes to be
reflected immediately during runtime, improving efficiency in
development and deployment processes.

## What is this change?

- Addition of new constants in `DabConfigEvents.cs` for GraphQL schema
events.
- Updates to `HotReloadEventHandler.cs` to include handlers for GraphQL
schema-related events.
- Modifications in `RuntimeConfigLoader.cs` to ensure proper firing
order of events for schema eviction and creation.
- Updates to `GraphQLSchemaCreator.cs` to handle configuration changes
and update runtime entities accordingly.
- Modifications in `Startup.cs` to register hot reload event handlers
and methods for refreshing and evicting the GraphQL schema.

## How was this tested?

- [X] Integration Tests
- [X] Manual Tests

## Sample Request(s)


https://github.com/user-attachments/assets/0a2e6353-d466-453e-b1d3-d2f64b3f1d75

src/Config/DabConfigEvents.cs

@@ -15,4 +15,8 @@ public static class DabConfigEvents
     public const string POSTGRESQL_QUERY_EXECUTOR_ON_CONFIG_CHANGED = "POSTGRESQL_QUERY_EXECUTOR_ON_CONFIG_CHANGED";
     public const string DOCUMENTOR_ON_CONFIG_CHANGED = "DOCUMENTOR_ON_CONFIG_CHANGED";
     public const string AUTHZ_RESOLVER_ON_CONFIG_CHANGED = "AUTHZ_RESOLVER_ON_CONFIG_CHANGED";
+    public const string GRAPHQL_SCHEMA_REFRESH_ON_CONFIG_CHANGED = "GRAPHQL_SCHEMA_REFRESH_ON_CONFIG_CHANGED";
+    public const string GRAPHQL_SCHEMA_EVICTION_ON_CONFIG_CHANGED = "GRAPHQL_SCHEMA_EVICTION_ON_CONFIG_CHANGED";
+    public const string GRAPHQL_SCHEMA_CREATOR_ON_CONFIG_CHANGED = "GRAPHQL_SCHEMA_CREATOR_ON_CONFIG_CHANGED";
+
 }

src/Config/HotReloadEventHandler.cs

@@ -30,7 +30,10 @@ public HotReloadEventHandler()
             { QUERY_ENGINE_FACTORY_ON_CONFIG_CHANGED, null },
             { MUTATION_ENGINE_FACTORY_ON_CONFIG_CHANGED, null },
             { DOCUMENTOR_ON_CONFIG_CHANGED, null },
-            { AUTHZ_RESOLVER_ON_CONFIG_CHANGED, null }
+            { AUTHZ_RESOLVER_ON_CONFIG_CHANGED, null },
+            { GRAPHQL_SCHEMA_CREATOR_ON_CONFIG_CHANGED, null },
+            { GRAPHQL_SCHEMA_REFRESH_ON_CONFIG_CHANGED, null },
+            { GRAPHQL_SCHEMA_EVICTION_ON_CONFIG_CHANGED, null }
         };
     }
 

src/Config/RuntimeConfigLoader.cs

@@ -100,6 +100,11 @@ protected void SignalConfigChanged(string message = "")
         // RuntimeConfig must already be updated and is implied to have been updated by the time
         // this function is called.
         OnConfigChangedEvent(new HotReloadEventArgs(AUTHZ_RESOLVER_ON_CONFIG_CHANGED, message));
+
+        // Order of event firing matters: Eviction must be done before creating a new schema and then updating the schema.
+        OnConfigChangedEvent(new HotReloadEventArgs(GRAPHQL_SCHEMA_EVICTION_ON_CONFIG_CHANGED, message));
+        OnConfigChangedEvent(new HotReloadEventArgs(GRAPHQL_SCHEMA_CREATOR_ON_CONFIG_CHANGED, message));
+        OnConfigChangedEvent(new HotReloadEventArgs(GRAPHQL_SCHEMA_REFRESH_ON_CONFIG_CHANGED, message));
     }
 
     /// <summary>

src/Core/Services/GraphQLSchemaCreator.cs

@@ -5,6 +5,7 @@
 using System.Collections.ObjectModel;
 using System.Net;
 using Azure.DataApiBuilder.Auth;
+using Azure.DataApiBuilder.Config;
 using Azure.DataApiBuilder.Config.DatabasePrimitives;
 using Azure.DataApiBuilder.Config.ObjectModel;
 using Azure.DataApiBuilder.Core.Configurations;
@@ -39,7 +40,7 @@ public class GraphQLSchemaCreator
         private readonly IQueryEngineFactory _queryEngineFactory;
         private readonly IMutationEngineFactory _mutationEngineFactory;
         private readonly IMetadataProviderFactory _metadataProviderFactory;
-        private readonly RuntimeEntities _entities;
+        private RuntimeEntities _entities;
         private readonly IAuthorizationResolver _authorizationResolver;
         private readonly RuntimeConfigProvider _runtimeConfigProvider;
         private bool _isMultipleCreateOperationEnabled;
@@ -52,13 +53,16 @@ public class GraphQLSchemaCreator
         /// <param name="mutationEngineFactory">MutationEngineFactory to retreive mutation engine to be used by resolvers.</param>
         /// <param name="metadataProviderFactory">MetadataProviderFactory to get metadata provider used when generating the SQL-based GraphQL schema. Ignored if the runtime is Cosmos.</param>
         /// <param name="authorizationResolver">Authorization information for the runtime, to be applied to the GraphQL schema.</param>
+        /// <param name="handler">Optional hot-reload event handler to subscribe to the config change event.</param>
         public GraphQLSchemaCreator(
             RuntimeConfigProvider runtimeConfigProvider,
             IQueryEngineFactory queryEngineFactory,
             IMutationEngineFactory mutationEngineFactory,
             IMetadataProviderFactory metadataProviderFactory,
-            IAuthorizationResolver authorizationResolver)
+            IAuthorizationResolver authorizationResolver,
+            HotReloadEventHandler<HotReloadEventArgs>? handler = null)
         {
+            handler?.Subscribe(DabConfigEvents.GRAPHQL_SCHEMA_CREATOR_ON_CONFIG_CHANGED, OnConfigChanged);
             RuntimeConfig runtimeConfig = runtimeConfigProvider.GetConfig();
 
             _isMultipleCreateOperationEnabled = runtimeConfig.IsMultipleCreateOperationEnabled();
@@ -70,6 +74,18 @@ public GraphQLSchemaCreator(
             _runtimeConfigProvider = runtimeConfigProvider;
         }
 
+        /// <summary>
+        /// Executed when a hot-reload event occurs. Pulls the latest
+        /// runtimeconfig object from the provider and updates the flag indicating
+        /// whether multiple create operations are enabled, and the entities based on the new config.
+        /// </summary>
+        protected void OnConfigChanged(object? sender, HotReloadEventArgs args)
+        {
+            RuntimeConfig runtimeConfig = _runtimeConfigProvider.GetConfig();
+            _isMultipleCreateOperationEnabled = runtimeConfig.IsMultipleCreateOperationEnabled();
+            _entities = runtimeConfig.Entities;
+        }
+
         /// <summary>
         /// Take the raw GraphQL objects and generate the full schema from them.
         /// At this point, we're somewhat agnostic to whether the runtime is Cosmos or SQL