Host ASP.NET Core in a web farm

By Luke Latham and Chris Ross

A web farm is a group of two or more web servers (or nodes) that host multiple instances of an app. When requests from users arrive to a web farm, a load balancer distributes the requests to the web farm's nodes. Web farms improve:

  • Reliability/availability – When one or more nodes fail, the load balancer can route requests to other functioning nodes to continue processing requests.
  • Capacity/performance – Multiple nodes can process more requests than a single server. The load balancer balances the workload by distributing requests to the nodes.
  • Scalability – When more or less capacity is required, the number of active nodes can be increased or decreased to match the workload. Web farm platform technologies, such as Azure App Service, can automatically add or remove nodes at the request of the system administrator or automatically without human intervention.
  • Maintainability – Nodes of a web farm can rely on a set of shared services, which results in easier system management. For example, the nodes of a web farm can rely upon a single database server and a common network location for static resources, such as images and downloadable files.

This topic describes configuration and dependencies for ASP.NET core apps hosted in a web farm that rely upon shared resources.

General configuration

Host and deploy ASP.NET Core
Learn how to set up hosting environments and deploy ASP.NET Core apps. Configure a process manager on each node of the web farm to automate app starts and restarts. Each node requires the ASP.NET Core runtime. For more information, see the topics in the Host and deploy area of the documentation.

Configure ASP.NET Core to work with proxy servers and load balancers
Learn about configuration for apps hosted behind proxy servers and load balancers, which often obscure important request information.

Deploy ASP.NET Core apps to Azure App Service
Azure App Service is a Microsoft cloud computing platform service for hosting web apps, including ASP.NET Core. App Service is a fully managed platform that provides automatic scaling, load balancing, patching, and continuous deployment.

App data

When an app is scaled to multiple instances, there might be app state that requires sharing across nodes. If the state is transient, consider sharing an IDistributedCache. If the shared state requires persistence, consider storing the shared state in a database.

Required configuration

Data Protection and Caching require configuration for apps deployed to a web farm.

Data Protection

The ASP.NET Core Data Protection system is used by apps to protect data. Data Protection relies upon a set of cryptographic keys stored in a key ring. When the Data Protection system is initialized, it applies default settings that store the key ring locally. Under the default configuration, a unique key ring is stored on each node of the web farm. Consequently, each web farm node can't decrypt data that's encrypted by an app on any other node. The default configuration isn't generally appropriate for hosting apps in a web farm. An alternative to implementing a shared key ring is to always route user requests to the same node. For more information on Data Protection system configuration for web farm deployments, see Configure ASP.NET Core Data Protection.

Caching

In a web farm environment, the caching mechanism must share cached items across the web farm's nodes. Caching must either rely upon a common Redis cache, a shared SQL Server database, or a custom caching implementation that shares cached items across the web farm. For more information, see Work with a distributed cache in ASP.NET Core.

Dependent components

The following scenarios don't require additional configuration, but they depend on technologies that require configuration for web farms.

Scenario Depends on …
Authentication Data Protection (see Configure ASP.NET Core Data Protection).

For more information, see Use cookie authentication without ASP.NET Core Identity and Share cookies among apps with ASP.NET and ASP.NET Core.
Identity Authentication and database configuration.

For more information, see Introduction to Identity on ASP.NET Core.
Session Data Protection (encrypted cookies) (see Configure ASP.NET Core Data Protection) and Caching (see Work with a distributed cache in ASP.NET Core).

For more information, see Session and app state: Session state.
TempData Data Protection (encrypted cookies) (see Configure ASP.NET Core Data Protection) or Session (see Session and app state: Session state).

For more information, see Session and app state: TempData.
Anti-forgery Data Protection (see Configure ASP.NET Core Data Protection).

For more information, see Prevent Cross-Site Request Forgery (XSRF/CSRF) attacks in ASP.NET Core.

Troubleshoot

When Data Protection or Caching isn't configured for a web farm environment, intermittent errors occur when requests are processed. This occurs because nodes don't share the same resources and user requests aren't always routed back to the same node.

Consider a user who signs into the app using cookie authentication. The user signs into the app on one web farm node. If their next request arrives at the same node where they signed in, the app is able to decrypt the authentication cookie and allows access to the app's resource. If their next request arrives at a different node, the app can't decrypt the authentication cookie from the node where the user signed in, and authorization for the requested resource fails.

When any of the following symptoms occur intermittently, the problem is usually traced to improper Data Protection or Caching configuration for a web farm environment:

  • Authentication breaks – The authentication cookie is misconfigured or can't be decrypted. OAuth (Facebook, Microsoft, Twitter) or OpenIdConnect logins fail with the error "Correlation failed."
  • Authorization breaks – Identity is lost.
  • Session state loses data.
  • Cached items disappear.
  • TempData fails.
  • POSTs fail – The anti-forgery check fails.

For more information on Data Protection configuration for web farm deployments, see Configure ASP.NET Core Data Protection. For more information on Caching configuration for web farm deployments, see Work with a distributed cache in ASP.NET Core.