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## CDN settings

The purpose with a content delivery network or content distribution network (CDN) is to ensure high availability when serving content to visitors. CDN consists of a globally distributed network of proxy servers deployed in multiple data centers. CDNs deliver content from the fastest and closest server available.

The CDN servers are designed to cache content according to the cache rules you define in the HTTP headers from your application. It is critical that these caching rules are correctly set for your solution to scale properly.  See [CDN Recommendations](🔗).

## Site startup

Proper [site warmup](🔗) is crucial for cloud-based environments. Since a node may be brought out for maintenance at any time, and then put back in during peak hours, a node that gets a full share of traffic without being warmed up first will cause response-time spikes and increasing risk of outages.

Use custom scripts, or the built-in ASP.NET feature that automatically starts up and initializes a web application, to warm up the server and getting data caches ready. See [Initialization](🔗).

Limiting the number of content types is a good practice. During a startup, assemblies are scanned and views are cached. A large number (200+) of content types significantly affects the startup time. You also should keep the Web App below 1 GB at all times. This includes binaries and .cshtml views, but _not_ media assets and logs that should be written to a BLOB storage container.

## Output cache

Cloud-based solutions are more likely to scale out the web servers rather than to scale them up. This means that each front-end node also contributes to a constant load to the database. In other words, if you go from two front-end servers (a typical on-premises setup) to four front-end servers, while keeping the total throughput the same, the load on the database server increases.

When scaling out, it is important to ensure that the machines that spend most effort building a page are the front-end servers. Caching in _multiple layers_ (object caches, partial HTML caches such as for complex menus and/or full output cache), helps avoid a "cache stampede", especially when combined with warmup.

By default when a page is published, all output caches are immediately invalidated for all sites. This causes all output cached pages to be re-rendered using the lower-level caches. Most of these lower-level caches remain valid after a publish, except the caches for the page that was published. It is therefore important to ensure that proper multi-layer/partial caching is implemented for rendered pages with heavy data processing. See [Caching](🔗)

## Entity tags (ETags)

The _ETag_ or _entity tag_ is part of HTTP protocol, and is used to determine web cache validation. See [CDN Recommendations](🔗) for information about using ETags.

## Resilience

In a cloud environment, _retry policies_ become increasingly important. Transient errors may occur due to network issues, or maintenance of infrastructure elements, and retry policies let the application gracefully recover from such errors without propagating the error to the end user.

Retry mechanisms for Azure services differ, because each service has different requirements and characteristics. Therefore, each retry mechanism is tuned to a specific service. See information about [transient faults](🔗) and [retry policies](🔗), and the [Azure SDK for .NET](🔗) for guidelines.

## Storage

Because the virtual machines hosting a Web App may be restarted at any time, you risk losing any information stored in the file system. Also, if you have large media volumes, you should store assets in a BLOB storage instead of in the Web App, because this will limit scalability. Optimizely provides access to BLOB storage through a [BlobProvider interface](🔗).


Some third-party components such as Lucene.NET that use file shares or files local to the web server, may have problems with high traffic in a cloud environment, and are therefore not supported.