Full-Site Delivery

These articles describe how to use Fastly's CDN.


Caching

These articles describe configuration settings and changes you can make to your cache settings when setting up Fastly services.

To ensure optimum origin performance during times of increased demand or during scheduled downtime for your servers, consider the following…

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Checking the cache status of an object on your website can help when troubleshooting problems. You can use the web interface or the curl…

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When we store your content in cache, we calculate a Time to Live (TTL). The TTL is the maximum amount of time we will use the content to…

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Application Programming Interfaces (APIs) allow you to retrieve data from a variety of web services. Fastly makes it possible for you to…

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This guide explains how to implement API cache control. Once you've enabled API caching , and ensured purging works properly with your…

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In certain situations you may want to conditionally apply a different caching policy based on a backend response. In this particular case…

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Fastly service configurations may be vulnerable to cache poisoning if they do not take into consideration the interaction between HTTP "X…

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Fastly’s Segmented Caching feature allows you to cache resources of any size. Segmented Caching works by breaking resources into smaller…

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Caching can be disabled: at the individual URL level, at the browser level, and at the site level. Disabling caching at the individual URL…

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Conditions

These articles describe conditions and how to use them in VCL and the Fastly web interface.

Conditions are a configuration mechanism that you can add to your service to control how particular requests are processed. Like conditions…

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If you are having problems using conditions, here are some common things to look for. Check the Apply if field for if statements Most…

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Conditions use the Varnish Configuration Language (VCL) to define when a configuration object should be applied while processing requests…

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Custom VCL

These articles describe how to create your own VCL files with specialized configurations.

VCL Snippets are short blocks of VCL logic that can be included directly in your service configurations. They're ideal for adding small…

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Basic authentication is a simple way of protecting a website at the edge. Users enter a username and password combination to access pages…

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Fastly can send custom responses for certain requests that you don't want to hit your origin servers. Creating a quick response Fastly…

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Best practices in Fastly VCL have changed over time to help address expectation gaps and improve maintainability. This page covers some of the most common use cases in edge logic and demonstrates how to avoid bad code, reduce risk, improve safety, and take steps to make the codebase more maintainable for large teams.

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In normal operation, a request directed to a Fastly POP will be handled by two separate cache servers acting together in a process we call clustering.

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If a fatal error occurs in your service while processing your VCL logic, Fastly will generate an error response with a 503 HTTP status, instead of using a response from an upstream server or cache, and unless intercepted by code running in vcl_error, this generated error may be served to the end user.

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VCL services offer an efficient mechanism to manage and control traffic to your domains, powered by Fastly's variant of the Varnish Configuration Language (VCL).

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Fastly VCL is a domain specific programming language which has evolved from the Varnish proxy cache, which is part of Fastly's platform architecture.

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Fastly’s Edge Cloud services use the Fastly Varnish Configuration Language (VCL), a scripting language used to configure and add logic to…

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Fastly supports the ability to extract header subfield values without regular expressions in a human-readable way. Headers subfields are…

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Before you begin If your origin uses special values (e.g., request headers) to select content for users or to otherwise direct requests to…

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The traditional way to read response cookies in VCL is to inspect either the beresp.http.Set-Cookie or the resp.http.Set-Cookie…

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Being able to track information related to your origin can be helpful in troubleshooting errors and making sure requests are processed as…

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Fastly allows you create your own Varnish Configuration Language (VCL) files with specialized configurations. By uploading custom VCL files…

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Passing with a request setting and with a cache setting triggers very different behavior in Varnish . Within VCL, passing with a…

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Dynamic VCL Snippets are one of two types of snippets that allow you to insert small sections of VCL logic into your service configuration…

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Regular VCL Snippets are one of two types of snippets that allow you to insert small sections of VCL logic into your service configuration…

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Fastly VCL uses a subset of Perl Compatible Regular Expression (PCRE) syntax. This is case sensitive and forward slashes don't need to be…

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Dictionaries

These articles describe how to move rapid key/value pair decision logic to the edge using dictionaries.

Dictionaries are a type of container that allow you to store data as key-value pairs that can be used in a service without being attached…

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Dictionaries are a type of container that allow you to store data as key-value pairs that can be used in a service without being attached…

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Domains and origins

These articles describe configuration settings and changes you can make to your domains and origins when setting up Fastly services.

This guide describes how to automatically load balance between two or more origin servers. Load balancing distributes requests across…

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Connection timeouts to your origin server control how long Fastly will wait for a response from your origin server before exiting with an…

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Fastly allows you to change origin servers based on the user's geographic location. This is useful when you need to serve different content…

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This guide describes how to configure failover origin servers. Failover (backup) servers ensure you can maintain availability of your…

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Fastly has integrated IPv6 into its technology stack. By enabling IPv6, visitors on IPv6 connections can access your websites and…

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It is common to use the same origin web application to serve both HTTP and HTTPS requests and let the application determine which actions to…

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Sometimes you want to set up two different origin servers, one as a primary and one as a backup in case the primary becomes unavailable. You…

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To rewrite the Host header being sent to your origin regardless of the Host used in the initial request, specify an override host. Use this…

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Some customers use only their second-level or apex domain (e.g., example.com rather than www.example.com ) as their canonical domain. Due…

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Headers

These articles describe configuration settings and changes you can make to your headers when setting up Fastly services.

HTTP header fields are components of the header section of request and response messages in the Hypertext Transfer Protocol (HTTP). They…

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Enabling Cross-Origin Resource Sharing (CORS) allows a server to indicate that other origins can request sub-resources, like scripts and…

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You can remove headers from any backend response. This may be necessary if your application automatically sets headers. For example, Drupal…

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In some situations you may want to override the content type that a backend returns. To do that you will need to create a new header object…

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Image optimization

These articles provide basic instructions for and examples of setting up and beginning to use the Fastly Image Optimizer.

Fastly's Image Optimizer (Fastly IO) is an image optimization service that manipulates and transforms your images in real time and caches…

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Observability

These articles provide information about monitoring your services via the web interface.

When an observed metric either exceeds or falls below a threshold, the Alerts feature displays a notification in the Fastly application and…

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From the Observability tab, you can access dashboards that contain collections of metrics represented as charts. Depending on the…

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Performance

These articles describe how to adjust the performance of Fastly's services beyond standard configuration methods.

We think the mtr tool offers a great way to test network speed, evaluate performance, and perform connection diagnostics. The mtr tool…

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Compression can help you reduce the size of your assets so traffic can flow faster. You can use Fastly to compress data automatically on our…

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This guide describes how to enable HTTP/3 for your Fastly services. About HTTP/3 HTTP/3 uses a web transport protocol standard called QUIC…

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If you haven't enabled Segmented Caching , you may encounter the following failure modes when working with large objects. If you created…

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Fastly continuously monitors the performance and status of our global network . We constantly monitor the health of all its related…

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HTTP/2 server push allows you to set up rules that enable Fastly to pre-emptively load and then send responses to an HTTP/2-compliant client…

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Under normal circumstances, Fastly would consider these URLs different objects that are cached separately: http://example.com http://example…

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Fastly can automatically detect and, in real time, route around transient connection problems that occur when fetching content from your…

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Fastly can optionally serve stale content when there is a problem with your origin server or if new content is taking a long time to fetch…

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When fetching an object from the origin, the Streaming Miss feature ensures the response is streamed back to the client immediately and is…

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Purging

These articles describe how to purge cache.

Fastly's URL purge feature allows you to purge individual URLs on your website. By default, authentication is not required to purge a URL…

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Fastly provides several levels of cache purging. You can use the Purge URL option to purge a single URL via the web interface. Before you…

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Fastly provides several levels of cache purging. You can use the Purge all option to purge all content under a service. Before you begin…

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Fastly provides several levels of cache purging and choosing the right purging method is essential to keeping your website fast. While…

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Fastly provides a Soft Purge feature that allows you to mark content as outdated ( stale ). Stale objects remain available to use in some…

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Surrogate keys are unique identifiers that you assign to groups of content for processing. While there are many use cases for surrogate keys…

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Requests

These articles describe configuration settings and changes you can make to requests when working with Fastly services.

Using a restart is a good option to check multiple backends for a single request. This can be created using a cache setting rule and…

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To conditionally change a URL based on the domain, include VCL that looks something like this: If you have shielding enabled, however, add…

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Request settings are applied based on the Action you select in the Create a new request setting page. You can choose any one of the…

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Responses

These articles describe configuration settings and changes you can make to your response settings when setting up Fastly services.

The robots.txt file tells web robots how to crawl webpages on your website. You can use Fastly's web interface to create and configure a…

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The default error responses served by Fastly can be jarring for your users, especially when using Fastly for consumer applications. To…

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You can use the Redirect traffic to www subdomains setting to redirect traffic for apex domains, wildcard domains, or subdomains to a www…

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Fastly allows you to create custom HTTP responses that are served directly from the cache without storing the page on a server. Responses…

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Video

These articles describe setup and configuration guidelines for setting up live stream delivery or video on-demand.

Fastly's On-the-Fly Packaging (OTFP) service supports any directory structure you might use to store different quality levels of a video…

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Fastly allows you to collect and process On-the-Fly Packaging (OTFP) service metrics for analysis using a combination of custom VCL…

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This page lists some possible error values that Fastly's On-The-Fly Packaging service (OTFP) service will send in the X-Fastly-Package…

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The Fastly network can deliver live streams for any HTTP streaming technology , archived or recorded, on any public or private cloud…

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