Code examples in VCL
These code examples have an implementation in VCL, the language supported by our VCL platform. To learn more see using VCL.
Geo-IP API at the edge
Create an API endpoint for fetching GeoIP data for the requesting browser, implemented 100% at the edge. The response should show your current approximate location, but no requests to any origin servers.
Tag requests with geolocation data
Add GeoIP data about the client browser as extra headers in any requests from Fastly to your origin.
Apply feature flags from an origin API
Park request, make a different request first, use the response to annotate the real origin request (or make decisions about how to route it).
Enforce a paywall using an origin API
Use a custom Paywall header to trigger preflight requests to authenticate every article view with a backend paywall service.
Detect leaked passwords
Detect requests that contain submitted passwords and use a service to determine whether the password has leaked before allowing the request to proceed to origin (data from haveibeenpwned).
Convert timestamps to relative times like '2 hours ago'
Add www. to apex hostname and subdomains
Detect requests that don't include a www. prefix, and redirect to the equivalent path on a hostname that starts with www., usually to make sure there's only one canonical location for your content.
Serve stale to search crawlers for better ranking
Prioritize human traffic over search crawlers by serving stale content to crawlers.
Support caching of OPTIONS requests
Normally OPTIONS requests aren't cacheable. Allow caching of OPTIONS by converting the request to a GET and back to OPTIONS if it's a cache miss.
Override TTLs based on content type
Set TTLs at the edge based on the type of resource. Better done at origin, but this can be a great 'quick fix' or a solution if you don't control the origin.
Override TTLs for path prefixes
Set TTLs at the edge based on looking up a path prefix in an edge dictionary.
Push prices or breaking news in real time with Server-Sent-Events
Stream responses to the browser while still receiving data from the origin and also saving it to cache. Great for spreading out server-sent-events streams to millions of users from a single source stream.
Map IPv6 addresses to IPv4 address space
Create an identifier that looks like an IPv4 address if the user has made their request using IPv6. Useful for origin servers that require an IPv4 address for some reason.
Capture and aggregate log data from client devices
Collect and aggregate log data submitted from browsers directly into S3 or another log store without having to handle the traffic at your origin.
Use microservices to divide up a domain
Send request to different origin servers based on the URL path.
Replace origin errors with 'safe' responses
Detect specified response statuses from backends and instead serve a precomposed error page or error content generated at the edge.
Answer CORS OPTIONS preflight requests at the edge
Browsers send OPTIONS requests before performing cross-origin POSTs. You can answer these requests directly from the edge.
Apply HTTP basic auth to private endpoints
Store username/password list in an edge dictionary, authorize user at the edge, reject requests that don't have correct credentials.
Serve robots.txt from the edge
Serve full text of robots.txt as a synthetic response to avoid requests hitting your origin.
Serve stale content from cache while origins are offline
Deal with all potential scenarios for using stale content to satisfy requests when origin is unhealthy or misbehaving.
Load balance randomly across multiple backends
Load balance requests randomly across multiple backends, dropping them automatically if they become unhealthy.
Load balance to optimise upstream caching with hash directors
Map requests to backends consistently, which can be useful to improve your internal cache and replication efficiency.
Cycle through backends until a healthy server is found
Try backends in turn until one is healthy (also known as an 'active-standby' strategy).
Sticky sessions using client directors
Map requests to backends based on user ID (a.k.a., "sticky sessions").
Log the data you need for actionable insights
Emit logging data to your chosen log endpoint from any VCL stage, not just vcl_log.
Support video scrubbing or download resumption using Range requests
Range headers sent from client are stripped by Fastly so we can cache the full object at the edge.
Serve small binary assets direct from edge
Serve binary objects, such as images, directly from edge configuration.
Make very large payloads visible in VCL using custom headers
Create self-destructing URLs for time-limited access
Make URLs expire after a configurable period.
Transform and optimize images on the fly
Use Fastly Image Optimizer to transform and serve images at the edge, closer to your users.
Schedule a cache invalidation at a predefined time
Purge your edge cache automatically at a certain time. This may cause an inrush of traffic to origin at the scheduled time and should be used with care.
Enable modern web security headers to all responses
Go from an F to an A grade on securityheaders.io by adding security policy headers to your responses at the edge.
Configuration version discrepancy detection
Detect situations when a shield server might have a different version of your code than the edge server, and avoid errors that this might cause.
Add, remove or change HTTP headers
Fastly can easily read and write HTTP headers at multiple stages of the request/response cycle.
Rewrite URL path
Receive a request for one path but request a different path from origin, without a redirect.
Filter query string parameters
Add, remove, and sort querystring parameters.
Change request method
Change PUT, DELETE, OPTIONS and others to POST, or vice versa, to help integrate incompatible client and server apps.
Compress content before caching
Compress HTML, SVG, and other compressible formats at the edge and store and serve both compressed and uncompressed versions.
Client public IP API at the edge
Quickly fetch the user's public IP from an API endpoint on your own domain, with no origin.
Add or remove cookies
Read individual cookies, set new cookies in response.
Filter cookies or other structured headers
Rewrite headers to keep only keys that you want to allow, similar to `querystring.filter_except` but for headers rather than querystrings.
Remove trailing slashes to normalize URLs
Treat URLs with and without suffixed slashes as equivalent, or redirect URLs with slashes to the version without.
Caching responses to POST requests
By default, Fastly does not cache responses to POST requests. But you can enable this if you wish.
Regular expression capturing patterns
Capture regex pattern groups to use in matching url segments and more.
Ban bad IPs for a fixed period
Block a list of IP addresses from accessing your service and include an expiry time.
ACL based IP block list
Block a list of IP address ranges from accessing your service.
Apply CAPTCHA to high risk requests
Intercept suspicious traffic and display a CAPTCHA challenge. If the user passes, allow the request to go to the origin server.
Normalize requests to increase cache efficiency
Improve cache performance by normalizing requests. Filter and reorder query params, convert to lowercase, filter headers, and more.
Search and replace in strings
Use regular expression substitution functions to map paths, strip extraneous slashes, and more.
Google Cloud Storage origin (public)
Use a public GCS bucket as a backend for your Fastly service.
Google Cloud Storage origin (private)
Use AWS compat mode to make authenticated requests to your GCS bucket.
AWS S3 bucket origin (private)
Use AWS authenticated requests (signature version 4) to protect communication between your Fastly service and AWS.
Azure blob storage bucket origin (private)
Use Microsoft Azure authenticated requests to protect communication between your Fastly service and Azure.
Alibaba Object Storage Service origin (private)
Use authenticated requests to protect communication between your Fastly service and Alibaba's Object Storage Service.
Log request data to Google BigQuery
Build raw JSON strings matching your BigQuery table schema to send log data to BigQuery.
Support matching on unicode in VCL. すごい！
Represent non-ASCII characters in VCL using unicode escapes.
Base64 POST body
Access the body of a POST request in Base64-encoded form.
Generate random whole numbers in a range.
Extract a substring from a string value
Isolate a portion of a string identified by a range of characters.
POST to GET rewrite using a Base64-encoded querystring
To allow caching of POST requests, consider rewriting them as GET requests at the edge.
Early expiry of cached objects
Cached a large number of objects for too long and want to update and shorten their TTLs.
Smoke test a new origin
Send a copy of your traffic to a test origin before returning a response from production.
Compute intersection of two lists
Useful for comparing capabilities with required permissions.
Prevent hotlinking of product images by third party websites
Detect and reject requests from third party websites that attempt to embed your images on their pages.
Prohibit browser caching
Ensure resources are not cached on the front end, while allowing caching within Fastly.
Assemble content at the edge with Edge-side includes (ESI)
Use Fastly's support for ESI to combine multiple origin-hosted objects into a single response at the edge.
Clean backend responses
Remove headers added by backends that you don't want to emit to the browser, like amz- or goog- headers.
Redirect insecure HTTP to TLS (SSL, HTTPS) in VCL
Redirect any requests that come into a VCL service on insecure HTTP, to the equivalent TLS endpoint (Compute@Edge does this automatically).
Follow redirects at the edge
Protect clients from redirects by chasing them internally at the edge, and then return the eventual non-redirect response.
Cache '429' rate-limiter responses per IP
If a backend returns a 429, cache it for the requesting IP but continue to allow other clients to use origin.
Vary based on a cookie
Return different objects based on the presence of a cookie.
Remove querystring from static assets
Use the new Sec-Fetch-Dest header or URL patterns to identify assets that should not allow querystrings to be part of the cache key.
Geofence / block access to content by region
Group countries to cache content by custom regions or reject requests from some regions entirely.
Cache 'near you' content for each area using lat/long grid
Bucket users into small grid squares to allow for hyper-local content caching (e.g., "stores near you", "local offers").
Set Google Analytics _ga cookie
Auto retry a secondary backend
If primary backend fails, retry with a different backend without caching the failure or reducing cache efficiency.
Fastly offers a myriad of different variables that you can log. See and test a large collection here.
Enable features gradually
Avoid a huge inrush of traffic to origin caused by gradually applying changes that affect the cache key, like segmented caching, over hours or days.
Create image transform presets
Use custom, predefined classnames like large, medium, small, teaser, thumb, or article to control Fastly Image Optimizer and optionally prevent end-user access to native properties like 'width'.
Use shielding to funnel origin requests though one Fastly POP
Shielding directs requests through two Fastly POPs instead of one, improving cache hit ratio and reducing traffic to your origin.
Create montage poster frame images
Combine multiple source images into a single image and then optimize and serve the result.
Anonymize client IPs for logging
Strip the last octet or compute a hash of client IP address for anonymization.
Log the IP version (IPv4/IPv6)
Identify which type of IP address was used by the client connecting to your Fastly service.
Send HTTP Basic Auth in request to origin
Convert a password sent by the client in the querystring into a Authorization header to your origin server.
Cache GraphQL query response
GraphQL query requests are POSTs, but normally responses to POST can't be cached. Normalize the query and convert it to a querystring on a GET request to allow Fastly to cache these responses.
Base64 URL path segments
Unknown data in URL paths can result in invalid URLs, but base64url is designed to be URL-safe.
Check validity of inputs using a non-crypto hash
Block or identify syntactically invalid requests at the edge by using a hash function of your choice.
Authenticate JSON Web Tokens at the edge
Decode the popular JWT format to verify user session tokens before forwarding trusted authentication data to your origin.
Use A/B testing to personalize responses
Serve different responses to separate user cohorts.
Redirect old URLs at the edge
Use a dictionary of URL mappings to serve your redirects at lightning speed.
Flatten the curve of major traffic spikes with a waiting room
A totally stateless solution to hold back new users for a minimum waiting period to smooth out spikes in traffic.
Add time zone offset to requests
Divide the world into time bands of custom size and forward time zone data to your origin server.
Perform redirects with wildcard patterns using an edge dictionary
Match URL prefixes and make use of configurable response status and querystring preservation.
Throttling per country over configurable time periods
Adjust the maximum TCP socket pacing for connections at peak times of day in busy regions.
CenturyLink-compatible token validation
Validate your CenturyLink tokens for access to video stream playlists.
Slowing down responses (tarpit)
Force a response to be delivered very slowly to reduce the rate at which an attacker can send requests.
Support for Edge-Control header
Use custom edge code to implement support for the proprietary Edge-Control cache directive.
Bot detection using ratecounter
Rate counters are normally used for detecting high volume DoS-style attacks, but you can also use them to measure lower rates, to ensure that navigation between pages is happening at human speed.
Rate limit requests
Use ratecounters and penalty boxes to stop high-volume automated attacks against your website.
Validate domain on query string
Check the query string if it contains a valid domain.
Backend health status API
Create an HTTP API for real time backend health status
All code on this page is provided under both the BSD and MIT open source licenses.