The document provides a detailed guide on using Fastly's VCL for handling requests, focusing on processes like restarting on 404 responses and following redirects. Key lessons include checking primary/secondary backends, altering headers, and managing request hashes effectively. Additional resources and support options are also mentioned for further learning about VCL.

1. Main parts of Fastly request
vcl_recv vcl_hash vcl_delivervcl_fetch End user

2. What is a restart?
vcl_recv vcl_hash vcl_delivervcl_fetch
restart

3. Why restart?

4. Why restart?
•Checking Primary/Secondary Backends
•Following Redirects

5. Checking a secondary origin based on a
404 response from the primary origin
Today’s lessons
Lesson 1 – Checking Primary/Secondary Origins

6. Checking primary/secondary origins

7. “File not found” restart in fetch
vcl_recv vcl_hash vcl_fetch

8. “File not found” restart in fetch
vcl_fetch {
if (beresp.status == 404 && req.restarts == 0) {
restart;
}
#FASTLY FETCH
…}
^ if the origin response = 404 then restart.
Restart – resets the vcl state machine (back to recv)
vcl_recv
vcl_hash
vcl_fetch

9. Catch the restart
vcl_recv vcl_hash vcl_fetch
restart

10. Catch the restart
vcl_recv
vcl_hash
vcl_fetch
vcl_recv {
if (req.restarts > 0) {
set req.backend = F_secondary;
set req.http.Fastly-Force-Shield = "1";
return(lookup);
}
#FASTLY RECV
…}
^ if we restarted, switch backends, re-enable clustering, and
lookup the content

11. Checking primary/secondary origins

12. Checking primary/secondary origins

13. Why restart?
•Checking Primary/Secondary Backends
•Following Redirects

14. Following a 301 redirect and rewriting the request so
we can remove the 301 from the cached result.
Today’s lessons
Lesson 2 – Following redirects

15. Following redirects

16. Creating new headers in deliver
vcl_recv vcl_hash vcl_delivervcl_fetch

17. Creating new headers in deliver
set req.http.OrigHost = req.http.host;
set req.http.OrigURL = req.url;
^ stores the initial request’s host and url into headers
that we can use later
vcl_recv
vcl_hash
vcl_deliver
vcl_fetch

18. Use regex
set req.url =
regsub(resp.http.Location,"^https?://
[^/]+(.*)","1");
^ pulls out /path/* from www.example.com/path/*

19. Final result
vcl_recv
vcl_hash
vcl_deliver
vcl_fetch
vcl_deliver {
#FASTLY deliver
if (resp.status == 301 && req.restarts == 0) {
set req.http.OrigHost = req.http.host;
set req.http.OrigURL = req.url;
set req.url =
regsub(resp.http.Location,"^https?://[^/]+
(.*)","1");
restart;
}
…}

20. Catch the restart
vcl_recv vcl_hash vcl_delivervcl_fetch
restart

21. Catch the restart
vcl_recv
vcl_hash
vcl_deliver
vcl_fetch
vcl_recv
if (req.http.OrigHost) {
set req.http.Fastly-Force-Shield = "1";
set req.backend = F_redirectBackend;
set req.http.host = "www.example.com";
return(lookup);
}
…}

22. Fix the hash
vcl_recv vcl_hash

23. Fix the hash
vcl_hash {
if(req.http.OrigHost) {
set req.hash += req.http.OrigURL;
set req.hash += req.http.OrigHost;
set req.hash += "#####GENERATION#####";
return (hash);
}
…}

24. Following redirects

25. Summary
1.Restart in fetch or deliver (if you alter headers)
2.Catch the restart in recv
3.Optional: change the hash
4.Enjoy VCL

26. A word of caution

27. Learning VCL
We have plenty to offer:
Check out our Docs page: https://docs.fastly.com/guides/vcl/
Send questions to support@fastly.com
#fastly in irc.freenode.net

28. Questions