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[vcl] don't set TTL to 0 #28927

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Merged
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Jul 29, 2021
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[vcl] don't set TTL to 0 #28927

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2 changes: 1 addition & 1 deletion app/code/Magento/PageCache/etc/varnish4.vcl
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Original file line number Diff line number Diff line change
Expand Up @@ -166,7 +166,7 @@ sub vcl_backend_response {

# cache only successfully responses and 404s
if (beresp.status != 200 && beresp.status != 404) {
set beresp.ttl = 0s;
set beresp.ttl = 120s;
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What is the purpose of caching 500 / 503 / 403?

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it doesn't cache them because of the following set beresp.uncacheable = true;. Instead Varnish will remember that these objects are uncacheable, and will disable request collapsing for them.

If you keep the TTL to 0, you get into something like this:

  • 20 users request with the same object
  • varnish only goes to the backend once (request coalescing)
  • varnish realizes it cannot reuse the object, so it only serves it to one client
  • we now have 19 users request with the same object
  • varnish only goes to the backend once
  • varnish realizes ....
    rinse, repeat. Ultimately, the last request needs to wait 20 round-trip to the backend before being delivered.

If you set the TTL to some positive value AND the uncacheable bit to true:

  • 20 users request with the same object
  • varnish only goes to the backend once (request coalescing)
  • varnish realizes it cannot reuse the object, so it only serves it to one client, but remembers it as uncacheable
  • the 19 remaining requests are woken up, and can all go to the backend in parallel

it's one of the trickiest Varnish gotchas, so please let me know if I wasn't clear.

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@gquintard Your explanation sounds fine for me. ✔️

set beresp.uncacheable = true;
return (deliver);
} elsif (beresp.http.Cache-Control ~ "private") {
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8 changes: 3 additions & 5 deletions app/code/Magento/PageCache/etc/varnish5.vcl
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Original file line number Diff line number Diff line change
Expand Up @@ -166,11 +166,9 @@ sub vcl_backend_response {
}

# cache only successfully responses and 404s
if (beresp.status != 200 && beresp.status != 404) {
set beresp.ttl = 0s;
set beresp.uncacheable = true;
return (deliver);
} elsif (beresp.http.Cache-Control ~ "private") {
if (beresp.status != 200 &&
beresp.status != 404 &&
beresp.http.Cache-Control ~ "private") {
set beresp.uncacheable = true;
set beresp.ttl = 86400s;
return (deliver);
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10 changes: 4 additions & 6 deletions app/code/Magento/PageCache/etc/varnish6.vcl
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Original file line number Diff line number Diff line change
Expand Up @@ -165,12 +165,10 @@ sub vcl_backend_response {
set beresp.http.X-Magento-Cache-Control = beresp.http.Cache-Control;
}

# cache only successfully responses and 404s
if (beresp.status != 200 && beresp.status != 404) {
set beresp.ttl = 0s;
set beresp.uncacheable = true;
return (deliver);
} elsif (beresp.http.Cache-Control ~ "private") {
# cache only successfully responses and 404s that are not marked as private
if (beresp.status != 200 &&
beresp.status != 404 &&
beresp.http.Cache-Control ~ "private") {
set beresp.uncacheable = true;
set beresp.ttl = 86400s;
return (deliver);
Expand Down