Running Varnish on Fly#

The goal: run Varnish in a Fly container as a caching proxy in front of another Fly application.

I ended up switching to Cloudflare for running a Varnish-style cache in front of my Fly application (purely to reduce the number of moving parts I needed to maintain), but I’m publishing my notes on how I got Varnish working here in case I need them in the future.

The Dockerfile#

Fly apps run Docker containers. Here’s the minimal Dockerfile that worked for me:

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FROM varnish
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COPY default.vcl /etc/varnish/
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USER varnish
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ENTRYPOINT [ \
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    "varnishd", "-F", \
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    "-f", "/etc/varnish/default.vcl", \
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    "-a", "http=:8000,HTTP", \
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    "-s", "malloc,256M", \
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    "-T", "none" \
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]

default.vcl#

The default.vcl file is the Varnish configuration file. Here’s the one I used:

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vcl 4.1;
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backend default {
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  .host = "my-underlying-app.fly.dev";
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  .port = "80";
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}
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sub vcl_recv {
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    unset req.http.x-cache;
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}
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sub vcl_hit {
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    set req.http.x-cache = "hit";
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}
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sub vcl_miss {
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    set req.http.x-cache = "miss";
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}
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sub vcl_pass {
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    set req.http.x-cache = "pass";
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}
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sub vcl_pipe {
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    set req.http.x-cache = "pipe uncacheable";
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}
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sub vcl_synth {
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    set req.http.x-cache = "synth synth";
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    set resp.http.x-cache = req.http.x-cache;
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}
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sub vcl_deliver {
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    if (obj.uncacheable) {
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        set req.http.x-cache = req.http.x-cache + " uncacheable" ;
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    } else {
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        set req.http.x-cache = req.http.x-cache + " cached" ;
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    }
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    set resp.http.x-cache = req.http.x-cache;
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}

fly.toml#

The Fly configuration file for the application:

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app = "my-varnish-app"
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primary_region = "lax"
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kill_signal = "SIGINT"
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kill_timeout = "5s"
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[[services]]
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  internal_port = 8000
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  protocol = "tcp"
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  [services.concurrency]
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    hard_limit = 25
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    soft_limit = 20
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  [[services.ports]]
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    handlers = ["http"]
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    port = 80
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  [[services.ports]]
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    handlers = ["tls", "http"]
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    port = 443
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  [[services.tcp_checks]]
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    interval = 10000
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    timeout = 2000
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    grace_period = "10s"

I’m using internal_port = 8000 here because I ran Varnish using -a "http=:8000,HTTP" in the ENTRYPOINT in the Dockerfile.

Deploying the application#

With all of the above files in a folder, the deploy command looks like this:

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flyctl deploy

The result ends up running at https://my-varnish-app.fly.dev/, and will serve pages from the underlying app - only caching them if those pages include a cache-control: s-maxage=15 header or similar.

Newsletter

Running Varnish on Fly#

The Dockerfile#

default.vcl#

fly.toml#

Deploying the application#

Further reading#