dns-watch

This is a simple helper utility that performs a DNS lookup of a list of hostnames and uses them to instantiate a handlebars template.

See handlebars-rust for details of the supported helpers.

Pair this with a HAProxy reload script and you can have a HAProxy docker image automatically discover docker-compose scaled services.

Example

If we have a handlebars template file like this

frontend app
    use_backend api if { path_beg /api }
    use_backend ui if { path_beg /ui }

backend api
    mode http
{{#each backend}}
    server {{{this}}} {{{this}}}:8080 check
{{/each}}

backend ui
    mode http
{{#each frontend}}
    server {{{this}}} {{{this}}}:8888 check
{{/each}}

Then running:

$ dns-watch -v frontend:frontend-service -v backend:backend-service -o haproxy.conf haproxy.conf.mustache

Will perform two DNS lookups, one for frontend-service and one for backend-service and write haproxy.conf and we will end up with something that looks like:

frontend app
    use_backend api if { path_beg /api }
    use_backend ui if { path_beg /ui }

backend api
    mode http
    server 10.0.0.4 10.0.0.4:8080 check
    server 10.0.0.7 10.0.0.7:8080 check
    server 10.0.0.8 10.0.0.8:8080 check

backend ui
    mode http
    server 10.0.0.5 10.0.0.5:8888 check
    server 10.0.0.6 10.0.0.6:8888 check

If you want to watch for changes then you can specify a command to execute every time there is a change:

$ dns-watch -v frontend:frontend-service -v backend:backend-service -o haproxy.conf -w reload_haproxy.sh haproxy.conf.hbs

Which watch for DNS changes every second and run the reload_haproxy.sh if the template is written.