I run 4 dynos with nodejs app on it under same subdomain (Example: app.website.com). I need to send to all my dynos request from other dyno app (other domain: admin.website.com), also nodejs app. All dynos should remove some cached data in memory on command from admin server.
If I send http request to app.website.com only one of the dynos get it (because of the load balancer).
In Herokus "common runtime" its neither possible to connect to individual dynos from the outside, nor can you send http requests from one dyno to another. (See https://devcenter.heroku.com/articles/dynos#common-runtime-networking). In Herokus private space on the other hand the later would be possible, because all dynos form a VPN. (See https://devcenter.heroku.com/articles/dynos#private-spaces-runtime-networking)
Instead of trying to send a cache purge event to all dynos using http, it would probably be a better idea to change the setup. You could use a central component, e.g. a message queue, to which both your admin dynos and your app dynos can connect. The admin app could then dispatch cache purge events, which your app consumes. If you don't want to expand your infrastructure unnecessarily, but you already have any other central storage (e.g. a database) connected, you could also use that.