Resources for Developers

TCP Gateway Addon

Introduction

By default, Scalingo will only let you run applications that communicate via HTTP or HTTPS. Sometimes this is not enough and you need to have applications that can accept raw TCP connections.

This addon lets you deploy applications that can accept raw TCP connections.

Setup of the Addon

Provision the Addon

First, you need to add the TCP addon to your application. This can be done through the dashboard on the addons section of your app under the Network category.

Setup Configuration

In order to use the Scalingo TCP addon, you need to define a new container type. This can be done in your Procfile.

Here is a sample Procfile for a Node.js application:

web: npm start
tcp: npm run start-tcp

Contact your Application

When your application starts, a PORT environment variable is defined in your container. Your application must listen on this port in order to be accessible from the outside. This is the exact same mechanism and the exact same environment variable that the one used to expose a HTTP server in a web container.

Once up and running your application will be accessible at the URL defined by the TCP_URL environment variable.

This TCP_URL looks like: tcp://tcp-gateway-1.eu-fr-agora-1.scalingo.com:20003

You can find this environment variable on our dashboard or by using our CLI:

scalingo --app my-app env | grep TCP_URL

This URL is also accessible on the addon dashboard.

If you need to contact a tcp container from a web container, you must also use this URL, there is no direct link between your containers, every communication must pass via our load balancer. It’s the exact same thing if you need to contact a web container from a tcp container, you must use the public URL of the web container.

Application Restart and New Deployments

When a new version of the application is pushed or when the tcp containers are restarted, newly created tcp containers are started. TCP handshakes are then sent to your applications. Once a TCP handshake is successful, newly incoming TCP connections will be sent to this container and old containers will receive a SIGTERM signal, notifying them to stop, see container-management for more informations.

The server application is responsible for closing opened connections with its clients once the signal SIGTERM is caught. If the server does not close the connections, the new rules will be updated and next packets will be dropped causing the clients to timeout.

Load Balancing

If you have multiple tcp containers running, Scalingo distributes connections to those containers. Our load balancer distributes connections during the TCP handshake. When established, the communication is always routed to the same server.

Rate Limiting

There’s no rate limiting in place. It may be put in place later.

Limitations

To be able to load balance TCP connections, connections are proxied to your containers via NAT. This process comes with a downside: the TCP source IP needs to be replaced by our front servers internal IP, this mean that a tcp container wont be able to access the real client IP. Thus, the device identification should be part of the protocol you designed over the TCP connection.

Samples

You can find a sample Node.js application that uses this addon to expose a MQTT server built with Mosca on GitHub.


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