Mastodon

Aside from the various external dependencies, Mastodon has three main applications:

Web

The Mastodon Web tier consists of the Mastodon Web UI/API and the separate Streaming API service. Our Web tier runs on the DigitalOcean managed Kubernetes platform.

Puma

What users perceive as "Mastodon" is a Ruby on Rails and React application (with Puma running as the web/presentation layer) providing both ActivityPub/Federation and the web user experience.

Our Puma pods are set to scale with Kubernetes and within Puma itself. We have Puma configured in .env.production and as follows:

WEB_CONCURRENCY=4
MAX_THREADS=16

Streaming

The Streaming API is a separate Node.js application which provides a background WebSockets connection between your browser session and the Mastodon server to provide real-time "streaming" updates as new posts are loaded to your timeline, to send notifications, etc.

As explained more in-depth in another section, the connection to the DigitalOcean-managed Redis database must be done via TLS. For the Streaming API, there are additional configuration options that must be set to allow Node.js to connect when it expects a non-encrypted connection by default.

Example of .env.production configuration settings relevant to Streaming:

# Streaming
STREAMING_API_BASE_URL=wss://streaming.vmst.io
DB_SSLMODE=require
NODE_EXTRA_CA_CERTS=/path/to/certs/do-internal.crt

The DB_SSLMODE and NODE_EXTRA_CA_CERTS settings are not there by default. The DigitalOcean databases use self-signed/private certificates, but the variable set will tell the Streaming API to trust that connection based on the CA certs that are downloaded from DigitalOcean and uploaded to the server.