Discord's Internal Infrastructure

Disclaimer

This has some degree of speculation as Discord's backend is closed.

Scylla for storage
- They used Cassandra for such but that changed
- They used MongoDB as primary storage, but moved to Cassandra, source).
It is still unknown what role MongoDB has on the stack, but it is still a primary part of the service: source and source.

Elixir for the Gateway/WebSockets API, source.
Python for HTTP/REST API, The best guesses to which framework are Flask.
Go:
- The embed servers, confirmed by Discord developers informally.
- Note that the embedding servers and mediaproxy, while able to be its own logical unit, as experimented in litecord's [mediaproxy], can be treated separately.
- Punt, replacing Logstash in ELK (Elasticsearch, Logstash, Kibana) setups.
- Lilliput, which is a main part of their media proxy. Lillput is also written in C++. source
Rust for parts of the Discord Store, most commonly:
- GameSDK, to make Discord integrations for any game.
- Dispatch, which is a tool to send assets to Discord's servers.
- It is known Rust is used for lazy guilds. source
- Read States service, rewritten from Go. source
C++ for the Discord RPC library. (deprecated in favor of GameSDK)

Punt in favour of Logstash for logging.
Elasticsearch that powers the search feature for users and powers logging.
- Sources about logging: Punt and this issue
- Source about message search

Google's Cloud Platform for their infrastructure, source
Cloudflare as a proxy to almost all services, voice servers need to be direct connections, so they don't pass through CF. More on voice servers in the WebRTC source.
"[...]we use Algolia to power some search features at Discord." source, by jhgg

WebRTC for voice and video. Source, which also goes on more detail on how the voice server architecture works.