By early 2019, a mix of JavaSCript, ES6, and React powered the web app, with the desktop app shipping in Electron. Java and Kotlin powered the Android app with Objective-C and Swift powering iOS.

CTO Cal Henderson goes on to describe the backend: “we have our core application which powers and our API, which is written in PHP/Hacklang running on HHVM. We store data in MySQL using Vitess. For caching, we use Memcached and MCRouter. Our search service is based on SolrCloud, with various Java services for ranking. Our real-time messaging system uses WebSockets and is comprised of many services written in Java and Go.

“We use HAproxy for load balancing and Consul for configuration and some service discovery. Most of our services talk to each other over gRPC, though we have some Thrift and JSON-over-HTTP too. Our voice and video calling service is built in Elixir. A few different services are also written in Node. Our async task queue system is built on Kafka and Redis.

“Our data warehouse is built on open source tools, including Presto, Spark, Airflow, Hadoop and Kafka. For server configuration and management we use Terraform, Chef and Kubernetes. We use Prometheus for time series metrics and ELK for logging. Slack is largely hosted in AWS, in many regions globally.”