Hack

Since the beginning, Cal Henderson has been the CTO of Slack. Earlier this year, he commented on a Quora question summarizing their current stack.

Apps

• Web: a mix of JavaScript/ES6 and React.
• Desktop: And Electron to ship it as a desktop application.
• Android: a mix of Java and Kotlin.
• iOS: written in a mix of Objective C and Swift.

Backend

• The core application and the API written in PHP/Hack that runs on HHVM.
• The data is stored in MySQL using Vitess.
• Caching is done using Memcached and MCRouter.
• The search service takes help from SolrCloud, with various Java services.
• The messaging system uses WebSockets with many services in Java and Go.
• Load balancing is done using HAproxy with Consul for configuration.
• Most services talk to each other over gRPC,
• Some Thrift and JSON-over-HTTP
• Voice and video calling service was built in Elixir.

Data warehouse

• Built using open source tools including Presto, Spark, Airflow, Hadoop and Kafka.

Etc

• For server configuration and management we use Terraform, Chef and Kubernetes.
• We use Prometheus for time series metrics and ELK for logging.

Throughout 2016, Slack began migrating from PHP5 to Hack. They cite several well-known challenges inherent to PHP, including surprise type conversions, inconsistency around reference semantics, inconsistencies in the standard library, and the fact that “PHP tries very, very hard to keep the request running, even if it has done something deeply strange.”

To overcome these challenges while maintaining the unique values of PHP, Slack turned to Hack, a gradual typing system for PHP. Hack runs on the HipHop Virtual Machine, or HHVM, an open source just-in-time (JIT) environment for PHP.