This document describes Siddhi, an open source complex event processing (CEP) engine. It discusses the objectives of developing Siddhi as a high performing CEP engine. It outlines Siddhi's architecture including its input/output adapters, compiler, Siddhi manager, and processors. It provides details on Siddhi's implementation and capabilities such as filtering, pattern matching, joins, windows, and aggregators. It also evaluates Siddhi's performance compared to other CEP engines and discusses its deployments, website, documentation, and future plans.