Liftbridge is a server that implements a durable, replicated message log for NATS. Clients create a named stream which is attached to a NATS subject. The stream then records messages on that subject to a replicated write-ahead log. Multiple consumers can read back from the same stream, and multiple streams can be attached to the same subject. Liftbridge provides a Kafka-like API in front of NATS.
Liftbridge streams are partitioned for horizontal scalability. By default, streams have a single partition. Each partition maps to a separate NATS subject derived from the stream subject. Each has a leader and is replicated to some set of followers for fault-tolerance.
NATS is a lightweight, high-performance pub/sub messaging system. It differs from traditional messaging middleware in that it does not provide queuing or message durability. Instead, NATS is a fire-and-forget system with at-most-once delivery semantics. This allows it to be extremely lightweight, highly scalable, and—above all—dead simple.
While NATS aims to be the always-on dial tone for cloud-native systems, Liftbridge is the voicemail. It provides an immutable, append-only log of NATS messages. Unlike a message queue where consumers remove messages from a queue, with Liftbridge, consumers read back the log. In this sense, the design is very similar to Apache Kafka.
Liftbridge was designed to bridge the gap between sophisticated log-based messaging systems like Apache Kafka and Apache Pulsar and simpler, cloud-native systems. There is no ZooKeeper or other unwieldy dependencies, no JVM, no complicated API, and client libraries are just gRPC. More importantly, Liftbridge aims to extend NATS with a durable, at-least-once delivery mechanism that upholds the NATS tenets of simplicity, performance, and scalability. Unlike NATS Streaming, it uses the core NATS protocol with optional extensions. This means it can be added to an existing NATS deployment to provide message durability with no code changes.
The high-level data flow in Liftbridge is shown in the diagram below.