Consul by HashiCorp

SmartStack is a tool which tackles the service discovery problem. It has a rather unique architecture and has 4 major components: ZooKeeper, HAProxy, Synapse, and Nerve. The ZooKeeper servers are responsible for storing cluster state in a consistent and fault-tolerant manner. Each node in the SmartStack cluster then runs both Nerves and Synapses. The Nerve is responsible for running health checks against a service and registering with the ZooKeeper servers. Synapse queries ZooKeeper for service providers and dynamically configures HAProxy. Finally, clients speak to HAProxy, which does health checking and load balancing across service providers.

Consul is a much simpler and more contained system as it does not rely on any external components. Consul uses an integrated gossip protocol to track all nodes and perform server discovery. This means that server addresses do not need to be hardcoded and updated fleet-wide on changes, unlike SmartStack.

Service registration for both Consul and Nerves can be done with a configuration file, but Consul also supports an API to dynamically change the services and checks that are in use.

For discovery, SmartStack clients must use HAProxy, requiring that Synapse be configured with all desired endpoints in advance. Consul clients instead use the DNS or HTTP APIs without any configuration needed in advance. Consul also provides a "tag" abstraction, allowing services to provide metadata such as versions, primary/secondary designations, or opaque labels that can be used for filtering. Clients can then request only the service providers which have matching tags.

The systems also differ in how they manage health checking. Nerve performs local health checks in a manner similar to Consul agents. However, Consul maintains separate catalog and health systems. This division allows operators to see which nodes are in each service pool and provides insight into failing checks. Nerve simply deregisters nodes on failed checks, providing limited operational insight. Synapse also configures HAProxy to perform additional health checks. This causes all potential service clients to check for liveness. With large fleets, this N-to-N style health checking may be prohibitively expensive.

Consul generally provides a much richer health checking system. Consul supports Nagios-style plugins, enabling a vast catalog of checks to be used. Consul allows for both service- and host-level checks. There is even a "dead man's switch" check that allows applications to easily integrate custom health checks. Finally, all of this is integrated into a Health and Catalog system with APIs enabling operators to gain insight into the broader system.

In addition to the service discovery and health checking, Consul also provides an integrated key/value store for configuration and multi-datacenter support. While it may be possible to configure SmartStack for multiple datacenters, the central ZooKeeper cluster would be a serious impediment to a fault-tolerant deployment.