open-consul/website/pages/intro/vs/smartstack.mdx

---
layout: intro
page_title: Consul vs. SmartStack
sidebar_title: 'SmartStack'
sidebar_current: vs-other-smartstack
description: >-
  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 vs. SmartStack

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](/docs/internals/gossip.html)
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.