mirror of https://github.com/hashicorp/consul
66 lines
3.7 KiB
Markdown
66 lines
3.7 KiB
Markdown
---
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layout: docs
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page_title: Consul vs. SmartStack
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sidebar_title: SmartStack
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description: >-
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SmartStack is a tool which tackles the service discovery problem. It has a
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rather unique architecture and has 4 major components: ZooKeeper, HAProxy,
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Synapse, and Nerve. The ZooKeeper servers are responsible for storing cluster
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state in a consistent and fault-tolerant manner. Each node in the SmartStack
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cluster then runs both Nerves and Synapses. The Nerve is responsible for
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running health checks against a service and registering with the ZooKeeper
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servers. Synapse queries ZooKeeper for service providers and dynamically
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configures HAProxy. Finally, clients speak to HAProxy, which does health
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checking and load balancing across service providers.
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---
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# Consul vs. SmartStack
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SmartStack is a tool which tackles the service discovery problem. It has a rather
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unique architecture and has 4 major components: ZooKeeper, HAProxy, Synapse, and Nerve.
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The ZooKeeper servers are responsible for storing cluster state in a consistent and
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fault-tolerant manner. Each node in the SmartStack cluster then runs both Nerves and
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Synapses. The Nerve is responsible for running health checks against a service and
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registering with the ZooKeeper servers. Synapse queries ZooKeeper for service providers
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and dynamically configures HAProxy. Finally, clients speak to HAProxy, which does
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health checking and load balancing across service providers.
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Consul is a much simpler and more contained system as it does not rely on any external
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components. Consul uses an integrated [gossip protocol](/docs/internals/gossip)
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to track all nodes and perform server discovery. This means that server addresses
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do not need to be hardcoded and updated fleet-wide on changes, unlike SmartStack.
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Service registration for both Consul and Nerves can be done with a configuration file,
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but Consul also supports an API to dynamically change the services and checks that are
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in use.
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For discovery, SmartStack clients must use HAProxy, requiring that Synapse be
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configured with all desired endpoints in advance. Consul clients instead
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use the DNS or HTTP APIs without any configuration needed in advance. Consul
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also provides a "tag" abstraction, allowing services to provide metadata such
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as versions, primary/secondary designations, or opaque labels that can be used for
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filtering. Clients can then request only the service providers which have
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matching tags.
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The systems also differ in how they manage health checking. Nerve performs local health
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checks in a manner similar to Consul agents. However, Consul maintains separate catalog
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and health systems. This division allows operators to see which nodes are in each service
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pool and provides insight into failing checks. Nerve simply deregisters nodes on failed
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checks, providing limited operational insight. Synapse also configures HAProxy to perform
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additional health checks. This causes all potential service clients to check for
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liveness. With large fleets, this N-to-N style health checking may be prohibitively
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expensive.
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Consul generally provides a much richer health checking system. Consul supports
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Nagios-style plugins, enabling a vast catalog of checks to be used. Consul allows for
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both service- and host-level checks. There is even a "dead man's switch" check that allows
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applications to easily integrate custom health checks. Finally, all of this is integrated
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into a Health and Catalog system with APIs enabling operators to gain insight into the
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broader system.
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In addition to the service discovery and health checking, Consul also provides
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an integrated key/value store for configuration and multi-datacenter support.
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While it may be possible to configure SmartStack for multiple datacenters,
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the central ZooKeeper cluster would be a serious impediment to a fault-tolerant
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deployment.
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