mirror of https://github.com/status-im/consul.git
63 lines
4.2 KiB
Markdown
63 lines
4.2 KiB
Markdown
---
|
|
layout: "intro"
|
|
page_title: "Consul vs. ZooKeeper, doozerd, etcd"
|
|
sidebar_current: "vs-other-zk"
|
|
description: |-
|
|
ZooKeeper, doozerd and etcd are all similar in their architecture. All three have server nodes that require a quorum of nodes to operate (usually a simple majority). They are strongly consistent, and expose various primitives that can be used through client libraries within applications to build complex distributed systems.
|
|
---
|
|
|
|
# Consul vs. ZooKeeper, doozerd, etcd
|
|
|
|
ZooKeeper, doozerd and etcd are all similar in their architecture.
|
|
All three have server nodes that require a quorum of nodes to operate (usually a simple majority).
|
|
They are strongly consistent, and expose various primitives that can be used
|
|
through client libraries within applications to build complex distributed systems.
|
|
|
|
Consul works in a similar way within a single datacenter with only server nodes.
|
|
In each datacenter, Consul servers require a quorum to operate
|
|
and provide strong consistency. However, Consul has native support for multiple datacenters,
|
|
as well as a more complex gossip system that links server nodes and clients.
|
|
|
|
If any of these systems are used for pure key/value storage, then they all
|
|
roughly provide the same semantics. Reads are strongly consistent, and availability
|
|
is sacrificed for consistency in the face of a network partition. However, the differences
|
|
become more apparent when these systems are used for advanced cases.
|
|
|
|
The semantics provided by these systems are attractive for building
|
|
service discovery systems. ZooKeeper et al. provide only a primitive K/V store,
|
|
and require that application developers build their own system to provide service
|
|
discovery. Consul provides an opinionated framework for service discovery, and
|
|
eliminates the guess work and development effort. Clients simply register services
|
|
and then perform discovery using a DNS or HTTP interface. Other systems
|
|
require a home-rolled solution.
|
|
|
|
A compelling service discovery framework must incorporate health checking and the
|
|
possibility of failures as well. It is not useful to know that Node A
|
|
provides the Foo service if that node has failed or the service crashed. Naive systems
|
|
make use of heartbeating, using periodic updates and TTLs. These schemes require work linear
|
|
to the number of nodes and place the demand on a fixed number of servers. Additionally, the
|
|
failure detection window is at least as long as the TTL. ZooKeeper provides ephemeral
|
|
nodes which are K/V entries that are removed when a client disconnects. These are more
|
|
sophisticated than a heartbeat system, but also have inherent scalability issues and add
|
|
client side complexity. All clients must maintain active connections to the ZooKeeper servers,
|
|
and perform keep-alives. Additionally, this requires "thick clients", which are difficult
|
|
to write and often result in difficult to debug issues.
|
|
|
|
Consul uses a very different architecture for health checking. Instead of only
|
|
having server nodes, Consul clients run on every node in the cluster.
|
|
These clients are part of a [gossip pool](/docs/internals/gossip.html), which
|
|
serves several functions including distributed health checking. The gossip protocol implements
|
|
an efficient failure detector that can scale to clusters of any size without concentrating
|
|
the work on any select group of servers. The clients also enable a much richer set of health checks to be run locally,
|
|
whereas ZooKeeper ephemeral nodes are a very primitive check of liveness. Clients can check that
|
|
a web server is returning 200 status codes, that memory utilization is not critical, there is sufficient
|
|
disk space, etc. The Consul clients expose a simple HTTP interface and avoid exposing the complexity
|
|
of the system to clients in the same way as ZooKeeper.
|
|
|
|
Consul provides first class support for service discovery, health checking,
|
|
K/V storage, and multiple datacenters. To support anything more than simple K/V storage,
|
|
all these other systems require additional tools and libraries to be built on
|
|
top. By using client nodes, Consul provides a simple API that only requires thin clients.
|
|
Additionally, the API can be avoided entirely by using configuration files and the
|
|
DNS interface to have a complete service discovery solution with no development at all.
|