»Consul vs. SkyDNS

SkyDNS is a tool designed to provide service discovery. It uses multiple central servers that are strongly-consistent and fault-tolerant. Nodes register services using an HTTP API, and queries can be made over HTTP or DNS to perform discovery.

Consul is very similar but provides a superset of features. Consul also relies on multiple central servers to provide strong consistency and fault tolerance. Nodes can use an HTTP API or use an agent to register services, and queries are made over HTTP or DNS.

However, the systems differ in many ways. Consul provides a much richer health checking framework with support for arbitrary checks and a highly scalable failure detection scheme. SkyDNS relies on naive heartbeating and TTLs, an approach which has known scalability issues. Additionally, the heartbeat only provides a limited liveness check versus the rich health checks that Consul performs.

Multiple datacenters can be supported by using "regions" in SkyDNS; however, the data is managed and queried from a single cluster. If servers are split between datacenters, the replication protocol will suffer from very long commit times. If all the SkyDNS servers are in a central datacenter, then connectivity issues can cause entire datacenters to lose availability. Additionally, even without a connectivity issue, query performance will suffer as requests must always be performed in a remote datacenter.

Consul supports multiple datacenters out of the box, and it purposely scopes the managed data to be per-datacenter. This means each datacenter runs an independent cluster of servers. Requests are forwarded to remote datacenters if necessary; requests for services within a datacenter never go over the WAN, and connectivity issues between datacenters do not affect availability within a datacenter. Additionally, the unavailability of one datacenter does not affect the discovery of services in any other datacenter.