SkyDNS vs etcd: What are the differences?

### Introduction
In this Markdown code, we will highlight the key differences between SkyDNS and etcd.

1. **Functionality**: SkyDNS is a lightweight DNS server for Kubernetes that allows for service discovery within the cluster. It maps DNS names to Kubernetes services, making it easier for applications to locate and communicate with each other. On the other hand, etcd is a distributed key-value store used for shared configuration and service discovery across distributed systems. It provides a reliable way to store and manage critical data used by applications.

2. **Use Case**: SkyDNS is primarily used for DNS-based service discovery within Kubernetes clusters. It helps services locate each other by providing a DNS interface for easy communication. In contrast, etcd is more versatile and can be used for a wide range of use cases, including distributed coordination, leader election, and storing configuration data. It is not limited to a specific environment like SkyDNS.

3. **Consistency Model**: SkyDNS uses eventually consistent DNS records, which may introduce some latency in service discovery as changes propagate through the system. On the other hand, etcd provides strong consistency guarantees using the Raft consensus algorithm, ensuring that all nodes in the cluster have a consistent view of the data at all times. This makes etcd more suitable for critical data that requires strong consistency.

4. **Scalability**: SkyDNS is designed to scale horizontally by adding more instances to handle increased query loads and service discovery requests. However, it may not scale as seamlessly as etcd, which is built to be distributed and scalable out of the box. Etcd can handle large amounts of data and traffic efficiently across multiple nodes in a cluster.

5. **Data Model**: SkyDNS uses a simple key-value data model with DNS names mapped to Kubernetes services and endpoints. It is optimized for service discovery within Kubernetes clusters. In contrast, etcd supports complex data structures like directories, leasing, and watch capabilities, making it more flexible for various data storage and coordination tasks beyond just service discovery.

6. **Consensus Algorithm**: SkyDNS does not have its built-in consensus algorithm since its primary goal is to serve as a DNS server for service discovery. On the other hand, etcd uses the Raft consensus algorithm to ensure data consistency and fault tolerance in distributed systems. This makes etcd more suitable for critical data storage where consistency and fault tolerance are paramount.

In Summary, the key differences between SkyDNS and etcd lie in their functionality, use cases, consistency models, scalability, data models, and consensus algorithms.