What is etcd?

In this world of different types of database systems, the distributed key-value store is a type of database system that provides a simple key-value data model. This type of database system stores the data in key-value pairs, each with a unique key associated with it. In this data model, there aren't any specifications on the data type that can be stored. Comparing this to the traditional databases, this allows more flexibility in data processing. Moreover, this database system offers better performance for reads and writes as it only has to identify a single key.

Now, etcd is a distributed key-value store, also described as a reliable system for storing and managing data. The CoreOS team developed this store to create an open-source secure storage space for critical data in distributed applications.

In layman's terms, etcd is a data store typically used for cluster and containerized application management. Most notably, it is used in the popular container orchestration platform, Kubernetes.

With all that in mind, let's focus on how etcd is related to Kubernetes and its significance in that system. 

Advantages of etcd

Being a distributed key-value store, etcd has many advantages over other types of data storing systems. The advantages are stated below:

Fully replicated: In an etcd cluster, every node has a complete copy of the data so that all nodes can access the same information.

Highly available: etcd is designed to keep working even if some nodes fail or there are network issues. It avoids having a single point of failure, so the system remains available.

Reliably consistent: When you read data from etcd, you always get the latest information written to etcd, no matter which node you read from. It ensures that everyone sees the same up-to-date data.

Fast: etcd can handle a large number of write operations per second, up to 10,000 writes. It performs well and responds quickly.

Secure: etcd supports secure communication using encryption (TLS) and can require client certificates for authentication. This helps protect sensitive configuration data. It's important to control access to etcd with appropriate permissions to ensure only authorized users can interact.

Simple: etcd can be easily used by any application, whether it's a simple web app or a complex system like Kubernetes. You can read from and write to etcd using standard tools and protocols like HTTP and JSON.

Etcd and Kubernetes

As Kubernetes is the most well-known container orchestration tool available on the market, we have to discuss how it manages the data. Moreover, in Kubernetes, the complexities of management increase as the workload scales, but it efficiently manages these workloads by coordinating all the tasks across all the clusters. This coordination is the key to simplifying the management process. However, this coordination is only achievable by having one data store providing the truth about the system's status. This refers to all the clusters, pods, and application instances at any time. The data regarding the states of the system are stored in the data store we discussed, ‘etcd’.

As we have seen, the data store can be the most essential aspect of the Kubernetes system. Due to its significance, etcd is one of the core components and can only be accessed by the API server. The API server is responsible for storing the state data of the system in etcd. This essentially creates a functioning, fault-tolerant Kubernetes cluster. 

What etcd also stores is the actual state of the system and the desired state of the system. This storing is crucial for the overall synchronization and coordination of the system. etcd uses the ‘watch’ function to monitor both states and if any divergence is identified, Kubernetes makes changes to reconcile the states.  

Moreover, as previously mentioned, etcd can only be accessed by the API server, and other components communicate with etcd through the API server.

What is the Raft consensus algorithm?

etcd relies on the Raft consensus algorithm to ensure consistent data across all nodes in a cluster, which is essential for a fault-tolerant distributed system.

Raft achieves this consistency by electing a leader node responsible for managing replication among the other nodes, known as followers. Clients send requests to the leader, which forwards them to the followers. The leader ensures that most followers have stored each request as a log entry. Once confirmed, the leader applies the entry to its state machineAn entity that executes a sequence of commands to transition between different states based on the agreed-upon log entries., executes it, and returns the result to the client.

If followers crash or network packets are lost, the leader persistently retries until all followers have consistently stored all log entries.

An election is triggered if a follower node doesn't receive messages from the leader within a defined timeframe. The follower becomes a candidate and seeks votes from other followers or nodes based on availability. Once a new leader is elected, it takes over replication management, and the process continues. This ensures that all etcd nodes maintain highly available and consistently replicated copies of the data store.

Alright, let's put your knowledge about etcd to the test with a quick quiz!

Summary 

etcd is a distributed key-value store that is an essential part of Kubernetes. It stores crucial cluster information, like configuration and state, in a reliable and fault-tolerant manner. With its seamless integration, etcd helps Kubernetes ensure smooth operations and provides features like service discovery. It can be managed using tools called etcd operators, simplifying tasks such as scaling and backup. In a nutshell, etcd is a crucial component that helps Kubernetes manage and maintain its clusters effectively.