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Last Friday, a team at Kubernetes announced the release of etcd 3.4 version. etcd 3.4 focuses on stability, performance and ease of operation. It includes features like pre-vote and non-voting member and improvements to storage backend and client balancer.

Key features and improvements in etcd v3.4

Better backend storage

etcd v3.4 includes a number of performance improvements for large scale Kubernetes workloads. In particular, etcd experienced performance issues with a large number of concurrent read transactions even when there is no write (e.g. “read-only range request … took too long to execute”). Previously, the storage backend commit operation on pending writes, blocks incoming read transactions, even when there was no pending write. Now, the commit does not block reads which improve long-running read transaction performance.

The team has further made backend read transactions fully concurrent. Previously, ongoing long-running read transactions block writes and upcoming reads. With this change, write throughput is increased by 70% and P99 write latency is reduced by 90% in the presence of long-running reads. They also ran Kubernetes 5000-node scalability test on GCE with this change and observed similar improvements.

Improved raft voting process

etcd server implements Raft consensus algorithm for data replication. Raft is a leader-based protocol. Data is replicated from leader to follower; a follower forwards proposals to a leader, and the leader decides what to commit or not. Leader persists and replicates an entry, once it has been agreed by the quorum of cluster. The cluster members elect a single leader, and all other members become followers. The elected leader periodically sends heartbeats to its followers to maintain its leadership, and expects responses from each follower to keep track of its progress.

In its simplest form, a Raft leader steps down to a follower when it receives a message with higher terms without any further cluster-wide health checks. This behavior can affect the overall cluster availability.

For instance, a flaky (or rejoining) member drops in and out, and starts campaign. This member ends up with higher terms, ignores all incoming messages with lower terms, and sends out messages with higher terms. When the leader receives this message of a higher term, it reverts back to follower.

This becomes more disruptive when there’s a network partition. Whenever the partitioned node regains its connectivity, it can possibly trigger the leader re-election. To address this issue, etcd Raft introduces a new node state pre-candidate with the pre-vote feature. The pre-candidate first asks other servers whether it’s up-to-date enough to get votes. Only if it can get votes from the majority, it increments its term and starts an election. This extra phase improves the robustness of leader election in general. And helps the leader remain stable as long as it maintains its connectivity with the quorum of its peers.

Introducing a new raft non-voting member, “Learner”

The challenge with membership reconfiguration is that it often leads to quorum size changes, which are prone to cluster unavailabilities. Even if it does not alter the quorum, clusters with membership change are more likely to experience other underlying problems.

In order to address failure modes, etcd introduced a new node state “Learner”, which joins the cluster as a non-voting member until it catches up to leader’s logs. This means the learner still receives all updates from leader, while it does not count towards the quorum, which is used by the leader to evaluate peer activeness. The learner only serves as a standby node until promoted. This relaxed requirements for quorum provides the better availability during membership reconfiguration and operational safety.

Improvements to client balancer failover logic

etcd is designed to tolerate various system and network faults. By design, even if one node goes down, the cluster “appears” to be working normally, by providing one logical cluster view of multiple servers. But, this does not guarantee the liveness of the client. Thus, etcd client has implemented a different set of intricate protocols to guarantee its correctness and high availability under faulty conditions.

Historically, etcd client balancer heavily relied on old gRPC interface: every gRPC dependency upgrade broke client behavior. A majority of development and debugging efforts were devoted to fixing those client behavior changes. As a result, its implementation has become overly complicated with bad assumptions on server connectivity. The primary goal in this release was to simplify balancer failover logic in etcd v3.4 client; instead of maintaining a list of unhealthy endpoints, whenever client gets disconnected from the current endpoint.

To know more about this release, check out the Changelog page on GitHub.

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