Service Fabric (continued)

Part 1 introduced the Service Fabric. This article continues the discussion with a comparison of the messaging protocols:

Service Fabric is not like Kubernetes which is a heavily centralized system relying on an API server, multiple Kubelets, a central etcd cluster repository and heartbeats collected every period. Service Fabric avoids this with a decentralized ring and failure detection. It is motivated by the adage that distributed consensus is at the heart of numerous co-ordination problems, but it has a trivially simple solution if there is a reliable failure detection service. Service Fabric does not use a distributed consensus protocol like Raft or Paxos or a centralized store for cluster state. Instead, it proposes a Federation subsystem which answers the most important question on membership which is whether a specific node is part of the system. The nodes are organized in rings, and the heartbeats are only sent to a small subset of nodes called the neighborhood. The arbitration procedure involves more nodes besides the monitor but it is only executed on missed heartbeats. A quorum of privileged nodes in an arbitration group helps resolve the possible failure detection conflicts, isolate appropriate nodes and maintain a consistent view of the ring.

Kubernetes requires distributed consensus, but Service Fabric relies on its ring. Distributed consensus algorithms have evolved into many forms but two primarily dominate the market – Paxos and Raft. Both take a similar approach differing only in their approach for leader election. Raft only allows servers with up-to-date logs to become leaders, whereas Paxos allows any server to be leader if it then updates its logs to ensure that it remains up-to-date. Raft is more efficient since it does not involve log entries to be exchanged during leader elections.

Some of the other differences can be compared along the following dimensions:

1.       How does it ensure that each term has at most one leader?

a.       Paxos: A server s can only be candidate in a term t if t mod n = s. There will be only one candidate per term so there will be only one leader per term.            

b.       Raft: A follower can become a candidate in any term. Each follower will only vote for one candidate per term, so only one candidate can get a majority of votes and become the leader.

2.       How does it ensure that a new leader’s log contains all committed entries?

a.       Paxos: Each RequestVote reply includes the followers’s log entries. Once a candidate has received RequestVote responses from a majority of followers, it adds the entries with the highest term to its log.

b.       Raft: A vote is granted only if the candidate’s log is at least as up-to-date as the followers’. This ensures that a candidate only becomes a leader if it’s log is at least as up-to-date as a majority of its followers.

3.       How does it ensure that the leader safely commits log entries from previous terms?

a.       Paxos: Log entries from the previous terms are added to the leader’s log with the leader’s terms. The leader then replicates the log entries as if they were from the leader’s terms.

b.       Raft: The leader replicates the log entries to the other servers without changing the term. The leader cannot consider those entries committed until it has replicated a subsequent log entry from its own term.

Raft algorithm was proposed to address the long-standing issues with understandability of the widely studied Paxos algorithm. It has a clear abstraction and presentation and can be a simplified version of the original Paxos algorithm. Specifically, Paxos divides terms between servers whereas Raft allows a follower to become a candidate in any term, but followers will vote for only one candidate per term. Paxos followers will vote for any candidate, whereas Raft followers will only vote for a candidate if the candidate’s log is at least as up to date. If a leader has uncommitted log entries from a previous term, Paxos will replicate them in the current term whereas Raft will replicate them in their original term. Raft’s leader election is therefore lightweight when compared to Paxos.