Chain Replication is a paper from ‘04 by Renesse and Schneider. The system is interesting, because it is a primary-backup system with an unconventional architecture that aimed to achieve high throughput and availability while maintaining strong consistency.

## What is Chain Replication

Chain replication is a special primary backup system, in which the servers are linearly ordered to form a chain with the primary at the end. In “classic” primary backup systems, the topology resembles a star with the primary at the center. The goal of this design is, in the authors’ own words, “to achieve high throughput and availability without sacrificing strong consistency”.

## The Chain replication protocol

The chain replication protocol is quite simple: All read requests are sent to the tail (primary) of the chain as in normal primary-backup systems, all write requests are sent to the head (a backup), which then passes the update along the chain. To avoid unnecessary work, only the result of the write is passed down the chain. Strong consistency naturally follows, because all requests are ordered by the primary at the tail of the chain.

The protocol considers only three failure cases, all of which are fail-stop failures:

2. Fail-stop of the tail (primary)
3. Fail-stop of a middle server

Failures are detected by a Paxos-service. The simplest failure is fail-stop of the head, where the next server in the chain takes over as head. Failure of the tail is equally simple.

To cope with a middle server failure, servers need to keep a history of requests that they have processed. If a middle server fails, its two neighbors bypass it and connect. The later server sends the other one the list of requests it has seen, so that the other server can forward any requests that were dropped when the middle server failed. To keep the size of this history short, the tail of the chain acks requests and the ack is passed up the chain.

The protocol also allows for chains to be extended. This is done by copying the state of the tail to a new server and then making the new server the tail.

## Performance evaluation

Renesse and Schneider compare their protocol with ordinary primary backup. While they admit that detecting a failed server will take much longer than fixing the failure once it’s detected, they discuss at length how many message round-trips the recovery will take in both systems. Which system performs better actually depends on the mix of reads vs. writes, because while primary failure is easier to fix in chain replication, backup failure is easier to fix under a classic primary-backup scheme.

A big part of the paper consists of evaluating the performance of chain replication with different setups under multiple load scenarios. The interesting take-away for systems with a single chain is that weak chain replication (where one can read (possibly stale) data from any server in the chain) may actually perform worse than strong chain replication with a ratio of updates to writes as small as $\frac{1}{10}$. They explain this with the fact that processing reads uses resources at the head that could otherwise be used for updates.

Renesse and Schneider also evaluate the performance of their system when the data is spread in volumes across multiple chains. The interesting thing to note there is that read throughput is permanently lowered after a failure, even when a new server is added to the system, because that new server will become the tail of all the chains that the failed server was involved in. Also interesting to note is that work throughput increases temporarily after a failure, because the shorter length of chains means that less work has to be done for each write until the failed server is replaced.

## Discussion

It was interesting to read a paper about a different kind of primary-backup system, and the simplicity of chain-replication was quite surprising. While the system is interesting from a conceptual point of view, it doesn’t seem very suitable for any real-world scenarios. Firstly, the system is designed to operate with equal latency between all nodes and thus doesn’t scale beyond a LAN. Secondly, the authors assume uniform popularity of stored items. The current system would perform very poorly if a few items were the target of the majority of the requests, because replication is not used to spread the load.

The system still has some potential for improvement. As the experiments clearly show, performance decreases with every failure. This could easily be fixed by allowing for servers to be added not only as tail, but also as head or middle server.