Designing Distributed Systems Using Gauge Synchrony Inwards Information Gist Networks

in our Zoom Reading Group, nosotros had read the "Scalable State Machine Replication" newspaper inward our group. SSMR built on meridian of a "slow" atomic multicast (implemented with Multi-Ring Paxos) as well as imposed a total guild for all messages before SMR. 

This newspaper introduces SpecPaxos (Speculative Paxos), which every bit good tries to impose an ordering of requests messages before SMR processing. SpecPaxos builds on a carefully constructed network to accept most messages ordered correctly to start with as well as provides a agency to bargain with messages reorderings every bit needed. The relaxing of the ordering requirement allows Speculative Paxos to plow over skillful (average) latency as well as high throughput, as well as escape the heavy toll of atomic multicast implementation. (To last fair, SpecPaxos doesn't see the work of partitioning SMR as well as however providing strict serializability over transactions on them similar SSMR newspaper did.)

MOMs against out of guild delivery 

SpecPaxos introduces a Mostly-Ordered Multicast primitive (MOM), which provides a best-effort guarantee that all receivers volition have messages from dissimilar senders inward a consistent order. MOMs leverage the structured topology of a information pump network as well as the forwarding flexibility provided past times software-defined networking. There are 3 increasingly costly/high-maintenance agency of achieving ordering at the network. 

1. Topology-aware multicast: Ensure that all multicast messages traverse the same issue of links. This eliminates reordering due to path dilation.
2. High-priority multicast: Use topology-aware multi- cast, only every bit good assign high QoS priorities to multicasts. This essentially eliminates drops due to congestion, as well as every bit good reduces reordering due to queuing delays.
3. In-network serialization: Use high-priority multicast, only road all packets through a unmarried beginning switch. This eliminates all remaining non-failure related reordering.

The mutual intuition behind all these designs is that messages tin last sent along predictable paths through the information pump network topology with depression latency as well as high reliability inward the mutual case.

SpecPaxos uses the commencement 2 approaches. In a datacenter setup, it sets upwards unopen to beginning switches as well as partitions keyspaces with them. To plow over topology-aware multicast, it makes all customer requests to larn upwards to the beginning switch as well as starts the multicasts from there. (Setting upwards the beginning nodes with this consistent partitioning at the beginning switches is a trusted consensus base of operations to bootstrap the residual of the protocol. Modification of that for charge balancing or failures would require help every bit well.)

Dan Ports, the commencement writer of the paper, published a follow-up run on SpecPaxos the adjacent twelvemonth at OSDI 2016 called NOPaxos (Scalable SMR is an instance of the guild commencement approach. But this tin backfire, every bit the ordering could create dependencies which delay the subject messages until the delivery of the before messages is completed, as well as this tin cascade, similar inward virtual synchrony solutions.  NoPaxos uses switch hardware to alleviate unopen to of these concerns. 

Best endeavour ordering first. SpecPaxos is an instance of order-first best effort. We tin fifty-fifty consider  EPaxos similar leaderless solutions nether this, because they optimistically assume no conflicts, as well as bargain with conflicts via a recovery protocol.

Central ordering at the leader. Paxos, MultiPaxos, multi-leader versions of Paxos such every bit WPaxos orders centrally at the leader or leaders (if considering partitioned keyspace) first.

Central lazy ordering at the leader. S-Paxos and SDPaxos order centrally at the sequential afterward replication

Are at that topographic point whatsoever unexplored options? How most lazy ordering at the leader(s)? 

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