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flowerinthenight authored Sep 28, 2024
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### On payload size

One of zgroup's main goal is to be able to track clusters with sizes that can change dynamically overtime (e.g. [Kubernetes Deployments](https://kubernetes.io/docs/concepts/workloads/controllers/deployment/), [GCP Instance Groups](https://cloud.google.com/compute/docs/instance-groups), [AWS Autoscaling Groups](https://docs.aws.amazon.com/autoscaling/ec2/userguide/auto-scaling-groups.html), etc.) with minimal dependencies and network load. All my previous related works so far, depend on some external service (see [spindle](https://github.com/flowerinthenight/spindle), [hedge](https://github.com/flowerinthenight/hedge)), using traditional heartbeating, to achieve this. This heartbeating technique usually suffers from increasing payload sizes (proportional to cluster sizes) as clusters get bigger. But I wanted a system that doesn't suffer from that side effect. Enter [SWIM](https://www.cs.cornell.edu/projects/Quicksilver/public_pdfs/SWIM.pdf)'s infection-style information dissemination. It can use a constant payload size regardless of the cluster size. SWIM uses a combination of `PING`s, `INDIRECT-PING`s, and `ACK`s to detect member failures while piggybacking on these same messages to propagate membership updates (gossip protocol). At the moment, zgroup only uses SWIM's direct probing protocol; it doesn't fully implement the Suspicion sub-protocol (yet).
One of zgroup's main goal is to be able to track clusters with sizes that can change dynamically overtime (e.g. [Kubernetes Deployments](https://kubernetes.io/docs/concepts/workloads/controllers/deployment/), [GCP Instance Groups](https://cloud.google.com/compute/docs/instance-groups), [AWS Autoscaling Groups](https://docs.aws.amazon.com/autoscaling/ec2/userguide/auto-scaling-groups.html), etc.) with minimal dependencies and network load. All my previous related works so far, depend on some external service (see [spindle](https://github.com/flowerinthenight/spindle), [hedge](https://github.com/flowerinthenight/hedge)), using traditional heartbeating, to achieve this. This heartbeating technique usually suffers from increasing payload sizes (proportional to cluster sizes) as clusters get bigger. But I wanted a system that doesn't suffer from that side effect. Enter [SWIM](https://www.cs.cornell.edu/projects/Quicksilver/public_pdfs/SWIM.pdf)'s infection-style information dissemination. It can use a constant payload size regardless of the cluster size. SWIM uses a combination of `PING`s, `INDIRECT-PING`s, and `ACK`s to detect member failures while piggybacking on these same messages to propagate membership updates (gossip protocol). Currently, zgroup only uses SWIM's direct probing protocol; it doesn't fully implement the Suspicion sub-protocol (yet).

At the moment, zgroup uses a single, 64-byte payload message for all its messages, including leader election (see below).

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