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GPU plotting was one of the items on the roadmap last week and that turned into a week+ long side quest, so let me share some details about that.

As mentioned on last week’s status update, one of the key pieces that I was missing to have the detailed operation of plot membership allocation was the impact of sector expiration on the protocol. By having a unique plot identifier, we are able to uniquely link sectors to plots, but these sectors need to expire in a way that does not require farmers to re-plot while archiving the most recent history. Fortunately, we can leverage the current expiration mechanism of the Subspace protocol, and build a layer on top of it to adapt it to the sharded version while maintaining the original guarantees in terms of plot expiration, sector re-plotting, and history archiving.

Last week I shared a model that can help us reason about the security of shards assuming an honest majority in the beacon chain. The model evaluates what are the trade-offs in terms of the number of shards, the number of farmers per shard, and the proportion of malicious farmers in the system. But if you recall from the overall design of the system, shards are periodically submitting segments and blocks to the upper layers of the hierarchy and to the beacon chain. We need to verify that these are valid, available, and correctly encoded before they are included in super segments and the global history of the system. Can we do so without relying on fraud proofs? This has been one of my focuses for the week, let’s jump right into it.

This was a lighter week on meaningful changes, but there are still few things to share. First of all, last week’s block import was (and still is) and incomplete prototype, but this week it was extended to become a bit more complete. I was also working on bringing up more components frm Subspace, including farmer, which lead me to attempt GPU plotting in Rust, so let’s get into it.

This week has been all about objectively assessing the security of the system. After all of the work around the membership allocation protocol and its security there are still two questions that we need to answer to understand the feasibility of the protocol: (i) what is the security bound of the protocol as a whole (from beacon chain to shards), and (ii) how can we ensure that plots are uniquely identified and that farmers cannot cheat by committing the same plot to different history sizes to try and game the shard allocation mechanism.

There was no update last week again since I didn’t feel like there was enough to share at the time, but now that more things have settled I’d like to share what I’ve been busy with.

Last week I shared a high-level of how I was thinking farmer membership selection should work. After some discussions early in the week, we realised there were still some blind spots and attacks that we weren’t protecting against (or if we were, we didn’t have an objective measure of how robust they were). Thus, this week has been exclusively focused on modeling the membership selection protocol so we can reason objectively about its design.

There have not been an update from me last week, how come? Well, it didn’t seem like there was anything particularly substantial to share, mostly some refactoring, so I decided to skip it. This week though I have a few words to say about the path towards block production (not there yet, but getting closer). Some this is a boring process, I procrastinated some too, diving into various topics with some interesting performance improvements and new learnings.

This week has been another of those weeks where I am pretty happy with the progress made. The highlights of the week are the following: (i) we now have a pretty good sense of the end-to-end operation to commit shard segments into the global history of the beacon chain (as described in the discussion of PR267); (ii) and Nazar had an idea to tackle the verification of the availability and correctness of shard segments without requiring an independent data availability mechanism, by leveraging the longest-chain rule and farmers membership allocation (which was an issue that was really bugging me). Let’s jump into the details of these two topics.

I started the week thinking about the mechanics of shard segment commitment into the global history of the beacon chain. If you recall from previous updates, we already had a pretty good idea of how the information about child shard segments flow up to the beacon chain, but there were still a few questions that were really bugging me.