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Synchronization

Sat Feb 10, 2018

One of the problems I'm still wrestling with in taking the proposed massively-distributed system you saw demoed in The Router is syncronization among cell tiles. And I'm not exactly sure where to begin implementing this thing.

So, you know what time it is.

There's a few different ways of dealing with this, and I'm not sure what we'll want to do. Because of how these systems are structured, we're solidly in CAP theorem territory and I don't think we can leave very easily. Although, there is a trade I'm thinking of that might make a decent compromise. The problem we're dealing with is

How do we provide an event window to the active cell despite the fact that constituent neighbor cells might be network-remote resources?

There's a few possible answers.

Don't

In this approach, we make the tile boundary explicit, and expose it to cells as well as their running machines. We're punting the decision of how to treat them differently up to the application developer. There are two different types of cells in the neighborhood a running machine might have access to; local neighbors and remote neighbors. local as in hosted on the same tile as the current cell, and remote as in hosted elsewhere. The neighborhood a running machine would get access to in this universe would be composed of zero or more remote cells and some number of local cells, depending on its exact position on a tile.

The availability/consistency decision here is pushed off to the application layer because some programs might be able to behave equivalently with respect to both cell types, but others might have to treat remote cells very differently indeed from local ones. The tiles get to remain simple this way, but the cells and machines may end up being more complicated, depending on the behavior requirements of a particular program.

Use Locks

When a particular cells' turn comes up, lock its entire neighborhood and give it write access. Lock its entire neighborhood, even if this involves remote locks which are a pain in the ass to implement correctly at the best of times, and even then, might behave unexpectedly in the face of extensive hardware and network failures.

This is slower, because we have to implement remote locking. Which at once restricts computations at a number of cell sites depending on the size of the event window, and involves some number of network calls just to establish that lock. It's Consistent and not Available because the locked cells don't get to do anything while locked, so they're guaranteed-ish to have consistent state across the turn. Guaranteed-ish because downed tiles or network partitions cause remote locks to make hard decisions between eternal deadlock and pre-maturely invalidating locks, thereby introducing inconsistency.

Now that I've kind of said this one out loud, it's pretty intuitively obvious that we don't want this solution naively, for various reasons. But it might still end up being the least evil, so we'll keep it in our back pocket. It also might work half-way decently with some prospective execution stuff bolted on.

Use No Locks

To avoid using locks, we need to have locally cached copies of remote cells. We refresh them from source by issuing a read on the affected cells at the beginning of each turn, running the selected local cell while giving it access to the proxy cells, and send remote writes/changes back to the appropriate remote tiles after the turn is complete. Depending on how much network bandwidth we feel comfortable using, we might report local changes in border regions too, so as to potentially reduce traffic.

This approach might be a bit faster but less consistent than locking. Because no locking is happening, we don't have to keep cells idle while waiting on remote computations to complete, but it does mean that their contents might change in a way that doesn't get communicated across to its proxy cells in time to make the current turn consistent. The drawback is that tiles get more complicated. The communication protocol between them becomes a bit meatier than just naive cell changes; there also need to be communications that talk about effects on proxy cells. Each tile also needs to have additional cells to store an event-window-wide section of the border region of surrounding tiles. As a result, there's also potentially a more elaborate tile-connection procedure that we need to go through when hooking up new tiles. In particular, it would have to communicate with its neighbors to figure out what its border region looks like to start with.

This sounds like the way to go, if I'm being honest. The real question here is "how much inconsistency are we actually introducing with the border-region proxy scheme?" My gut initially tells me that it's a tolerable amount, especially since this approach would let us gracefully deal with tile failure without busting our locks' consistency guarantees. Given that the goal of the whole system is to be robust in the face of radical inconsistency, this might just be the right tradeoff here.

General Meditations

No matter which of the above approaches we take, we'll still need to be able to serialize cell contents somehow. This implies that we'll need to construct our cell-internal language at least somewhat carefully1. The bottom line for tiles is that they need to know how to talk to each other with messages that let them communicate about the status and possible mutations of cell contents. If we go the cell-language route, this will need to include a serialized listing for the cells' behavior, but even if we don't, we need to be able to communicate state mutations. Depending on how much or little we care about reducing network traffic, we might want to make that protocol more elaborate to include minimal delta communication, rather than just outright serialization.

If we take either the "Locks" or "No Locks" appraches, we'll also need enough space locally to instantiate some number of proxy cells. Units that represent cells on other tiles for the purposes of computing the effects of a turn. Again, we might make choices here depending on how much we value local space and network transfer2. So something that would be nice to build regardless of what decision we actually end up making regarding tile behavior are a serialization format for cell contents (including behavior and state), and a protocol that sends that format to remote machines.

On a completely different topic, depending on how we end up setting up cells, we might have some interesting ways around currency. Or rather, around additional types of currency. One specific thing I've got in mind: if we make the decision to develop a language for expressing machines, and we make that machine language sufficiently restricted we can handicap certain operations to prevent the rise of grey-goo-likes. The reason we'd want a currency/resource system around is as a check against unlimited growth and hypercompetition. A concrete case is the cell that infinitely spawns more copies of itself. One way to stop this tactic dead is to ensure that there's some small number of times a cell can call spawn! in a single turn. The extreme case is "your turn ends as soon as you successfully spawn". A less extreme, but slightly more complex, case is "you may only take some very small number of 'big' actions (like spawn!) before your turn ends". That way, if you had an accidental grey goo roaming around, it wouldn't be fast enough to outcompete its' neighbors outright. It would at least be plausible to mount a counter-offensive.

Whether that could be effective enough will be the subject of a few prototypes, and possibly some very interesting conversation.

  1. We could avoid this by trying to build a known set of machines that are built into each tile and instantiated locally, at which point we could just send labels and state contents across. I'm honestly not sure which of these is preferable, and it's a deeper question that I hope to tackle in a future standalone blog-post. Because of Silverman's work, I know that there are n-state cellular automata that end up being turing complete (which means we could encode larger programs in them), but I'm not sure whether this gets us to anything resembling "robustness" as we mean it in this context. It's not necessarily the case that we can apply existing CA research directly, and it may in fact be a red herring; our goals may be different enough that the similarities between Robust-First and something like Wireworld are valid but unproductive comparisons. In other words, we might not technically lose expressiveness because we could implement turing machines with a very small subset of all possible cells, but it might be complicated and I'm not sure we'd keep any inherent robustness (in fact, it seems very likely that we wouldn't).
  2. For instance, if we really care about minimizing network traffic, we might do something like add a surrounding buffer of cells wide enough to accomodate 1/2 the event window size. This would let us refresh those cells before running the tile, and we could then act on a quasi-consistent external neighborhood without making additional queries during a turn. If we don't care, we might take the approach of refreshing right before issuing a lock (which would lead to the same cells being refreshed multiple times during a set of turns), or maybe even on each modification (which would lead to the same cells being refreshed multiple times during a single turn). If we take the resource-minimizing approach, we kinda want the first of these.


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