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Design Exercise: Distributing (Consistent) Data at Scale

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Design Exercise: Distributing (Consistent) Data at Scale

Explore a design exercise that deals with distributing consistent data at scale.

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The Question

Before I start discussing this topic, I want to talk a bit about the speed of light. That pesky limit basically means that there is an inherent lag in passing information between any two points in space. In your daily life, you can mostly ignore it. The human brain is far too slow to perceive it, and even if you are working with computers, you can usually ignore the speed of light for anything less than about 500 miles.

But the speed of light is merely the hard upper limit of our ability to send information from one location to another. In practice, the lag time between any two computers connected to a network is much higher. In fact, if you are a gamer, you are very well acquainted with that fact.

Let’s assume that we have a need to do the following:

  • Hold a mutable state of some kind.
  • Modify it in a consistent manner.
  • Distribute it to many locations, where many are at least hundreds and potentially tens of thousands of locations.

Let’s break this up a bit to its component parts. A mutable state basically means that we can modify it over time and that these modifications show up in all locations. The speed of light (and network lag) ensures that we can’t have this happen instantaneously. And, of course, the killer requirement here is that we need to do this in a consistent manner.

I find it really hard to talk about abstract problems concretely, so let’s use an example. We have a set of tax rules that determine how certain products should be taxed. The ruleset is big, changes on a regular basis and it is quite important to get things right. Oh, and we also need to do push those rules to tens of thousands of locations that would independently compute taxes for purchases.

In this case, the solution is quite easy. We have a single source of truth that all the locations can pull the tax ruleset from (directly or through mirrors). We’ll also ensure that tax rules are applied only at some future date from their creation, to allow time for all these locations to be updated. If a location hasn’t had an updated ruleset for over a certain amount of time, they can refuse to process any payments until they get an update. This example has really very little for us to do in term of design.

A better example would have multiple actors changing the data as we work with it. An actual example for when this is required can be when we have an ad provider that needs to run ads in many (physical locations). Each location is actually an IoT device that includes a computer, a screen, and a camera. Each such device decides independently what ads should be shown (for example, if the camera sees a baby stroller, they show ads for baby toys).

I actually had to search hard for an example that would work for this scenario but I think this is a good one. We obviously have a lot of activity on the ad provider with many people registering and modifying ads settings. We need that portion of our business to be consistent (this is what we are charging people for, after all). However, given the probably high amount of ad devices spread all over, we can’t rely on them always being connected to a central server or even on them being connected at all.

And even if there is no connectivity and nothing new to show, we must show something. Even if we aren’t getting paid for showing the ad, not showing something or (actually worse) showing an error is something that we can’t tolerate.

Given all of these constraints, how do would you build such a system?

The Answer

The first thing to realize about this task that this is basically a synchronization problem over anything else. We define a certain location as the primary one, the one that will accept all the modifications to the data. Each of the nodes will then simply need to connect to that location every now and then and get all the updates.

“Basically a synchronization problem” is like saying that getting a P1 emergency bug at Friday night just before the movie starts is a “tad annoying.” The problem is that to do synchronization properly, you have to model your data properly, make sure that you keep track of changes and be able to send partial changes down the wire efficiently. That is not a simple task at all.

In this post, I want to offer another option to handle this. Using Raft. This is a strange use case for a consensus algorithm, I’ll admit. I guess that technically you could run a Raft consensus over 10,000 nodes. Just don’t expect it to be making any sort of decisions. So why am I offering Raft for a scenario when we have that many nodes? Because Raft, at its core, is a way to achieve consensus on a distributed log, that is all. And no one says that you must get that distributed log only via Raft directly.

The idea is basically to have a single source of truth. This can be a single server or it can be a Raft cluster with 3 – 7 nodes in it. All writes in the system are going to go there. The actual process is very well understood and there are multiple ways to do that. The simplest one to consume is likely rqlite. The log, in the case of rqlite, is going to be the SQL statements that are going to be applied to a SQLite database.

But how does this solve the problem of distributing the data? The answer is simple, we already have a way to disseminate distributed state, the log itself. What is going to happen is that you’ll have each of the nodes in the edge connect to the cluster and ask for a copy of the log as of the last committed entry that they have. When they get that, they can apply these statements to their own local copy of SQLite and know that they are now up to date with the state of the system at that time frame.

This approach skips over the need to architect your data for sync (which is hard) and push all of that complexity down the stack to your infrastructure. If the number of nodes you have is large enough, you might need to introduce mirrors to reduce the load. But that fits very nicely into the architecture without really needing to change something.

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database ,database design ,data consistency ,distributed data ,data at scale

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