StarCounter and CAP

Question

I have been reading about a database named Starcounter. It makes a claim that it can handle loads that a "NoSql"-database only can handle without dropping consistency. As far as I understand the CAP-theorem, if you keep consistency, you lose availability or partition tolerance. So what trick makes StarCounter work?

I can imagine that StarCounter is fast, but the claim that NoSql needs to drop consistency to keep up seems a little bit strange to me. Can anyone please explain?

Thanks in advance Roland

Answer 1

The short answer

The CAP theorem (aka Brewers theorem) cannot be beaten for a single piece of information (like a consistent database). If you have a horizontally scaled database, you won't get consistency and performance. This conclusion comes from the laws of physics and can be deducted from Brewers theorem and Einsteins theories of relativity. You need to scale-in/up, not out. Not very "cloudy", but as the enemies of Galileo would probably confess if they were alive today, nature does a poor job at honouring human fashion.

Scaling consistent data

I'm sure there are other approaches, but Starcounter works by hosting the database image in RAM. Instead of moving database data to the application code, parts of the application code is moved to the database. Only data in the final response gets moved from the original place in RAM memory (where the data was in the first place). This makes most of the data stay put even if there are millions of requests processed every second. The downside is that the database needs to know the programming language of your application logic. The upside, however, is obvious if you have ever tried to serve millions of HTTP requests/sec, each requiring extensive database access.

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A more thourough answer

The question is a good one. It is no wonder you find it strange as it was only a few years back that CAP was proven (turned into a theorem). Many developers are as disappointed as a kid would be when theoretical physicist tells him to stop looking for the perpetual motion machine because it cannot work. We still want the scale-out consistent database, don't we?

The CAP theorem

The CAP theorem gives that any piece of information cannot have consistency (C), availability (A) and partition tolerance (P). It applies to a unit of information (such as a database). You can of course have independent pieces of information that operates differently. One piece could be AP, another could be CA and a third could be CP. You just cant have the same information being CAP.

The problem with the impossibility of the 'P' in a consistent and available database results in how a scaled-out database MUST do signalling between the nodes. The conclusion must be, that even in a hundred years from now, CAP gives that a single piece of consistent data will have to live on hardware interconnected using hard wires or light beams.

The problem with the P in CAP

The problem lies in performance if you apply horizontal scaling to an available consistent database. A good performance was the very reason to do horizontally scaling in the first place, this is a very bad thing. As every node needs communicate with the other nodes whenever there is database access to achieve consistency, and given the fact that signalling is ultimately limited by the speed of light, you are left with sad but true fact that database scientist (as well as CPU scientists) are not just being stubborn for failing to see scale-out as a a magical silver bullet. It will not happen because it cannot happen (however, parts of your database could be placed in a AP set, so remember, we are talking about consistent data here). Adding the theories of Einstein to the CAP theorem and the small box wins of the cloudy data-center for consistent data.

Perpetual machines and CAP

The state of things in the database community is a little bit like the state of perpetual motion machines when horse and carriage was the way to get to work. Without any theoretical evidence against it, the patent offices granted hundreds of patents for impossible perpetual machines. Today, we may laugh at this, but we have a similar situations in the database industry with consistent scale-out databases. When you hear somebody claim that they have a scale-out ACID database, be cautious. It was only after the dot com crash mathematicians at MIT proved Brewer right at the CAP theorem was officially born, so the hunt for the impossible has unfortunately not died off just yet. You can compare this, if you want, to the way laggards kept trying to invent the perpetual machine for years after modern theoretical physics should reasonably have put a stop to it. Old habits die hard (my apologies to anyone on Stack overflow still making drawings of bearings and arms moving ad finitum on their own accord - I don't mean to be offensive).

CAP and performance

All is not lost however. Not all pieces of information needs to be consistent. Not all pieces needs to scale-out. You just have the accept Brewers theorem and make the best out of it.

For applications such as Facebook, consistency is dropped. This is okay as data is entered once and then is manipulated by a single users. Still we can experience the side effects in everyday Facebook usage such as things popping in and out of existence for a while.

However, in most business applications, data needs to be correct. The sum of all accounts in your bookkeeping needs to amount to zero. Your stock inventory must equal to 8 if you sold 2 out of 10 items even if there are multiple users buying from the same stock.

The problem with scaling out available data is that you have to make do without partition tolerance. This fancy word simply means that you have to signal between the nodes in your cloud at all times. And as it takes light a few nanoseconds to travel a single meter, this becomes impossible without making your scale-out result in less performance rather than more performance. Of course, this is only true for consistent data. The implications of this has been known by the engineers of Intel, AMD, Oracle et. al for a long time. It is not their scientist haven't heard of scale-out. It is just that they have come to accept the world as Einstein described it.

Some comfort in the gloom

If you do the math, you find that a single PC has instructions to spare on each human being living on Earth for each second it is running (google on 'modern CPU' and 'MIPS'). If you do some more math, like taking the total turnover of Amazon.com (you can find it at wwww.nasdaq.com) divided by the price of an average book, you will find that the total number of sales transactions can fit in RAM of a single modern PC. The cool thing is that the number of items, customers, orders, products etc. occupies the same amount of space in 2012 as it did in 1950. Images, video and audio has increased in size, but numeric and textual information does not grow per item. Sure the number of transactions grows, but not in the same phase as computer power grows. So the logical solution is to scale out read-only and AP data and "scale-in/up" business data.

"Scale-in" instead of "scale-out"

Database engines and business logic running in a VM (like the Java VM or the .NET CLR) typically use fairly effective machine code. This means that moving memory is the overshadowing bottleneck of total throughput for a consistent database. This is often referred to as the memory wall (wikipedia has some useful information).

The trick is to transfer code to the database image instead of data from the database image to the code (if using a MVC or a MVVM pattern). This means that the consuming code executes in the same address space as the database image and that data is never moved (and the disk is merely securing transactions and images). Data can stay in the original database image and does not have to be copied into the memory of the application. Instead of treating the database as a RAM database, the database is treated as primary memory. Everything stays put.

Only data that is part of the final user response is moved out of the database image. For a large scale applications with hundreds of millions of simultaneous users this typically amounts to only a few million requests per second, something that a single PC has no problem with handling given that the HTTP packaging is done on gateway servers. Fortunately, such servers scales out beautifully as they don't need to share data.

As it turns out, the disk is fast at sequential writes so a raided disk can persist terabytes or changes every minute.

Horizontal scaling in Starcounter

Normally you do not scale a Starcounter node. It scales-in rather than out. This works well for a few million simultaneous users. To go above that, you need to add more Starcounter nodes. They can be used to partition data (but then you lose consistency and Starcounter is not designed for partitioning so it is less elegant than solutions such as Volt DB). So a better alternative is to use the additional Starcounter nodes as gateway servers. These servers simple accumulates all incoming HTTP requests for a millisecond at a time. This might sound like a short amount of time, but it is enough to accumulate thousands of request if you decided you need to scale Starcounter. The batch of requests are then sent to the ZLATAN node (Zero LATency Atomicity Node) a thousand times a second. Each such batch can contain thousands of requests. In this way, a few hundred million user sessions can be served by a single ZLATAN node. Although you can have several ZLATAN nodes, there is only one active ZLATAN node at a time. This is how the CAP theorem is honored. To go above that, you need to consider the same tradeoff as Facebook and others.

Another important note is that the ZLATAN node does not serve applications with data. Instead, the applications controller code is run by the ZLATAN node. The cost of serializing/deserializing and sending data to an application is far greater than to process the controller logic cycles. I.e. the code is sent to the database instead of the other way around (a traditional approach is that the applications asks for data or sends data).

Making the "shared-everything" node faster by doing less

The use of the database as a "heap" for the programming language instead of a remote system for serialization and deserialization is a trick that Starcounter calls VMDBMS. If the database is in RAM, you should not move data from one place in RAM to another place in RAM which is the case with most RAM databases.