Wednesday, June 2, 2010

Trends in programming

There is a dizzying wealth of new options for web programmers. I am trying to understand why there are so many changes and which of those changes I should be choosing to learn. I have found that the answer to this sort of question often comes from looking backward and trying to judge current trends in an historical context.
Hardware: Multicore is the future
This is an inevitable trend, as carefully explained in The Future of Computing by Anant Agarwal to MIT freshman. The power consumption of a CPU scales as the cube of the voltage. So trying to improve performance by brute force is a loosing game. Instead, it is much better to build smaller, lower power cores. Prof. Agarwal is among those who are laying the foundation for a future of RISC cores arranged in a tiled multi core architecture. As a programmer, you can get much better performance if you can leverage concurrency. We can leave the hardware issues to the likes of MIT, but the programming challenges will affect everyone in who programs.
To better understand hardware and how it affects software, listen to A Crash Course in Modern Hardware by Cliff Click of Azul. Azul has systems with 864 processor cores, so they are on the front lines of this revolution. He explains how modern computers, with caches and pipelining, are not particularly well described by the von Neumann architecture. The entire talk seems like an explanation of how hardware engineers have been addressing the von Neumann bottleneck between CPU and memory. Even here, we find that each physical core is running concurrent processes.
So, we are seeing horizontal scaling of computers in clusters, with each computer having more cores and with each core handing more concurrent tasks. I can spot a trend - concurrent processing is the future. But I HATE threading and locks.
Functional Programming and Concurrency
Erlang was developed for high reliability in Ericsson telephone switches. Joe Armstrong describes how they tried to develop Systems that Never Stop and why that lead to Message Passing Concurrency in Erlang. Armstrong came up with a list of six laws for reliable systems. The laws/requirements are:
  1. isolation (the probability of isolated systems failing is independent. So processes need to be isolated.)
  2. concurrency (do you need to worry about how many concurrent processes can you support? You need at least two systems to make a non-stop system. Even a two computer system is concurrent. You can't avoid concurrency)
  3. failure detection (if you can't detect a failure, you can't fix it. the thing that crashed can be the thing that detects failures.)
  4. fault identification (You need to figure out why there was a failure if you want to fix it.)
  5. live code upgrade (Systems that should never stop, need live code upgrade)
  6. stable storage (no backups, implies multiple copies. must keep crash logs)
Erlang is a functional language, so there are no side effects to calculations, Joe Armstrong discusses the importance of Functional Programming in Functions + Messages + Concurrency = Erlang. There really is no point in arguing about if he is right. Erlang is the proof. It is reliable, Ericsson has the AXD 301 switch with two million lines of code: BT operates these switches at a 'nine-nines' reliability - it can fail for 32 ms each year. BT has run switches for six years without stopping. Erlang also allows for high concurrency, this is clearly illustrated in Apache vs. Yaws: while Apache and Yaws have similar throughputs for score of concurrent clients, Apache bogs down at around 4 thousand parallel sessions. By contrast, the Erlang powered Yaws can serve over 80 thousand parallel sessions on Linux with similar hardware. Apache is a well written server, Netcraft shows that Apache is easily the #1 server on the World Wide Web in May 2010. But Yaws beat it handily in this measure. In RESTful Services with Erlang and Yaws, Steve Vinoski examines Yaws in more detail.
But perhaps Erlang is hard to use and expensive to code? According to Ulf Wigner, Ericsson recorded a Four-fold increase in Productivity and Quality on the ADX 301 project. Joe Armstrong is very optimistic about the future of Erlang, and I think he is right.
When I looked into this some more, I realized that Erlang was not the 'bolt of the the blue' that is first seemed to me. Some of the greatest minds in programming have been noticing that imperative programming has been at something of a standstill. Read John Backur's 1977 ACM Turing Award Lecture, Can Programming Be Liberated from the von Neumann Style? A Functional Style and Its Algebra of Programs. This is a broadside attack on imperative programming is all the more remarkable because it is coming from the author of FORTRAN. He then goes on to explain what he sees are the advantages of functional programming. This must have been a controversial paper, Edsger Dijkstra critique of Backur's lecture ended with:
In short, the article is a progress report on a valid effort but suffers badly from agressive overselling of its significance, long before convincing results have been reached.
I tend to believe both Backur and Dijkstra. I resolve any apparent conflict by admitting that Backur was ahead of his time.

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