When a dog owner wants to train his dog, the procedure is well-known and quite simple. The owner runs two loops: one of positive feedback and one of negative ditto. Whenever the dog does something right, the positive feedback loop is invoked and the dog is treated with a snack. Whenever the dog does something wrong, the dog is scolded and the negative feedback loop is used.
The result is positive and negative reinforcement of the dogs behavior. The dog will over time automatically behave as the owner wants and never even think of misbehaving.
When a programming language trains its leashed programmer, it likewise uses positive and negative feedback. Whenever a problem is easily solvable in the constructs and features of said language, it reinforces the use of those features and constructs. And also in the same vein, if something is hard to do in the language, the programmer will shy away from thinking the idea, since it may be too hard to do in the language. Another negative feedback loop is when resource usage of a program is bad. Either it will use too much memory of too many CPU resources to carry out its work. This discourages the programmer from using that solution again.
The important point is that while all practical general purpose languages are Turing complete, the way they train programmers to behave as they want is quite different. In an Object Oriented language for instance, the programmer is trained to reframe most - if not all - questions as objects of bundled fields and methods. A functional programmer is trained to envision programs as transformations of data from the form X into the form Y through the application of a function. And so on.
Which brings us to how ZeroMQ and Erlang will train its programmers into doing vastly different things:
In ZeroMQ, the programmer will configure a messaging network. That network will probably be fairly static. It is not the case that there is one configuration, invoked in the beginning of the system, which stays there for all of the systems execution, but chances are that the configuration is still fairly rigid and not subject to much change.
Around this messaging network, we now add processes which operate on the network. We send and receive messages on the network, processing them in various different programming languages. A process is also fairly static in the sense that the process is started, and then tend to use the messaging network as a queue: It will pick off some work, look at the data, process the data and push it back into the messaging network again. Taken to the extreme, ZeroMQ applications are often queue processors like this.
Incoming requests are transformed to a message and then sent into the messaging network of ZeroMQ. Then it will be processed by some vacant process in the other end and it will be moved from queue to queue until it finally completes. Note ZeroMQ is more complex than a simple point-to-point messaging framework, but the gist of the idea here is that many applications will have the above mentioned system design.
In Erlang, the programmer will take the request and turn the request into a process. Erlang programmers are trained by the language to like having a large amount of small processes like this, because the language encourages that behaviour. ZeroMQ-solutions on the other hand will be discouraged to dequeue a message and create a process out of it - if the processing language uses a traditional heavyweight thread for a process.
Now, given that the request is a process, an Erlang programmer will do the dual to the ZeroMQ programmer. Since the process is now the request, whenever the request needs to do some processing, it will do so itself. If it needs some external information, the request/process will ask for it. The preemptive scheduler will ensure that no request can stall the pipeline. Also, since the scheduler in Erlang is parallel, it is relatively easy to get all cores to work, but by having enough requests/processes in the run-queue.
Note, also, how the messaging network in this solution is fairly dynamic. Point-to-point exchanges are formed on demand when a request need some information.
To the Erlang programmer, the representation of the request is the process. There is no "message" living in a queue which is the representation of the request at any point. There are less need to tune fan-in/fan-out at the end of queues to make parallel work go through, since the schedulers will automatically parallelize.
Both styles have their advantages and disadvantages. But that is not the interesting point. The interesting point is how the ZeroMQ idea encourages one system design, whereas the Erlang solution encourages another. In Erlang a process is cheap, so it is totally feasible to turn each request into one. Furthermore, since there is no concept of a message channel, sending a message to a target is all about knowing or finding the name of the target (see for instance Ulf Wiger/Erlang Solutions gproc framework for an efficient name registry). On the contrary, since ZeroMQ has the concept of a generalized socket as a channel endpoint and assuming the programming language we use for processing does not have cheap processes. Then the only feasible solution is to build a static channel network with processors.
Another interesting point is that most languages with static type systems have latched onto the channel solution. It is easy because a channel can be annotated with the type traversing the channel. But it also trains programmers to think in that model. It would be fun to see a language where process Id's - that is names of processes - have a type of what that process receives.
The concluding remark: ZeroMQ/Erlang above is just an example. You can find this kind of reinforcement with basically any language design choice. It is rarely that a feature is ubiquitously good or bad. Rather, most choices are a trade-off. Haskells lazy evaluation enables some optimizations but disallow some other optimizations. Ocaml, being eager, can do the optimization that Haskell can't and can't do the ones that Haskell can. Similarly, for a programmer, getting easier access to some feature often means less control of something else, or that something else become much harder to pull off. The next time you enter in a discussion on the merits of a given language feature, make sure there is not a programming language holding your leash and telling you what is right or wrong!
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- Jesper Louis Andersen
- Lambda-loving CS Geek. Likes metal music. Likes dogs. Likes cats. Does not like pictures of dogs and cats (unless they are lambdacats!)Has an unhealthy coffee addiction. Calls himself the coffee zombie in the morning (BEEEEANS!)Has a neverending curiosity gene. Likes intelligence.