Tuesday, June 1, 2021

Towards a ToDoMVC Backend in Objective-S

A couple of weeks ago, I showed a little http backend. Well, tiny is probably a more apt description, and also aptly describes its functionality, which is almost non-existent. All it does is define a simplistic Task class, create an array with two sample instances and then serves that array of tasks over http. And it serves the -description of those tasks rather than anything usefuk like a JSON encoding.

For reference, this is the original code, hacked up in maybe 15 minutes:


#!env stsh
framework:ObjectiveHTTPD load.

class Task {
   var <bool> done.
   var title.
   -description { "Task: {this:title} done: {this:done}". }
}

taskList ← #( #Task{ #title: 'Clean my room', #done: false }, #Task{ #title: 'Check twitter feed', #done: true } ).

scheme todo {
   var taskList.
   /tasks { 
      |= { 
         this:taskList.
      }
   }
}.

todo := #todo{ #taskList: taskList }.
server := #MPWSchemeHttpServer{ #scheme: todo, #port: 8082 }.
server start.
shell runInteractiveLoop.

What would it take to make this borderline useful? First, we would probably need to encode the result as JSON, rather than serving a description. This is where Storage Combinators come in. We (now) have a MPWJSONConverterStore that's a mapping store, it passes its "REST" requests through while performing certain transformations on the data and/or the references. In this case the transformation is serializing or deserialzing objects from/to JSON, depending on which way the request is going and which way the converter is pointing.

In this case, the converter is pointing "up", that is it serializes objects read from its source to JSON and deserializes data written to its source from JSON to objects. We also tell it that it is dealing with Task objects. When we have the converter we connect it to our todo scheme and tell the HTTP server to talk to the json converter (which talks to our todo scheme):


todo := #todo{ #taskList: taskList, #store: persistence }.
json := #MPWJSONConverterStore{  #up: true, #class: class:Task }.
json → todo.
server := #MPWSchemeHttpServer{ #scheme: json, #port: 8082 }.

Second, we also want to be to interact with individual tasks. No problem, just add a /task/:id proprerty path to our store/scheme handler, along with GET ("|=") and PUT ("=|") handlers. I am not fully sold yet on the "|=" syntax for this, but I would like to avoid names for this sort of structural component. Maybe arrows?
	/task/:id {
		|= {
			this:taskDict at:id .
		}
		=| {
			this:taskDict at:id put:newValue.
		}

In order to facilitate this, the taskList was changed to a dictionary. Once we make changes to our data, we probably also want to persist it. One easy way to do this is to store the tasks as JSON on disk. This allows us to reuse the JSON converter from above, but this time pointing "down". We connect this converter to the filesystem at the directory /tmp/tasks and to the store:
json → todo → #MPWJSONConverterStore{  #class: class:Task } → ref:file:/tmp/tasks/ asScheme.

In addition, we need to trigger saving in the PUT handler:
		=| {
			this:taskDict at:id put:newValue.
			self persist.
		}
	-persist {
		source:tasks := this:taskDict allValues.
	}
}

This will (synchronously) write the entire task list on every PUT. The full code is here:
#!env stsh
framework:ObjectiveHTTPD load.

class Task {
	var id.
	var  done.
	var title.
	-description { "Task: {this:title} done: {this:done} id: {this:id}". }
	-writeOnJSONStream:aStream {
		aStream writeDictionaryLikeObject:self withContentBlock:{ :writer |
			writer writeInteger: this:id forKey:'id'.
			writer writeString: this:title forKey:'title'.
			writer writeInteger: this:done forKey:'done'.
		}.
	}
}

taskList ← #( #Task{ #id: '1', #title: 'Clean Room', #done: false }, #Task{ #id: '2', #title: 'Check Twitter', #done: true } ).

scheme todo : MPWMappingStore {
	var taskDict.
	-setTaskList:aList {
		this:taskDict := NSMutableDictionary dictionaryWithObjects: aList forKeys: aList collect id.
	}
	/tasks { 
		|= { 
			this:taskDict allValues.
		}
	}
	/task/:id {
		|= {
			this:taskDict at:id .
		}
		=| {
			this:taskDict at:id put:newValue.
			self persist.
		}
	}
	-persist {
		source:tasks := this:taskDict allValues.
	}
}.

todo := #todo{ #taskList: taskList }.
json := #MPWJSONConverterStore{  #up: true, #class: class:Task }.
json → todo → #MPWJSONConverterStore{  #class: class:Task } → ref:file:/tmp/tasks/ asScheme.
server := #MPWSchemeHttpServer{ #scheme: json, #port: 8082 }.
server start.
shell runInteractiveLoop.

The writeOnJSONStream: method is currently still needed by the serializer to encode the task object as JSON. The parser doesn't need any support, it can figure things out by itself for simple mappings. Yes, this makes no sense, as serializing is easier than parsing, but I haven't gotten around to the automation for serializing yet.

Analysis

So there you have it, an almost functional Todo backend, in refreshingly little code, and with refreshingly little magic. What I find particularly pleasing is that this conciseness can be achieved while keeping the architecture fully visible and maintaining a hexagonal/ports-and-adapters style.

What is the architecture of this app? It says so right at the end: the server is parametrized by its scheme, and that scheme is a JSON serializer hooked up to my todo scheme handler, hooked up to another JSON serializer hooked up to the directory /tmp/tasks.

Although a Rails app contains comparably little code, this code is scattered over different classes and is only comprehensible as a plugin to Rails. All the architecture is hidden inside Rails, it is not at all visible in the code and simply cannot be divined from looking at the code. Although there are many reasons for this, one fundamental one is that Ruby is a call/return language, and Rails does its best to translate from the REST architectural style to something that is more natural in the call/return style. And it does an admirable job at it.

I do think that this examples gives us a little glimpse into what I believe to be the power of Architecture Oriented Programming: the power and succinctness of frameworks, but with the simplicity, straightforwardness and reusability of more library-oriented styles.

Performance

I obviously couldn't resist benchmarking this, and to my great joy found that wrk now works on the M1. Since the interpreter isn't thread safe, I had to restrict it to a single connection and thread. My expectations were that it requests/s would be in the double to low triple digits, my fear was that it would be single digits. (The reason for that fear is the writeOnJSONStream: method that is called for every object serialized and is in interpreted Objective-S, probably one of the slowest language implementations currently in existence). To say I was surprised is an understatement. Stunned is more like it:
wrk -c 1 -t 1 http://localhost:8082/task/1 
Running 10s test @ http://localhost:8082/task/1
  1 threads and 1 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   133.62us   14.45us   0.97ms   98.52%
    Req/Sec     7.50k   311.09     7.62k    99.01%
  75326 requests in 10.10s, 12.28MB read
Requests/sec:   7458.60
Transfer/sec:      1.22MBTransfer/sec:      1.97MB

More than 7K requests per second! Those M1 Macs really are fast. I wonder what it will be once I remove the need for the manually written writeOnJSONStream: method.

(NOTE: previous version said >12K requests/s, which is even more insane, but was with an incorrect URL that had the server returning 404s)

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