Programming, Software, Web Applications, Work

Prettier in anger

I’ve generally found linting to be a pretty horrible experience and Javascript/ES haven’t been any exception to the rule. One thing that I do agree with the Prettier project is that historically linters have tried to perform two tasks to mixed success: formatting code to conventions and performing static analysis.

Really only the latter is useful and the former is mostly wasted cycles except for dealing with language beginners and eccentrics.

Recently at work we adopted Prettier to avoid having to deal with things like line-lengths and space-based tab sizes. Running Prettier over the codebase left us with terrible-looking cramped two-space tabbed code but at least it was consistent.

However having started to live with Prettier I’ve been getting less satisfied with the way it works and Prettier ignore statements have been creeping into my code.

The biggest problem I have is that Prettier has managed its own specific type of scope creep out of the formatting space. It rewrites way too much code based on line-size limits and weird things like precedent rules in boolean statements. So for example if you have a list with only one entry and you want to place the single entry on a separate line to make it clear where you intend developers to extend the list Prettier will put the whole thing on a single line if it fits.

If you bracket a logical expression to help humans parse the meaning of the statements but the precedent rules mean that brackets are superfluous then Prettier removes them.

High-level code is primarily written for humans, I understand that the code is then transformed to make it run efficiently and all kinds of layers of indirection are stripped out at that point. Prettier isn’t a compiler though, it’s a formatter with ideas beyond its station.

Prettier has also benefited from the Facebook/React hype cycle so we, like others I suspect, are using it before it’s really ready. It hides behind the brand of being “opinionated” to avoid giving control over some of its behaviour to the user.

This makes using Prettier a kind of take it or leave it proposition. I’m personally in a leave it place but I don’t feel strongly enough to make an argument to remove from the work codebase. For me currently tell Prettier to ignore code, while an inaccurate expression of what I want it to do, is fine for now while another generation of Javascript tooling is produced.

Programming, Python

403 Forbidden errors with Flask and Zappa

One thing that tripped me up when creating applications with Zappa was an error I encountered with form posting that seems to have also caught out several other developers.

The tldr is that if you are getting a 403 Forbidden error but your application is working locally then you probably have a URL error due to the stage segment that Zappa adds to the URL of the deployed application. You need to make sure you are using url_for and not trying to write an absolute path.

The stage segment

Zappa’s url structure is surprisingly complicated because it allows you to have different versions of the code deployed under different aliases such as dev, staging and production.

When running locally your code doesn’t have the stage prefix so it is natural to use a bare path, something like flask.redirect(‘/’) for example.

If you’re using the standard form sequence of GET – POST – Redirect then everything works fine locally and remotely until the raw redirect occurs remotely and instead of getting a 404 error (which might tip you off to the real problem more quickly) you get a 403 forbidden because you are outside the deployed URL space.

If you bind a DNS name to a particular stage (e.g. then the bare path will work again because the stage is hidden behind the CloudFront origin binding.

Always use url_for

The only safe way to handle URLs is for you to delegate all the path management and prefixing to Zappa. Fortunately Flask’s in-built url_for function, in conjunction with the Zappa wrapper can take care of all the grunt work for you. As long as all your urls (both in the template and the handlers) use url_for then the resulting URLs will work locally, on the API Gateway stages and if you bind a DNS name to the stage.

If this is already your development habit then great, this post is irrelevant to you but as I’ve mostly been using Heroku and App Engine for my hobby projects I’d found myself to be in the habit of writing the URLs as strings, as you do when you write the route bindings.

Then when the error occurred I was checking the URL against my code, seeing that they matched and then getting confused about the error because mentally I’d glossed over the stage.


Google Cloud Functions

I managed to get onto the Google Cloud Functions (GCF) alpha so I’ve had a chance to experiment with it for a while. The functionality is now in beta and seems to be generally available.

GCF is a cloud functions, functions as a service, AWS Lambda competitor. However thanks to launching after Lambda it has the advantage of being able to refine the offering rather than cloning it.

The major difference between GCF and Lambda is that GCF allows functions to be bound to HTTP triggers trivially and exposes HTTPS endpoints almost without configuration. There’s no messing around with API Gateway here.

The best way I can describe the product is that it brings together the developer experience of App Engine with the on-demand model of Lambda.

Implementing a Cloud Function

The basic HTTP-triggered cloud function is based on Express request handling. Essentially the function is just a single handler. Therefore creating a new endpoint is trivial.

Dependencies are automagically handled by use of a package.json file in the root of the function code.

I haven’t really bothered with local testing, partly because I’ve been hobby-programming but also because each function is so dedicated the functionality should be trivial.

For JSON endpoints you write a module that takes input and generates a JSON-compatible object and test that. You then marshal the arguments in the Express handler and use the standard JSON response to send the result of the module call back to the user.


Using Google App Engine with Pyenv

I recently started using PyEnv to control my Python installations and make it easier to try to move more of my code to Python 3.

Google App Engine though is unapologetically Python 2.7. Google wants people to move away from the platform in favour of Google Compute custom environments and therefore has little incentive to upgrade the App Engine SDK and environments to support Python 3.

When I set my default Python to be Python 3 with PyEnv I found that despite setting a local version of Python 2.7 my App Engine instance was failing to run with an execfile is not defined exception.

The App Engine Python scripts use #!/usr/bin/env python to invoke the interpreter and for some reason PyEnv doesn’t seem to override the global setting for this despite it being correct when you check it in your shell.

After a lot of frustration and googling for an answer I haven’t found anything elegant. Instead I found this Stack Overflow answer which helpful explained that you can use #!/usr/bin/env/python2 to invoke a specific language interpreter.

Manually changing the shebang line in and solved the problem for me and got me running locally.

Obviously this is a pain if I upgrade and I feel it might be better for Google to change the scripts since they don’t have a plan to move to Python3.


/dev/winter 2015

The Dev Sessions are a Cambridge tech conference organised by the same people who do FPDays. The conference was free, held on a Saturday and was based in the Moeller Centre near the Churchill College campus. The only practical way to and from the station was via taxi (befriend those on expenses, thank you John Stevenson).

The talks were on broad topics relating to development. I had pitched a talk on Developer Autonomy, something I'm engaged with in the day job.

Misjudging the train times I arrived a little late and jumped in to the talk on using graph databases in game design. This turned out to be a much more general talk about how the speaker had created tooling to support the game designers in his job. Being a fellow tool provider my interest was immediately piqued.

The game the team were building was some weird monster trapping game, something like Pokemon but more complicated. To trap monsters you need a trap, a lure or bait and you would need to craft both so acquiring recipes and components. Trapped animals provide you with components for other baits and traps and a monetary reward.

The talk was pretty wide-ranging, they were using Neo4J to analyse circular dependencies in "quests" to capture monsters. When designers changed the game data it would get loaded into the graph and all the dependencies checked that they are like a tree (flowing forward) rather than having inter-dependencies (circular references).

It was also possible to generate a "map" of everything in the game and what elements of the game were central and which were on the periphery (which should be the high-level monsters near the end of the game).

All the game data is in text files that are stored in Git, the developers had built a tool over the VCS that simplified the presentation of the many JSON files but it was also possibly for designers to edit them directly with whatever editor they favoured.

All the game data then gets built, validated and packed so it can be shipped off to the servers to power the game.

I think, if I understood the talk correctly, that the build also includes the localised text which is then powered from the server rather than updating a binary datafile on the client.

The final really interesting part of the talk involved the use of genetic algorithms to try and create game data. Data is captured from the game indicating what percentage of the players have captured a particular monster. The designer can then enter the percentage that they intend to capture the monster and the program goes off and tries to generate variations on the monster stats and trap requirements that it predicts will be more achievable by players. If any suitable combinations are found the designer can review them and choose the one they prefer.

Again having selected some changes these are applied to the data files via the tool and then packed and shipped.

It was a really interesting talk about how engineers can make a real difference by building tools and was completely undersold by its title.

The Mixcloud talk on scaling on a bootstrap budget was very interesting as most talks on scaling are about reliability, volume and throughput. It is very rare to get one that focuses purely on trying to create the lowest cost stack.

One of the key things they do to achieve this is a lot of capacity planning with just-in-time rental, buying capacity just ahead of rising usage, something that is much easier when you have a focused product with a limited scope that all your engineers can focus on.

They were also using some interesting hacks like ruthlessly using their right to renew contracts to make sure their applications ran on the newest hardware that was being brought into the datacentre instead of staying on the older blades. A few of the other things I'd heard of before: like setting your requirements so you require individual boxes and therefore do not share your infrastructure with someone else instead of building smaller services with numerous deployments.

There were a few blanket statements that I didn't agree with. For example S3 was condemned as being "expensive" when its really not the more nuanced statement is that S3 bandwidth is expensive and it really is more of a storage solution than something you use to directly serve the public at scale.

One of the big domain specific issues was around streaming audio files, of which, intriguingly was the idea that when you serve the files the connection is so fast you serve the whole asset to the browser when the user is perhaps only going to listen to ten seconds to see if they like it.

A lot of the talk was really about building a single point of presence CDN on the cheap. I did wonder if there wasn't something smart to be done with servers that regulated the downloads more evenly or using a customer player and streaming format.

I stopped by the Julia introduction and there was some interesting points but it was very slow. Julia is quite an interesting language though and I should spend more time with it.

The final talk of the day was on "smells" in automated testing. I thought this would be an interesting topic because I think automated testing was hard but a combination of obscure slide illustrations, fairly old testing strategies and dodgy OO-code examples at the end of the day resulted in a talk that was side-tracked. Testing is hard, and since test code is code then it does not seem worth calling out tests as something special within a codebase. Writing good test code means writing good code and applying the same scrutiny of solution design to the test code just makes sense.

Two things that were not mentioned in the talk but which I think matter when you are talking about the subject as a whole are monitoring and generative testing. I think any talk about testing now needs to cover an approach to generative testing, the old world of testing examples and specifications might be helpful for illustrating code but should not be considered as really being proper test code.

Things that can be extremely difficult to test might be trivial to monitor. Time spent understanding the performance of code in production can be just as valuable as investing a lot of time in creating complex test code.

The whole day was full of interesting talks and bits and pieces and I'm definitely interested in trying to make the trip to the summer version of the event.


Trading performance for asynchronicity

An unusual conversation came up at one of the discussion groups in the day job recently. One of the interesting things that the Javascript language specification provides is a very good description of asynchronous execution that is then embodied in execution environments like NodeJS. Asynchronicity on the JVM is  emulated by an event loop mechanism on top of the usual threaded execution environment. In general if you run JVM code in a single-thread environment bad things will happen I would prefer to do it on at least two cores.

So I made the argument that if you want asynchronous code you would be better off executing code on NodeJS rather than emulating via something like Akka.

Some of my colleagues shot back that execution on NodeJS would be inferior and I didn’t disagree. Just like Erlang sometimes you want to trade raw execution performance to get something more useful out of the execution environment.

However people felt that you would never really want to trade performance for a pure asynchronous environment, which I found very odd. Most of the apps we write in the Guardian are not that performant because they don’t really need to be. The majority of our volume is actually handled by caching and a lot of the internal workloads are handled by frameworks like Elasticsearch that we haven’t written.

In follow up discussion I realised that people hadn’t understood the fundamental advantage of asynchronous execution which is that it is easier to reason about than concurrent code. Asynchronous execution contexts on NodeJS provide a guarantee that only one scope is executing at a time so whenever you come to look at an individual function you know that scope is limited entirely the block you are looking at.

Not many programmers are good at parsing and understanding concurrent code. Having used things like Clojure I have come to the conclusion that I don’t want to do concurrency without excellent language support. In this context switching to asynchronous code can be massively helpful.

Another common situation is where you want to try and achieve data locality. With concurrent code it is really easy to actually end up with net poorly performing code due to contention on contexts. Performing a logical and cohesive unit of work is arguably a lot easier in asynchronous code blocks. It should be easier to establish a context, complete a set of operations and then throw away the whole context, knowing that you won’t need to reload that context again as the task will now be complete.

It is hard to make definite statements of what appropriate solutions are for in particular situations. I do know though that performance is a poor place to start in terms of solution design. Understanding the pros and cons of execution modes matters considerably more.

Clojure, Programming

Transducers at the November London Clojure Dojo 2014

One of the topics for the November ThoughtWorks dojo was transducers (something I’ve looked at before and singularly failed to get working). Tranducers will be coming to clojure.core in 1.7, the code is already in Clojurescript and core.async.

There were two teams looking at transducers, one looked more at the foundations of how transducers are implemented and the other at their performance. These are my notes of what they presented back at the dojo.

How do transducers work?

One of the key ideas underpinning transducers (and their forebears reducers) is that most of the sequence operations can be implemented in terms of reduce. Let’s look at map and filter.

(defn my-map-1 [f coll]
     (fn [acc el] (conj acc (f el))) [] coll))

(defn my-filter-1 [pred coll]
     (fn [acc el]
       (if (pred el)
         (conj acc el)
   [] coll))

Now these functions consist of two parts: the purpose of the function (transformation or selection of values) and the part that assembles the new sequence representing the output. Here I am using conj but conj can also be replaced by an implementation that uses reduce if you want to be purist about it.

If we replace conj with a reducing function (rf) that can supplied to the rest of the function we create these abstractions.

(defn my-map-2 [f]
  (fn [rf]
    (fn [acc el]
      (rf acc (f el))))

(defn my-filter-2 [pred]
  (fn [rf]
    (fn [acc el]
      (if (pred el)
        (rf acc el)

And this is pretty much what is happening when we call the single-arity versions of map and filter; in tranducers. We pass a function that is the main purpose of the operation, then a reducing function and then finally we need to do the actual transducing, here I am using reduce again but transduce does the same thing.

((my-map-2 inc) conj) ; fn
(reduce ((my-map-2 inc) conj) [] (range 3)) ; [1 2 3]

(reduce ((my-filter-2 odd?) conj) [] (range 7)) ; [1 3 5 7 9]

The team’s notes have been posted online.

How do transducers perform?

The team that was working on the performance checking compared a transduced set of functions that were composed with comp to the execution of the same functions pipelined via the right-threading macro (->>).

The results were interesting, for two or three functions performance was very similar between both approaches. However the more functions that are in the chain then the better the transduced version performs until in the pathological case there is a massive difference.

That seems to fit the promises of transducer performance as the elimination of intermediate sequences would suggest that performance stays flat as you add transforms.

There was some discussion during the dojo as to whether rewriting the historical sequence functions was the right approach and whether it would have been better to either make transducers the default or allow programmers to opt into them explicitly by importing the library like you do for reducers. The team showed that performance was consistently better with transducers (if sometimes by small margins) but also that existing code does not really need to be modified unless you previously had performance issues in which case transducers allows a simpler, direct approach to transformation chaining than was previously possible.

Closing thoughts

I suggested the transducers topic as I had singly failed to get to grips with them by myself and I was glad it sparked so much investigation and discussion. I certainly got a much better understanding of the library as a result. My thanks got to the dojo participants, particularly James Henderson.