My take on an AJAX re-frame effect handler

There’s already an AJAX re-frame effect handler provided by Day8, the same guy who made re-frame and there’s nothing wrong with it. From the documentation, this is how you use it:

(re-frame/reg-event-fx ::http-post
  (fn [_world [_ val]]
    {:http-xhrio {:method          :post
                  :uri             "https://httpbin.org/post"
                  :params          data
                  :format          (ajax/json-request-format)
                  :response-format (ajax/json-response-format {:keywords? true})
                  :on-success      [::good-post-result]
                  :on-failure      [::bad-post-result]}}))

That felt a little to verbose for my taste, so I made my own that you use like this:


(re-frame/reg-event-fx ::http-post
  (fn [_world [_ val]]
    {:ajax {:post            "https://httpbin.org/post"
            :params          data
            :on-success      [::good-post-result]
            :on-failure      [::bad-post-result]}}))

If you are familiar with cljs-ajax, you’ll notice the pattern. Day8’s solution uses the raw cljs-ajax API, while I use the higher level one. Day8’s is more versatile, mine is more compact.

This is my solution:

(ns ajax-fx-handler
  (:require [ajax.core :as ajax]
            [re-frame.core :as re-frame]
            [clojure.set :as set]))

(re-frame/reg-fx :ajax
  (fn http-effect [request]
    (let [request-verb (set/intersection #{:get :post :put :patch :delete :head :options :trace :purge}
                                         (set (keys request)))]
      (if (not= 1 (count request-verb))
        (throw (js/Error (str "When specifying an AJAX request, one and only one verb should be specified. Found: " request-verb)))
        (let [request-verb (first request-verb)
              url (get request request-verb)
              request (if (contains? request :on-success)
                        (assoc request :handler #(re-frame/dispatch (conj (:on-success request) %)))
                        request)
              request (if (contains? request :on-failure)
                        (assoc request :error-handler #(re-frame/dispatch (conj (:on-failure request) %)))
                        request)
              request (-> request
                          (dissoc request-verb)
                          (dissoc :on-success)
                          (dissoc :on-failure))
              ajax-fn (cond
                        (= request-verb :get) ajax/GET
                        (= request-verb :post) ajax/POST
                        (= request-verb :put) ajax/PUT
                        (= request-verb :patch) ajax/PATCH
                        (= request-verb :delete) ajax/DELETE
                        (= request-verb :head) ajax/HEAD
                        (= request-verb :options) ajax/OPTIONS
                        (= request-verb :trace) ajax/TRACE
                        (= request-verb :purge) ajax/PURGE)]
          (ajax-fn url request))))))

Feel free to use it, but, is it worth releasing it as a library? does anybody want that?

Pre-signed S3 URLs with Clojure and ClojureScript

Dashman uses S3 for storing the encrypted screenshots. They can go directly from the renderer application to S3 and from there to the displayer application as there’s no need for the server application to ever see them. They are end-to-end encrypted anyway.

My needs in this regard were a bit unique. I have a server component that can generate whatever tokens and signatures are needed and then send it to both applications. The applications are written in ClojureScript, so, essentially, JavaScript, but unlike most direct-to-S3 upload solutions, I don’t have a web site with a form, so, I don’t need to fiddle with CORS or forms.

The two libraries

I found two libraries that could have helped me. The big one is amazonica, a Clojure library to interact with AWS. It looks very complete but when I tried to use the com.amazonaws.services.s3/generate-presigned-url function, I got this error:

The authorization mechanism you have provided is not supported. Please use AWS4-HMAC-SHA256.

There are two versions of signatures, version 2 and 4. Version 2 is deprecated, it might or might not work on some data centers but it definitely doesn’t work on all. I refused to believe it at first because I was using very recent version of all libraries, but yup, it was generating version 2 of the signature with signed URLs that look something like this:

https://bucket-name.s3.amazonaws.com/key-name?AWSAccessKeyId=AKI...OA&Expires=1494190327&Signature=ct...3D

Digging further I found this snippet of code:

(System/setProperty SDKGlobalConfiguration/ENFORCE_S3_SIGV4_SYSTEM_PROPERTY "true")

But for me, it made no difference in my case. For the record I reported the issue and it is supposed to work, but it doesn’t for me.

Then there’s s3-beam, that looked promising, but it seems targeted towards forms, not just plain PUT requests from the clients. On top of that, I didn’t want to mess with async, I didn’t need it and it would be over-engineering for my case. Lastly, it uses it’s own custom way of creating the signatures instead of Amazon’s SDK and that code looks like it’s generating the old signatures (sha1 instead of sha256). I ended up not even trying it.

Instead, I decided to do my own custom option, which was not hard and worked straight away. This post is about sharing that code.

Custom solution using the AWS Java SDK

Starting on the server/Clojure side of things, I added the dependency:

[com.amazonaws/aws-java-sdk "1.11.126"]

With only that I was getting this unexpected error when trying to use the AWS SDK:

ERROR com.fasterxml.jackson.databind.ObjectMapper.enable([Lcom/fasterxml/jackson/core/JsonParser$Feature;)Lcom/fasterxml/jackson/databind/ObjectMapper;
java.lang.NoSuchMethodError: com.fasterxml.jackson.databind.ObjectMapper.enable([Lcom/fasterxml/jackson/core/JsonParser$Feature;)Lcom/fasterxml/jackson/databind/ObjectMapper;
ERROR Could not initialize class com.amazonaws.partitions.PartitionsLoader
which I resolved by adding these other dependencies as well (mostly about getting the latest version):
[com.fasterxml.jackson.core/jackson-core "2.8.8"]
[com.fasterxml.jackson.core/jackson-databind "2.8.8"]
[com.fasterxml.jackson.core/jackson-annotations "2.8.8"]

I wrote a function to generate the S3 client:

(defn aws-s3-client []
  (let [region (Regions/fromName (config/env :aws-s3-region))
        credentials (BasicAWSCredentials. (config/env :aws-access-key-id)
                                          (config/env :aws-secret-access-key))]
    (-> (AmazonS3ClientBuilder/standard)
        (.withRegion region)
        (.withCredentials (AWSStaticCredentialsProvider. credentials))
        .build)))

config/env just fetches configuration values, powered by cprop and mount. This is just standard stuff from Luminus. With

(defn- pre-signed-url [key & [verb]]
  (let [expiration (tc/to-date (-> 24 t/hours t/from-now))
        generate-presigned-url-request (doto (GeneratePresignedUrlRequest. (config/env :aws-s3-bucket-name) key)
                                         (.setMethod (if (= verb :put)
                                                       HttpMethod/PUT
                                                       HttpMethod/GET))
                                         (.setExpiration expiration))]
    (.toString (.generatePresignedUrl (aws-s3-client) generate-presigned-url-request))))

The function generates either a URL for reading, :get, or for writing, :put, depending on the parameter verb. key is just the path part of the URL (remember, S3 is actually a key/value storage). All URLs will last for 24hs, something you can tweak as well.

And that’s it, generating the URL is done, now we have to use it.

Reading and writing from ClojureScript

If you have a form, then, you might need to deal with CORS and other issues. I suggest you look at s3-beam at least for inspiration. For me, it was just a matter of generating the requests from the ClojureScript app, which uses cljs-ajax. Reading was straightforward enough:

(ajax/GET get-url
          {:handler (fn [value]
                      (do-something value))})

although the handler might have to deal with encoding issues. Writing is similar, but again, you may need to fiddle with encoding issues. Since what I was sending is a JSON payload (an encrypted screenshot), this worked for me:

(ajax/PUT put-url 
          {:format :json
           :params payload})

If you are uploading binary data, I recommend you play with :format :raw and the :body attribute.

How to work with a private library in ClojureScript

Dashman is composed of many components  including three desktop apps written in ClojureScript using Electron that share code through a private library (as in, it’s not open source).

To have continuous integration and a set up that is easy to boostrap, that library should be deployed to a private repository. I achieved that using the plug in s3-wagon-private so that the project.clj of the common library looks like this:

:repositories [["private" {:url           "s3p://s3-bucket-for-my-project/releases/"
                           :username      "not-telling-you"
                           :passphrase    "not-telling-you-either"
                           :sign-releases false}]]

:plugins [;...
          [s3-wagon-private "1.3.0"]]

You should never have production or even staging credentials on your source code repository; but those credentials are neither. They allow to compile the project so, pretty much everybody that has access to the source code also needs access to that private repository. Having them in project.clj simplifies CI, on-boarding developers, etc.

One you have that you can deploy the library by executing:

lein deploy private

The project of the apps using the common library look similar:

:repositories [["private" {:url           "s3p://s3-bucket-for-my-project/releases/"
                           :username      "not-telling-you"
                           :passphrase    "not-telling-you-either"
                           :sign-releases false}]]
:dependencies [;...
               [tech.dashman/clientcommon "0.1.0-SNAPSHOT"]]

:plugins [;...
          [s3-wagon-private "1.3.0"]]

Yes… very similar.

The next issue is compiling and re-loading code. You could modify the common library, install it, re-compile the apps, and re-run the apps. And you could do it while wearing a mullet and watching MacGyver (the original one). This is not the 80s! You want code reloading and all the goodies.

If you are using Clojure, you should look into checkouts and maybe lein-checkout. In ClojureScript you can just add the path of the source code to your project even if it’s outside the project, with one gotcha. You might be used to a config like this:

:cljsbuild {:builds {:app {:source-paths ["src/app"]}}}

which would imply this might work:

:cljsbuild {:builds {:app {:source-paths ["src/app" "../clientcommon/src"]}}}

It might on non-Windows platforms, on Windows it doesn’t. On Windows you want this:

:cljsbuild {:builds {:app {:source-paths ["src/app" "..\\clientcommon\\src"]}}}

Since you should probably never commit that anyway, it’s not a big deal.

ClojureScript debouncer

I’m developing Dashman in ClojureScript (and Clojure) and as I was developing the cookie handler I found sites that constantly modify the cookies. As in, many times per second. Obviously I don’t want to encrypt the cookies and send them to the server that often, but even without taking that into account, just storing them to keep track of them in an atom, as in:

(reset! cookies new-cookies)

was slowing down the app considerably. I found myself needing to debounce so many calls as there’s no value in the middle calls, only the last one. Let’s backtrack a little bit here.

The term to debounce comes from electronics:

To remove the small ripple of current that forms when a mechanical switch is pushed in an electrical circuit and makes a series of short contacts.

In programming it refers to a signal that gets emitted often and you want to process it but you don’t care about processing every intermediate value. To tie into electronics imagine you have a program that receives a message every time the temperature changes and the message contains all the temperature changes for the day. Clearly each message doesn’t have to be processed, only one, only the latest.

Here’s where a software debouncer can help. It’s a function that gets a function and returns another function. When you call the resulting function 10 times, it will only call the original function once, with the last value it received.

I found this debouncer written by David Nolan: https://gist.github.com/swannodette/5888989 and in the comments you can start to see that writing a debouncer might not be a trivial as you may think. One of the comments point to this other version: https://gist.github.com/scttnlsn/9744501. The comment implies this is a more correct version but I haven’t verified it myself. In that last page you can see someone found a bug and posted an improved version.

For me, when a supposedly simple code snippet generates so many comments and versions, it’s better to find a library that implements it than to copy and paste and never maintain it again. Unfortunately, I couldn’t find a ClojureScript debouncer library but I got pointed to goog.function.debounce which you can use this way;

(ns whatever
  (:require [goog.functions]))

(def debounced-println (goog.functions.debounce println))

The Highlander query with Rails’ ActiveRecord

For those cases in which there can be one and only one record on the database with certain fields and I don’t just want to get the first one and silently get the wrong one. I want to make sure there’s one and only one, so, I wrote this little extension to ActiveRecord that does exactly that:

module ActiveRecordExtension
  extend ActiveSupport::Concern

  class_methods do
    def one_and_only
      records = limit(2).all.to_a
      if records.count > 1
        raise "#{self} generated more than one record when expecting only one."
      else
        records.first
      end
    end

    def one_and_only!
      one_and_only.tap do |record|
        if record.nil?
          raise "#{self} didn't generate any records."
        end
      end
    end
  end
end

ActiveRecord::Base.send(:include, ActiveRecordExtension)

The first method, one_and_only, will raise an exception if there’s more than one item but it’ll return null if there aren’t any. one_and_only! will fail if there isn’t exactly one and only one record in the database.

If you don’t know why I’m calling this The Highlander query, you should go and watch Christopher Lambert’s masterpiece.

Screensaver Ninja might be coming back

Since discontinuing Screensaver Ninja, I have received many messages asking when it is coming back: over Twitter, Facebook, email, and even one person tracking me down on Reddit..

For those of you who don’t know what Screensaver Ninja is, here is the old explainer video:

It has been very painful to read these messages for a couple of reasons. Firstly, I strongly believe in the product. I want to have it; use it; and enable others to use it. I constantly see expensive and badly designed dashboards or wasted screens, which my product will address. Secondly, judging by the requests, other people want this just as much as I do. Not proceeding with Screensaver ninja could be a wasted business opportunity; although it is hard to tell if the demand is enough to support its development right now.

I set up a landing page explaining what happened to Screensaver Ninja and a form for people to register for notifications of its potential comeback. This was a way to save everybody’s time and frustration; for those emailing requests for up to date information when I could only say, with sadness, “it’s over”. To my surprise, this form has been gathering five or so leads a week, which is rather a lot for an abandoned product.

I have started playing with the idea that I might revive Ninja: This time I have designed a bigger system that covers many more use cases and allows me to support both Windows and Mac OS as well as other platforms just as easily.

During this process I identified the technological bottlenecks; the aspects to product creation that can take months to negotiate and solve, such as hacking Apple’s cookie jar or packaging Chromium. In doing so, I have built a selection of prototypes testing my choices – and everything is working beautifully.

So that’s it: I have decided to revive Screensaver Ninja. I have emailed all of you whom have shown interest to tell you the good news, and have received an overwhelmingly positive response from both individuals and corporations; some wanting to run hundreds of instances.

I want to be completely transparent with my supporters; I am building Screensaver Ninja by myself in my spare time between long days and after hours work at two different consultant gigs. Whilst I am looking into the options of partnerships, developers, and marketers, I have decided not to wait for these additions to the team in order to make progress. I’m very excited about this phase both from the technical as well as the business points of view so Screensaver Ninja is moving forward and I will have frequent updates.

Going into the property business 

I’m a tech entrepreneur and that has not changed, but after six or seven years of trying to have at least moderate success, I’m starting to hedge my bets.

On the side, I want to have some passive income and it looks to me like buy and hold properties, that is, buying them and renting them out, also known as buy to let, is the way to move forward. I identified some very profitable areas and they are in the least expected places. For example, in London you can expect a return on investment of around 3% while I’m getting something around 16%.

If you want to listen to my journey, I’m documenting it as I go with Clayton Morris on his podcast about investing in property. He just published the second episode in which I talk about getting the money for my first two deals.  I just pulled the trigger on starting the business. I’m super excited and I can’t wait to share more good news in the next episode.