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))

Emacs is hurting Clojure 

Emacs is a very powerful text editor and its popularity amongst Clojurians is easily understood. Emacs has a long tradition in the Lisp communities as it’s written, in a big part, in a flavor of Lisp called Emacs Lisp.

Because of its history, it handles Lisp code wonderfully, with automatic correct indentation, paraedit, integration with REPLs, etc. But Emacs is really hard to use.

Yeah, most Clojurians know how to use it by now and they suffer from bias: “it’s not that hard” they say. Learning Emacs or Clojure is hard enough. Combining them is insane.

Many Clojurians also say it’s worth it. And again, I think they are biased. Human brains are very good at forgetting pain. Other editors these days are also very powerful and although not as much as Emacs, their usage is intuitive so you can achieve a higher level of proficiency just by using it, without spending time and effort in becoming better at it.

The way Emacs is hurting Clojure is by Clojurians maintaining this myth that you need to use Emacs for Clojure. This is not done by simple statements but by a general culture of jokes saying things such as “you are wrong if you don’t use emacs”.

Me, personally, I don’t care what editor you use. If you want to learn Emacs, go for it. Intellij and Cursive is much easier to use and almost as powerful. When I compare myself to another clojurian, productivity is generally decided by knowledge of the language and libraries, not the editor. If you want to use another editor, so be it. It’s better if they understand Lisp code but it’s not a deal breaker for learning Clojure.

I do care about the success and popularity of Clojure. Coupling the growth of the language to the use of an editor that is hard to use and non intuitive makes no sense. It’s hurting us. Even if you are an Emacs power user, when you talk to a Clojure newbie, please, don’t push it down their throats.

Thank you.

to-jdbc-uri 0.5.0 released

We just released to-jdbc-uri 0.5.0 with support for URL parameters. Courtesy of Joe Kutner.

I’d like to note, sometimes I hear that the Clojure community doesn’t care about testing, but to-jdbc-uri has a very complete testing suite and so far, this and other contributions came fully tested.

Free-form version 0.2.0 released

We are very happy to announce version 0.2.0 of our form building library Free-form. This version includes:

The Bootstrap 3 support means that you can have whole fields defined as succinctly as:

[:free-form/field {:type        :email
                   :key         :email
                   :label       "Email"}]]

Enjoy!

 

Prerenderer 0.2.0 released

We are proud to announce the release of version 0.2.0 of our ClojureScript library Prerenderer, a library to do server side pre-rendering of single page applications. In this release, we include:

The two first items in the changelog came hand in hand and they are the biggest changes to keep in mind if you are upgrading. We are very happy that we no longer need a fork of re-frame and we would like to extend our gratitude to Mike Thompson for working with us on having the appropriate API to make this happen.

The change in API means that your Prerenderer module now would look something like this:

(ns projectx.node
  (:require [prerenderer.core :as prerenderer]))

(defn render-and-send [page-path send-to-browser]
  (send-to-browser (render page-path)))

(set! *main-cli-fn* (prerenderer/create render-and-send "ProjectX"))

instead of:

(ns projectx.node
  (:require [cljs.nodejs :as nodejs]
            [prerenderer.core :as prerenderer]))

(defn render [req res]
  (let [page-path (.-path (.parse url (.-url (.-query req))))]
    (.send res (render page-path))))

(set! *main-cli-fn* (prerenderer/create render "ProjectX"))

Enjoy!