added solution description

README.md

@@ -1,5 +1,10 @@
+
 # Assignment
 
+This assignment is designed to take a day's worth of work (8 hours).
+if you find that some things are not possible to complete in that timeframe,
+that's okay – prioritize as you see fit.
+
 ## Datawrapper Test Project: Backend
 ### Overview
 
@@ -29,6 +34,113 @@ containerized application, but other technologies are allowed as well)
 The project should be delivered as a Github repository.
 It should be possible to run the service locally.
 
+# Solution
+
+## Service boilerplate
+
+I'd say that this is a very odd test assignment, since it requires one to
+create an entirely new service, which involves a lot of boilerplate, and takes
+most of the allotted time.
+This assignment effectively tests whether the applicant is capable of a new
+service on their own, from scratch, which is (hopefully) very unrepresentative
+of the actual job responsibilities; typically, companies have established
+procedures for creating new services, often they have templates and starters,
+and they definitely have established stack and dependencies.
+
+At my current company we're using `tsoa` for microservices, with our own
+templates and starters.
+However, creating a new tsoa project from scratch, without using these
+templates, and without looking at the code belonging to the company,
+would demand a lot of time.
+So I decided to search for an alternative, and settled on NestJS for the
+purpose of this assignment.
+
+Note that this is my first experience with NestJS, so I may violate some best
+practices, just because I'm not aware of them.
+NestJS also turned out to be surprisingly powerful for creating small
+proof-of-concept services.
+
+## Scalability
+
+The requirement that the service should be scalable and handle high load calls
+for an obvious solution: queues.
+If this was an actual service and not an MVP, the architecture would probably
+look like this:
+* Public service accepts a screenshotting job, stores it in queue;
+* A number of private workers (serverless functions, or actual private services)
+pick up jobs from the queue, perform computation-heavy page rendering and
+screenshotting, and store screenshots in the same queue/DB;
+* Public service, when queried the status of a specific job, returns its status
+along with the screenshot (if the job is completed already).
+
+This way, quality of service would never degrade for the public service;
+since it just adds jobs to queue and checks their status (which are relatively
+lightweight tasks), it would always be responsible.
+
+And, depending on the load (and budget considerations), new workers could be
+added to reduce the queue length, and the amount of time clients would need to
+wait for their jobs to be processed.
+
+In order to further reduce load on the public service, it would make sense to
+use the push model instead of the polling; when submitting a new job, clients
+could provide their callback URL to be notified once the job is completed,
+instead of constantly checking its status.
+(Unfortunately, I was unable to implement this within the 8-hour time limit).
+
+The architecture of this proof-of-concept project roughly resembles the ideal
+architecture as described above, except that private workers live in the same
+public service, negating all the potential scalability benefits.
+However, it seems that with NestJS, workers could be trivially extracted from
+the public service, as the communication between public controllers and workers
+is done via Redis in this project.
+
+## Screenshotting
+
+First idea that came to my mind was that there ought to be headless Chrome
+browser intended to be used for testing.
+And sure there is; `puppeteer` package comes with a headless `Chrome`, and
+allows to make screenshots.
+This should be enough for the proof-of-concept.
+
+In order to showcase the API capabilities, I originally intended to allow
+clients to set additional options for screenshotting.
+Unfortunately, I only had time to implement support for `imageType` (which
+could be jpeg or png).
+
+## Tests
+
+Unfortunately, I didn't have enough time to ensure reasonable coverage, as I had
+to spend most of these 8 hours getting myself acquainted with NestJS.
+
+There is an end-to-end test that covers the basic positive scenario.
+Unfortunately, it requires Redis to be up and running; I did not have enough
+time to decouple it from Redis.
+
+Also note that tests check some of the results against image snapshots stored in
+repository, and of course screenshots on your platform may look different than
+on mine, which can cause tests to fail (unless snapshots are recreated).
+Tests pass on my system.
+
+## Usage
+
+In addition to using this project as an API, you can also use the included
+single-page application (available under `http://localhost:3000/spa` or similar).
+
+Just enter the target URL, create the job, check its status.
+Once the status changes to `'completed'`, you will see the screenshot.
+
+## Other considerations
+
+Since I did not have enough time to actually write the code (instead spending
+most of it basically on learning NestJS), there are some issues with the code.
+
+In particular, there are basically no tests besides one e2e test for the most
+basic scenario, and there is some unneeded code duplication.
+Additionally, classes naming could be better; and `ScreenshotsController`
+contains a lot of code that does not belong to it (ideally, it would all be in
+some `ScreenshotsService`, with `ScreenshotsController` just being an adapter
+between that service and NestJS/API interface).
+
 # Commands
 
 ## Configuration
@@ -54,6 +166,10 @@ $ npm run start:dev
 $ npm run start:prod
 ```
 
+After starting the app, navigate to the displayed URL (`https://localhost:3000`)
+in order to check out the API docs and the basic single-page screenshotting
+application.
+
 ## Test
 
 ```bash

client/spa/index.html

@@ -66,7 +66,7 @@
         </tr>
         <tr>
           <td>&nbsp;</td>
-          <td><input type="submit"></td>
+          <td><input type="submit" value="Create job"></td>
         </tr>
       </table>
     </form>
@@ -80,7 +80,7 @@
         </tr>
         <tr>
           <td>&nbsp;</td>
-          <td><input type="submit"></td>
+          <td><input type="submit" value="Check status / load image"></td>
         </tr>
         <tr>
           <td>Status</td>