Raspberry Pi printer server

A few months ago, my wife needed to print a couple of documents. I had an old HP printer (Officejet 4355) sitting around, but I couldn’t use it in her laptop - it’s a Chromebook, which doesn’t support printer drivers. It can only print over the network, and my printer doesn’t have network connection, just USB. The only way I could make that work was installing the printer in another computer and making it share the printer over the network. But I’d have to keep the other computer on and connected to the printer every time she wanted to print something, which isn’t convenient.

So I decided to remove the dust from an old Raspberry Pi (the original model B) I hadn’t used for a while, and use it as a print server! Now I can print from any device, including her Chromebook and my Android phone.


First install Raspbian (without graphical interface, as you’ll use it only as a server), and set a static IP for it. Then connect the printer to one of its USB ports and install the driver - for my case, I found HP’s Linux Imaging and Printing software (HPLIP). It’s an official HP driver for Linux that supports many old printers, including mine. To install it:

sudo apt-get install hplip

The next step is installing CUPS and adding your user (default user is pi) to lpadmin group:

sudo apt-get install cups
sudo usermod -a -G lpadmin pi

To add a printer to CUPS, open its web interface in http://<pi IP address>:631/admin and click Add Printer. Your printer should be discovered automatically. After selecting it, you can add a meaningful name to it. Also, don’t forget to check Share This Printer before continuing.

With the print server fully configured, you need to setup the printer in each device. In a laptop running Ubuntu, my “new” printer could be automatically discovered. If you have any problem with it, check Debugging Printing Problems. In Chromebook, I installed IPP/CUPS printing extension. And in Android, I’m using CUPS printing app. They all work great!

Code review is an amazing tool to improve code quality. It brings many benefits, to the reviewer, to the reviewee and to the team.

How we do code reviews

To add code reviews to your team workflow, first you need to use a version control tool. If you don’t, start with that.

Basically, the idea is never committing directly to the master branch - or whatever other branch you choose as the main one, but usually it’s the master branch. To start any changes to the source code, be it a new feature, refactorings or bugfixes, you need to create a new branch. You can also create conventions for the branch names, but this is not required.

After you finish working on your own branch, you can trigger the code review process. If you use a repository manager like GitHub or GitLab, they already provide tools to help on this process: GitHub calls them pull requests, and GitLab, merge requests - to make it simpler, I’ll call them change requests from now on. You can add some comments explaining the purpose of the proposed changes, and maybe links to other related change requests or issues. These are very important to make the objective of that change request clear for everyone that might review your code. When in doubt, be as specific and clear as you can. Here’s an example from tsuru, the project I currently work in: a pull request with a detailed description.

A pull request with a detailed description

As soon as a change request is created, it’s available for any other member of the team to start reviewing it. You can also assign it to a specific person, if you think his/her opinion is important - maybe when you change a specific and obscure part of the project that not everyone dominates.

As a reviewer, you job is to check many aspects of the code. Here’s a non exaustive reference list:

  • accuracy and completeness: does the code do what it’s supposed to do?
  • bug free: doesn’t it introduce new bugs (like not handling corner cases)?
  • expressiveness: is it clear on its intentions? Can a reader clearly understand the coder’s objectives?
  • scalability: could it handle the expected load?
  • conventions and standards: does it follow the agreed conventions for code style, file and directory structure, etc?
  • tests: do the tests cover all the use cases, including important corner cases? Is there a specific use case that deserves more tests?

After checking the code, the reviewer can basically accept or reject it - GitHub also allows you to add review comments without explicitly approving or rejecting. When you approve, that means you think those changes could be promptly merged to the main branch. But when you reject, that could mean you doesn’t agree with that change at all (e.g. you don’t think this software should have the proposed feature), or that you’re requesting some changes to the code. That’s what review comments are meant for.

Usually a review comment can be added to a specific line or code block. That could be a typo, a function you think should be named differently, a bug you spotted or a missing test or documentation. The comments should make this clear to the reviewee, who can reply to the comments or make requested changes. Then, this process repeats until the change request gets approved.

When the reviewer accepts the proposed changes, the reviewee has two options: merge/rebase the changes to the main branch and finish the code review process, or ask for another developer review. This is the case when the change is very complex and you’re insecure you may be missing something. When this happens, the review process doesn’t finish until all reviewers accept the changes.

To illustrate, here’s another example of code review from tsuru: in this pull request, I received a couple of comments with request changes. For most of them I changed the code according the suggestions, and one of them generated a small discussion. What’s best is that the discussion is documented in the pull request, for anyone to read and participate.

A pull request discussion

Benefits of the code review

A code review process brings a lot of benefits to the reviewer, to the reviewee and to the team as a whole:

For the reviewer

  • learn about the changes: if someone commits directly on the main branch, only the author would know the details about it. When you review, you also get to know what those changes do, how and why they were written. And if you don’t understand, you have the opportunity to ask for more information
  • learn about the project: if you are new in the team, code review is a great tool to obtain knowledge about the project. Even if you still don’t understand the project, go ahead and review everyone’s code, just to start understanding it a little better
  • learn about the technologies involved (languages, frameworks, libraries): just like when you’re new to the team, if you are getting started with the tech stack used in the project, you’ll benefit even more from reviewing people’s code. You’ll learn about the language’s features, new libraries to solve common problems and features of the frameworks the project uses
  • develop a critical view of someone else’s code: if you are an inexperienced developer, you probably have a hard time trying to spot problems in someone else’s code. After code reviewing a couple of times (and checking other developer’s reviews), you’ll learn how to be more critic. And what’s more important, you’ll learn that criticizing someone else’s code is not offensive, it’s actually a good thing, an opportunity for them to learn. And you’ll learn with the other’s mistakes. That’s why junior devs should review seniors’ commits

For the reviewee

  • learn about the project: just like the reviewer, a great way to start in a new project is asking for feedback from the more experienced in the team. Here’s another example
  • learn about the technologies involved (languages, frameworks, libraries): the same comments from the reviewer part are worth here. Here’s one more example
  • learn other ways to solve problems: when you face a bug, you may already have the complete solution in your head; that solution could really work after you implement it. But that doesn’t mean this is the only way to solve the problem. Probably there are other solutions out there, and they may be simpler, clearer, safer or perform better. Other developers could show these other solutions in their reviews
  • learn to accept critics: sometimes we get so attached to the code we wrote that we could get offended when someone criticizes it. Code reviews help us learn to get over this, because we are explicitly asking for people to give feedback on our code, and they’ll answer. Over time you’ll learn these feedbacks are great ways of learning

For the team

  • shared code ownership: now two or more people are responsible for the code one of them wrote - by the way, that could also be reached with pair programming. Shared code ownership is great after a bug is released into production: the developer who wrote the code is no longer the only responsible for that. This also helps reaching a blameless culture
  • helps keeping standards: if the team agreed on specific conventions, like code style rules, tests for every line of code or document all the APIs, code review is a great way to make people monitor each other on these

Common problems and concerns

Won’t this slow me down?

When we suggest that our team starts working with code reviews, people usually get apprehensive about this process making the team slower. If they’re used to finish their work, push the code and deploy to production, they may have this concern.

In fact, you won’t have that fast dev-to-production cycle. But that is a small downside against the many benefits presented above. It’s a trade-off, like every decision you make in your project.

One common concern is having to wait for a long time for another dev to start the code review process. If people still don’t see value in this process, they indeed may not prioritize this. But as soon as you get to be on the other side - waiting for other people’s reviews -, you’ll start giving it more attention. It’s an organic process that usually regulates itself.

How to handle large change requests?

Another problem is when the change request is very large. This will require more time from the reviewers, and it may be harder to analyze: you may lose focus during the review.

Just like on the previous item, this should be self regulated. If you start making very large change requests, you’ll learn they aren’t much productive when someone else asks for you to review a large change request.

There isn’t an ideal size of change request. But you should make them as small as possible. If you’re working on a complex feature, you may try splitting it in a couple of small change requests, independent from each other. But if you can’t make them independent, an alternative is starting with a branch forked from the main one - you may call it feature-something, then creating other branches from it. As you finish each part, make the change requests to merge to you feature branch, not the main branch. And only when you finish every “sub-feature” change request, only then you merge your feature branch to the main one.

When not to use code reviews?

Finally, another common question is: are there exceptions to the “never commit to the main branch” rule? If the change is very simple and small, won’t the code review process be just a formality?

In fact, there are a couple of situations where you may bypass the code review process. One example is updating a dependency version. But I still think it’s worth opening the change request, to make other team members aware of the changes. But this kind of decision is up to the team, and should be accorded among them.

To finish, I suggest reading this great post about how to conduct effective code reviews.

Tmux is a fantastic tool for improving productivity when working with a terminal. One of the first things people configure when start using tmux is changing the prefix key. The default value is control+b, which is not very confortable to press with a single hand. And as you’ll end up pressing it a lot, for every tmux command, the most common used configuration is changing it to control+a.

This is much better, but you still need to press two keys simultaneously before typing any tmux command. After using this configuration for some time, I decided to change it to a single key, to make it even easier.

I though about changing the prefix to caps lock. Besides being rarely used, it’s very well positioned. However, you can’t set caps lock as prefix in tmux. An alternative solution is mapping the caps lock key to something else. In OSX, you can set it to another modifier key, like control, shift or esc: go to System Preferences => Keyboard => Modifier keys. First I tried mapping it to esc, and setting esc as tmux prefix. It works, but this setup brought another problem: as a vim user, I use the esc key a lot (to alternate between vim modes), so now I had to type esc/caps lock twice to send the esc key to vim. It was ok, but not ideal.

Then I tried another solution: I installed Karabiner-Elements, a Mac app which allows you to completely customize your keyboard. So I mapped the caps lock key to Home (which doesn’t exist in Mac keyboard), and changed tmux prefix key to Home:

set -g prefix Home
unbind C-b
bind-key Home send-prefix

Now I have a great configuration: I use a single key (caps lock) as prefix, and without losing any key functionality.

UPDATE: to do this same configuration in Linux, you just need to open the /usr/share/X11/xkb/symbols/pc file and change the line that starts with key <CAPS> to this:

key <CAPS> {        [ Home          ]       };

Incidents like WannaCry ransomware expose the importance of doing backups, which is usually forgotten by many people.

When someone talks about backing up our personal files, we usually think about services like Dropbox and Google Drive. But they have a reasonable cost if you have more than a couple of GB of data. There are a lot of other solutions available. Most of them are cheaper, but not always reliable - imagine if you backup all your personal data to a small and unknown service, and a few months later, the company breaks. Or a security flaw exposes all your personal data! Of course this could also happen with Dropbox and Google Drive, but it’s much less likely, being two large and serious companies.

One alternative to them is Amazon Glacier. It’s a not so popular Amazon service for data archiving. You should notice it works differently from the usual backup solutions. When you sign up to Dropbox, for instance, you can install an app to your computer or mobile phone, or use the web interface to instantly access your files and upload new ones. Glacier is much more low level. It doesn’t have a web interface, app or even command line tool! There’s only an API, which you use to check your files, download or upload.

And there’s more: the download and upload rates are very slow. And to download a file, you first have to ask for a file retrieval job; the download will be available in a couple of hours!!!

This seems like a terrible service, so why use it? Because it’s very, very cheap! You only pay US$ 0.004 per GB per month for storage, besides additional costs for requests. And even being slow and hard to use, it’s a service offered by Amazon, which gives you confidence it won’t suddenly disappear.

Having said that, Glacier isn’t a service to keep data you may need immediately. But it’s ideal for something you probably won’t need to access anytime soon. Think about your family pictures: when you want to access them, you probably doesn’t need them right away; you’re fine waiting a couple of hours for that.

Glacier is also a great option for “backups of backups”. If you want to be neurotic about backups (and you should!), you can archive a copy of your backups there.


The easiest way to use Glacier is with a third party client. I like amazon-glacier-cmd-interface. After setting up the basic configuration, you can create a vault and upload you files:

$ glacier-cmd mkvault my-disaster-backup
$ glacier-cmd upload my-disaster-backup my-file1 my-file2 ...

To list archives in a vault:

$ glacier-cmd inventory <vaultname>

The inventory retrieval job takes a couple of hours to be processed. You can check its status with:

$ glacier-cmd listjobs <vaultname>
|                      VaultARN                        |    Job ID     | Archive ID   |       Action       |        Initiated         |   Status   |
| arn:aws:glacier:us-west-2:483413266890:vaults/backup | QYqdvM4k8q... |    None      | InventoryRetrieval | 2017-07-24T15:47:48.310Z | InProgress |

When the job status change to Succeeded, run the inventory command again to check the archive list.

To download an archive, first you need to check its id in the inventory:

$ glacier-cmd inventory <vaultname>
Inventory of vault: arn:aws:glacier:us-west-2:483413266890:vaults/backup
Inventory Date: 2017-07-05T11:22:15Z

|  Archive ID   | Archive Description |       Uploaded       | SHA256 tree hash |    Size    |
| uFg2FE_guu... | file1.tar.gz        | 2017-03-31T14:29:17Z | b41922e1a2...    | 1342622251 |
| 43Wjk63Dcu... | file2.tar.gz        | 2017-03-31T17:18:28Z | 2346170d22...    | 2347810677 |
This vault contains 2 items, total size 2.5 GB.

Then, create an archive retrieval job using the archive id:

$ glacier-cmd getarchive <vaultname> <archive id>
|   Header  |    Value      |
|   JobId   | Xa17IAadQG... |
| RequestId | cPcomv_vTf... |

When the download is available (you can check its status with glacier-cmd listjobs <vaultname>), download it with:

$ glacier-cmd download <vaultname> <archive id> --outfile <filename>

Globosat Play is a video product for pay TV subscribers, where you can catch up programs you missed on TV. It’s like an umbrella for a couple of different channels. One of the most popular of them is SporTV, one of the largest sports channels in Brazil.

Last year we had the Olympic Games, a major sports event here in Rio. As SporTV channel was going to have a large coverage for the event, we decided to rethink the user experience for SporTV Play (SporTV channel offers inside Globosat Play). In this case, we were going to focus in improving the live TV experience, which is responsible for most of the audience.

Besides rethinking the user experience, we decided to also rethink our front-end architecture, and address a couple of the issues we had.

I’ve already written about Globosat Play architecture before. Its front-end is basically a couple of Rails apps using regular erb templates. These apps share components through a component library called globotv-ui. It’s about 4 years old, way before newer component technologies arised and got popular.

As descripted in the previous post, this component library solution allowed us to avoid rewriting the same components again and again for each app. We were able to share our components, but the main problem is that this was an in-house solution. We defined our components structure to attend our needs, so it was really hard to share them outside our product.

Also, we had a few problems with our front-end architecture. When we updated a component that was used on pages served by different apps, we needed to “synchronize” the deploys. If we deployed one app and took too long before deploying another, in the mean time, both would have different versions of that component. That could generate an inconsistency for our product, and it was one of the issues we wanted to address with the new solution.

In late 2015, we started a couple of technical discussions and some proofs of concept, and decided to adopt React in our front-end. A couple of arguments led the way to our decision:

Standardize components structure

Even after creating a dozen components in globotv-ui, we still could find some differences among them. That’s because the structure is loose and not well documented. Usually we start a new component looking at another one, and replicate that structure. But a lot of different developers worked on that library, each one with his own preferences. So we really didn’t have a pattern for components. They were well organized and tested, but in the end, they were just a couple of JS and CSS files, sometimes with a template for generating the HTML (in Handlebars for client-side rendering, or ERB templates with Rails helpers for server-side).

The clear and well known React component structure helps keeping a pattern among components. The component lifecycle lets us manage how our components should behave. It allows us to not only open source a couple of generic components, but also search for ready components instead of recreating everything we needed.

As an example of this, we wanted to keep our header sticky after the user scrolls down the page. Instead of implementing this behavior, we used react-headroom. Problem solved!

Declarative programming model

Another benefit that React brings is its declarative programming model, instead of the traditional imperative model. Here is a simple example: a text area with a “Tweet” button, which should be disabled while the text area field is empty. Here is the imperative implementation using jQuery:

// Initially disable the button
$("button").prop("disabled", true)

// When the value of the text area changes...
$("textarea").on("input", function() {
  // If there's at least one character...
  if ($(this).val().length > 0) {
    // Enable the button.
    $("button").prop("disabled", false)
  } else {
    // Else, disable the button.
    $("button").prop("disabled", true)

Now the React version:

class TweetBox extends Component {
  state = {
    text: ""

  handleChange(event) {
    this.setState({ text: event.target.value })

  render() {
    return (
        <textarea onChange={this.handleChange}></textarea>
        <button disabled={this.state.text.length === 0}>Tweet</button>

Globo Play release

The third strong argument in favor of React was the release of Globo Play, in late 2015. It’s another video product developed here at Globo.com. It’s very similar to Globosat Play, and it already used React. So when we started developing the new interface for SporTV Play at the Olympic Games, the team that developed Globo Play already had a great experience with it to help our adoption.

The new architecture

As descripted in a previous post, our architecture was already microservices-based:

Globosat Play original architecture
Globosat Play original architecture

In the front-end, we had a couple of different apps to serve different pages in our product, with an nginx server in front of them. The nginx server proxies the requests to each app, according to the request path. Our idea was adding a new rule to it, to forward all requests to SporTV Play home and live signals pages to a new app. This new project was going to use React, and ideally share components with Globo Play.

The first step was thinking about the API for this new front-end app. We already had a couple of APIs serving our current apps, so we didn’t need to create a new API. We could just use what we already had, but we decided to follow the Back-end for Front-end pattern.

The APIs we already had served well many of our apps, and some of them are legacy. They have a lot of services, and most of them are fine-grained. To serve the new front-end app, we would probably need to make many requests to group all the data we needed.

Also, we thought it was a good idea to separate the new app from the old ones. Doing this, we would have less chance of coupling old and new apps. Suppose both of them consumed the same services; that would create a coupling between them. If the new app required a change in this service contract, we wouldn’t be able to do that without the risk of breaking the old app.

Besides that, we know the mobile consumption is raising, and sometimes we suffer with terrible 3/4g connections. With a new API specifically designed to attend the needs of the new app, we could deliver the smallest possible payload (just the data we were going to use). Also, we could create more specific services, reducing the number of requests to a minimum.

We followed two principles from the Back-end for Front-end (BFF) pattern: the front-end consumes services from its BFF and nothing more; and the front-end is the only BFF client. They are tightly coupled together, but this is not a problem, because they should both be maintained by the same team. It’s like we are splitting our app in two. The BFF is responsible for orchestrating requests from internal, fine-grained services, apply some business rules and deliver data ready to be consumed by the front-end, which just consumes these coase-grained services and takes care of the presentation layer.

BFF architecture
BFF architecture

One downside of the BFF is code replication. Some of the services we created in the BFF were new and very specific to our new SporTV Play front-end app (like the current schedule and the list of live signals). Others were already available in our internal APIs. But to follow the rule were the front-end can’t access any service outside its BFF, we needed to add a new route to the BFF, and basically make a proxy pass to our internal APIs. As any decision in software engineering, it’s a tradeoff. There is no perfect solution for everything, and the benefits overcome this issue.

For more details and discussions about the BFF pattern, check out Sam Newman’s and Phil Calçado’s articles.

This is the first post of a series. In the next ones, I intend to write a little more about React components, state management, CSS architecture and components sharing.