Recently, I’ve had an unfortunate opportunity to deal with a very unexpected issue with Kafka that had quite terrible consequences. The exact origin of the issue is yet to be discovered; nevertheless, the process leading to the final solution to the problem and the fix itself were interesting enough that I decided to write a quick blog post about it as it might be potentially valuable to someone who also encounters a similar problem.
If you’ve ever had a need to implement an audit log to track all the changes that get persisted for all or at least some models in your application, there is a good chance that you’ve encountered PaperTrail gem that makes it trivial to track all the changes - it might be as easy as adding has_paper_trail to the desired models.
Imagine that you are implementing a payment processing system for the e-commerce system and discover that multiple customers were charged twice for exactly the same order. Ouch… Sounds like a real nightmare to deal with! And the next day, you see that something is not right with the credits system in which users were able to pay using special credits instead of using credit card - somehow instead of getting charged $100 in total for two orders of $25 and $75, they were charged just 25$! And to make it even more epic, it turned out that the uniqueness validation you added didn’t work at all, and now you have three users with the same email address!
Communication between two or more applications is often everyday stuff, and it might seem that there is not too much to add there as this subject has been covered pretty well in the last years. Thanks to that, multiple patterns and standards have emerged. You no longer need to think about how the response format should look like for your REST API (go with JSONAPI and stick to the conventions) or figure out the authentication/authorization protocol (go with OAuth and the security headaches won’t bother you).
Imagine that you are working on a large legacy application that also contains the dreaded ActiveRecord callbacks in the models handling most of the business logic. At some point, and under a certain level complexity, the mess caused by that choice might become hard to keep under control, the risk of introducing bugs will increase and the teams(s) working on the application will be way less productive. That will most likely lead to an attempt to find a better way of designing the application. The problem, though, might be that the scope of the application is so huge that introducing any meaningful changes to the application will take weeks, if not months.
In the first part of this series, we were exploring some potential options for communication between services - what their advantages and disadvantages are, why HTTP API is not necessarily the best possible choice and suggesting that asynchronous messaging might be a better solution, using, e.g. RabbitMQ and Kafka. We’ve already covered Kafka in the part 2, now it’s the time for RabbitMQ.
In the first part of this series, we were exploring some potential options for communication between services - what their advantages and disadvantages are, why HTTP API is not necessarily the best possible choice and suggesting that asynchronous messaging might be a better solution, using, e.g. RabbitMQ and Kafka. Let’s focus this time entirely on the latter.
Microservices, Service-Oriented Architecture (SOA) and in general, distributed ecosystems, have been on hype in the last several years. And that’s for a good reason! At certain point, The Majestic Monolith “pattern” might start causing issues, both from the purely technical reasons like scalability, tight coupling of the code if you don’t follow Domain-Driven Design or some other practices improving modularity, maintenance overhead, and also from organizational perspective since working in smaller teams on smaller apps is more efficient than working with huge team on an even bigger monolith which suffers from tight coupling and low cohesion. However, this is only true if the overall architecture addresses the potential problems that are common in the micro/macro-services world. One of these problems I would like to focus on is communication between apps and how the data flows between them.
In the typical Rails application, you can find the most of the validations in the ActiveRecord models, which is nothing surprising - ActiveRecord models are used for multiple things. Whether it is a good thing, or a bad thing (in most cases it’s the latter) deserves a separate book or at least blog post-series as it’s not a simple problem, there is one specific thing that can cause a lot of issues that are difficult to solve and go beyond design decisions and ease of maintenance of the application, something that impacts the behavior of the model - the validations.
Optimizing database queries is arguably one of the fastest ways to improve the performance of the Rails applications. There are multiple ways how you can approach it, depending on the kind of a problem. N+1 queries seem to be a pretty common issue, which is, fortunately, easy to address. However, sometimes you have some relatively simple-looking queries that seem to take way longer than they should be, indicating that they might require some optimization. The best way to improve such queries is adding a proper index.
Few months ago I wrote a blog post about ActiveRecordbefore_validation callback and how it is used for wrong reasons and concluded that in most cases this is not something we should be using routinely. However, I missed one appropriate use case for it which might be quite common in Rails apps, so this might be an excellent opportunity to get back to before_validation callback and show its other side.
TouchingActiveRecord models is quite a common thing in most of the Rails applications, especially useful for cache invalidation. By default, it updates updated_at timestamp with the current time, Here’s a typical example of using touch in a model:
One very useful feature of ActiveRecord is automatically defining attribute readers and writers for all the columns for given tables. For the ones with boolean type, however, there is one more addition - defining an alias of the method with a question mark. Sometimes it might be useful to override this method and add some extra requirements for a given condition. However, this might not be such a good idea.
It’s nothing new that ActiveRecord callbacks are abused in many projects and used for the wrong reasons for many use cases where they can be easily avoided in favor of a much better alternative, like service objects. There is one callback though that is special and quite often used for pretty exotic reasons that have nothing to do with the process when it gets executed - it’s the before_validate callback.
Object#try is quite a commonly used method in Rails applications to cover cases where there is a possibility of dealing with a nil value or to provide flexible interface for handling cases where some kind of object doesn’t necessarily implement given method. Thanks to try, we may avoid getting NoMethodError. So it seems like it’s perfect, right? No NoMethodError exception, no problem?
As hard as it is for me to believe, I already have over 5 years of professional experience in Ruby and Rails. Throughout all these years my attitude towards Rails has been fluctuating between going from blind love to harsh critic (ActiveRecord, I’m looking at you) ending with a bit more balanced but certainly a positive approach. Such time is long enough to have a meaningful opinion about the overall experience using any framework, so here are few points about Rails that I would particularly like to focus on in my reflections.
Rails 5.0 is without a doubt a great release with plenty of useful changes and additions. The most notable change was probably ActionCable - the layer responsible for integrating your app with websockets. However, there were also other additions that could bring some substantial improvements to your Rails apps, but were a bit outshined by bigger changes. One of such features is Attributes API.
Every now and then I discover some features in Rails that are not that (arguably) commonly used, but there are some use cases when they turn out to be super useful and the best tool for the job. One of them would definitely be a nice addition to ActiveRecord::QueryMethods - extending method. Let’s see how it could be used in the Rails apps.
I was recently asked what is secret key base used for in Rails applications and why not secure value of it (or even worse - the public one!) creates a security issue. That was a really good question, I remember how it was a serious threat years ago, especially before introducing secrets.yml in Rails 4.1 - at that time by default secret_token initializer was generated and the secret key was directly stored there. The result was that in many open source projects secret key was publicly available creating a great security risk. Let's take a look how exposed secret key base could be exploited.
If you happen to develop API for non-trivial app with complex business logic beyond CRUD directly mapped to the database tables (i.e. typical Active Record pattern) you were probably wondering many times how to handle these cases and what's the best way to do it. There are quite a few solutions to this problem: you could add another endpoint for handling given use case, add non-RESTful action to already existing endpoint or you can add a magic param to the payload that would force non-standard scenario in the API. Let's take a closer look at the these solutions and discuss some advantages and disadvantages of each of them.
Having some kind of type attribute in your models is a pretty common thing, especially in applications with more complex domain. How do you handle such cases? Is it with multiple conditionals / case statements in every method dealing with the type attribute? If you've been struggling with organizing and maintaing code for such use cases then say hello to your new friend: type delegation.
Recently I've been repeatedly asked how to get started with programming, especially Ruby and Rails. To keep things DRY and make sure I always include all great resources for learning, I decided to write this blog post. Most of people who asked me had never programmed before or had done some coding before in other languages, but are not (yet) proficient developers, so if you are a total beginner or just started learning Ruby / Rails and you are not sure what the next step is then you came to the right place.
In many Rails applications the modeling is limited only to creating classes inheritng from ActiveRecord::Base which are 1:1 mapped to database tables. Having AR models like User and Project doesn't tell much about the domain of your application. Well, you can add some domain logic to the models but that way you will easily end up having giant classes with thousands lines of code, so let's just use models for database-related stuff only. Other option would be to create service objects for every usecase. That's a good and clean way, but it's merely the interaction layer between different parts of your application. You may end up easily with all logic encapsulated within service objects that will start looking more like procedural programming rather than proper OOP and not real domain API at all. The good news is that you can easily counteract it: time to use composed models.
There've been a lot of discussions for several months about "The Rails Way" and problems associated with it - huge models with several thousands lines of code and no distinguishable responsibilities (User class which does everything related to users doesn't tell much what the app does), unmaintainable callback hell and no domain objects at all. Fortunately, service objects (or usecases) and other forms of extracting logic from models have become quite mainstream recently. However, it's not always clear how to use them all together without creating huge classes with tens of private methods and too many responsibilities. Here are some strategies you can use to solve these problems.
You are working currently on that awesome app and just started thinking about implementing new feature, let's call it feature X. What's the first thing you do? Rolling your own solution or... maybe checking if there's a magical gem that can help you solve that problem? Ok, it turns out there's already a gem Y that does what you expect. Also, it does tons of other things and is really complex. After some time your app breaks, something is definitively not working and it seems that gem Y is responsible for that. So you read all the issues on Github, pull requests and even read the source code and finally find a little bug. You managed to do some monkeypatching first and then send pull request for a small fix and solved a problem, which took you a few hours. Looks like a problem is solved. And then, you try to update Rails to2 the current version. Seems like there's a dependency problem - gem Y depends on previous version of Rails...
Last time, in part 1, I was giving some advice about testing - why to test at all, which tests are valuable and which are not, when to write acceptance tests and in what cases aim for the maximum code coverage. It brought about some serious discussion about testing ideas and if you haven't read it yet, you should probably check (it) it out. Giving some general point of view about such broad topic like Test Driven Development / Behavior Driven Development is definetely not enough so I will try to apply these techniques by implementing a concrete feature. I wanted to choose some popular usecase so that most developers will have an opinion how they would approach it. In most applications you will probably need:
Testing is still one of the hottest topics when it comes to developing maintainable and business-critical applications. Ruby on Rails community embraces the importance of writings tests, yet there are so little resources about the Test-Driven Development or Behavior-Driven Development in Rails applications from a wider perspective. How should we test our application? Which tests are valuable and which don’t provide any value? What scenarios should be included in acceptance tests?
There’ve been a lot of discussions recently about applying Object Oriented Programming in Rails applications, how ActiveRecord callbacks make testing painful and how Rails makes it hard to do OOP the right way. Is it really true? Rails makes everything easy - you can easily write terrible code, which will be a maintenance nightmare, but is also easy to apply good practices, especially with available gems. What is the good way then to extract logic in Rails applications and the best place to put it?
Server setup with the entire environment for Rails applications can be quite tricky, especially when you do it for the first time. Here is step by step guide how to setup CentOS 6.4 server with a basic environment for deploying Rails applications. I encourage you to choose CentOS Linux - it is a reliable distro (well, based on Red Hat Enterprise Linux), easy to handle and doesn't require advanced Unix knowledge like Gentoo (especially while updating system related stuff).