In my last article, I started to look at multitenant Web applications. These are applications that run a single time, but that can be retrieved via a variety of hostnames. As I explained in that article, even a simple application can be made multitenant by having it check the hostname used to connect to the HTTP server, and then by displaying a different set of content based on that.

For a simple set of sites, that technique can work well. But if you are working on a multitenant system, you more likely will need a more sophisticated set of techniques.

For example, I recently have been working on a set of sites that help people practice their language skills. Each site uses the same software but displays a different interface, as well as (obviously) a different set of words. Similarly, one of my clients has long operated a set of several dozen geographically targeted sites. Each site uses the same software and database, but appears to the outside world to be completely separate. Yet another reason to use a multitenant architecture is if you allow users to create their own sites—and, perhaps, add users to those private sites.

In this article, I describe how to set up all of the above types of sites. I hope you will see that creating such a multitenant system doesn't have to be too complex, and that, on the contrary, it can be a relatively easy way to provide a single software service to a variety of audiences.

Identifying the Site

In my last article, I explained how to modify /etc/passwd such that more than one hostname would be associated with the same IP address. Every multitenant site uses this same idea. A limited set of IP addresses (and sometimes only a single IP address) can be mapped to a larger number of hostnames and/or domain names. When a request comes in, the application first checks to see which site has been requested, and then decides what to do based on it.

The examples in last month's article used Sinatra, a lightweight framework for Web development. It's true that you can do sophisticated things with Sinatra, but when it comes to working with databases and large-scale projects, I prefer to use Ruby on Rails. So here I'm using Rails, along with a back end in PostgreSQL.

In order to do that, you first need to create a simple Rails application:

rails new -d postgresql multiatf

Then create a "multiatf" user in your PostgreSQL installation:

createuser multiatf

Finally, go into the multiatf directory, and create the database:

rake db:create

With this in place, you now have a working (if trivially simple) Rails application. Make sure you still have the following two lines in your /etc/hosts file:

127.0.0.1 atf1
127.0.0.1 atf2

And when you start up the Rails application:

rails s

you can go to http://atf1:3000 or http://atf2:3000, and you should see the same results—namely, the basic "hello" that you get from a Rails application before you have done anything.

The next step then is to create a default controller, which will provide actual content for your users. You can do this by saying:

rails g controller welcome

Now that you have a "welcome" controller, you should uncomment the appropriate route in config/routes.rb:

root 'welcome#index'

If you start your server again and go to http://atf1:3000, you'll now get an error message, because Rails knows to go to the "welcome" controller and invoke the "index" action, but no such action exists. So, you'll have to go into your controller and add an action:

def index
  render text: "Hello!"
end

With that in place, going to your home page gives you the text.

So far, that's not very exciting, and it doesn't add to what I explored in my last article. You can, of course, take advantage of the fact that your "index" method is rendering text, and that you can interpolate values into your text dynamically:

def index
  render text: "Hello, visitor to #{request.host}!"
end

But again, this is not what you're likely to want. You will want to use the hostname in multiple places in your application, which means that you'll repeatedly end up calling "request.host" in your application. A better solution is to assign a @hostname variable in a before_action declaration, which will ensure that it takes place for everyone in the system. You could create this "before" filter in your welcome controller, but given that this is something you'll want for all controllers and all actions, I think it would be wiser to put it in the application controller.

Thus, you should open app/controllers/application_controller.rb, and add the following:

before_action :get_hostname

def get_hostname
  @hostname = request.host
end

Then, in your welcome controller, you can change the "index" action to be:

def index
  render text: "Hello, visitor to #{@hostname}!"
end

Sure enough, your hostname now will be available as @hostname and can be used anywhere on your site.

Moving to the Database

In most cases, you'll want to move beyond this simple scheme. In order to do that, you should create a "hosts" table in the database. The idea is that the "hosts" table will contain a list of hostnames and IDs. It also might contain additional configuration information (I discuss that below). But for now, you can just add a new resource to the system. I even would suggest using the built-in scaffolding mechanism that Rails provides:

rails g scaffold hosts name:string

Why use a scaffold? I know that it's very popular among Rails developers to hate scaffolds, but I actually love them when I start a simple project. True, I'll eventually need to remove and rewrite parts, but I like being able to move ahead quickly and being able to poke and prod at my application from the very first moments.

Creating a scaffold in Rails means creating a resource (that is, a model, a controller that handles the seven basic RESTful actions and views for each of them), as well as the basic tests needed to ensure that the actions work correctly. Now, it's true that on a production system, you probably won't want to allow anyone and everyone with an Internet connection to create and modify existing hosts. And indeed, you'll fix this in a little bit. But for now, this is a good and easy way to set things up.

You will need to run the new migration that was created:

rake db:migrate

And then you will want to add your two sites into the database. One way to do this is to modify db/seeds.rb, which contains the initial data that you'll want in the database. You can use plain-old Active Record method calls in there, such as:

Host.create([{name: 'atf1'}, {name: 'atf2'}])

Before you add the seeded data, make sure the model will enforce some constraints. For example, in app/models/host.rb, I add the following:

validates :name, {:uniqueness => true}

This ensures that each hostname will appear only once in the "hosts" table. Moreover, it ensures that when you run rake db:seed, only new hosts will be added; errors (including attempts to enter the same data twice) will be ignored.

With the above in place, you can add the seeded data:

rake db:seed

Now, you should have two records in your "hosts" table:

[local]/multiatf_development=# select name from hosts;
--------
| name |
--------
| atf1 |
--------
| atf2 |
--------
(2 rows)

With this in place, you now can change your application controller:

before_action :get_host

def get_host
  @requested_host = Host.where(name: request.host).first

  if @requested_host.nil?
    render text: "No such host '#{request.host}'.", status: 500
    return false
  end

end

(By the way, I use @requested_host here, so as not to collide with the @host variable that will be set in hosts_controller.)

@requested_host is no longer a string, but rather an object. It, like @requested_host before, is an instance variable set in a before filter, so it is available in all of your controllers and views. Notice that it is now potentially possible for someone to access your site via a hostname that is not in your "hosts" table. If and when that happens, @requested_host will be nil, and you give an appropriate error message.

This also means that you now have to change your "welcome" controller, ever so slightly:

def index
  render text: "Hello, visitor to #{@requested_host.name}!"
end

This change, from the string @requested_host to the object @requested_host, is about much more than just textual strings. For one, you now can restrict access to your site, such that only those hosts that are active can now be seen. For example, let's add a new boolean column, is_active, to the "hosts" table:

rails g migration add_is_active_to_hosts

On my machine, I then edit the new migration:

class AddIsActiveToHosts < ActiveRecord::Migration
  def change
    add_column :hosts, :is_active, :boolean, default: true, 
     ↪null: false
  end
end

According to this definition, sites are active by default, and every site must have a value for is_active. You now can change your application controller's get_host method:

def get_host
  @requested_host = Host.where(name: request.host).first

  if @requested_host.nil?
    render text: "No such host '#{request.host}'.", status: 500
    return false
  end

  if !@requested_host.is_active?
    render text: "Sorry, but '#{@requested_host.name}' 
     ↪is not active.", status: 500
    return false
  end

end

Notice how even a simple database now allows you to check two conditions that were not previously possible. You want to restrict the hostnames that can be used on your system, and you want to be able to turn hosts on and off via the database. If I change is_active to false for the "atf1" site:

UPDATE Hosts SET is_active = 'f' WHERE name = 'atf1';

immediately, I'm unable to access the "atf1" site, but the "atf2" site works just fine.

This also means that you now can add any number of sites—without regard to host or domain—so long as they all have DNS entries that point to your IP addresses. Adding a new site is as simple as registering the domain (if it hasn't been registered already), configuring its DNS entries such that the hostname points to your IP address, and then adding a new entry in your Hosts table.

Users and Permissions

Things become truly interesting when you use this technique to allow users to create and manage their own sites. Suddenly, it is not just a matter of displaying different text to different users, but allowing different users to log in to different sites. The above shows how you can have a set of top-level administrators and users who can log in to each site. However, there often are times when you will want to restrict users to be on a particular site.

There are a variety of ways to handle this. No matter what, you need to create a "users" table and a model that will handle your users and their ability to register and log in. I used to make the foolish mistake of implementing such login systems on my own; nowadays, I just use "Devise", the amazing Ruby gem that handles nearly anything you can imagine having to do with registration and authentication.

I add the following line to my project's Gemfile:

gem 'devise'

Next, I run bundle install, and then:

rails g devise:install

on the command line. Now that I have Devise installed, I'll create a user model:

rails g devise user

This creates a new "user" model, with all of the Devise goodies in it. But before running the migrations that Devise has provided, let's make a quick change to the Devise migration.

In the migration, you're going to add an is_admin column, which indicates whether the user in question is an administrator. This line should go just before the t.timestamps line at the bottom, and it indicates that users are not administrators by default:

t.boolean :is_admin, default: false, null: false

With this in place, you now can run the migrations. This means that users can log in to your system, but they don't have to. It also means that you can designate users as administrators. Devise provides a method that you can use to restrict access to particular areas of a site to logged-in users. This is not generally something you want to put in the application controller, since that would restrict people from logging in. However, you can say that your "welcome" and "host" controllers are open only to registered and logged-in users by putting the following at the top of these controllers:

before_action :authenticate_user!

With the above, you already have made it such that only registered and logged-in users are able to see your "welcome" controller. You could argue that this is a foolish decision, but it's one that I'm comfortable with for now, and its wisdom depends on the type of application you're running. (SaaS applications, such as Basecamp and Harvest, do this, for example.) Thanks to Devise, I can register and log in, and then...well, I can do anything I want, including adding and removing hosts.

It's probably a good idea to restrict your users, such that only administrators can see or modify the hosts controller. You can do that with another before_action at the top of that controller:

before_action :authenticate_user!
before_action :only_allow_admins
before_action :set_host, only: [:show, :edit, :update, :destroy]

Then you can define only_allow_admins:

def only_allow_admins
  if !current_user.is_admin?
    render text: "Sorry, but you aren't allowed there", 
     ↪status: 403
    return false
  end
end

Notice that the above before_action filter assumes that current_user already has been set, and that it contains a user object. You can be sure that this is true, because your call to only_allow_admins will take place only if authenticate_user! has fired and has allowed the execution to continue.

That's actually not much of a problem. You can create a "memberships" table that joins "users" and "hosts" in a many-to-many relationship. Each user thus can be a member of any number of hosts. You then can create a before_action routine that checks to be sure not only whether users are logged in, but also whether they are a member of the host they're currently trying to access. If you want to provide administrative rights to users within their site only, you can put such a column (for example, "is_host_admin") on the memberships table. This allows users to be a member of as many sites as they might want, but to administer only those that have been specifically approved.

Additional Considerations

Multitenant sites raise a number of additional questions and possibilities. Perhaps you want to have a different style for each site. That's fine. You can add a new "styles" table, which has two columns: "host_id" (a number, pointing to a row in the host table) and "style", text containing CSS, which you can read into your program at runtime. In this way, you can let users style and restyle things to their heart's content.

In the architecture described here, the assumption is that all data is in the same database. I tend to prefer to use this architecture, because I believe that it makes life easier for the administrators. But if you're particularly worried about data security, or if you are being crushed by a great load, you might want to consider a different approach, such as firing up a new cloud server for each new tenant site.

Also note that with this system, a user has to register only once on the entire site. In some cases, it's not desirable for end users to share logins across different sites. Moreover, there are cases (such as with medical records) that might require separating information into different databases. In such situations, you might be able to get away with a single database anyway, but use different "schemas", or namespaces, within it. PostgreSQL has long offered this capability, and it's something that more sites might be able to exploit.

Conclusion

Creating a multitenant site, including separate administrators and permissions, can be a quick-and-easy process. I have created several such sites for my clients through the years, and it has only gotten easier during that time. However, at the end of the day, the combination of HTTP, IP addresses and a database is truly what allows me to create such flexible SaaS applications.

Resources

The Devise home page is at https://github.com/plataformatec/devise.

For information and ideas about multitenant sites in Ruby on Rails, you might want to read Multitenancy with Rails, an e-book written by Ryan Bigg and available at https://leanpub.com/multi-tenancy-rails. While the book specifically addresses multitenancy with Rails, it offers many ideas and approaches that are appropriate for other software systems.

Now Available: Practice Makes Python by Reuven M. Lerner

My new e-book, Practice Makes Python, is now available for purchase. The book is aimed at people who have taken a Python course or learned it on their own, but want to feel more comfortable with the "Pythonic" way of doing things—using built-in data structures, writing functions, using functional techniques, such as comprehensions, and working with objects.

Practice Makes Python contains 50 exercises that I have used in nearly a decade of on-site training classes in the US, Europe, Israel and China. Each exercise comes with a solution, as well as a detailed description of why the solution works, often along with alternatives. All are aimed at improving your proficiency with Python, so that you can use it effectively in your work.

You can read more about the book at http://lerner.co.il/practice-makes-python.

Linux Journal readers can get 10% off the purchase price by using the coupon code LINUXJOURNAL at checkout. Questions or comments can be sent to me by e-mail at reuven@lerner.co.il or @reuvenmlerner on Twitter.