How server team @Gett used Opal (opalrb.org) to write isomorphic Ruby code that ran on the server and smartphone clients.

1. Isomorphic Server/Client Ruby with Opal
Max Rozenoer
November, 2015

2. Our R&D problems started when we decided to
remove a couple letters from the company name.
You’d think it would be easy! We used to be called
GetTaxi, you see - and we provided only on-
demand taxis. Then we changed the name to Gett,
and now provide many kinds of on-demand services
(salads, flowers, pizzas, etc).
Intro

3. To make it possible to launch these new services
quickly, we did a lot of work in the last year. Part of
this work had to do with how each order can be
priced, since the pricing business logic has become
a lot more complex. This is what this talk is about.
Intro

4. Intro
Supplier App
iOS / Android
Customer App
iOS / Android
Server

5. So, customer has our Customer app installed,
Supplier has our supplier app installed.
Customer makes an order for something, it arrives
to the Supplier app, Supplier delivers the goods (in
case of a taxi ride the goods are the Customer
him/herself :))
Intro

6. So, how do we determine the price of an order?
Well, there is a Pricing Calculator library with
complicated business logic which dynamically
calculates the price of the order based on its current
properties.
Intro

7. Intro
Server
Pricing
Calculator
Supplier App
iOS / Android
Price: $XX

8. Normally this code runs on the server, and gets
pushed to or fetched by the client. However,
sometimes the client loses network connection right
at the end of an order which needs to be paid with
cash. Since in this case the supplier must
immediately collect the money from the client, the
ride must be priced right away, and without
network.
Problem

9. Problem
Server
Pricing
Calculator
Supplier App
iOS / Android
Price: ???

10. For this reason, we want to be able to run the
Pricing Calculator library locally on the client as a
fallback.
Problem

11. Problem
Server
Pricing
Calculator
Supplier App
iOS / Android
Pricing
Calculator

12. Initially, we ported this library to native
iOS/Android. But the library has complicated
business logic which changes often, so this
approach was costly, time-consuming and difficult
to maintain.
Problem

13. Solution
Isomorphic CodeServer
Client
Isomorphic code is code that runs on both the server
and client.

14. You probably heard of isomorphic code in the
context of Node.js web development (if not, see
isomorphic.net for more details).
Well, it’s easy there because in the context of
Node.js Javascript runs both on server and on the
client (browser).
Solution

15. But we love Ruby and want to develop and
maintain code in Ruby!
Alas, currently nothing beats Javascript when
executing on clients.
What to do? Opal: Ruby to Javascript compiler.
Solution

16. Solution: Opal!
Isomorphic CodeServer
Client
Ruby JavascriptOpal
opalrb.org

17. The Opal Open-source Project is a source-to-source
Ruby-to-Javascript compiler. It is similar to
Coffeescript in that it allows a web developer to
write her client code in something nicer than JS,
which then compiles to JS.
Unlike Coffeescript, Opal code needs a runtime to
run.
Opal

18. So, what is our flow for developing isomorphic
Ruby? It ships as a gem containing both Ruby code
and JS code.
Development Flow

19. Development Flow: Pricing Calculator Gem
1. Write Ruby Code
2. Run
+ Ruby Specs .
JS Code
3. Opal-Compile
5. Run
Minified JS Code
6. Minify
JS Specs
4. Opal -Compile

20. Truth be told, I didn’t have the time to quite get
steps 4 and 5 above to work, but in theory it should
be like this :)
Now, this seems like a lot. But steps 2-6 are done for
us automatically using the amazing Guard gem, so
all we have to do is write Ruby code + Ruby specs.
Development Flow

21. Development Flow: Pricing Calculator Gem
1. Write Ruby Code
2. Run
+ Ruby Specs .
JS Code
3. Compile
5. Run
Minified JS Code
6. Minify
Guard!JS Specs
4. Compile
7. Push!

22. In production, the Pricing Calculator gem is
deployed inside an App Server container (in our
case, Rails), which exposes several APIs.
Runtime Flow

23. Runtime Flow
App Server Container (Rails)
Pricing Calculator Gem
JS
Code
Ruby
Code

24. One API simply serves the static Opal Pricing
Calculator lib file, which is loaded by the client at
the beginning of a session, and loaded in a JS
Runtime.
On Android, the JS runtime we use is J2V8, on iOS
it’s an SDK Framework called JavascriptCore.
Runtime Flow

25. Supplier App
iOS / Android
Runtime Flow
App Server Container (Rails)
Pricing Calculator Gem
JS
Code
Ruby
Code
JS Runtime
JS Pricing
Calculator
Load JS
Pricing Calculator

26. The other exposed API is the regular API requesting
to perform server-side pricing calculation.
Runtime Flow

27. Supplier App
iOS / Android
Runtime Flow
App Server Container (Rails)
Pricing Calculator Gem
JS
Code
Ruby
Code
JS Runtime
JS Pricing
Calculator
Load JS
Pricing Calculator
Server-side
Calculation API

28. Generally the server-side calculation is the authority,
and the client-side calculation is a fallback in case
server is not available.
But also, during a taxi ride in some cases we want to
display a constantly updating taxi meter, and don’t
want to make so many calls to the server, so the
client-side library is also used for that.
Runtime Flow

29. So now we got the same code running on server and
on client. Will it produce the same results?
Wait, the capabilities of the execution contexts in
which the code runs might be different. For one thing,
we assumed that the client execution context might
not have any network. What if the Pricing Calculator
needs to make network calls?
Execution Context

30. Different Capabilities of Execution Context
Supplier App
Pricing
Calculator
(JS)
Pricing
Calculator
(Ruby)
Server
Network Call
Network Call

31. To solve this problem, we don’t make any network
calls from the library itself.
Instead, the library assumes that the runtime context
will provide a layer that produces the resources the
library needs.
Execution Context

32. In case of server execution context, it simply makes a
network call to fetch the resources.
In case of client execution context, it pre-fetches data
from the server so that (in some cases) it’s able to
“simulate” a network call without any network.
Execution Context

33. Runtime Context Layer
Pricing
Calculator
(Ruby)
Server
Runtime
Context
Layer
Supplier App
Pricing
Calculator
(JS)
Runtime
Context
Layer
Simulated
network call
via pre-
loaded
data
Network Call

34. Lessons Learned
● Solution is running in production for several
months
● Opal JS code is reliable and works well
Downsides:
○ Our JS lib weighs ~310Kb (60Kb zipped)
(80% is Opal base runtime)

35. Downsides (continued):
Isomorphic Ruby you write needs to be “Opal-
friendly”:
● Opal’s Ruby stdlib support is still far from
complete (e.g., “Time” has only a few
methods).
● Your Ruby needs to have no external gem
dependencies, since most gems are probably
not Opal-friendly.

36. Downsides (continued):
● Debugging is tough. Production JS exceptions
from minified code are unreadable - need to
write defensive code and raise your own
exceptions!

37. That’s all, Folks!
Max Rozenoer
we’re hiring!maxr@gett.com
This presentation on SlideShare: bit.ly/gett_isomorphic