Monthly Archives: January 2012

Moving contexts and debuggers between images with Fuel

Hi guys. During ESUG 2011, at the Awards, I was showing Fuel. The week before such event I was thinking what I could show to the people. This was a challenge because showing a serializer can be plain boring. I was working at home that afternoon, and suddenly I thought: “What happens if I try to serialize a living debugger and materialize it in another image?” After 5 minutes, really, you will see it takes only 5 minutes, I notice that such crazy idea was working OUT OF THE BOX. Even if I knew Fuel supported serialization of methods, contexts, closures, classes, etc…I was surprised that it worked from the first try. I was so happy that I tried to explain to my poor wife what I had just done hahahah. Unfortunately, she told me it was too abstract and that understanding the garbage collector was easier (I promise she really understands what the garbage collector does hahhahaha).

Well….several months has passed, but I would like to show you how to do it because I think it may be of help for real systems 😉  So…the idea is the following: whenever there is an error, you can get the context from it, and such context is what is usually written down into a log file (in Pharo this is PharoDebug.log). I will show you two things: 1) how to serialize a debugger in one image and materialize it another one and; 2) how to write down the context into a Fuel file when there is an error so that you can materialize it later in another image.

Installing Fuel

The first step is, of course, install Fuel. The latest stable release is 1.7 but to have better results with this example, I would recommend 1.8. Fuel 1.8 is not released yet it is because we plan to write some stuff in the website. The code is almost finish, so you should load Fuel 1.8 beta1. In my case I am using a normal Pharo 1.3 image:

Gofer it
url: '';
package: 'ConfigurationOfFuel';
((Smalltalk at: #ConfigurationOfFuel) project version: '1.8-beta1') load.

Once you have finished loading Fuel, save the image. Let’s call it Fuel.image.

Serializing and materializing a debugger

Now its time to do something hacky in the image so that to open a debugger. Or you can just take a piece of code and debug it. In my example, I opened a workspace and wrote the following:

| a |
a := 'Hello Smalltalk hackers. The universal answer is '.
a := a , '42!'.
Transcript show: a.

Then I select the whole code, right click -> “debug it”. Then I do one “step over” and I stop there before the concatenation with ’42!’.

I am sure there could be better ways, but the simpler way I found to get the debugger instance for this example, is to do a Debugger allInstances first 😉  so… be sure not to have another debugger opened hahaha.  Now…let’s serialize the debugger:

Smalltalk garbageCollect.
serialize: Debugger allInstances first
toFileNamed: 'debugger.fuel'.

After that, you should have a ‘debugger.fuel’ created in the same directory where the image is. Now close your image (without saving it) and re open it. If everything is fine, we should be able to materialize our debugger and continue debugging. So, let’s try it:

| newDebugger |
newDebugger := FLMaterializer materializeFromFileNamed: 'debugger.fuel'.
newDebugger openFullMorphicLabel: 'Materialized debugger ;)'.

So????  Did it work??  are you as happy as me when I first saw it? 🙂  if you check this new opened debugger, you will see its state is correct. For example, the instVar ‘a’ has the correct state. You can now open a Transcript and continue with the debugger as if were the original one.

Of course that even if this simple example works, there are a lot of problems. But I will explain them at the end of the post.

Serializing and materializing errors

In the previous example we have serialized the debugger manually. But imagine the following: you have a production application running. There is an error, and PharoDebug.log is written with all the stack. The user/client send you by email the .log and you open your favorite text editor to try to understand what happened. Now imagine the following: you have a production application running. There is an error, and a PharoDebug.fuel is written with all the stack. The user/client send you by email the file and you open an image, and then materialize and open a debugger. How does it sound? 🙂 magical?

For this example, we will just change the place where Pharo writes PharoDebug.log when there is an error. That method is #logError:inContext:. What we will do is to add just 2 lines at the beginning to serialize the context:

logError: errMsg inContext: aContext

" we should think about integrating a toothpick here someday"
serialize: aContext
toFileNamed: 'PharoDebug.fuel'.

self logDuring: [:logger |
nextPutAll: 'THERE_BE_DRAGONS_HERE'; cr;
nextPutAll: errMsg; cr.

aContext errorReportOn: logger.

"wks 9-09 - write some type of separator"
logger nextPutAll: (String new: 60 withAll: $- ); cr; cr.

Now yes, let’s execute something that causes an error. What I did is to evaluate 1/0. After that, you should see the file PharoDebug.fuel in the same directory where the image is. You can now close the image and reopen it. And then, let’s reopen de debugger:

| aContext |
aContext := FLMaterializer materializeFromFileNamed: 'PharoDebug.fuel'.
Debugger openContext: aContext label:  'This is the new debugger!' contents: nil

Et voilà! Hopefully that worked 🙂   Notice that in this example and the previous one, there is nothing in special with the Fuel serialization. You are using the normal API, and all you do is to serialize a debugger or a context as if you were serializing any normal object. Of course, you can also apply this idea to other places. For example, in Seaside you have an error handler. You may want to serialize the error with Fuel there.

Limitation and known problems

  • Even if Fuel can fully serialize methods, classes, traits, etc., it is recommended that the image were the contexts/debuggers are serialized and materialized are equal. If you are doing this in a production application, then you can have the same image running locally. The idea is that both images have the same classes and methods installed. This is because, by default, if the object graph to serialize includes compiled methods, classes, class variables, etc., they are all considered as “globals”, which means that we only serialize its global name and then during materialization it is searched in Smalltalk globals. Hence, classes and methods have to be present. Otherwise you have to use Fuel in a way that it serializes classes as well, but that’s more complicated.
  • There may be things that affects the debugger which are part of the image and not the serialization, and they may have changed. Imagine for example, a class variable which has changed its value in the image where you serialize. Then it will have a different value in the image where you materialize. Most of these problems also happens if even if you open the debugger later in the same image…some state may have changed…
  • The graph reachable from the contexts can be very big. For example, Esteban Lorenzano was doing this for an application and one of the problems is that from the context it was reachable the whole UI…which means lots and lots of objects. In such a case, you can always use Fuel hooks to prune the object graph to serialize.
  • Be aware to use exactly the same version of Fuel in both images 😉


All in all, I think that as a very first step, it is very nice that we can serialize this kind of stuff like contexts and debuggers out of the box with Fuel. This could be the infrastructure for a lot of fancy stuff. I don’t think that the debugger materialization can be as reliable as if you were debugging in the original image. I don’t think either that it should replace PharoDebug.log. However, what I do think is that you can add the Fuel serialization just as another way of getting more information about the problem. It’s one more tool you can add to your Smalltalk toolbox 🙂