Software has two ingredients: opinions and logic (=programming). The second ingredient is rare and is typically replaced by the first.
I blog about code correctness, maintainability, testability, and functional programming.
This blog does not represent views or opinions of my employer.

Saturday, October 18, 2014

I don't like Hibernate/Grails part 10: Repeatable finder, lessons learned

Repeatable finder (concurrency issue described in part 2) is what started/motivated this series. I had hoped that that this issue will draw some reaction from the community. It did not. Why? Tallying up all the answers/responses from the last 2 moths amounts to: 6, none of them useful or even correct. What does that mean?

In this series I tried to sneak in some things that interest me like FP and logical reasoning of code correctness.  I will use repeatable finder problem as a way to sneak in a bit more of this stuff later in this post.

Denial isn't just a river in Egypt?
This has been a twilight-zone.
To refresh you memory:  if more than one query is executed in a single Hibernate session and the result sets intersect then the second query returns a weird combination of old and new data.  That can break the logic in your code,  for example:

can return records with nickName != 'bob'. Other things can go wrong too: Maybe you have used a DB unique key to define equals()?  Or maybe you have used a DB unique key as a key in a Map? Any of this could go very wrong.

At first, I thought that the issue must be well know and I am missing some way of handling it. This, unfortunately, is not the case. Very recently, I came across this blog from 2009: orm-sucks-hibernate-sucks-even-more
"... take a look how even a silly CRUD application would suffer, once you've got "not-very-recent" object from the session"
that quote points to a (now non-existing) page on the hibernate website. Did we know more in 2009 that we know now?  If we did know, why have we allowed for this issue to stay unresolved? Well, this is all speculation.

I tried my best to do 2 things:  make the community aware and persuade Hibernate to fix it. I have failed miserably on both accounts. Here are the results of my efforts (as of Oct 17, 2014, tallied after a bit over 2 months since I started my crusade):
  • post part 2:  effectively no replies, but over 1100 reads.
  • Grails JIRA: incorrect comments and then ignored
  • Hibernate JIRA:  rejected (works as intended) with suggested work-around which is incorrect
  • Stack Overflow question:  a whooping +4 score (started at -1) and bunch of incorrect or meaningless answers
  • Grails forum: 0 replies
Hibernate ticket was the weirdest experience.  It got rejected very fast (not a bug) with a comment to just use refresh().  After pointing out that this workaround is a total nonsense, I was sent to read some completely not relevant documentation about concurrency.  After that, my (and Tim's) comments have been ignored.

What can I conclude from these 3 facts?:
  • nobody seems to know how to resolve or even work-around this issue
  • experts provide advice that is incorrect
  • there is no interest in solving, discussing or even acknowledging it as a problem
I do not know, but probably nothing good. I think it is interesting to try to puzzle out the few responses that the problem did generate. I will try to do that here.

The replies I got from the expects fall into 2 categories. The first category are answers like this:
  • It is any ORM issue
  • Any database application will have an issue like this 
It is true that the issue can be resolved with DB locking.  In particular, I could prevent repeatable finder by having all HTTP requests wrapped in long transactions and configuring higher (repeatable read) isolation level. Indeed, it is a big framework design failure, if we need to resort to things like this.
It is NOT true that any ORM and any database application will have this problem.  The most likely explanation for this type of response is that developers do not think about side-effects. They see Hibernate query and think of a SELECT statement only.  If I see a problem, it must be from the SELECT, where else would it come from?  This is consistent with the point I tried to make in my previous posts.

The second category are answers that suggest using refresh() or discard() to fix the problem:
'To fix your problem'
  • add refresh() to your code
  • or:  add discard()/evict() to your code
My first reaction was: Grrr, my second:  Hmm.  If I could only continue this conversation I am sure it would go like this:
Me: Where do I add these?  Expert's Reply: Add them where you have that problem. 

If you have been following various Hibernate discussion forums, you must have noticed that the same type of advice (either to add refresh() or to add evict()) shows up very frequently. This advice is never right.

Grails and Hibernate experts:  I am very disappointed in you.

Add refresh()...  add evict()... Thinking in Hibernate.
(Here is where I sneak-in some interesting stuff.)
This is how we typically reason about our code: the problem is on line 57 because variable xyz is ... and then on line 89 we do that..., and then on line 127 we have an if statement that goes like that...
We reason about our code by examining chains of programming instructions.

This is called imperative thinking and imperative programming.  If you read my previous posts you may assume that I consider such programs not logical,  they are logical, only the logic is very cumbersome and complex.
A well designed OO program is where lines 57, 89 and 127 are all in the same class and the chains of instructions we need to examine are relatively short.  In a procedural program lines 57, 89, and 127 can be anywhere and chains are long.  Badly designed OO programs behave like procedural programs.

Repeatable finder is a great example where imperative reasoning fails.  The problem is not something between lines 57 and 89 or something on line 115.  The problem is (or can be) anywhere.
Answer 'add refresh()' or 'add discard()' is a very imperative thinking: it assumes I can add it on line 89.  (I can only conclude that this expert advice is not to sprinkle refresh() all over my code just for fun ... and because my code will run too fast without it.)

So what is the alternative?  The idea is to think about a block of code in a way that can prove certain behavior of that block. If we know that a code block 'A' exhibits the same behavior no matter where it is placed or how is used, then we no longer need to think about lines 57, 89, and 127 or about chains of computing instructions.

This is called declarative thinking and programming.  Logical reasoning is now simplified, I no longer need to follow chains of programming steps to reason about the correctness.

Declarative thinking works great, except, if I cannot trust any property, even that:
    Users.findAllByNickName('bob').every{it.nickName == 'bob'}
then I am stuck.
That may sound like a limitation of declarative programming:  I can still keep going using imperative approach. That is true, and we all 'keep going'.  I did not stop programming my project and no, I did not add refresh() all over my code. That is why our applications are so buggy: we ignore logical problems unless we can pin them to line 127.

Side Note: The fun starts when I start combining my declarative code blocks into bigger blocks. Code needs to be logically composable.  I want a bigger block (composed of smaller blocks) to have properties too. Some like to call it programming with combinators.

I would like to suggest this as a new rule of thumb: 
  the answer to use Hibernate/GORM refresh() or evict()/discard() is wrong regardless of the question.
(with exception of Functional Tests - which may need to refresh some records used in asserts).  Please comment below if you find a counter example to this rule.

I am not claiming that I know the solution to Repeatable Finder.  Maintaining Hibernate cache synchronized with the DB is hard, maybe impossible.  One way of dealing with hard problems is: make them somebody else's.  If GORM/Hibernate just told me when the query is lying (returns stale data even if it has new data) or allowed me to request/configure the query to refresh all records... That would go a long way.

It looks like the community has decided to not acknowledge Repeatable Finder as a problem. There is really no good solution for it and acknowledging it would be admitting to that fact. This issue is likely to remain unsolved and ignored. More complex Grails apps are doomed to work incorrectly under heavier concurrent use.

I have added a label to my posts (which does not work so do not click on it - and that convinces me that blogger must be using Hibernate):  'Stop thinking in C++ and Java'.  I think we need to stop thinking imperative or at least stop thinking only in imperative terms.

Next post:  I need to do one more to wrap-up. I will be finishing this series next week.

I have a busy period ahead of me.  I started going over a set of courses published online by University of Oregon (Oregon Programming Languages Summer School) and that will be many hours of not very easy listening and learning.  I also have to start preparing/training for a ski camp in early November (I live in CO and skiing has started here already).  No, this will not be a SKI calculus camp;) - but then, believe it or not, technical skiing is (or should be) a fun intellectual activity too.