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This chapter is from the book

Topic 26: How to Balance Resources

  • To light a candle is to cast a shadow...

  • Ursula K. Le Guin, A Wizard of Earthsea

We all manage resources whenever we code: memory, transactions, threads, network connections, files, timers—all kinds of things with limited availability. Most of the time, resource usage follows a predictable pattern: you allocate the resource, use it, and then deallocate it.

However, many developers have no consistent plan for dealing with resource allocation and deallocation. So let us suggest a simple tip:

This tip is easy to apply in most circumstances. It simply means that the function or object that allocates a resource should be responsible for deallocating it. Let’s see how it applies by looking at an example of some bad code—part of a Ruby program that opens a file, reads customer information from it, updates a field, and writes the result back. We’ve eliminated error handling to make the example clearer:

def read_customer
  @customer_file = File.open(@name + ".rec", "r+")
  @balance       = BigDecimal(@customer_file.gets)
end
def write_customer
  @customer_file.rewind
  @customer_file.puts @balance.to_s
  @customer_file.close
end
def update_customer(transaction_amount)
  read_customer
  @balance = @balance.add(transaction_amount,2)
  write_customer
end

At first sight, the routine update_customer looks reasonable. It seems to implement the logic we require—reading a record, updating the balance, and writing the record back out. However, this tidiness hides a major problem. The routines read_customer and write_customer are tightly coupled4—they share the instance variable customer_file. read_customer opens the file and stores the file reference in customer_file, and then write_customer uses that stored reference to close the file when it finishes. This shared variable doesn’t even appear in the update_customer routine.

Why is this bad? Let’s consider the unfortunate maintenance programmer who is told that the specification has changed—the balance should be updated only if the new value is not negative. They go into the source and change update_customer:

def update_customer(transaction_amount)
  read_customer
  if (transaction_amount >= 0.00)
    @balance = @balance.add(transaction_amount,2)
    write_customer
  end
end

All seems fine during testing. However, when the code goes into production, it collapses after several hours, complaining of too many open files. It turns out that write_customer is not getting called in some circumstances. When that happens, the file is not getting closed.

A very bad solution to this problem would be to deal with the special case in update_customer:.

def update_customer(transaction_amount)
  read_customer
  if (transaction_amount >= 0.00)
    @balance += BigDecimal(transaction_amount, 2)
    write_customer
  else
    @customer_file.close # Bad idea!
  end
end

This will fix the problem—the file will now get closed regardless of the new balance—but the fix now means that three routines are coupled through the shared variable customer_file, and keeping track of when the file is open or not is going to start to get messy. We’re falling into a trap, and things are going to start going downhill rapidly if we continue on this course. This is not balanced!

The finish what you start tip tells us that, ideally, the routine that allocates a resource should also free it. We can apply it here by refactoring the code slightly:

def read_customer(file)
  @balance=BigDecimal(file.gets)
end
def write_customer(file)
  file.rewind
  file.puts @balance.to_s
end
def update_customer(transaction_amount)
  file=File.open(@name + ".rec", "r+")          # >--
  read_customer(file)                           #    |
  @balance = @balance.add(transaction_amount,2) #    |
  file.close                                    # <--
end

Instead of holding on to the file reference, we’ve changed the code to pass it as a parameter.5 Now all the responsibility for the file is in the update_customer routine. It opens the file and (finishing what it starts) closes it before returning. The routine balances the use of the file: the open and close are in the same place, and it is apparent that for every open there will be a corresponding close. The refactoring also removes an ugly shared variable.

There’s another small but important improvement we can make. In many modern languages, you can scope the lifetime of a resource to an enclosed block of some sort. In Ruby, there’s a variation of the file open that passes in the open file reference to a block, shown here between the do and the end:

def update_customer(transaction_amount)
  File.open(@name + ".rec", "r+") do |file|       # >--
    read_customer(file)                           #    |
    @balance = @balance.add(transaction_amount,2) #    |
    write_customer(file)                          #    |
  end                                             # <--
end

In this case, at the end of the block the file variable goes out of scope and the external file is closed. Period. No need to remember to close the file and release the source, it is guaranteed to happen for you.

When in doubt, it always pays to reduce scope.

Nest Allocations

The basic pattern for resource allocation can be extended for routines that need more than one resource at a time. There are just two more suggestions:

  • Deallocate resources in the opposite order to that in which you allocate them. That way you won’t orphan resources if one resource contains references to another.

  • When allocating the same set of resources in different places in your code, always allocate them in the same order. This will reduce the possibility of deadlock. (If process A claims resource1 and is about to claim resource2, while process B has claimed resource2 and is trying to get resource1, the two processes will wait forever.)

It doesn’t matter what kind of resources we’re using—transactions, network connections, memory, files, threads, windows—the basic pattern applies: whoever allocates a resource should be responsible for deallocating it. However, in some languages we can develop the concept further.

Objects and Exceptions

The equilibrium between allocations and deallocations is reminiscent of an object-oriented class’s constructor and destructor. The class represents a resource, the constructor gives you a particular object of that resource type, and the destructor removes it from your scope.

If you are programming in an object-oriented language, you may find it useful to encapsulate resources in classes. Each time you need a particular resource type, you instantiate an object of that class. When the object goes out of scope, or is reclaimed by the garbage collector, the object’s destructor then deallocates the wrapped resource.

This approach has particular benefits when you’re working with languages where exceptions can interfere with resource deallocation.

Balancing and Exceptions

Languages that support exceptions can make resource deallocation tricky. If an exception is thrown, how do you guarantee that everything allocated prior to the exception is tidied up? The answer depends to some extent on the language support. You generally have two choices:

  1. Use variable scope (for example, stack variables in C++ or Rust)

  2. Use a finally clause in a trycatch block

With usual scoping rules in languages such as C++ or Rust, the variable’s memory will be reclaimed when the variable goes out of scope via a return, block exit, or exception. But you can also hook in to the variable’s destructor to cleanup any external resources. In this example, the Rust variable named accounts will automatically close the associated file when it goes out of scope:

{
  let mut accounts = File::open("mydata.txt")?; // >--
  // use 'accounts'                             //    |
  ...                                           //    |
}                                               // <--
// 'accounts' is now out of scope, and the file is
// automatically closed

The other option, if the language supports it, is the finally clause. A finally clause will ensure that the specified code will run whether or not an exception was raised in the trycatch block:

try
  // some dodgy stuff
catch
  // exception was raised
finally
  // clean up in either case

However, there is a catch.

An Exception Antipattern

We commonly see folks writing something like this:

begin
   thing = allocate_resource()
   process(thing)
finally
   deallocate(thing)
end

Can you see what’s wrong?

What happens if the resource allocation fails and raises an exception? The finally clause will catch it, and try to deallocate a thing that was never allocated.

The correct pattern for handling resource deallocation in an environment with exceptions is

thing = allocate_resource()
begin
   process(thing)
finally
   deallocate(thing)
end

When You Can’t Balance Resources

There are times when the basic resource allocation pattern just isn’t appropriate. Commonly this is found in programs that use dynamic data structures. One routine will allocate an area of memory and link it into some larger structure, where it may stay for some time.

The trick here is to establish a semantic invariant for memory allocation. You need to decide who is responsible for data in an aggregate data structure. What happens when you deallocate the top-level structure? You have three main options:

  • The top-level structure is also responsible for freeing any substructures that it contains. These structures then recursively delete data they contain, and so on.

  • The top-level structure is simply deallocated. Any structures that it pointed to (that are not referenced elsewhere) are orphaned.

  • The top-level structure refuses to deallocate itself if it contains any substructures.

The choice here depends on the circumstances of each individual data structure. However, you need to make it explicit for each, and implement your decision consistently. Implementing any of these options in a procedural language such as C can be a problem: data structures themselves are not active. Our preference in these circumstances is to write a module for each major structure that provides standard allocation and deallocation facilities for that structure. (This module can also provide facilities such as debug printing, serialization, deserialization, and traversal hooks.)

Checking the Balance

Because Pragmatic Programmers trust no one, including ourselves, we feel that it is always a good idea to build code that actually checks that resources are indeed freed appropriately. For most applications, this normally means producing wrappers for each type of resource, and using these wrappers to keep track of all allocations and deallocations. At certain points in your code, the program logic will dictate that the resources will be in a certain state: use the wrappers to check this. For example, a long-running program that services requests will probably have a single point at the top of its main processing loop where it waits for the next request to arrive. This is a good place to ensure that resource usage has not increased since the last execution of the loop.

At a lower, but no less useful level, you can invest in tools that (among other things) check your running programs for memory leaks.

Related Sections Include

  • Topic 24, Dead Programs Tell No Lies

  • Topic 30, Transforming Programming

  • Topic 33, Breaking Temporal Coupling

Challenges

  • Although there are no guaranteed ways of ensuring that you always free resources, certain design techniques, when applied consistently, will help. In the text we discussed how establishing a semantic invariant for major data structures could direct memory deallocation decisions. Consider how Topic 23, Design by Contract, could help refine this idea.

Exercise 17 (possible answer)

Some C and C++ developers make a point of setting a pointer to NULL after they deallocate the memory it references. Why is this a good idea?

Exercise 18 (possible answer)

Some Java developers make a point of setting an object variable to NULL after they have finished using the object. Why is this a good idea?

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