Transactions and in-memory data (part 2 of n)

Buffering the opertations wasn't particularly successful.

Let's look at maintaining a rollback log.

Seems pretty simple.  You can do something like:

typedef void (*rollback_function_ptr_t)(void *context);
typedef struct _rollback_record {
rollback_function_ptr_t rollbackFunction;
struct _rollback_record *next;
} rollback_record_t;

void do_rollback(rollback_record_t *head) {
while (head != NULL) {
rollback_record_t *next = head->next;
(*head->rollbackFunction)(head + 1);
free(head);
head = next;
}
}

That's pretty simple, eh?  Since rollbacks probably have to be done in reverse order of changes, you have to maintain the list with a stack discipline (always adding to the head).  Whenever you're going to do anything which mutates global in-memory state, you first allocate a rollback record plus space for your own data, fill it in and then put it on the head of the list.

Sounds good at one level, but simultaneously it totally breaks the model of folks like STL who were trying to do the right thing by providing a few functions like std::map::erase() which do not fail.  Now these destructive actions have to be able to fail since they can allocate a rollback record which can fail.

Next time I'll go into this model a little more and point out some of its problems but since I hadn't written since Thursday I wanted to get the start of this out at least.

Comments

• Anonymous
  May 16, 2005
  url=http://www.oxford-english.com 英语培训
• Anonymous
  May 16, 2005
  url=http://www.oxford-english.com 英语培训
• Anonymous
  May 17, 2005
  Im not sure Im following this. Are you asking for erase methods that can fail or erase methods that are undoable?
  Why do you want the former? I dont think the latter is possible without going down 'the functional route' copying the data structure, working on the copy and replacing the original with the copy at commit time.
• Anonymous
  May 17, 2005
  I'm not wanting anything; I'm exploring the various routes available to achieve reliability for in-memory caches/stores.
• Anonymous
  May 17, 2005
  "Now these destructive actions have to be able to fail"
  
  I dont understand this sentence. Do you mean they have to be undoable?
• Anonymous
  July 06, 2005
  The comment has been removed