C++ bindings for libpmemobj (part 6) - transactions

Posted May 25, 2016         « Previous post     Next post »

As I mentioned in my previous blog post, transactions are the heart of libpmemobj. That is why we had to take utmost care while designing their C++ versions, so that they are as easy to use as possible. There are, however, a couple of compromises we had to make due to the inadequacies of the C++11 standard. That is why we encourage using the lambda, until the C++17 standard is more widely implemented. Frankly, even after C++17, the lambda transactions are more user friendly.

Closure-like transactions

This is the type of transactions we encourage everyone to use. It automatically handles all of the commit/abort semantics behind the scenes, so you don’t have to worry about it. As the worker for the transactions, it accepts a std::function<void()> object, so you don’t have to use lambda expressions if you don’t want to. The transactions however look really nice and clear, so don’t be discouraged by the peculiar C++ lambda syntax. Here is an example:

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auto pop = pool_base::create(...);
persistent_ptr<entry> pentry;
transaction::exec_tx(pop, [&] {
  pentry = make_persistent<entry>();
  // make other changes inside the transaction
});

Of course, if you need to take locks for the whole durations of the transaction, you can do that. The transaction::exec_tx, takes a locks variadic template parameter:

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auto pop = pool_base::create(...);
transaction::exec_tx(pop, transaction_fn, locks...);

By locks, I of course mean the C++ persistent memory resident locks which are available in the libpmemobj bindings.

The closure-like transactions handle cases where there are exceptions thrown inside the transaction. The transaction is then aborted and the original exception is rethrown. That way you never loose the original exception and at the same time, the transaction state is handled properly by our library. If the transaction aborts because of internal errors (such as out of memory errors), you will get an nvml::transaction_error exception.

This is the preferred way of handling transactions, because every aspect of the transaction is handled automatically by the library. If you however find the lambda functions cumbersome to write, you have another option.

Manual transactions

The manual transactions are a little tricky to use, because they abort the transaction by default. What I mean by that is that the following example will abort:

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auto pop = pool_base::create(...);
{
  transaction::manual tx(pop);
  auto pentry = make_persistent<entry>();
} // here the transaction aborts

You might wonder, why did we at all decide to do manual transactions if the std::uncaught_exception is available in C++11? Why not go automatic from the start? Yes it is available, but with the way it is designed and implemented, it is not usable. You can read this nice article and the solution proposed by Herb Sutter to C++17. To sum it up (although I encourage you to read the articles), with the C++11 uncaught_exception you do not know if the object is being destroyed to perform stack unwinding or was the unwinding already in progress. So for example, if a transaction would be started as cleanup in an object’s destructor, you wouldn’t know whether to abort or commit the transaction - there would already be an active exception at the start of the transaction. The only way out of this predicament in C++11 is to manually commit the transaction:

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auto pop = pool_base::create(...);
{
  transaction::manual tx(pop);
  auto pentry = make_persistent<entry>();
  transaction::commit(); // here the transaction commits
}

By now, you are probably wondering

How will I know if the transaction aborted?

And that is a very good question. There is an API call transaction::get_last_tx_error() which tells you whether the last transaction errored. This makes this scoped RAII approach really tedious and fragile. Yet another reason to get used to the closure version of transactions.

Automatic transactions

The automatic scoped RAII transactions leverage the improved std::uncaught_exceptions from C++17, which is available if the __cpp_lib_uncaught_exceptions feature test macro is defined. It is available since GCC 6.1(libstdc++) and clang 3.7(libc++). The automatic version releases the developer from the burden of manually committing the transaction.

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auto pop = pool_base::create(...);
{
  transaction::automatic tx(pop);
  auto pentry = make_persistent<entry>();
} // here the transaction commits

However you still don’t know whether the transaction committed or aborted and still have to use the transaction::get_last_tx_error(). So if you really need the scoped transactions, try to use the automatic versions as they are less error prone. However, I would still recommend the closure ones.

Utils

Besides the closure and scoped transactions, the transaction class has a couple of utility methods that might come in handy.

There is one more thing I need to mention. There should be no code after the calls to transaction::abort or transaction::commit. There are a bunch of reasons why this is wrong, the main being something called undefined behavior.

We are reaching the end of this series of blog posts on C++ bindings. In the next post I will show you the persistent memory resident synchronization variables available in our C++ bindings for libpmemobj. After that I would like to do a short story on how to convert existing code to understand persistent memory using C++.



Posted by @tomaszkapela         « Previous post     Next post »