GEODE-6210: Add 'transaction' example for cpp

[moleske]

• demonstrates commit and rollback of transactions
• handles exceptions

[jake-at-work]

I’m approving with reservations. This example is very contrived and stinks of a unit test. Perhaps a better example shows an example that does some work, has an error (perhaps introduced randomly) and rolls back when the error occurs. We want to show examples of best practice implementation so doing work and then explicitly calling rollback doesn’t show an best practice, if you know you’re going to rollback best practice would be don’t do the work. ;)

[moleske]

I personally would be ok with those changes being requested, let's see if others have your reservations. Seems like it wouldn't be too crazy to throw some error and then rollback in something slightly more realistic.

[jake-at-work]

Something like:

main() {
  ...
  transactionManager.begin();
  try {
    for (auto& key : batch) {
      auto value = getValueFromExternalSystem(key);
      cache.put(key, value);
    }
    transactionManager.commit();
  } catch ( ... ) {
    transactionManager.rollback();
  }
}

int32_t getValuFromExternalSystem(int32_t key) {
  if (random(10) == 0) {
    throw "failed to get from external system";
  }

  return random(key);
}

[pdxcodemonkey]

I was getting stuck on the idea that we needed to force an exception external to the example code to have a "real" example. Charlie has suggested we implement a retry path as well, which also seems like a good suggestion. Thoughts?

[davebarnes97]

README correction, Line 1: Change 'Transaction Manger' to 'TransactionManager'.

[pdxcodemonkey]

Le sigh - is there a modern C++ version that's not way more complex? STL stuff seems like overkill

[jake-at-work]

This compiles? So we have non-static rd passed to a static e. At best here we allocating more rd on each call that we will never use.

[jake-at-work]

static thread_local std::default_random_engine generator(std::random_device{}());
auto value = std::uniform_int_distribution<int32_t>{0, 9}(generator);
...