`get_boundary_keys` in C API

Daniel-B-Smith · January 3, 2020, 6:10pm

The Python API has the useful function get_boundary_keys, but I don’t see an equivalent in the C API. Am I just misreading the docs? If not, I would be happy to add it with some pointers to get started.

ryanworl · January 3, 2020, 6:19pm

get_boundary_keys, and the equivalent functions in other bindings which have implemented it, is implemented by reading the system keyspace using a regular transaction. If you ported the equivalent code from Python to C, it would work.

ajbeamon · January 3, 2020, 6:26pm

Here is a link to the Python implementation, if that helps:

github.com

apple/foundationdb/blob/92c4a94ce7f0b14bc441a631165d7fa65a0a0278/bindings/python/fdb/locality.py#L59


                begin = kv.key[13:] + b'\x00'
            begin = end
        except _impl.FDBError as e:
            # if we get a transaction_too_old and *something* has happened, then we are no longer transactional
            if e.code == 1007 and begin != lastbegin:
                tr = tr.db.create_transaction()
            else:
                tr.on_error(e).wait()




def get_boundary_keys(db_or_tr, begin, end):
    begin = _impl.keyToBytes(begin)
    end = _impl.keyToBytes(end)


    gen = _get_boundary_keys(db_or_tr, begin, end)
    try:
        next(gen)
    except StopIteration:  # if _get_boundary_keys() never yields a value, e.g. begin > end
        return (x for x in list())
    return gen

I think it’s easy enough to build your own for use on top of the C API, though I think we initially opted not to add it to the C API itself because the C API is intended to be somewhat minimal and low level. The API for using get_boundary_keys would have probably looked somewhat similar to the range reading API (though complicated by its multi-transactional nature), and in the end it seemed simpler to implement the range reads directly in the bindings, even though I suppose it does require some awareness of the system keyspace.