Hi Teodor,
the confusion here maybe comes from the fact, that there are two
(logical) representations of an element in PCollection. One
representation is the never mutable (most probably serialized in a
binary form) form of a PCollection element, where no modifications are
possible. Once a PCollection is created (e.g. read from source, or
created by a PTransform) it cannot be modified further. The second form
is an SDK-dependent representation of each PCollection element in user
code. This representation is what UDFs work with. The same source
(binary) form of element can have (and will have) different
representation in Java SDK and in Python SDK. The Beam model says
nothing about mutability of this SDK-dependent form. Nevertheless, even
if you modify this element, it has no impact on the source
representation. But, it can lead to SDK-dependent errors, when the
element is mutated in a way that a runner might not expect.
Hope this helps.
Jan
On 10/29/20 1:58 PM, Teodor Spæren wrote:
Hey!
Just so I understand this correctly then, what does the following
quote from [1], section 3.2.3 mean:
A PCollection is immutable. Once created, you cannot add, remove, or
change individual elements. A Beam Transform might process each
element of a PCollection and generate new pipeline data (as a new
PCollection), *but it does not consume or modify the original input
collection.*
(Don't know what the normal way of highlighting is on mailing lists,
so I just put it between *)
I read this as meaning that it is the users responsibilty to make sure
that their transformations do not modify the input, but should I
rather read it as meaning the beam runner itself should make sure the
user cannot make such a mistake? I find this reading at odds with the
documentation about the direct runner and it's express purpose being
to make sure users doesn't rely on semantics the beam model doesn't
ensure. And modifying of input arguments being one of the constraints
listed. [2].
It doesn't change the outcome here, adding an opt out switch, but if
I've missunderstood the quote above, I think this might benefit by
being reworded, to make sure it is communicated that shooting yourself
in the foot is impossible and the direct runner testing of modifying
input should be removed, as there is no point in users making sure to
not modifying the input if all runners guarantee it.
Also, I ran the whole Flink test suite with a simple return instead of
the deep copy and all tests passed, so there is no such test in there.
Depending on the reading above, we should add such tests to all runners.
Best regards,
Teodor Spæren
On Thu, Oct 29, 2020 at 10:16:30AM +0100, Maximilian Michels wrote:
Ok then we are on the same page, but I disagree with your
conclusion. The reason Flink has to do the deep copy is that it
doesn't state that the inputs are immutable and should not be
changed, and so have to do the deep copy. In Beam, the user is not
supposed to modify the input collection and if they do, it's
undefined behavior. This is the reason the DirectRunner checks for
this, to make sure the users are not relying on it.
It's not written anywhere that the input cannot be mutated. A
DirectRunner test is not a proof. Any runner could add a test which
proves the opposite. In fact we may have one that checks copying for
Flink.
I prefer safety and correctness over performance because I've seen
too many cases where users shoot themselves in the foot. We should
make sure that, by default, the user cannot modify the input element.
An option to disable that is fine.
-Max
