While the Go SDK doesn't yet support a State API, Option 3) is what the Go SDK 
does for all non-standard coders (aka custom coders) anyway.

For wire transfer, the Java Runner also adds a LengthPrefixCoder for the coder and its subcomponents. The problem is that this is an implicit assumption made. In the Proto, we do not have this represented. This is why **for state requests**, we end up with a "LengthPrefixCoder[CustomCoder]" on the Runner and a "CustomCoder" on the SDK Harness side. Note that the Python Harness does wrap unknown coders in a LengthPrefixCoder for transferring regular elements, but the LengthPrefixCoder is not preserved for the state requests.

In that sense (3) is good because it follows this implicit notion of adding a LengthPrefixCoder for wire transfer, but applies it to state requests.

However, option (1) is most reliable because the LengthPrefixCoder is actually in the Proto. So "CustomCoder" will always be represented as "LengthPrefixCoder[CustomCoder]", and only standard coders will be added without a LengthPrefixCoder.

I'd really like to avoid implicit agreements about how the coder that
should be used differs from what's specified in the proto in different
contexts.

Option (2) would work on top of the existing logic because replacing a non-standard coder with a "NOOP coder" would just be used by the Runner to produce a serialized version of the key for partitioning. Flink always operates on the serialized key, be it standard or non-standard coder. It wouldn't be necessary to change any of the existing wire transfer logic or representation. I understand that it would be less ideal, but maybe easier to fix for the release.

The key concept here is not "standard coder" but "coder that the
runner does not understand." This knowledge is only in the runner.
Also has the downside of (2).

Yes, I had assumed "non-standard" and "unknown" are the same, but the latter can be a subset of the former, i.e. if a Runner does not support all of the standard coders for some reason.

This means that the wire format that the runner sends for the "key" represents 
the exact same wire format it will receive for state requests.

The wire format for the entire element is the same. Otherwise we wouldn't be able to process data between the Runner and the SDK Harness. However, the problem is that the way the Runner instantiates the key coder to partition elements, does not match how the SDK encodes the key when it sends a state request to the Runner. Conceptually, those two situations should be the same, but in practice they are not.


Now that I thought about it again option (1) is probably the most explicit and in that sense cleanest. However, option (3) is kind of fair because it would just replicate the implicit LengthPrefixCoder behavior we have for general wire transfer also for state requests. Option (2) I suppose is the most implicit and runner-specific, should probably be avoided in the long run.

So I'd probably opt for (1) and I would update the PR[1] rather soon because this currently blocks the release, as this is a regression from 2.16.0.[2]


-Max

[1] https://github.com/apache/beam/pull/9997
[2] (In 2.16.0 it worked for Python because the Runner used a ByteArrayCoder with the OUTER encoding context for the key which was basically option (2). Only problem that, for standard coders the Java SDK Harness produced non-matching state request keys, due to it using the NESTED context.)

On 07.11.19 18:01, Luke Cwik wrote:


On Thu, Nov 7, 2019 at 8:22 AM Robert Bradshaw <rober...@google.com <mailto:rober...@google.com>> wrote:

    On Thu, Nov 7, 2019 at 6:26 AM Maximilian Michels <m...@apache.org
    <mailto:m...@apache.org>> wrote:
     >
     > Thanks for the feedback thus far. Some more comments:
     >
     > > Instead, the runner knows ahead of time that it
     > > will need to instantiate this coder, and should update the bundle
     > > processor to specify KvCoder<LengthPrefixCoder<CustomCoder>,
     > > VarIntCoder> as the coder so both can pull it out in a
    consistent way.
     >
     > By "update the bundle processor", do you mean modifying the
     > ProcessBundleDescriptor's BagUserState with the correct key coder?
     > Conceptually that is possible, but the current implementation
    does not
     > allow for this to happen:
     >
    
https://github.com/apache/beam/blob/076a037e53ca39b61c1dfeb580527bc8d0371dc1/runners/java-fn-execution/src/main/java/org/apache/beam/runners/fnexecution/control/ProcessBundleDescriptors.java#L284
     > It enforces ByteString which does not tell the SDK Harness anything
     > about the desired encoding.

    I meant update the BundleProcessDescriptor proto that is sent to the
    SDK
    
https://github.com/apache/beam/blob/master/model/fn-execution/src/main/proto/beam_fn_api.proto#L140,
    essentially option (1).


For clarity, the "key" coder is specified by the stateful ParDo's main input PCollection. This means that the ProcessBundleDescriptor should have something that has the length prefix as part of the remote grpc port specification AND the PCollection that follows it which is the main input for the stateful ParDo. This means that the wire format that the runner sends for the "key" represents the exact same wire format it will receive for state requests.

I see what you mean Max, https://github.com/apache/beam/blob/076a037e53ca39b61c1dfeb580527bc8d0371dc1/runners/java-fn-execution/src/main/java/org/apache/beam/runners/fnexecution/control/ProcessBundleDescriptors.java#L284 could change to represent the actual coder (whether it be LengthPrefixCoder<BytesCoder> or some other model coder combination). Currently that logic assumes that the runner can perform the backwards mapping by decoding the bytestring with the appropriate coder.

     > Since the above does not seem feasible, I see the following options:
     >
     > (1) Modify the pipeline Proto before the translation and wrap a
     > LengthPrefixCoder around non-standard key coders for stateful
     > transforms. This would change the encoding for the entire
    element, to be
     > sure that the key coder for state requests contains a
    LengthPrefixCoder
     > for state requests from the SDK Harness. Not optimal.

    Yes. The contract should be that both the runner and SDK use the
    coders that are specified in the proto. The runner controls the proto,
    and should ensure it only sends protos it will be able to handle the
    SDK responding to. I'm not seeing why this is sub-optimal.

     > (2) Add a new method WireCoders#instantiateRunnerWireKeyCoder which
     > returns the correct key coder, i.e. for standard coders, the concrete
     > coder, and for non-standard coders a ByteArrayCoder. We also need to
     > ensure the key encoding on the Runner side is OUTER context, to avoid
     > adding a length prefix to the encoded bytes. Basically, the
    non-standard
     > coders result in a NOOP coder which does not touch the key bytes.

    I'd really like to avoid implicit agreements about how the coder that
    should be used differs from what's specified in the proto in different
    contexts.

     > (3) Patch the Python SDK to ensure non-standard state key coders are
     > always wrapped in a LengthPrefixCoder. That way, we can keep the
     > existing logic on the Runner side.

    The key concept here is not "standard coder" but "coder that the
    runner does not understand." This knowledge is only in the runner.
    Also has the downside of (2).

     > Option (2) seems like the most practical.
     >
     > -Max
     >
     > On 06.11.19 17:26, Robert Bradshaw wrote:
     > > On Wed, Nov 6, 2019 at 2:55 AM Maximilian Michels
    <m...@apache.org <mailto:m...@apache.org>> wrote:
     > >>
     > >> Let me try to clarify:
     > >>
     > >>> The Coder used for State/Timers in a StatefulDoFn is pulled
    out of the
     > >>> input PCollection. If a Runner needs to partition by this
    coder, it
     > >>> should ensure the coder of this PCollection matches with the
    Coder
     > >>> used to create the serialized bytes that are used for
    partitioning
     > >>> (whether or not this is length-prefixed).
     > >>
     > >> That is essentially what I had assumed when I wrote the code. The
     > >> problem is the coder can be "pulled out" in different ways.
     > >>
     > >> For example, let's say we have the following Proto PCollection
    coder
     > >> with non-standard coder "CustomCoder" as the key coder:
     > >>
     > >>     KvCoder<CustomCoder, VarIntCoder>
     > >>
     > >>   From the Runner side, this currently looks like the following:
     > >>
     > >>     PCol: KvCoder<LengthPrefixCoder<ByteArrayCoder>, VarIntCoder>
     > >>     Key:  LengthPrefixCoder<ByteArrayCoder>
     > >
     > > This is I think where the error is. When If the proto references
     > > KvCoder<CustomCoder, VarIntCoder> it should not be pulled out as
     > > KvCoder<LengthPrefixCoder<ByteArrayCoder>, VarIntCoder>; as that
     > > doesn't have the same encoding. Trying to do instantiate such a
    coder
     > > should be an error. Instead, the runner knows ahead of time that it
     > > will need to instantiate this coder, and should update the bundle
     > > processor to specify KvCoder<LengthPrefixCoder<CustomCoder>,
     > > VarIntCoder> as the coder so both can pull it out in a
    consistent way.
     > >
     > > When the coder is KvCoder<LengthPrefixCoder<CustomCoder>,
    VarIntCoder>
     > > instantiating it as KvCoder<ByteArrayCoder, VarIntCoder> on the
    runner
     > > is of course OK as they do have the same encoding.
     > >
     > >> At the SDK Harness, we have the coder available:
     > >>
     > >>     PCol: KvCoder<CustomCoder, VarIntCoder>
     > >>     Key:  CustomCoder
     > >>
     > >> Currently, when the SDK Harness serializes a key for a state
    request,
     > >> the custom coder may happen to add a length prefix, or it may
    not. It
     > >> depends on the coder used. The correct behavior would be to
    use the same
     > >> representation as on the Runner side.
     > >>
     > >>> Specifically, "We have no way of telling from the Runner
    side, if a length prefix has been used or not." seems false
     > >>
     > >> The Runner cannot inspect an unknown coder, it only has the
    opaque Proto
     > >> information available which does not allow introspection of
    non-standard
     > >> coders. With the current state, the Runner may think the coder
    adds a
     > >> length prefix but the Python SDK worker could choose to add
    none. This
     > >> produces an inconsistent key encoding. See above.
     > >
     > > I think what's being conflated here is "the Coder has been
    wrapped in
     > > a LengthPrefixCoder" vs. "the coder does length prefixing."
    These are
     > > two orthogonal concepts. The runner in general only knows the
    former.
     > >
     > >> It looks like the key encoding for state requests on the
    Python SDK
     > >> Harness side is broken. For transferring elements of a
    PCollection, the
     > >> coders are obviously working correctly, but for encoding
    solely the key
     > >> of an element, there is a consistency issue.
     > >>
     > >>
     > >> -Max
     > >>
     > >> On 06.11.19 05:35, Kenneth Knowles wrote:
     > >>> Specifically, "We have no way of telling from the Runner
    side, if a
     > >>> length prefix has been used or not." seems false. The runner
    has all the
     > >>> information since length prefix is a model coder. Didn't we
    agree that
     > >>> all coders should be self-delimiting in runner/SDK interactions,
     > >>> requiring length-prefix only when there is an opaque or
    dynamic-length
     > >>> value? I assume you mean that at runtime the worker for a
    given engine
     > >>> does not know?
     > >>>
     > >>> Kenn
     > >>>
     > >>> On Tue, Nov 5, 2019 at 3:19 PM Luke Cwik <lc...@google.com
    <mailto:lc...@google.com>
     > >>> <mailto:lc...@google.com <mailto:lc...@google.com>>> wrote:
     > >>>
     > >>>      +1 to what Robert said.
     > >>>
     > >>>      On Tue, Nov 5, 2019 at 2:36 PM Robert Bradshaw
    <rober...@google.com <mailto:rober...@google.com>
     > >>>      <mailto:rober...@google.com
    <mailto:rober...@google.com>>> wrote:
     > >>>
     > >>>          The Coder used for State/Timers in a StatefulDoFn is
    pulled out
     > >>>          of the
     > >>>          input PCollection. If a Runner needs to partition by
    this coder, it
     > >>>          should ensure the coder of this PCollection matches
    with the Coder
     > >>>          used to create the serialized bytes that are used
    for partitioning
     > >>>          (whether or not this is length-prefixed).
     > >>>
     > >>>          Concretely, the graph looks like
     > >>>
     > >>>
     > >>>          Runner                          SDK Harness
     > >>>
     > >>>          WriteToGbk
     > >>>               |
     > >>>          ReadFromGbk
     > >>>               |
     > >>>          RunnerMapFn.processKeyValue(key, value)
     > >>>               |
     > >>>               WriteToDataChannel
     > >>>                       ------------------------>
     > >>>                            ReadFromDataChannel
     > >>>                                          |
     > >>>                                      (pcIn)
     > >>>                                          |
     > >>>                             MyStatefulDoFn.process(key, value)
     > >>>
     > >>>          Now the (key part of the) Coder of pcIn, which comes
    from the proto
     > >>>          that the Runner sent to the SDK, must match the (key
    part of the)
     > >>>          encoding used in WriteToGbk and ReadFromGbk. If a
    LenthPrefix is
     > >>>          added
     > >>>          in one spot, it must be added in the other.
     > >>>
     > >>>
     > >>>          [1]
     > >>>
    
https://github.com/apache/beam/blob/release-2.17.0/sdks/python/apache_beam/runners/worker/bundle_processor.py#L1183
     > >>>
     > >>>          On Tue, Nov 5, 2019 at 1:25 PM Maximilian Michels
     > >>>          <m...@apache.org <mailto:m...@apache.org>
    <mailto:m...@apache.org <mailto:m...@apache.org>>> wrote:
     > >>>           >
     > >>>           > Hi,
     > >>>           >
     > >>>           > I wanted to get your opinion on something that I
    have been
     > >>>          struggling
     > >>>           > with. It is about the coders for state requests
    in portable
     > >>>          pipelines.
     > >>>           >
     > >>>           > In contrast to "classic" Beam, the Runner is not
    guaranteed
     > >>>          to know
     > >>>           > which coder is used by the SDK. If the SDK
    happens to use a
     > >>>          standard
     > >>>           > coder (also known as model coder), we will also
    have it
     > >>>          available at the
     > >>>           > Runner, i.e. if the Runner is written in one of
    the SDK
     > >>>          languages (e.g.
     > >>>           > Java). However, when we do not have a standard
    coder, we just
     > >>>          treat the
     > >>>           > data from the SDK as a blob and just pass it
    around as bytes.
     > >>>           >
     > >>>           > Problem
     > >>>           > =======
     > >>>           >
     > >>>           > In the case of state requests which the SDK
    Harness authors
     > >>>          to the
     > >>>           > Runner, we would like for the key associated with
    the state
     > >>>          request to
     > >>>           > match the key of the element which led to
    initiating the
     > >>>          state request.
     > >>>           >
     > >>>           > Example:
     > >>>           >
     > >>>           > Runner                 SDK Harness
     > >>>           > ------                 -----------
     > >>>           >
     > >>>           > KV["key","value"]  --> Process Element
     > >>>           >                                |
     > >>>           > LookupState("key") <-- Request state of "key"
     > >>>           >          |
     > >>>           >     State["key"]    --> Receive state
     > >>>           >
     > >>>           >
     > >>>           > For stateful DoFns, the Runner partitions the
    data based on
     > >>>          the key. In
     > >>>           > Flink, this partitioning must not change during
    the lifetime of a
     > >>>           > pipeline because the checkpointing otherwise
    breaks[0]. The
     > >>>          key is
     > >>>           > extracted from the element and stored encoded.
     > >>>           >
     > >>>           > If we have a standard coder, it is basically the
    same as in the
     > >>>           > "classic" Runner which takes the key and
    serializes it.
     > >>>          However, when we
     > >>>           > have an SDK-specific coder, we basically do not
    know how it
     > >>>          encodes. So
     > >>>           > far, we have been using the coder instantiated
    from the
     > >>>          Proto, which is
     > >>>           > basically a LengthPrefixCoder[ByteArrayCoder] or
    similar[1].
     > >>>          We have had
     > >>>           > problems with this because the key encoding of
    Java SDK state
     > >>>          requests
     > >>>           > did not match the key encoding on the Runner side
    [2]. In an
     > >>>          attempt to
     > >>>           > fix those, it is now partly broken for portable
    Python pipelines.
     > >>>           > Partly, because it "only" affects non-standard
    coders.
     > >>>           >
     > >>>           > Non-standard coders yield the aforementioned
     > >>>           > LengthPrefixCoder[ByteArrayCoder]. Now, following
    the usual
     > >>>          encoding
     > >>>           > scheme, we would simply encode the key using this
    coder.
     > >>>          However, for
     > >>>           > state requests, the Python SDK leaves out the
    length prefix
     > >>>          for certain
     > >>>           > coders, e.g. for primitives like int or byte. It
    is possible
     > >>>          that one
     > >>>           > coder uses a length prefix, while another
    doesn't. We have no
     > >>>          way of
     > >>>           > telling from the Runner side, if a length prefix
    has been
     > >>>          used or not.
     > >>>           > This results in the keys to not match on the
    Runner side and the
     > >>>           > partitioning to be broken.
     > >>>           >
     > >>>           >
     > >>>           > How to solve this?
     > >>>           > ==================
     > >>>           >
     > >>>           > (1) Should this simply be fixed on the Python SDK
    side? One
     > >>>          fix would be
     > >>>           > to always append a length prefix to the key in state
     > >>>          requests, even for
     > >>>           > primitive coders like VarInt which do not use one.
     > >>>           >
     > >>>           > OR
     > >>>           >
     > >>>           > (2) Should the Runner detect that a non-standard
    coder is
     > >>>          used? If so,
     > >>>           > it should just pass the bytes from the SDK
    Harness and never
     > >>>          make an
     > >>>           > attempt to construct a coder based on the Proto.
     > >>>           >
     > >>>           >
     > >>>           > Thinking about it now, it seems pretty obvious
    that (2) is
     > >>>          the most
     > >>>           > feasible way to avoid complications across all
    current and
     > >>>          future SDKs
     > >>>           > for key encodings. Still, it is odd that the
    Proto contains coder
     > >>>           > information which is not usable.
     > >>>           >
     > >>>           > What do you think?
     > >>>           >
     > >>>           >
     > >>>           > Thanks,
     > >>>           > Max
     > >>>           >
     > >>>           >
     > >>>           > [0] It is possible to restart the pipeline and
    repartition the
     > >>>           > checkpointed data.
     > >>>           > [1]
     > >>>           >
     > >>>
    
https://github.com/apache/beam/blob/c39752af5391fe698a2b4f1489c187ddd4d604c0/runners/flink/src/main/java/org/apache/beam/runners/flink/FlinkStreamingPortablePipelineTranslator.java#L682
     > >>>           > [2] https://issues.apache.org/jira/browse/BEAM-8157
     > >>>

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