Luke Cwik created BEAM-13930:
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             Summary: Address StateSpec inconsistency between Runner and Fn API
                 Key: BEAM-13930
                 URL: https://issues.apache.org/jira/browse/BEAM-13930
             Project: Beam
          Issue Type: Improvement
          Components: beam-model, sdk-java-core, sdk-py-core
            Reporter: Luke Cwik
            Assignee: Luke Cwik


The ability to mix and match runners and SDKs is accomplished through two 
portability layers:
1. The Runner API provides an SDK-and-runner-independent definition of a Beam 
pipeline
2. The Fn API allows a runner to invoke SDK-specific user-defined functions

Apache Beam pipelines support executing stateful DoFns[1]. To support this 
execution the Runner API defines multiple user state specifications:
* ReadModifyWriteStateSpec
* BagStateSpec
* OrderedListStateSpec
* CombiningStateSpec
* MapStateSpec
* SetStateSpec

The Fn API[2] defines APIs[3] to get, append and clear user state currently 
supporting a BagUserState and MultimapUserState protocol.

Since there is no clear mapping between the Runner API and Fn API state 
specifications, there is no way for a runner to know that it supports a given 
API necessary to support the execution of the pipeline. The Runner will also 
have to manage additional runtime metadata associated with which protocol was 
used for a type of state so that it can successfully manage the state’s 
lifetime once it can be garbage collected.

Please see the doc[4] for further details and a proposal on how to address this 
shortcoming.

1: https://beam.apache.org/blog/stateful-processing/
2: 
https://github.com/apache/beam/blob/3ad05523f4cdf5122fc319276fcb461f768af39d/model/fn-execution/src/main/proto/beam_fn_api.proto#L742
3: https://s.apache.org/beam-fn-state-api-and-bundle-processing
4: http://doc/1ELKTuRTV3C5jt_YoBBwPdsPa5eoXCCOSKQ3GPzZrK7Q




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