[GitHub] [beam] lostluck commented on a change in pull request #15639: [BEAM-12979, BEAM-11097] Change cache to store ReStreams, clean up to…

[GitBox <gi...@apache.org>]

lostluck commented on a change in pull request #15639:
URL: https://github.com/apache/beam/pull/15639#discussion_r720610725



##########
File path: sdks/go/pkg/beam/core/runtime/harness/statecache/statecache.go
##########
@@ -21,25 +21,45 @@
 package statecache
 
 import (
+	"io"
 	"sync"
 
+	"github.com/apache/beam/sdks/v2/go/pkg/beam/core/typex"
 	"github.com/apache/beam/sdks/v2/go/pkg/beam/internal/errors"
 	fnpb "github.com/apache/beam/sdks/v2/go/pkg/beam/model/fnexecution_v1"
 )
 
 type token string
 
-// ReusableInput is a resettable value, notably used to unwind iterators cheaply
-// and cache materialized side input across invocations.
+// FullValue represents the full runtime value for a data element, incl. the
+// implicit context. The result of a GBK or CoGBK is not a single FullValue.
+// The consumer is responsible for converting the values to the correct type.
+// To represent a nested KV with FullValues, assign a *FullValue to Elm/Elm2.
 //
-// Redefined from exec's input.go to avoid a cyclical dependency.
-type ReusableInput interface {
-	// Init initializes the value before use.
-	Init() error
-	// Value returns the side input value.
-	Value() interface{}
-	// Reset resets the value after use.
-	Reset() error
+// Copied from exec/fullvalue.go to avoid cyclical dependencies.
+type FullValue struct {
+	Elm  interface{} // Element or KV key.
+	Elm2 interface{} // KV value, if not invalid
+
+	Timestamp typex.EventTime
+	Windows   []typex.Window
+	Pane      typex.PaneInfo
+}
+
+// Stream is a FullValue reader. It returns io.EOF when complete, but can be
+// prematurely closed.
+//
+// Copied from exec/fullvalue.go to prevent cyclical dependencies.
+type Stream interface {
+	io.Closer
+	Read() (*FullValue, error)

Review comment:
       Copying this from exec will not work since exec.FullValue and statecache.FullValue are different types., so version of Stream won't match the exec.Stream version. Go doesn't have any co- or contra-variance in it's type system. (See https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)) 
   
   Instead this package should depend on the exec package, and if the exec package needs to interact with a cache-like object, it uses an interface on it's end instead. Afterall, that's how the SideInputAdapters, and state reader and similar work.




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