satellite/metabase/rangedloop: observer interface

New interface for parallel segment loop.

Closes https://github.com/storj/storj/issues/5239

Change-Id: I2bcce6f836f6625da8ceb4fc0fc030c0ea4380e7

satellite/metabase/rangedloop/observer.go

@@ -0,0 +1,34 @@
+// Copyright (C) 2022 Storj Labs, Inc.
+// See LICENSE for copying information.
+
+package rangedloop
+
+import (
+	"context"
+	"time"
+
+	"storj.io/storj/satellite/metabase"
+	"storj.io/storj/satellite/metabase/segmentloop"
+)
+
+// Observer subscribes to the parallel segment loop.
+// It is intended that a naïve implementation is threadsafe.
+type Observer interface {
+	Start(context.Context, time.Time, metabase.NodeAliasMap) error
+
+	// Fork creates a Partial to process a chunk of all the segments.
+	Fork(context.Context) (Partial, error)
+	// Join is called after the chunk for Partial is done.
+	// This gives the opportunity to merge the output like in a reduce step.
+	Join(context.Context, Partial) error
+
+	// Finish is called after all segments are processed by all observers.
+	Finish(context.Context) error
+}
+
+// Partial processes a part of the total range of segments.
+type Partial interface {
+	// Process is called repeatedly with batches of segments.
+	// It is not called concurrently on the same instance.
+	Process(context.Context, []segmentloop.Segment) error
+}