From ea6b6adfe029815d6f5cf82ba2c68128e5d8a1ed Mon Sep 17 00:00:00 2001
From: Evgenii Stratonikov <evgeniy@nspcc.ru>
Date: Fri, 27 Nov 2020 11:46:25 +0300
Subject: [PATCH] [#157] netmap: Fallback to using minimal backup factor

Use CBF=1 when strict policy can't be satisfied.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>
---
 pkg/netmap/selector.go      | 24 ++++++++++++++++--------
 pkg/netmap/selector_test.go | 30 +++++++++++++++++++++++++++++-
 2 files changed, 45 insertions(+), 9 deletions(-)

diff --git a/pkg/netmap/selector.go b/pkg/netmap/selector.go
index f12027c7..ec1aa3d2 100644
--- a/pkg/netmap/selector.go
+++ b/pkg/netmap/selector.go
@@ -36,14 +36,14 @@ func (c *Context) processSelectors(p *PlacementPolicy) error {
 	return nil
 }
 
-// GetNodesCount returns amount of buckets and nodes in every bucket
-// for a given placement policy.
-func GetNodesCount(p *PlacementPolicy, s *Selector) (int, int) {
+// GetNodesCount returns amount of buckets and minimum number of nodes in every bucket
+// for the given selector.
+func GetNodesCount(_ *PlacementPolicy, s *Selector) (int, int) {
 	switch s.Clause() {
 	case ClauseSame:
-		return 1, int(p.ContainerBackupFactor() * s.Count())
+		return 1, int(s.Count())
 	default:
-		return int(s.Count()), int(p.ContainerBackupFactor())
+		return int(s.Count()), 1
 	}
 }
 
@@ -69,17 +69,25 @@ func (c *Context) getSelection(p *PlacementPolicy, s *Selector) ([]Nodes, error)
 		}
 	}
 
+	maxNodesInBucket := nodesInBucket * int(p.ContainerBackupFactor())
 	nodes := make([]Nodes, 0, len(buckets))
+	fallback := make([]Nodes, 0, len(buckets))
 
 	for i := range buckets {
 		ns := buckets[i].nodes
-		if len(ns) >= nodesInBucket {
-			nodes = append(nodes, ns[:nodesInBucket])
+		if len(ns) >= maxNodesInBucket {
+			nodes = append(nodes, ns[:maxNodesInBucket])
+		} else if len(ns) >= nodesInBucket {
+			fallback = append(fallback, ns)
 		}
 	}
 
 	if len(nodes) < bucketCount {
-		return nil, fmt.Errorf("%w: '%s'", ErrNotEnoughNodes, s.Name())
+		// Fallback to using minimum allowed backup factor (1).
+		nodes = append(nodes, fallback...)
+		if len(nodes) < bucketCount {
+			return nil, fmt.Errorf("%w: '%s'", ErrNotEnoughNodes, s.Name())
+		}
 	}
 
 	if len(c.pivot) != 0 {
diff --git a/pkg/netmap/selector_test.go b/pkg/netmap/selector_test.go
index 7f74f21a..b1cf378e 100644
--- a/pkg/netmap/selector_test.go
+++ b/pkg/netmap/selector_test.go
@@ -92,6 +92,33 @@ func TestPlacementPolicy_GetPlacementVectors(t *testing.T) {
 	require.Equal(t, len(v.Replicas()[1]), len(ids), "not all nodes we distinct")
 }
 
+func TestPlacementPolicy_LowerBound(t *testing.T) {
+	p := newPlacementPolicy(
+		2, // backup factor
+		[]*Replica{
+			newReplica(1, "X"),
+		},
+		[]*Selector{
+			newSelector("X", "Country", ClauseSame, 2, "*"),
+		},
+		nil, // filters
+	)
+
+	nodes := []NodeInfo{
+		nodeInfoFromAttributes("ID", "1", "Country", "DE"),
+		nodeInfoFromAttributes("ID", "2", "Country", "DE"),
+		nodeInfoFromAttributes("ID", "3", "Country", "DE"),
+	}
+
+	nm, err := NewNetmap(NodesFromInfo(nodes))
+	require.NoError(t, err)
+
+	v, err := nm.GetContainerNodes(p, nil)
+	require.NoError(t, err)
+
+	require.Equal(t, 3, len(v.Flatten()))
+}
+
 func TestPlacementPolicy_ProcessSelectors(t *testing.T) {
 	p := newPlacementPolicy(2, nil,
 		[]*Selector{
@@ -128,6 +155,7 @@ func TestPlacementPolicy_ProcessSelectors(t *testing.T) {
 		sel := c.Selections[s.Name()]
 		s := c.Selectors[s.Name()]
 		bucketCount, nodesInBucket := GetNodesCount(p, s)
+		nodesInBucket *= int(p.ContainerBackupFactor())
 		targ := fmt.Sprintf("selector '%s'", s.Name())
 		require.Equal(t, bucketCount, len(sel), targ)
 		for _, res := range sel {
@@ -216,7 +244,7 @@ func TestPlacementPolicy_ProcessSelectorsInvalid(t *testing.T) {
 		{
 			"NotEnoughNodes (backup factor)",
 			newPlacementPolicy(2, nil,
-				[]*Selector{newSelector("MyStore", "Country", ClauseDistinct, 1, "FromRU")},
+				[]*Selector{newSelector("MyStore", "Country", ClauseDistinct, 2, "FromRU")},
 				[]*Filter{newFilter("FromRU", "Country", "Russia", OpEQ)}),
 			ErrNotEnoughNodes,
 		},