Add support for soft constraints

Signed-off-by: sivaram mothiki <sivaram@opdemand.com>

scheduler/filter/constraint.go

@@ -36,6 +36,9 @@ func (f *ConstraintFilter) Filter(config *dockerclient.ContainerConfig, nodes []
 			}
 		}
 		if len(candidates) == 0 {
+			if constraint.isSoft {
+				return nodes, nil
+			}
 			return nil, fmt.Errorf("unable to find a node that satisfies %s%s%s", constraint.key, OPERATORS[constraint.operator], constraint.value)
 		}
 		nodes = candidates

scheduler/filter/constraint_test.go

@@ -246,3 +246,35 @@ func TestUnsupportedOperators(t *testing.T) {
 	assert.Error(t, err)
 	assert.Len(t, result, 0)
 }
+
+func TestFilterSoftConstraint(t *testing.T) {
+	var (
+		f      = ConstraintFilter{}
+		nodes  = testFixtures()
+		result []cluster.Node
+		err    error
+	)
+
+	result, err = f.Filter(&dockerclient.ContainerConfig{Env: []string{"constraint:node==~node-1-name"}}, nodes)
+	assert.NoError(t, err)
+	assert.Len(t, result, 1)
+	assert.Equal(t, result[0], nodes[1])
+
+	result, err = f.Filter(&dockerclient.ContainerConfig{Env: []string{`constraint:name!=~/(?i)abc*/`}}, nodes)
+	assert.NoError(t, err)
+	assert.Len(t, result, 4)
+
+	// Check not with globber pattern
+	result, err = f.Filter(&dockerclient.ContainerConfig{Env: []string{"constraint:region!=~us*"}}, nodes)
+	assert.NoError(t, err)
+	assert.Len(t, result, 2)
+
+	result, err = f.Filter(&dockerclient.ContainerConfig{Env: []string{"constraint:region!=~can*"}}, nodes)
+	assert.NoError(t, err)
+	assert.Len(t, result, 4)
+
+	// Check matching
+	result, err = f.Filter(&dockerclient.ContainerConfig{Env: []string{"constraint:region==~us~"}}, nodes)
+	assert.Error(t, err)
+	assert.Len(t, result, 0)
+}