public static Optional<RemovalRule> getRule(String key) {
        return Optional.ofNullable(RULE_MAP.getOrDefault(key, rule));
    }
@Test
    public void testRule() {
        Optional<RemovalRule> removalRuleOptional = RuleCache.getRule(KEY);
        Assert.assertTrue(removalRuleOptional.isPresent());
        RemovalRule rule = removalRuleOptional.get();
        Assert.assertEquals(RATE, rule.getErrorRate(), 0.0);
        Assert.assertEquals(rule.getScaleUpLimit(), SCALE, 0.0);
        Assert.assertEquals(rule.getMinInstanceNum(), NUM);
    }