对 issue 进行合理的分类整理这件事情本身需要一定的经验，这篇 paper 对 issue 进行了多维度分类， 非常有参考价值。另外，paper 对 issue 出现的原因进行了统计学的分析，并给出他们自己的一些猜测， 这个统计结果对我们找 bug、设计测试用例具有指导性的意义。
读完整篇 paper，有利于拓展我们的思维，形成一些方法论，尤其是我们在 review bug 的时候：
- bug 出现原因，原因的原因
- hang 住
- data race
- bug 影响范围、类型
- bug 所属组件
一篇 paper 所能提供的内容总是有限的，我认为这篇 paper 主要精力还是花在统计和分类上， 但是从这个统计分类结果中，我们能领悟什么、学习到什么、总结出什么结论，就看我们自己的造化了。
摘要：我们回顾了 6 个的云系统(cloud system)（Hadoop MapReduce, HDFS, HBase, Cassandra, ZooKeeper） 三年间出现的 21399 个 issue。从这里面，选出了 3655 个重要的(vital) issues 并进行了分类， 并把结果整理成一个数据库
Cloud Bug Study database (CbsDB)。
cloud system 发展迅速，并且越来越重要。但这些系统都存在 bug，它们的稳定性还有很高的提升空间。
This paper was started with a simple question: why are cloud systems not 100% dependable? In our attempt to pro- vide an intelligent answer, the question has led us to many more intricate questions. Why is it hard to develop a fully reliable cloud systems? What bugs “live” in cloud systems? How should we properly classify bugs in cloud systems? Are there new classes of bugs unique to cloud systems? What types of bugs can only be found in deployment? Why existing tools (unit tests, model checkers, etc.) cannot capture those bugs prior to deployment? And finally, how should cloud dependability tools evolve in the near future?
为了定位上面这些问题，网上的一些博客并不足够，但好在开源系统的 issue 是公开的。
Cloud Bug Study
这篇论文展示了我们一年内对 6 个系统的 issues 研究成果。
总共有 2 万多个 issue，我们选了 3655 个重要的，类似代码重构、单元测试、文档内的 issue 都属于不重要的类型，不在他们的研究范围内。
对于每个重要的 issue，我们分析了 patch 和开发者对 issue 的回复。他们对 issue 进行了多种分类。
First, we categorize them by aspect such as reliability, performance, availability, security, data consistency, scalability, topology and QoS. Second, we introduce hardware type and failure mode labels for issues that involve hardware failures. Next, we dissect vital issues by a wide range of software bug types such as error handling, optimization, configuration, data race, hang, space, load and logic bugs. Fourth, we also study the issues by im- plication such as failed operations, performance problems, component downtimes, data loss, staleness, and corruption. In addition to all of these, we also add bug scope labels to measure bug impacts (a single machine, multiple machines, or the whole cluster).
New bugs on the block：如果按照 aspect 来分，可靠性占 45%，性能占 22%， 可用性占 16%，这三类占了绝大部分。除此以外，一些 cloud system 独有的 bug： 数据一致性 5%，可扩展性（scalability）2%，拓扑 1%。
Killer bugs：139 个。Their existence (139 issues in our study) implies that cascades of failures happen in subtle ways and the no single point of failure principle is not always upheld.
“killer bugs” (i.e., bugs that simultaneously affect multiple nodes or the entire cluster).
single point of failure: 单点故障。
Hardware can fail, but handling is not easy: Dealing with hardware failures at the software level remains a challenge
非常有意思的一个结论，这大概也是 Chaos Engineering 的意义所在吧。
Vexing software bugs: Cloud systems face a variety of software bugs: logic-specific (29%), error handling (18%), optimization (15%), configuration (14%), data race (12%), hang (4%), space (4%) and load (4%) issues.
逻辑错误排第一，这和我自己这一个月的工作感受也是相符的。另外，我觉得这里一个有趣的点是作者把 error handling 单独拿出来（如果是我，我可能觉得错误处理也算是代码逻辑的一部分）， 除了这篇 paper，另外有篇 paper 专门说了 error handling 的地方是多么容易出现 bug。 这可能是我们可以注意的一个点。
Availability first, correctness second
The need for multi-dimensional dependability tools: 多维度 + 维度组合
In this section, we describe our methodology (方法论), specifically our choice of target systems, the base issue repositories, our issue classifications and the resulting database.
- Hadoop MapReduce representing distributed computing frameworks
- Hadoop File System (HDFS) representing scalable storage systems
- HBase and Cassandra representing distributed key-value stores (also known as NoSQL systems)
- ZooKeeper representing synchronization services
- Flume representing streaming systems
这 6 个系统确实很经典了，也很有代表性。
Issue Classifications 我们做的第一个分类是 issue type (“miscellaneous” vs. “vital”).
他们也说了不能用开发者打的标签，因为一些琐碎的 bug 也会被开发者打上重要的标签， 恩，我们也确实遇到了相似的问题。
我们仔细的阅读了每个(vital) issue，包括 patch 和讨论。 对于每个重要的 issue，我们都打上了 aspect 标签。如果某个 issue 涉及到硬件问题， 我们还会添加 hardware type 和 failure mode 的标签。接着，我们准确详尽的描述 了 software bug types. 最后，我们加上了 implication 标签。作为一个说明， 一个 issue 可以有多个标签。有趣的是，我们发现一个 bug 可以影响多台机器甚至整个集群。 为了这个目的，我们使用了 bug scope 标签。除了一般的分类，它们还加了 component 标签来标记 bug 属于哪个组件。
Cloud Bug Study DB
Threats to validity: 为了保证有效性，每个 issue 都最少有两个人达成一致意见。
Reliability, Performance, and Availability Aspects
Reliability (45%), performance (22%) and availability (16%) are the three largest categories of aspect.
Data Consistency Aspect
In reality, there are several cases (5%) where data consistency is violated and users get stale data or the system’s behavior becomes erratic. The root causes mainly come from logic bugs in operational protocols (43%), data races (29%) and failure handling problems (10%).
Buggy logic in operational protocols esides the main read/write protocols, many other operational protocols (e.g., bootstrap, cloning, fsck) touch and modify data, and bugs within them can cause data inconsistency.
Concurrency bugs and node failures Intra-node data races are a major culprit of data inconsistency. Inter-node data races (§6.3) are also a major root cause.
data race: 竞态条件
Summary: Operational protocols modify data replicas, but they tend to be less tested than the main protocols, and thus often carry data inconsistency bugs. We also find an interesting scenario: when cloud systems detect data inconsistencies (via some assertions), they often decide to continue running even with incorrectness and potential catastrophes in the fu- ture. Availability seems to be more important than consistency; perhaps, a downtime “looks worse” than running with incorrectness.
有点同感：辅助工具也是非常重要的，而这些工具往往不那么被重视。置于 availability > correctness 这一点， 在我们这边，感受不是很明显。
Scalability aspect accounts for 2% of cloud issues. Although the number is small, scalability issues are interesting because they are hard to find in small-scale testing. Within this category, software optimization (29%), space (21%) and load (21%) problems are the dominant root causes. In terms of implications, performance problems (52%), component downtimes (27%), and failed operations (16%) are dominant. To understand deeper the root problems, we categorize scalability issues into four axes of scale: cluster size, data size, load, and failure.
Scale of cluster size
Scale of data size
Scale of request load
Scale of failure
Summary: Scalability problems surface late in deployment, and this is undesirable because users are affected. More research is needed to unearth scalability problems prior to deployment. We also find that main read/write protocols tend to be robust as they are implicitly tested all the time by live user workloads. On the other hand, operational protocols (recovery, boot, etc.) often carry scalability bugs. Therefore, operational protocols need to be tested frequently at scale (e.g., with “live drills”). Generic solutions such as loose coupling, decentralization, batching and throttling are popular but sometimes they are not enough; some problems require fixing domain-specific algorithms.
In several cases (1%), protocols of cloud systems do not work properly on some network topology. We call this topology bugs; they are also intriguing as they are typically unseen in predeployment. Below we describe three matters that pertain to topology bugs.
a Cross-DC awareness
New layering architecture
总结: 用户期望系统在不同的拓扑结构下都能正常工作。拓扑结构相关的测试和验证仍然很少， 可靠性工具需要注意这方面。另外值得强调的是：拓扑架构调整后，一些受影响的逻辑往往不会及时调整。
This particular study is important because although the “no-SPoF” principle has been preached extensively within the cloud community, our study reveals that SPoF still exists in many forms, which we present below.
Positive feedback loop
Repeated bugs after failover
A small window of SPoF
Buggy start-up code
Scalability and QoS bugs
Summary: The concept of no-SPoF is not just about a simple failover. Our study reveals many forms of killer bugs that can cripple an entire cluster (potentially hundreds or thousands of nodes). Killer bugs should receive more attention in both offline testing and online failure management.
另外，论文里面有张热力图，告诉我们导致 Killer Bugs 的原因占比，我概括一下，大概是：
- 硬件问题（Node, Net, Disk）里面 Node 占最大比 17/20
- 硬件失败（Stop, Limp, Corrupt）模式，Stop 占最大 18/20
- 软件问题（Logic, Err-handling, Load, Hang, Config, Race, Space, Opt)， logic 占最多 1/2
corruption 对应数据损坏，checksum 不 match。
Summary: As hardware can fail, “reliability must come from software” . Unfortunately, dealing with hardware failures is not trivial. Often, cloud systems focus on “what” can fail but not so much on the scale or the “when” (e.g., subsequent failures during recovery). Beyond the failstop model, cloud systems must also address other failure modes such as corruption and limpware. Reliability from software becomes more challenging.
不能分类的 bug 分为 logic bug，可以分的如下
Both hardware and software can fail, and thus error-handling code is a necessity. Unfortunately, it is a general knowledge that error-handling code introduces complexity and is prone to bugs. In our study, error-handling bugs are the 2nd largest category (18%) after logic bugs and can lead to all kinds of implications. Below, we break down error-handling bugs into three classes of problems.
感觉 golang 有点机智。
大概是说：单机 data race 很多人研究了很多年，分布式 data race 研究仍待进行。
Silent hangs Timeouts
Overlooked limp mode
Big data cleanup
Beyond limit -> OOM, backlogs
Operational loads -> too many error log
Throughout previous sections, we have implicitly presented how hardware and software failures can lead to a wide range of implications, specifically failed operations (42%), performance problems (23%), downtimes (18%), data loss (7%), corruption (5%), and staleness (5%), as shown in Figure 4b.
In reverse, Figure 5 also depicts how almost every implication can be caused by all kinds of hardware and software faults. As an implication, if a system attempts to ensure reliability on just one axis (e.g., no data loss), the system must deploy all bugfinding tools that can catch all software fault types and ensure the correctness of all handlings of hardware failures. Building a highly dependable cloud system seems to be a distant goal.
Other Use Cases of CBSDB
- Longest time to resolve and most commented
- Top 1% or 10%
- Per-component analysis
- Bug evolution
We perform the first bug study of cloud systems. Our study brings new insights on some of the most vexing problems in cloud systems. We show a wide range of intricate bugs, many of which are unique to distributed cloud systems (e.g., scalability, topology, and killer bugs). We believe our work is timely especially because cloud systems are considerably still “young” in their development. We hope (and believe) that our findings and the future availability of CBSDB can be beneficial for the cloud research community in diverse areas as well as to cloud system developers.