Community Blog A Must-Have for Emergency Handling: Troubleshooting and System Optimization Manual

A Must-Have for Emergency Handling: Troubleshooting and System Optimization Manual

In this article, Chuheng shares the common issues, processes, and tools for server troubleshooting, with reference to actual projects.


By Chuheng

There is a common understanding in the software engineering field that programmers spend more time maintaining existing code than they do writing code. It is also understood that the most challenging part in the whole process of code maintenance is troubleshooting.

Frontline server programmers who maintain online services on a 24/7 basis often tackle small and large problems online. If they are not careful enough, a small problem may lead to many other large problems. In this article, I will share my experience in server troubleshooting in terms of common issues, troubleshooting processes, and troubleshooting tools, with reference to some painful cases in actual projects.


Common Problems

Know Your Enemy: Know yourself, know your enemy, and you shall win a hundred battles without loss.

In my experience, most problems fall into the following categories:

  • Logical defects: Null Pointer Exception (NPE), infinite loop, and boundary condition not covered.
  • Performance bottleneck: sharp increase in the API response time (RT) but low throughput.
  • Memory exceptions: serious lag caused by garbage collection (GC), frequent full garbage collection (FGC), memory leakage, and out of memory (OOM).
  • Concurrent/distributed system: race conditions and clock out-of-synchronization.
  • Data issues: dirty data and serialization failure.
  • Security risks: distributed denial of service (DDoS) attack and data breach.
  • Environmental failure: host shutdown, network disconnection, and packet loss.
  • Operational errors: configuration push error and library deletion (Risky action. Do not try this.).

The preceding categories may not be complete or precise. You should create your own checklist. When you encounter a problem and cannot figure out a solution, go through the checklist and you may find the answer.

Troubleshooting Process


Quickly Stop the Bleeding

The first step in troubleshooting is to stop the bleeding as soon as you can. How can I quickly stop the bleeding? In other words, how can I prevent a problem from worsening during the troubleshooting process? The following lists some common methods:

  • What can I do if an error is reported during the release but everything is normal before the release? Roll back the system first and then consider troubleshooting.
  • What can I do if an application has been running stably for a long time but suddenly exits? It is very likely that a memory leakage occurred. Restart the server.
  • What can I do if errors occur only on a few specific machines? Try to isolate these machines (disable the incoming traffic).
  • What can I do if the service becomes unstable due to traffic spikes of single users? For common users, apply traffic limit rules.
  • What can I do if the service breaks down due to failure of the downstream dependency? Follow the downgrade plan.

Preserve the Scene

Is the bleeding stopped? Have you stopped the problem from worsening? If yes, congratulations. At least things will not get worse. Take a break. The next step is to find out the root cause of the problem based on the clues. A programmer experienced in troubleshooting should have the awareness to try to preserve the scene, for example:

  • Isolate one or two machines: Disable the incoming traffic of these machines for inspection.
  • Dump application snapshots: The common snapshot types are thread stacks and heap memory mappings.
  • What can I do if all the machines have been rolled back? Take it easy. If your application monitoring O&M system is robust enough, you can trace back multi-dimensional historical data such as application logs, middleware logs, GC logs, kernel logs, and metrics.

Identify the Cause

How can I find the specific cause by using historical data? This step will test your technical understanding, business familiarity, and practical operation experience. There could be many causes of a problem. You need to track and analyze the causes case by case. The following are several suggestions for troubleshooting:

  • Review recent changes: More than 90% of online problems are caused by changes. That is why Alibaba Group focuses on safe production and change control. Don't rush to deny that the error is caused by the code you just added. Believe in statistical probability and review the recent change history (start from the most recent change).
  • Track and analyze the entire process: As microservices and mid-end architectures are so popular, a Java business request is usually processed by many applications. Therefore, you need to expand the troubleshooting scope to the entire business process instead of just focusing on your own application.
  • Review the event timeline: Imagine yourself as a detective, Holmes or Conan, and you are faced with a crime scene. You need to link all the event clues at different time points to rebuild and review the entire crime process. Believe that timestamps do not lie.
  • Find the root cause: As you gain experience in troubleshooting, you will find that many suspected causes are often just the result of another deeper cause. As a detective, what you need to find most of all is the murderer behind the scene, not the hired murderer. Otherwise, another crime would occur.
  • Try to reproduce the problem: After you find the root cause, do not start fixing the bug in a hurry. If possible, reproduce the problem to make the root cause more convincing. However, never do this in the production environment (unless you really know what you are doing). Otherwise, you will make the users experience the problem again.

Resolve the Problem

Since the root cause has been identified, how can I resolve the problem perfectly? Follow several basic principles here:

  • The fix is also a change that requires complete regression testing and gray release. Do not fix the bug and put the system into production in such a hurry that more bugs are caused.
  • After the repaired version is released, be sure to perform online verification and keep observing the system for a period of time to make sure the bug is fixed successfully.
  • If the problem has worsened to a failure, call a retrospective meeting. There must be room for improvement in the whole process. Your experience and lessons are also a good input and self-examination opportunity for other colleagues.

Troubleshooting Tools

If all you have is a hammer, everything looks like a nail. As an engineer, you need a complete toolkit.



Troubleshooting is a process of continuously observing application behavior. To preserve key details, you need to make your application more observable.

There are three powerful tools for improving the observability of applications: logging, monitoring, and tracing. In my previous projects, logging was provided by Log Service (SLS), monitoring was provided by AliMonitor/AliMetrics/tsar, and tracing was provided by Eagle Eye. These are not described here in detail.

We also recommend that you use the Arthas tool, which is very practical and easy to use. I believe many of you have already used it.

System Optimization

It is not enough to learn how to troubleshoot problems. To be skilled in troubleshooting is only a temporary solution and cannot resolve the root cause. To avoid problems from the root, you need to start from the system itself: continue to optimize your system implementation from three aspects (performance, stability, and maintainability) and nip the problems in the bud.

Boss: "The system must be fast, stable, and excellent in performance. By the way, don't worry about the salary. You will definitely get it next month."


System optimization mainly focuses on three aspects: performance, stability, and maintainability. The three aspects are not completely independent. Instead, they act upon each other in a complex way.

The best software system does not require high performance, high stability, and high maintainability at the same time. The most important thing is to make reasonable trade-offs based on the actual business needs and scenarios and reach an optimal dynamic balance between the three aspects.

Therefore, optimization is not only a science, but also an art.

Performance Optimization

Q: In auto racing, which one is more important? The driver or the car?

A: Both are equally important.


No one dislikes high-performance sports cars or wants to suddenly get stuck while watching the live broadcast of an internet influencer.

Performance is the ultimate goal pursued by engineers from all walks of life.

Performance Indicators

An indicator is a scientific and quantitative means for measuring the quality of an object. The following indicators are generally used to evaluate performance:

  • Throughput: the workloads that a system can bear per unit time, for example, Queries Per Second (QPS)/Transactions Per Second (TPS) (for online web systems) and data volume that can be processed per second (for offline data analysis systems).
  • Response time (RT) : the time interval between sending a request to a service and receiving the first response from the service during web request processing. It generally consists of the network transmission latency, queuing latency, and actual processing time.
  • Scalability: the system's capability of bearing more workloads as machine resources are increased (vertically or horizontally). The higher the input-output ratio (linear scaling in an ideal situation), the better the system scalability.

In addition, the throughput and RT of the same system generally have the following correlations: When the throughput is less than a critical value, the RT is almost unchanged. Once the throughput exceeds the critical value, the system will enter an overloaded state, and the RT begins to increase linearly. For a system with stability requirements, you need to fully consider the critical value during performance stress testing and capacity planning.

Note: To put it in a more rigorous way, performance indicates only how "fast" a system is. Some of the preceding indicators are closely related to the system's operating speed, yet do not merely represent the operating speed.

Performance Analysis

As an old saying goes, if you cannot measure it, you cannot improve it.

To optimize the performance of a system (for example, the RT of web requests), you need to first accurately measure and analyze the performance of the current system and figure out the causes of poor performance: Is the request parsing or database query too slow? If the database query is too slow, is it too slow to scan the data entries or return the result sets? Or is the network latency between the application and database too long?

The handling process of any complex request can be split into a series of parallel or serial atomic operations. If you optimize the atomic operations at will, obviously, the efficiency will not be too high unless you are lucky. A more reasonable approach is to adhere to the 2/8 principle: Preferentially analyze and optimize system bottlenecks, that is, atomic operations with the greatest impact on system performance at present. They are likely to be the optimization points with the highest return on investment (ROI).

How exactly can I quantify and analyze performance? Here are some tools for your reference:

  • System level: tsar, top, iostat, and vmstat
  • Network level: iftop, tcpdump, and Wireshark
  • Database level: SQL explain and CloudDBA
  • Application code level: JProfiler, Arthas, and jstack

Many of these tools are also common troubleshooting tools. After all, both performance analysis and diagnostic analysis are intended to help you understand a system and the environment where the system resides. The operations to be performed are similar.

Optimization Principles

I have already mentioned a lot above, and here is another point. Like planning for functional requirements, performance optimization is also oriented at business. Therefore, during performance optimization, you should accommodate the target requirements and application scenarios in case that the optimization is not needed in the production environment. Moreover, you can actually customize some complex optimizations based on traffic characteristics, instead of making general optimizations.

You should not make premature optimization or over optimization. In general, performance optimization is not a free lunch. The more optimizations you do, the worse the maintainability.

Optimization Methods

What are the common methods for performance optimization? I have summarized eight methods here. The last one is an all-in-one method.

(1) Simplification

Simply put, you can omit some operations.

  • Business level: Streamline the process and simplify requirements.
  • Coding level: Reduce operations with high overheads in a loop.
  • Architecture level: Omit unnecessary abstraction/layering.
  • Data level: Cleanse, extract, and aggregate data.

(2) Parallel processing

Simply put, you can perform some operations together with other people.

Mode: parallel processing in standalone (multithreaded) or multi-node (distributed) mode.

Advantage: Machine resources (multiple cores and clusters) are fully utilized.

Disadvantage: synchronization overhead, thread overhead, and data skew.

  • Synchronization optimization: optimistic locking, fine-grained locking, and no locking.
  • Thread replacement (such as coroutine): Java WISP, goroutine, and Kotlin coroutine.
  • Data skew: load balancing (hash/RR/dynamic).

(3) Asynchronization

Simply put, you can ignore some operations without waiting.

Mode: message queue + task thread + notification mechanism.

Advantage: The throughput, component decoupling, and load shifting are improved.

Disadvantage: queuing delay (queue backlog).

Excessive backlog avoided: back-pressure (reactive thinking).

(4) Batch processing

Simply put, you can combine some operations.

Mode: multiple single operations combined into a single batch operation.

Example: TCP Nagel algorithm and batch read/write API of a database.

Advantage: Inherent overheads of single operations are avoided, reducing the total overheads.

Disadvantage: waiting delay + aggregation delay.

  • Waiting delay reduced: Batch processing is triggered by a timeout, to control the upper limit of the delay.

(5) Choose between time and space

You have to choose between time and space.

Exchange space for time: Avoids repeated computation, shortens the transmission distance, and reduces pressure through offloading.

  • Example: cache, CDN, index, and read-only replica.

Exchange time for space: This sometimes can make the system faster. For example, the data volume is reduced, and therefore the data transmission time is shortened.

  • Example: data compression (HTTP/2 header compression and bitmap).

(6) Data structure and algorithm optimization

Program = Data structure + Algorithm

  • Learn more about "less popular" data structures such as Skip List, Bloom Filter, and Time Wheel.
  • Know some "simple" algorithm techniques such as recursion, grouping, greedy, and dynamic programming.

(7) Pooling & localization

Sharing economy & community supermarket

Pooling: Reduces resource creation and deletion overheads.

  • Example: thread pool, memory pool, database connection pool, and socket connection pool.

Localization: Avoids overheads resulting from contention for shared resources.

  • Example: Thread Local Buffer (TLB) and multi-level caches (local cache and shared global cache).

(8) More optimization methods

  • Upgrade: kernel, Java runtime environment (JRE), dependency library, and protocol.
  • Parameter tuning: configuration, Java Virtual Machine (JVM), kernel, and network adapter.
  • SQL optimization: index, SELECT *, and LIMIT 1.
  • Custom optimization based on business characteristics: log rotation in off-peak hours of the morning.
  • Hybrid techniques: JDK sort() and Weex/RN.

Stability Optimization

"Hold on. We can win," a game player.

How to maintain stability is a major issue that programmers think and discuss every day.

What kind of system is stable? Is a tool that has never failed while running locally stable? Taobao is maintained by thousands of personnel. However, users often fail to place orders in the Double 11 Shopping Festival. So, is it unstable?

Stability is a relative concept. As the business scale expands and the scenarios become more complex, the system is more likely to become unstable, and the resulting impact becomes more severe.


Measurement Metrics

Different businesses provide different types of services. How can I measure the system stability with the same metrics? The standard practice is to define the service availability. Provided that the service is "available" to users, the system is currently stable; otherwise, it is unstable. In this way, after collecting and aggregating data, you can obtain the service availability ratio based on the service duration or number of service times, to monitor and quantify the stability of a system.

However, what can we use to define whether a service is currently available? This is related to specific businesses, but most businesses of the same type can be defined in a similar way. For example, if all API requests are responded to successfully and the total page loading time is less than 3s, a general website can be defined as available.

For cloud products provided by Alibaba Cloud, service availability must be especially valued and continuously improved. Many users on Alibaba Cloud use multiple cloud products at the same time, and an availability issue of any product will be directly perceived and magnified by users. Therefore, the underlying infrastructure has higher availability requirements. For more information about availability metrics and concepts such as the service level indicator (SLI), service level objective (SLO), and service level agreement (SLA), see the Cloud Intelligence SLA.

Availability Measurement

After defining the availability metrics, how can we accurately measure the availability of a system? In general, there are two ways.

(1) Probe simulation

Simulate users' calls on a client.

  • Advantage: real data based on the client.
  • Disadvantage: incomplete data of a single customer.

(2) Data collection on a server

Analyze logs and data directly on a server.

  • Advantage: All call data is covered.
  • Disadvantage: Client process data is missing.

You can use both methods in a system requiring high availability. We recommend that you choose a suitable method based on your business scenarios.

Optimization Principles

You should focus on the RT data distribution (for example, quantiles p50, p99, and p999) rather than the mean. In other words, you should care more about the feelings of 1% or 0.1% of your users.

You should not promise or attempt to optimize the availability to 100%. On the one hand, this is impossible because there are too many objective and uncontrollable factors. On the other hand, this is meaningless because customers can hardly discern the difference of 0.001% in availability.

Optimization Methods

What are the common methods for stability optimization? I have also summarized eight methods here.

(1) Avoiding single points of failure

It is like your parents telling you that it is time for you to settle down with someone.

What can I do to avoid single points of failure?

  • Cluster deployment
  • Data replica
  • Multi-data-center disaster recovery

Redundancy deployment is not enough. A failover capability is also required.

  • Access layer: DNS, VipServer, and SLB.
  • Service layer: service discovery, health check, and removal.
  • Application layer: stateless design, which allows fast failover at any time.

(2) Traffic control/throttling

Family planning, school adjustment, license plate number restriction, and scenic area restriction. Control is everywhere in our daily life.

  • Type: QPS throttling and concurrency throttling.
  • Tool: RateLimiter, semaphore, and Sentinel.
  • Granularity: global, user-level, and API-level.
  • Hotspot throttling: Avoids unexpected traffic bursts.

(3) Circuit breaker mechanism

Imagine a case where a circuit breaker is triggered in your stocks in the morning and your fuse at home is blown at night. Calm down. It's just for stopping the loss as soon as possible.

  • Purpose: Prevents cascading failures (avalanche effect).
  • Tool: Hystrix, Failsafe, and Resilience4j.
  • Functionality: Automatically bypasses abnormal services and detects the recovery status.
  • Process: close → open → half open.

(4) Downgrade

It's like you ordering takeaway food when you have no time to cook. For the sake of health, such downgrade is not preferred.

Cause: throttling, circuit breaker mechanism, or overload.

Common downgrade methods:

  • Disable non-core functionalities: Stop recording application logs.
  • Sacrifice data timeliness: Return old data in the cache.
  • Sacrifice data accuracy: Reduce the data sampling frequency.

(5) Timeout/retry

What can I do if I get no reply on DingTalk? Ping the target user every 10 minutes, and call the target user if you get no reply in one hour.

Timeout: Avoids permanent blocking on the caller.

  • Timeout period settings: end-to-end planning in the whole process.
  • Timeout vs. Deadline: absolute time preferred.

Retry: Ensures the idempotence of retries.

  • Message deduplication
  • Asynchronous retry
  • Exponential backoff

(6) Resource limit

What can I do to prevent my girlfriend from spending too much money during a massive online promotion? Lower your credit card limit in advance.

  • Purpose: Prevents resources from being exhausted by abnormal traffic.
  • Resource type: thread, queue, and database connection.
  • Restriction method: resource pooling and bounded queue.
  • Processing logic if the limit is exceeded: Return ServiceUnavailable or QuotaExceeded.

(7) Resource isolation

What if my girlfriend has a long shopping list for a massive online promotion? Probably, you can ask your girlfriend to use her own credit card instead of yours.

  • Purpose: Prevents resources from being exhausted by abnormal traffic and provides quality of service (QoS) assurance for VIP customers.
  • Isolation method: queue division and independent cluster. Pay attention to the processing priority and resource allocation proportion.

(8) Safe production

What if my girlfriend begs for my credit card for shopping? You'd better still say no to her.

Program dynamics: switch, configuration, and hot update.

  • Switch: safe and less intrusive.

Review mechanism: code review and release approval.

Gray release: batch deployment and rollback plan.

  • DUCT: Automatically or manually adjusts the weights of high-speed service framework (HSF) nodes.

Maintainability Optimization

If ancestors plant trees, descendants enjoy the shade.

If ancestors dig pits, descendants just have holes.

Maintenance also means preservation and supply. So, how important can software maintenance be? It acts as the lungs and mouth of the software system, and is necessary to maintain the life of the software.

The process of developing and releasing a software system is to "give birth to" it. How much value the software system can bring depends on the process after delivery. Does it thrive continuously and benefit users? Or does it gradually fade out and get abandoned by users? This does not depend on the short-term performance or stability of the system but depends on the future. It depends on whether the system can preserve good performance and stability in the changing market environment where customer needs and human factors are constantly changing as well.

Compared with performance and stability, maintainability creates the most long-term value, which is the most difficult to create in the short term. Therefore, maintainability is sacrificed in the early stage of many software projects. The consequences of such decisions normally cannot be remedied (or can be remedied with high costs), similar to those in architecture design. Many software projects are becoming increasingly unmaintainable (little space for code changes, numerous bugs to fix, and little space for adding features). They will eventually be abandoned.

Measurement Metrics

Compared with performance and stability, maintainability is more difficult to quantify because the artistic part is more emphasized than the scientific part. Here, I have selected the following metrics for qualitative analysis:

(1) Complexity: Is the complexity controllable?

  • Coding: simplicity, naming consistency, and number of code lines.
  • Architecture: component coupling, hierarchy clarity, and single responsibility.

(2) Extensibility: Is it easy to change?

  • This means whether it is easy to change code and configuration without mistakes.

(3) Operability: Is it convenient to operate and maintain?

  • This means whether logging and monitoring are properly enabled, and whether deployment and scaling are easy.


I will further emphasize the importance of maintainability from the following aspects.

  • Software lifecycle: The maintenance cycle is much longer than the development cycle.
  • Broken window effect and law of entropy increase: Maintainability tends to worsen.
  • Hazards of legacy systems: Legacy systems are difficult to understand and costly to change, with high change risks. You may have to repeat the process of fixing bugs, introducing new bugs, and fixing bugs again.

Optimization Principles

You should follow the KISS, DRY, code readability, and architecture design principles.

You should not introduce too much temporary and hack code, or focus only on the functionality without overall considerations.

Optimization Methods

What are the common methods for maintainability optimization? I have summarized four methods here.

(1) Coding specifications

Nothing can be accomplished without norms or standards.

  • Coding specifications: Java Development Manual and The Art of Readable Code.
  • Logs: no blind spots, no redundancy, and with the trace ID.
  • Testing: code coverage and automatic regression.

(2) Code refactoring

Do not give up. The code can be saved.

  • Refactoring timing: any time when you find a code bug.
  • Refactoring method: stepwise iteration and regression verification.
  • Refactoring or rewriting: depending on the costs, risks, concurrent version maintenance, and other factors.
  • Reference: Refactoring: Improving the Design of Existing Code.

(3) Driving force of data

Believe in the power of data.

  • System data: Monitor the system and collect system metrics, which is essential for understanding the system and troubleshooting.
  • Business data: Check data consistency and clear historical data. Believe that data tends to be more persistent than code.

(4) Technical evolution

Technology is the primary productive force.

  • Holding position or keeping up with the trend: depending on the risks, productivity, and learning costs.
  • Current direction: microservice- and container-based transformation.
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Adrian Peng

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