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Software Stability | Gigavoice Blog

Software Stability

Failures are inevitable.

The system can fail on a small or large scale many times in a row. What is important is that we make a system that will only fail on a small scale. Next it is very important how fast the system will recover and continue to work. The era of monolithic applications is over. Modern software applications has N Tires, meaning there are several pieces of code split across servers. They communicate between tires, and very often they use external services. That makes our system dependent on other services, and we must not let failures on other dependent services to make our system fail. here you will find short explanation on how to improve the stability of your system, and also what to avoid in order to improve the system itself. The following is inspired from the book “Release it, design and deploy production ready software” by Michael T.Nygard.

Stability Patterns

Fail fast

Imagine a traffic light. What will happen if it fails and green light is shown to all parts involved. A lot of accidents of course. In order to prevent this, the system detects the unusual state (with Conflict Monitor Unit), and goes in fail fast mode, a yellow light will blink meaning there is an error, so the drivers will drive carefully.

Same for the software system. Fail-fast systems are usually designed to stop normal operation rather than attempt to continue a possibly flawed process. The main idea is if the problem occurs, the system fails immediately and visibly. Also will log the problem and possibly send the error by mail to notify someone. I will demonstrate this principle by a single example. Lets say we have a configuration file and a code that reads it.

&amp;lt;br /&amp;gt;&lt;br /&gt;<br />
string getConfigSensitiveData()&amp;lt;br /&amp;gt;&lt;br /&gt;<br />
{&amp;lt;br /&amp;gt;&lt;br /&gt;<br />
    string property = getProperty(“some_key”);&amp;lt;br /&amp;gt;&lt;br /&gt;<br />
    if (property == null)&amp;lt;br /&amp;gt;&lt;br /&gt;<br />
        return ‘some_default_value’;&amp;lt;/p&amp;gt;&lt;br /&gt;<br />
&amp;lt;p&amp;gt;    return property;&amp;lt;br /&amp;gt;&lt;br /&gt;<br />
}&amp;lt;br /&amp;gt;&lt;br /&gt;<br />

The code seems fine. Technically nothing is wrong. But imagine there is a system update. There is typo in the key of the configuration file. You make the update. Everything works. But the customer notices unusual behaviour. You spend a lot of hours to find why the system has different behaviour tham the one the customer expects. Then you find out that the system reads default value instead of the value that customer has defined. Instead of leaving the system to work, just make it to fail right away, visibly, so you will detect the problem and fix it.

&amp;lt;br /&amp;gt;&lt;br /&gt;<br />
string getConfigSensitiveData()&amp;lt;br /&amp;gt;&lt;br /&gt;<br />
{&amp;lt;br /&amp;gt;&lt;br /&gt;<br />
    string property = getProperty(“some_key”);&amp;lt;br /&amp;gt;&lt;br /&gt;<br />
    if (property == null)&amp;lt;br /&amp;gt;&lt;br /&gt;<br />
       throw new Exception(“The setting some_key cannot be null”)&amp;lt;/p&amp;gt;&lt;br /&gt;<br />
&amp;lt;p&amp;gt;    return property;&amp;lt;br /&amp;gt;&lt;br /&gt;<br />
}

Simmilar problem happened to me. Instead of throwing an exception I initialized a collection to empty collection instead of setting some values, and one month later I received a bug that something strange is going on. Then I found out that the collection was initialized to default value instead of the real value, never got an error but got strange behavior. I should of throw an exception right away.

Timeouts

Now and forever, networks will always be unreliable. If you have sent request to get some data, no matter the system, only wait for a specific time and then timeout. Otherwise your resources will be hanging for a period of time, and your system will becoming slower and even may stop responding. This is very important if you use external services together with your system. So keep in mind, when communicating to external services, always define reasonable timeouts.

Circuit breaker

Once of a lifetime everyone has witnessed at home that the electrical circuit breaker has triggered (of burned fuse in older houses). The purpose of the electrical circuit breaker is to protect the electrical installation from burning, thus protecting the house also from burning. If there was no fuse or circuit breaker then the installation would burn and cut the current. But the software is different. There is no physical burning, but instead the software system will continue failing. So the circuit breaker pattern is used to detect failures and encapsulates logic of preventing a failure to reoccur constantly. You wrap a protected function call in a circuit breaker object, which monitors for failures. Once the failures reach a certain threshold, the circuit breaker trips, and all further calls to the circuit breaker return with an error, without the protected call being made at all. You may also want to set up some kind of monitor alert in case the circuit breaker trips. For more info please see reference.

Bulk heads

Will the ship definitely sink if it is damaged? Probably not if it has bulkheads. Bulkheads are metal partitions of the bottom of the ship they are sealed. So if you damage a small section of a boat, then the water will enter only in few bulkheads that are sealed, so the ship will not sink. Similarly in the software system, we can use virtualization to split the system into several compartments. Then if one virtual server goes down, the system will continue to work.

Steady state

System should run indefinitely in time without intervention. Steady state pattern says that for every mechanism that accumulates resources, there should be another mechanism that releases those resources. We are not talking about the growth of data in our database, but also databases can be purging from the unnecessary/outdated data. One example is log files. Do not leave all log files on production server. Copy them somewhere else. Also check what resources are occupied from your app, and release them if possible by cronjob or similar. When using cache, introduce expiration.

Health checks

For external services consider health checks. Check the services periodically. If they are not available do something, probably use circuit break pattern.

AntiPatterns

Accelerate cracks in the system. Avoiding them does not mean that crashes and mistakes will not happen, but it can help minimizing the damage.

Big Cookies

The primary purpose of the cookies is session management. Beside session management, you can use them for tracking some minor informations. But not for data storage. For example, some developers use them to store shopping cart or other data. There are many problems with this approach, starting from performance issues, ending with security issues. Simple rule. Only put minimal non-sensitive amount of data there.

Big Sessions

The idea is great. Keep everything in session, so you will not need to query the database. Yes, it is great if you have endless RAM memory. But in real case scenario the memory is not endless. If your system has few users at a time, no problem, but we are not talking about that kind of systems here. If session is kept in memory, you will reach the memory limit. If it is stored on disk or database, you have another problem. Serialization and de-serialization of big data. That will take certain time and resources. In both cases you have a problem. So keep the session minimal.

Ajax Overkill

Ajax is a technique, not a goal. The user experience should be primary. Apply Ajax only where needed. Ajax means more requests coming from the browser to the web server. Depends how heavy we use Ajax, we can reach the capacity of the system with just a small number of user requests multiplied by the number of Ajax calls. So, minimize ajax requests, and use them only where is really necessary. Keep in mind not to create too many Ajax requests from a single user because you will decrease the capacity of the system.

Handcrafted SQl

If you use some kind of ORM, like Hibernate or Doctrine, they will generate sql for you. They follow predictable patterns. So the DBA admins can follow the pattern and optimize the queries for the whole system. If you do a lot of handcrafted sql, they are unpredictable. So the admin may have difficulty optimizing the system because each manual query is unique, so applying rules for that query does not mean that also other queries will be optimized.

Reference

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