Troubleshooting Practices

Troubleshooting is important and inevitable for engineers when we try to figure out why the code doesn't work; it also improves our problem solving skills: what steps to take to quickly find the root cause.

This articles is to reflect the process that I took to find the root cause.

Can't deploy Spring-boot application to tomcat
- SpringBootServletInitializer.configure is not called at all

During dev, the team is running spring-boot application as java application. But later when we deploy to the cloud, we need run it as web application. The developer tried it but doesn't work.

We found out it's because the dev specifies 2.5 in web.xml unintentionally and Spring-boot requires Servlet 3.0 by default.
http://docs.spring.io/spring-boot/docs/current/reference/html/howto-traditional-deployment.html

jackson-module-scala: NoSuchMethodError due to intellij
Recently when I ran tests in intellij, it throws the following exception:
java.lang.NoSuchMethodError: scala.util.matching.Regex.unapplySeq(Ljava/lang/CharSequence;)Lscala/Option;
at com.fasterxml.jackson.module.scala.JacksonModule$.version$lzycompute(JacksonModule.scala:30)
at com.fasterxml.jackson.module.scala.JacksonModule$.version(JacksonModule.scala:26)
at com.fasterxml.jackson.module.scala.JacksonModule$class.version(JacksonModule.scala:49)
at com.fasterxml.jackson.module.scala.DefaultScalaModule.version(DefaultScalaModule.scala:19)
at com.fasterxml.jackson.databind.ObjectMapper.registerModule(ObjectMapper.java:701)

Seems it's caused by jackson-module-scala, but I use mvn dependency:tree and dependency:copy-dependencies to check all thing looks good: we use scala-2.11 and jackson-module-scala_2.11. Also if I ran the test in command line, it works.

This made me think maybe this is because of intellij. 
I added a breakpoint and run: scala.util.Properties.versionString in debug mode, it's 2.10.6. This is strange. Then I check external libraries in intellij, there are indeed multiple scala-library versions: 2.10.6 and 2.11.8.

In our current projects, we don't use scala-library-2.10.6 anywhere. Maybe previously we use scala 2.10.6, later we upgraded to 2.11. But intellij still keeps reference to old scala in external libraries.
- no effects when reimport all maven projects

The workaround is to delete .idea/libraries internals and restart IntelliJ and reimport projects.

ClassNotFoundException: StandardSessionIdGenerator because we overwrite tomcat.version
- Asked by team mate to help solve this issue
Usually this is because of version conflict. We use spring-boot 1.5, but the tomcat-embed-core is 7.0.65. This looks suspicious. Then I checked the pom.xml. This makes me check how spring-boot specifies its version. search tomcat-embed-core in https://github.com/spring-projects, found 
spring-boot-starters/spring-boot-starter-tomcat/pom.xml and spring-boot-dependencies/pom.xml:
<tomcat.version>8.5.6</tomcat.version>

But we also defined same property: tomcat.version to 7.0.50 in project's pom.xml for some tomcat dependencies we used. This overwrites spring-boot property. So now spring-boot use tomcat-embed-core 7.0.65 which doesn't have StandardSessionIdGenerator class.

The fix is to change our own property to xx. tomcat.version and also bumped its version to same as spring-boot.
- spring-boot should add prefix such as spring.boot to these properties.

Find code by running sample code and debugging it
Build Web Service APIs to Update Solr's Managed Resources (stop words, synonyms)
I need figure out how to get address of one of live solr nodes by using CloudSolrClient.  I know solrJ uses apache http client to send requests, somewhere it needs get addresses of solr nodes, so I write some sample code which sends invalid request, solrJ will throw exception, then I check the stack trace form the log and find out the code that I need at org.apache.solr.client.solrj.impl.CloudSolrClient.sendRequest(SolrRequest, String).
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