Issue running 2 Solr Instances, Solr 7.2.1 port 8983 and Solr 6.6.2 port 8984

Question

I have both Solr 7.2.1 with port 8983 and Solr 6.6.2 with port 8984 on my machine. My 9.1 instance is using solr 7.2.1 and working fine. But my Solr 6.6.2 for Sitecore 9.0.2, is showing this error.

Server Error in '/' Application.

Data at the root level is invalid. Line 1, position 1.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.Xml.XmlException: Data at the root level is invalid. Line 1, position 1.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 


[XmlException: Data at the root level is invalid. Line 1, position 1.]
   System.Xml.XmlTextReaderImpl.Throw(Exception e) +91
   System.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace() +408
   System.Xml.XmlTextReaderImpl.ParseDocumentContent() +487
   System.Xml.Linq.XDocument.Load(XmlReader reader, LoadOptions options) +60
   System.Xml.Linq.XDocument.Parse(String text, LoadOptions options) +156
   SolrNet.Impl.SolrCoreAdmin.SendAndParseHeader(ISolrCommand cmd) +40
   Sitecore.ContentSearch.SolrProvider.SolrNetIntegration.SolrPinger.Ping() +32
   Sitecore.ContentSearch.SolrProvider.SolrConnectors.SolrConnector.TestConnection() +70
   Sitecore.ContentSearch.SolrProvider.SolrStatus.OkSolrStatus() +99
   Sitecore.ContentSearch.SolrProvider.SolrStatus..cctor() +72

[TypeInitializationException: The type initializer for 'Sitecore.ContentSearch.SolrProvider.SolrStatus' threw an exception.]
   Sitecore.ContentSearch.SolrProvider.SolrStatus.SetIndexForInitialization(ISearchIndex solrIndex) +0
   Sitecore.ContentSearch.SolrProvider.SolrSearchIndex.SetForReinitialization() +202
   Sitecore.ContentSearch.SolrProvider.SolrSearchIndex.InitializeSolr() +190
   Sitecore.ContentSearch.SolrProvider.SolrSearchIndex.Initialize() +46
   Sitecore.ContentSearch.SolrProvider.SolrContentSearchManager.Initialize() +169
   (Object , Object ) +9
   Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args) +484
   Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain, Boolean failIfNotExists) +236
   Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain) +22
   Sitecore.Nexus.Web.HttpModule.Application_Start() +161
   Sitecore.Nexus.Web.HttpModule.Init(HttpApplication app) +767
   System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers) +581
   System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context) +168
   System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context) +277
   System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext) +369

[HttpException (0x80004005): The type initializer for 'Sitecore.ContentSearch.SolrProvider.SolrStatus' threw an exception.]
   System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +532
   System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +111
   System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +714

Answer 1

Solr 6.6.2 and Solr 7.2.1 schema definitions are not compatible. There were some breaking changes introduced in Solr 7 -> more details here.

Second problem was that you were trying to run both Solr servers on same 8983 port. This is not supported. You need to have one on port 8983 (default) and the other on other port e.g. 8984. Or to distinguish them better you can use 8622 port for Solr 6.2.2 and 8721 port for Solr 7.2.1

I recommend to have both Solr versions installed on your machine and create for each separate Windows service also to run it.

Then for each Sitecore version you will configure different Solr server: - For Sitecore 9 Update 2 instance you will use Solr 6.6.2 - For Sitecore 9.1 instance you will use Solr 7.2.1

This is perfectly fine setup and shouldn't be a problem to configure it with below scripts.

Jeremy Davis created really nice low effort Powershell script to install, configure SSL and create also windows service for Solr. You can find it here -> https://jermdavis.wordpress.com/2017/10/30/low-effort-solr-installs/ . By default it will install Solr 6.6.2 but you can easily modified that script to install also 7.2.1 version. I have already prepared that kind of script (for Solr 7.2.1 for Sitecore 9.1) here -> https://tothecore.sk/2018/11/28/sitecore-9-1-initial-release-installation-guide-for-development-machine

Just make sure that you have specified correct path to JRE before running script:

Also specify solrVersion, installFolder and solrPort parameters in that script.

After all these changes, just rebuild indexes on both instances and you should be fine.