Could not find configuration node: contentSearch/indexConfigurations/indexUpdateStrategies/syncMaster

Question

So, we're running Sitecore 9 on Azure.
For some reason, on our CD, we're seeing the above error. You can see the full error message below.
I haven't been able to find a lot of helpful information about this.
I did come across the following article: https://kb.sitecore.net/articles/347205 which sounds reasonable, except it doesn't look like Sitecore 9 uses SwitchMasterToWeb.config.

Any ideas what this could be caused by?

[InvalidOperationException: Could not find configuration node: contentSearch/indexConfigurations/indexUpdateStrategies/syncMaster]
       Sitecore.Configuration.DefaultFactory.GetConfigNode(String xpath, Boolean assert) +525
       Sitecore.Configuration.DefaultFactory.CreateObject(String configPath, String[] parameters, Boolean assert) +271
       Sitecore.Configuration.DefaultFactory.CreateFromReference(XmlNode configNode, String[] parameters, Boolean assert) +170
       Sitecore.Configuration.DefaultFactory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper) +116
       Sitecore.Configuration.DefaultFactory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert) +72
       Sitecore.Configuration.DefaultFactory.GetInnerObject(XmlNode paramNode, String[] parameters, Boolean assert) +929
       Sitecore.Configuration.DefaultFactory.AssignProperties(XmlNode configNode, String[] parameters, Object obj, Boolean assert, Boolean deferred, IFactoryHelper helper) +529
       Sitecore.Configuration.DefaultFactory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper) +320
       Sitecore.Configuration.DefaultFactory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert) +72
       Sitecore.Configuration.DefaultFactory.GetInnerObject(XmlNode paramNode, String[] parameters, Boolean assert) +929
       Sitecore.Configuration.DefaultFactory.AssignProperties(XmlNode configNode, String[] parameters, Object obj, Boolean assert, Boolean deferred, IFactoryHelper helper) +529
       Sitecore.Configuration.DefaultFactory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper) +320
       Sitecore.Configuration.DefaultFactory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert) +72
       Sitecore.Configuration.DefaultFactory.CreateObject(String configPath, String[] parameters, Boolean assert) +613
       Sitecore.ContentSearch.ContentSearchManager.get_SearchConfiguration() +305
       Sitecore.ContentSearch.ContentSearchManager.GetIndex(String name) +40
       Sitecore.Marketing.Search.Observers.Activation.UpdateSearchIndexObserver`1..ctor(ILogger`1 logger, String databaseName, String searchIndex) +302
       lambda_method(Closure , ServiceProvider ) +6857
       Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetService(IServiceProvider provider) +69
       Sitecore.Marketing.Definitions.DefinitionManagerFactory.GetDefinitionManager() +84
       Sitecore.Analytics.Data.<>c__DisplayClass6_0.<.ctor>b__4() +69
       System.Lazy`1.CreateValue() +727
       System.Lazy`1.LazyInitValue() +184
       Sitecore.Analytics.Data.TrackingField.NormalizeProfiles(IMarketingDefinitions marketingDefinitions) +64
       Sitecore.Analytics.Data.TrackingField..ctor(Field innerField) +133
       Sitecore.Analytics.Data.TrackingField.GetTrackingField(Item item) +106
       Sitecore.Analytics.Data.TrackingField.FindTrackingField(Item item) +37
       Sitecore.Analytics.Data.TrackingField.HasTracking(Item item) +9
       Sitecore.Analytics.Media.MediaRequestTrackingInformation.IsTrackedRequest() +38
       Sitecore.Analytics.RobotDetection.Media.MediaRequestSessionModule.IsSessionRequired() +162
       Sitecore.Analytics.Media.MediaRequestSessionModule.ContextPostMapRequestHandler(Object sender, EventArgs e) +17
       System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +136
       System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +195
       System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +88

Answer 1

It seems that you have disabled or removed syncMaster index update strategy on your CD server, but you have an index that is still referencing that strategy, So you need to search your index configuration files and find the the index that is still using it and remove it, it would look something like:

<strategy ref="contentSearch/indexConfigurations/indexUpdateStrategies/syncMaster"/>