Exception during Layout Service RenderItem - Could not find configuration node: databases/database[@id='core']

Had the following exception since core db was unreachable from cd instance as a result cd api host failed for any page using multilist. So, fixed the same as follows:

Exception:

8000 05:27:23 ERROR Exception during Layout Service RenderItem (configuration: default, item: {20D5306F-6FF1-4CD8-A82A-7668A5D2288F})
Exception: System.InvalidOperationException
Message: Could not find configuration node: databases/database[@id='core']
Source: Sitecore.Kernel
   at Sitecore.Configuration.DefaultFactory.GetConfigNode(String xpath, Boolean assert)
   at Sitecore.Configuration.DefaultFactory.CreateObject(String configPath, String[] parameters, Boolean assert)
   at Sitecore.Configuration.DefaultFactory.GetDatabase(String name, Boolean assert)
   at Sitecore.Configuration.DefaultFactory.GetDatabase(String name)
   at Sitecore.Web.UI.HtmlControls.Data.LookupSources.GetDatabase(String source)
   at Sitecore.Data.Fields.CustomField.GetDatabase()
   at Sitecore.Data.Fields.MultilistField.GetItems()
   at Sitecore.LayoutService.Serialization.FieldSerializers.MultilistFieldSerializer.WriteValue(Field field, JsonTextWriter writer)
   at Sitecore.LayoutService.Serialization.FieldSerializers.MultilistFieldSerializer.Serialize(Field field, JsonTextWriter writer)
   at Sitecore.LayoutService.Serialization.ItemSerializers.BaseItemSerializer.SerializeItem(Item item, SerializationOptions options)
   at Sitecore.LayoutService.Serialization.FieldSerializers.InternalLinkFieldSerializer.GetProperties(Item item, InternalLinkField field, Int32 depth)
   at Sitecore.LayoutService.Serialization.FieldSerializers.InternalLinkFieldSerializer.WriteValueObject(Item item, InternalLinkField field, JsonTextWriter writer, Int32 depth)
   at Sitecore.LayoutService.Serialization.FieldSerializers.InternalLinkFieldSerializer.WriteValue(Field field, JsonTextWriter writer)
   at Sitecore.LayoutService.Serialization.FieldSerializers.InternalLinkFieldSerializer.Serialize(Field field, JsonTextWriter writer)
   at Sitecore.LayoutService.Serialization.ItemSerializers.BaseItemSerializer.SerializeItem(Item item, SerializationOptions options)
   at Sitecore.LayoutService.Serialization.FieldSerializers.MultilistFieldSerializer.GetProperties(Item item, MultilistField field, Int32 depth)
   at Sitecore.LayoutService.Serialization.FieldSerializers.MultilistFieldSerializer.WriteValueObject(IEnumerable`1 items, MultilistField field, JsonTextWriter writer, Int32 depth)
   at Sitecore.LayoutService.Serialization.FieldSerializers.MultilistFieldSerializer.WriteValue(Field field, JsonTextWriter writer)
   at Sitecore.LayoutService.Serialization.FieldSerializers.MultilistFieldSerializer.Serialize(Field field, JsonTextWriter writer)
   at Sitecore.LayoutService.Serialization.ItemSerializers.BaseItemSerializer.SerializeItem(Item item, SerializationOptions options)
   at Sitecore.LayoutService.ItemRendering.LayoutService.Render(Item item, IRenderingConfiguration renderingConfiguration, RenderOptions renderOptions)
   at Sitecore.LayoutService.Mvc.Controllers.LayoutServiceController.RenderItem(String configuration, String item)

Solution:

Add core connection string to cd environment connectionstrings.config

Next, add a patch to cd instance under a folder from where you are sure the config will be deployed/patched to cd instance, since security in connectionstrings.config was pointing to core db, used the same in the patch(as highlighted below):

Sitecore.Core.ConnectionString.config

<?xml version="1.0" encoding="utf-8"?>

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/"

  xmlns:role="http://www.sitecore.net/xmlconfig/role/"

  xmlns:security="http://www.sitecore.net/xmlconfig/security/">

<sitecore> 

<databases>

<database id="core" singleInstance="true" type="Sitecore.Data.DefaultDatabase, Sitecore.Kernel"   role:require="ContentDelivery">

    <param desc="name">security</param>

    <connectionStringName>security</connectionStringName>

    </database>

</databases>

</sitecore>

</configuration>