Thanks for taking part, Eduard. According to this record:

2017-03-31 19:24:51 0 : USER1 is ready to be configured using USER1 and %DB_%DEFAULT

MainDB was neither undefined nor empty.

My post was mainly about %Installer's usage; doesn't it use transactions internally by itself? I didn't notice any transaction tags in it's language definition.

As to the log level, my other scripts (not this one) are intended to be used during Cache installation. I don't know how to adjust the log level in this case, therefore I used to deliberately set it equal to "0" to be sure that my debug messages will be logged; having been finished with debugging, I just change all 

Level="0"

to

Level="3"

I've got into another issue trying to write a code which should configure some global mapping to already existing namespace. Here is its skeleton: 

 Include %occInclude

/// Create simple global mapping (main DB and cachetemp only)
Class %z.Mapping Extends %Library.RegisteredObject
{

/// Application Definition
XData qMS [ XMLNamespace = INSTALLER ]
{
<Manifest>
<If Condition='$L("${NAMESPACE}")=0'>
<!-- Report an error if the namespace wasn't specified -->
<Error Status="$$$NamespaceDoesNotExist">
<Arg Value="${NAMESPACE}"/>
</Error>
</If>

<If Condition='$zcvt("${NAMESPACE}","U")="%SYS"'>
<!-- Report an error if the namespace = %SYS -->
<Error Status="$$$GeneralError">
<Arg Value="${NAMESPACE} should not be %SYS"/>
</Error>
</If>

<Invoke Class="%z.Mapping" Method="GetMainDB" CheckStatus="0" Return="MainDB" >
<Arg Value="${NAMESPACE}"/>
</Invoke>

<If Condition='$L("${MainDB}")=0'>
<!-- Report an error if the globals database was not detected -->
<Error Status="$$$GeneralError">
<Arg Value="${NAMESPACE} has no Globals database" />
</Error>
</If>

<Var Name="DBRESOURCE" Value="%DB_%DEFAULT"/>

<Namespace Name="${NAMESPACE}" Create="no" >

<Log Level="0" Text="${NAMESPACE} is ready to be configured using ${MainDB} and ${DBRESOURCE}" />
 
 <Configuration>

<!-- ??? Configure the database that should exist upto this step -->
  <Database Name="${MainDB}"
 Dir="${MainDB}"
 Create="no"
 Resource="${DBRESOURCE}"
 PublicPermissions=""
  />
<!-- ??? Configure the database that should exist upto this step -->

  <GlobalMapping Global="Q(&quot;0!&quot;):(END)"  From="CACHETEMP"/>
  <GlobalMapping Global="Q(&quot;0mseSTATUS&quot;):(&quot;0mseSTATUS~&quot;)"  From="${MainDB}"/>
  <GlobalMapping Global="Qa"     From="CACHETEMP"/>
  <GlobalMapping Global="Qi"     From="CACHETEMP"/>
  <GlobalMapping Global="Qi(&quot;MO&quot;)"     From="${MainDB}"/>
 
  </Configuration>

</Namespace>

</Manifest>
}

/// This is a method generator whose code is generated by XGL.
ClassMethod setup(ByRef pVars, pLogLevel As %Integer, pInstaller As %Installer.Installer, pLogger As %Installer.AbstractLogger) As %Status [ CodeMode = objectgenerator, Internal ]
{
#; Let our XGL document generate code for this method. 
Quit ##class(%Installer.Manifest).%Generate(%compiledclass, %code, "qMS")
}

ClassMethod GetMainDB(pNamespace) As %String
{
new $namespace s $namespace="%SYS"
s sc=##class(Config.Namespaces).Get(pNamespace,.p)
if 'sc d $system.OBJ.DisplayError(sc) set ret="" g gmdbq
s ret=p("Globals")
gmdbq
quit ret
}

/// Invoke the installer passing in some variables
/// zn "USER1" s sc=$system.OBJ.Load("mapping.xml","ck") d:'sc $system.Status.DisplayError(sc)
/// s sc=##class(%z.Mapping).RunInstallWithLog($zu(12)_"mapping.log",2) d:'sc $system.Status.DisplayError(sc)
/// 
ClassMethod RunInstallWithLog(pLogfile As %String, pLogLevel = 1) As %Status
{
#dim tVars
#dim tStatus As %Status

Set tVars("NAMESPACE") = $zu(5)

// Construct a file logger
#dim tLogger As %Installer.FileLogger = ##class(%Installer.FileLogger).%New(1,pLogfile)

// Invoke the installer
Set tStatus = ..setup(.tVars,pLogLevel,,tLogger)
Do:$$$ISERR(tStatus) $system.OBJ.DisplayError(tStatus)
Quit tStatus
}

}

I've started it at the same Cache instance where I wrote it with following commands:

zn "USER1" s sc=##class(%z.Mapping).RunInstallWithLog($zu(12)_"mapping.log",2) d:'sc $system.Status.DisplayError(sc)

And it works. But if I'm excluding an [unneeded] code fragment that resides between two lines:

<!-- ??? Configure the database that should exist upto this step -->

I'm getting several errors, please see below. Why this fragment is still needed nevertheless I marked my namespace with Create="no" attribute?

​2017-03-31 19:24:51 0 %z.Mapping: Installation starting at 2017-03-31 19:24:51, LogLevel=2
2017-03-31 19:24:51 0 : USER1 is ready to be configured using USER1 and %DB_%DEFAULT
2017-03-31 19:24:51 1 CreateNamespace: Creating namespace USER1 using /
2017-03-31 19:24:51 2 CreateNamespace: Modifying namespace USER1
2017-03-31 19:24:51 0 %z.Mapping: ERROR #5002: Cache error: <SUBSCRIPT>%LoadData+6^Config.Databases.1 ^SYS("CONFIG","CACHE","Databases","")
ERROR #5659: Property 'Config.Namespaces::Globals(3@Config.Namespaces,ID=CACHE||Namespaces||USER1)' required
ERROR #7202: Datatype value '' length less than MINLEN allowed of 1
ERROR #5659: Property 'Config.Namespaces::Routines(3@Config.Namespaces,ID=CACHE||Namespaces||USER1)' required
ERROR #7202: Datatype value '' length less than MINLEN allowed of 1
2017-03-31 19:24:51 0 %z.Mapping: ERROR #ConfigFailed: Unknown status code: <Ins>ConfigFailed (,,,,,,,)
  > ERROR #5002: Cache error: <SUBSCRIPT>%LoadData+6^Config.Databases.1 ^SYS("CONFIG","CACHE","Databases","")
  > ERROR #5659: Property 'Config.Namespaces::Globals(3@Config.Namespaces,ID=CACHE||Namespaces||USER1)' required
  > ERROR #7202: Datatype value '' length less than MINLEN allowed of 1
  > ERROR #5659: Property 'Config.Namespaces::Routines(3@Config.Namespaces,ID=CACHE||Namespaces||USER1)' required
  > ERROR #7202: Datatype value '' length less than MINLEN allowed of 1
2017-03-31 19:24:51 0 %z.Mapping: Installation failed at 2017-03-31 19:24:51
2017-03-31 19:24:51 0 %Installer: Elapsed time .178622s

Timothy,

Both options succeded. Thank you so much! 

Just FYI, your 1st option

<Var Name="AddClassesErrors" Value="&quot;,5202,5373,&quot;" />

generated a code:

 Do tInstaller.SetVariable("AddClassesErrors",""",5202,5373,""")

and the 2nd one

<Var Name="AddClassesErrors" Value="#{&quot;5202,5373&quot;}" />

generated:

 Do tInstaller.SetVariable("AddClassesErrors",tInstaller.Evaluate("#{""5202,5373""}"))

Timothy,

Both options succeded. Thank you so much! 

Just FYI, your 1st option

<Var Name="AddClassesErrors" Value="&quot;,5202,5373,&quot;" />

generated a code:

 Do tInstaller.SetVariable("AddClassesErrors",""",5202,5373,""")

and the 2nd one

<Var Name="AddClassesErrors" Value="#{&quot;5202,5373&quot;}" />

generated:

 Do tInstaller.SetVariable("AddClassesErrors",tInstaller.Evaluate("#{""5202,5373""}"))

Hi Timothy and Sergey,

Timothy, putting the literal codes list inside a variable doesn't help much: I'm getting the same error. 

Thank you anyway.

P.S. The reason of the issue is a wrong code for variable evaluation, e.g. 

 <Var Name="AddClassesErrors" Value="5202,5373" />

is transformed to

 Do tInstaller.SetVariable("AddClassesErrors","5202","5373")