Using the Callout Gateway
Invoking Callout Library Functions
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The Callout Gateway provides three different interfaces that can be used to load an InterSystems Callout library at runtime and call functions from that library. This chapter covers the following topics:

Shared libraries and Callout libraries
In this book, the term shared library refers to a dynamically linked file (a DLL file on Windows or an SO file on UNIX® and related operating systems). A Callout library is a shared library that includes hooks to the Callout Gateway, allowing various $ZF functions to load it at runtime and invoke its functions. Instructions for writing Callout libraries are provided in Creating an InterSystems Callout Library.
Callout Library Interfaces
The $ZF(-3), $ZF(-5), and $ZF(-6) functions are all used to invoke Callout library functions, but each of these functions has its own strengths and weaknesses.
The $ZF(-3) Interface
The $ZF(-3) function loads a Callout library and invokes a library function by specifying the library file path and the function name. It is simple to use, but can only have one library at a time in virtual memory. Unlike the other interfaces, it does not require any initialization before a library function can be invoked. See Using $ZF(-3) for Simple Library Function Calls for details.
The $ZF(-5) Interface
The $ZF(-5) function invokes Callout library functions by specifying system-defined numeric identifiers for the Callout library and the library functions. It can keep several libraries in memory at the same time, and individual function calls are much more efficient than $ZF(-3). The $ZF(-5) interface also includes utility functions $ZF(-4,1), $ZF(-4,2), and $ZF(-4,3), which must be used to obtain library and function IDs and to load or unload libraries. See Using $ZF(-5) to Access Libraries by System ID for details.
The $ZF(-6) Interface
The $ZF(-6) interface is similar to $ZF(-5) except that Callout applications can load libraries without hard-coding the actual library filenames. Instead, a separate index table contains that information, and each indexed library is given a unique, user-defined index number. Once the index table is defined in an instance of Caché, it is available to all Callout applications in that instance. The $ZF(-6) function invokes a library function by specifying the index number and a function ID. If necessary, it automatically reads the index table and loads the specified library. The $ZF(-6) interface also includes utility functions $ZF(-4,4) through $ZF(-4,8), which must be used to unload libraries and create or maintain indexes. See Using $ZF(-6) to Access Libraries by User Index for details.
Using $ZF(-3) for Simple Library Function Calls
The $ZF(-3) function is used to load a Callout library and execute a specified function from that library. $ZF(-3) is most useful if you are only using one library, or aren’t making enough calls to worry about the overhead of loading libraries. It allows you to call any available library function by specifying the library name, the function name, and a comma-separated list of function arguments:
   result = $ZF(-3, library_name[, function_name[, arguments]])
The specified library is loaded if it hasn’t already been loaded by a previous call to $ZF(-3). Only one library can be loaded at a time. When a subsequent $ZF(-3) call specifies a different library, the old library is unloaded and the new one replaces it. The library stays loaded as long as subsequent $ZF(-3) calls specify the same library. After a library has been loaded, the library name can be specified as a null string ("") in subsequent calls.
You can load or unload a library without calling a function. To load a new library, specify only the library name. To unload the current library without loading a new one, specify only a null string. In either case, $ZF(-3) returns a status code indicating whether the load or unload was successful.
The following ObjectScript code calls two different functions from each of two different libraries, and then unloads the current library:
Using $ZF(-3) to load libraries and call functions
   // define Callout library paths
   set libOne = "c:\intersystems\cache\bin\myfirstlibrary.dll"
   set libTwo = "c:\intersystems\cache\bin\anotherlibrary.dll"

   //load and call
   SET result1=$ZF(-3,libOne,"FuncA",123)   // loads libOne and calls FuncA
   SET result2=$ZF(-3,"","FuncB","xyz")   // calls FuncB from same library

   //load, then call with null name
   SET status=$ZF(-3,libTwo)   // unloads libOne, loads libTwo
   SET result1=$ZF(-3,"","FunctionOne","arg1")
   SET result2=$ZF(-3,"","FunctionTwo","argA", "argB")

   SET status=$ZF(-3,"")   // unloads libTwo
The following sections of this chapter describe $ZF functions that can load more than one library at a time. These functions will not conflict with $ZF(-3). You can always use $ZF(-3) as if it were loading and unloading its own private copy of a library.
Using $ZF(-5) to Access Libraries by System ID
The $ZF(-5) function uses system-defined library and function identifiers to invoke library functions. Utility functions $ZF(-4,1), $ZF(-4,2) and $ZF(-4,3) are used to get the required identifiers and to load or unload libraries. Multiple libraries can be open at the same time. Unlike $ZF(-3) (see Using $ZF(-3) for Simple Library Function Calls), $ZF(-5) can not be used until the utility functions have been called to load libraries and get identifiers. However, each library only needs to be loaded once, and each library or function identifier only has to be generated once. In applications that make many library function calls, this can significantly reduce processing overhead.
The following $ZF functions are discussed in this section:
The $ZF(-4,1) and $ZF(-4,3) functions are used to load Callout libraries and get library and function identifiers. The syntax for $ZF(-4,1) is:
   lib_id = $ZF(-4,1,lib_name)   // get library ID
where lib_name is the full name and path of the shared library file, and lib_id is the returned library ID. The syntax for $ZF(-4,3) is:
   func_id=$ZF(-4,3,lib_id, func_name)   // get function ID
where lib_id is the library ID, func_name is the library function name, and func_id is the returned function ID value.
The following ObjectScript code loads Callout library mylibrary.dll and gets the library ID, then gets the function ID for "MyFunction" and invokes it with $ZF(-5):
Loading a library and invoking a function with $ZF(-5)
   set libID = $ZF(-4,1,"C:\calloutlibs\mylibrary.dll")
   set funcID = $ZF(-4,3,libID, "MyFunction")
   set x = $ZF(-5,libID, funcID, "arg1")
Once the identifiers have been defined, the library will remain loaded until unloaded by $ZF(-4,2), and the identifiers can be used without any further calls to $ZF(-4,1) or $ZF(-4,3). This eliminates a significant amount of processing overhead when functions from several libraries are invoked many times.
The following ObjectScript code loads two different libraries and invokes functions from both libraries in long loops. A function in inputlibrary.dll acquires data, and functions in outputlibrary.dll plot and store the data:
Using $ZF(-5) with multiple libraries and many function calls
Method GraphSomeData(loopsize As %Integer=100000) As %Status
   // load libraries and get system-defined ID values
   set InputLibID = $ZF(-4,1,"c:\intersystems\cache\bin\inputlibrary.dll")
   set OutputLibID = $ZF(-4,1,"c:\intersystems\cache\bin\outputlibrary.dll")
   set fnGetData = $ZF(-4,3,InputLibID,"GetData")
   set fnAnalyzeData = $ZF(-4,3,OutputLibID,"AnalyzeData")
   set fnPlotPoint = $ZF(-4,3,OutputLibID,"PlotPoint")
   set fnWriteData = $ZF(-4,3,OutputLibID,"WriteData")

   // call functions from each library until we have 100000 good data items
   do {
      set datapoint = $ZF(-5,InputLibID,fnGetData)
      set normalized = $ZF(-5,OutputLibID,fnAnalyzeData,datapoint)
      if (normalized>"") { set flatdata($INCREMENT(count)) = normalized }
   } while (count<loopsize)
   set status = $ZF(-4,2,InputLibID)   //unload "inputlibrary.dll"

   // plot results of the previous loop and write to output
   for point=1:1:loopsize {
      set list = $ZF(-5,OutputLibID,fnPlotPoint,flatdata(point))
      set x = $PIECE(list,",",1)
      set y = $PIECE(list,",",2)
      set sc = $ZF(-5,OutputLibID,fnWriteData,flatdata(point),x,y,"outputfile.dat")
   set status = $ZF(-4,2,OutputLibID)   //unload "outputlibrary.dll"
   quit 0
The following section describes the $ZF(-6) interface, which loads libraries into the same virtual memory space as the $ZF(-5) interface.
Using $ZF(-6) to Access Libraries by User Index
The $ZF(-6) function provides an efficient interface that allows access to Callout libraries through a globally defined index, usable even by applications that do not know the location of the shared library files. The user-defined index table stores a key/value pair consisting of a library ID number and a corresponding library filename. The filename associated with a given library ID can be changed when a library file is renamed or relocated. This change will be transparent to applications that load the library by index number. Other $ZF functions are provided to create and maintain index tables, and to unload libraries loaded by $ZF(-6).
The following $ZF functions are discussed in this section:
The $ZF(-6) interface is similar to the one used by $ZF(-5) (see Using $ZF(-5) to Access Libraries by System ID) with the following differences:
The following examples demonstrate how the $ZF(-6) interface is used. The first example defines a library ID in the system index table, and the second example (which may be called from a different application) uses the library ID to invoke a library function:
Defining a system index entry with $ZF(-4,5) and $ZF(-4,6)
This example sets 100 as the library ID for mylibrary.dll in the system index table. If a definition already exists for that number, it is deleted and replaced.
   set LibID = 100
   set status = $ZF(-4,6,LibID)  // clear any old entries with this ID value
   set status = $ZF(-4,5,LibID,"C:\calloutlibs\mylibrary.dll")  // set system ID
Once the library ID is defined in the system index table, it is globally available to all processes within the current instance of Caché.
Invoking a function with $ZF(-6)
This example uses the system index table created in the previous example. It uses $ZF(-6) to load the library and invoke a library function, then unloads the library. This code does not have to be called from the same application that defined the library ID in the system index:
   set LibID = 100   // library ID in system index table
   set FuncID = 2    // second function in library ZFEntry table
   set x = $ZF(-6,LibID, FuncID, "arg1")   // call function 2
   set status = $ZF(-4,4,LibID)            // unload the library
Using the $ZF(-6) Interface to Encapsulate Library Functions
It would be simple to write an example for the $ZF(-6) interface that works just like the example for the $ZF(-5) interface (see Using $ZF(-5) to Access Libraries by System ID earlier in this chapter), but this would not demonstrate the advantages of using $ZF(-6). Instead, this section will present ObjectScript classes that allow an end user to perform exactly the same task without knowing anything about the contents or location of the Callout libraries.
The $ZF(-5) example invoked functions from Callout libraries inputlibrary.dll and outputlibrary.dll to process some experimental data and produce a two-dimensional array that could be used to draw a graph. The examples in this section perform the same tasks using the following ObjectScript code:
The User.SystemIndex class allows applications that use the Callout libraries to create and access system index entries without hard coding index numbers or file locations:
ObjectScript Class User.SystemIndex
Class User.SystemIndex Extends %Persistent
/// Defines system index table entries for the User.GraphData libraries
ClassMethod InitGraphData() As %Status
 // For each library, delete any existing system index entry and add a new one
    set sc = $ZF(-4,6,..#InputLibraryID)
    set sc = $ZF(-4,5,..#InputLibraryID,"c:\intersystems\cache\bin\inputlibrary.dll")
    set sc = $ZF(-4,6,..#OutputLibraryID)
    set sc = $ZF(-4,5,..#OutputLibraryID,"c:\intersystems\cache\bin\outputlibrary.dll")
    quit 0

Parameter InputLibraryID = 100;
Parameter OutputLibraryID = 200;
The User.GraphData class allows end users to invoke library functions without knowing anything about the actual Callout libraries.
ObjectScript Class User.GraphData
Class User.GraphData Extends %Persistent
/// Gets library IDs and updates the system index table for both libraries.
Method Init() As %Status
    set InLibID = ##class(User.GraphDataIndex).%GetParameter("InputLibraryID")
    set OutLibID = ##class(User.GraphDataIndex).%GetParameter("OutputLibraryID")
    quit ##class(User.SystemIndex).InitGraphData()
Property InLibID As %Integer [Private]; 
Property OutLibID As %Integer [Private]; 

/// Calls function "FormatData" in library "inputlibrary.dll"
Method FormatData(rawdata As %Double) As %String
    quit $ZF(-6,..InLibID,1,rawdata)
/// Calls function "RefineData" in library "outputlibrary.dll"
Method RefineData(midvalue As %String) As %String
    quit $ZF(-6,..OutLibID,1,midvalue)
/// Calls function "PlotGraph" in library "outputlibrary.dll"
Method PlotGraph(datapoint As %String, xvalue As %Integer) As %String
    quit $ZF(-6,..OutLibID,2,datapoint,xvalue)
/// Unloads both libraries 
Method Unload() As %String
    set sc = $ZF(-4,4,..InLibID)   // unload "inputlibrary.dll"
    set sc = $ZF(-4,4,..OutLibID)   // unload "outputlibrary.dll"
    quit 0
The following example demonstrates how an end user might use the methods in User.GraphData. The GetGraph() method uses the Callout libraries to perform exactly the same task as the GraphSomeData() method in the $ZF(-5) interface example (see Using $ZF(-5) to Access Libraries by System ID earlier in this chapter), but it does not directly call any $ZF functions:
Method GetGraph()
Method GetGraph(loopsize As %Integer = 100000) As %Status
   // Get an instance of class GraphData and initialize the system index
   set graphlib = ##class(User.GraphData).%New()
   set sc = graphlib.Init()

   // call functions from both libraries repeatedly
   // each library is loaded automatically on first call
   for count=1:1:loopsize {
      set midvalue = graphlib.FormatData(^rawdata(count))
      set flatdata(count) = graphlib.RefineData(midvalue)

   // plot results of the previous loop
   for count=1:1:loopsize {
      set x = graphlib.PlotGraph(flatdata(count),0)
      set y = graphlib.PlotGraph(flatdata(count),x)
      set ^graph(x,y) = flatdata(count)

   //return after unloading all libraries loaded by $ZF(-6)
   set status = graphlib.Unload()
   quit 0
Using a Process Index for Testing
As previously mentioned, a process index table is searched before the system index table, so it can be used within a process to override system-wide definitions. The following example creates a process index that is used to test a new version of one of the libraries used in the previous section.
Using a process index to test a new version of "inputlibrary.dll"
   // Initialize the system index and generate output from standard library
   set testlib = ##class(User.GraphData).%New()
   set sc = testlib.Init()
   set sc = graphgen.GetGraph()   // get 100000 data items by default
   merge testgraph1 = ^graph
   kill ^graph

   // create process index and test new library with same instance of testproc
   set sc = $ZF(-4,4,100)   // unload current copy of inputlib
   set sc = $ZF(-4,8)   // delete existing process index, if any
   set sc = $ZF(-4,7,100, "c:\testfiles\newinputlibrary.dll")  // override system index
   set sc = graphgen.GetGraph()
   merge testgraph2 = ^graph

   // Now compare testdata1 and testdata2

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