Exactly - you haven't used the Gateways before. Why? IMO, the Gateways (mostly Java and DotNet) were a bit cumbersome to deal with. With ELS, we have default servers that can be easily managed and discovered, they require little or no configuration to get started (some ELS's do require some configuration if the language platform support is not discoverable by IRIS), and the interface is simple and direct.

One quick example, using Python, is to use the os module to get the current working directory:

USER>set python = $system.external.getPythonGateway()

USER>write python.invoke("os","getcwd")
/opt/intersystems/iris/xdbc/mgr

All that is required is a gateway connection to an external server, your code needs to be visible to that external server, either by direct placement into the default path for that language platform or by explicitly adding it by calling addToPath(), and public interfaces in your external code. By "external", I mean code that isn't written inside of the IRIS Server - ObjectScript.

When your external code writes to the "system output" device, that output is redirected to the IRIS current device. In my above example, the renderTable() function simply constructs a formatted string using the AsciiTable library (got it from GitHub) and writes it using System.out.println(formattedstring). I simply copied the output that was displayed in my IRIS session terminal window and pasted it in the original post. No extra work involved.

If your external code returns an object then you can indeed make use of that object as if it were a local IRIS Object - because it is. It is actually a network proxy object that communicates with the original external language object. That communication can actually be full duplex - your external code can talk to IRIS and IRIS can talk to your external code.

One really simple example that I think is quite profound is using JDBC in IRIS. This isn't JDBC Gateway - that still exists and it is what it is. I am talking about using a Java JAR file that contains a JDBC driver and you want to use it. I'll try a simple demo here. I have MariaDB installed on my system so I'll just use it to query a table I have defined there.

First - using mariaDB from the command line interface:

 ~ % mysql -u myusername -p
Enter password:
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 4
Server version: 10.5.8-MariaDB Homebrew

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> show databases
    -> ;
+--------------------+
| Database           |
+--------------------+
| SAMPLES            |
| information_schema |
| mysql              |
| performance_schema |
| test               |
+--------------------+
5 rows in set (0.010 sec)

MariaDB [(none)]> use SAMPLES
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
MariaDB [SAMPLES]> show tables;
+-------------------+
| Tables_in_SAMPLES |
+-------------------+
| person            |
+-------------------+
1 row in set (0.000 sec)

MariaDB [SAMPLES]> select * from person limit 5
    -> ;
+-----------------------+-------------+------------+-----------------------+------------+------------+----------+
| name                  | ssn         | dob        | home_street           | home_city  | home_state | home_zip |
+-----------------------+-------------+------------+-----------------------+------------+------------+----------+
| Basile,Molly M.       | 452-57-8033 | 1994-06-02 | 1153 First Street     | Xavier     | SD         | 98033    |
| Cooke,Howard F.       | 131-62-3894 | 2017-12-09 | 5172 Washington Place | Zanesville | NE         | 44980    |
| Donaldson,Phil R.     | 480-79-5019 | 1990-01-23 | 4429 Elm Street       | Miami      | WA         | 67638    |
| Eisenstien,Michael D. | 655-11-6334 | 1948-08-14 | 4676 Elm Avenue       | Reston     | KY         | 52729    |
| Faust,Mo E.           | 772-42-3921 | 2018-01-10 | 826 Maple Avenue      | Youngstown | OH         | 37180    |
+-----------------------+-------------+------------+-----------------------+------------+------------+----------+
5 rows in set (0.016 sec)

MariaDB [SAMPLES]>

Next, I'll follow up with an example of doing this from within IRIS.

You are right, of course. I was puzzled by the lack of documentation myself. I did some research and I've been told that there will be documentation available for the External Language Servers and $system.external.

The $system.external.Help() feature is something that has been part of the product for many years. It is not a substitute for proper documentation but serves to aid the command line user when questions regarding function names and arguments need a quick answer.

The ELS features are based on what is known as "Dynamic Object Gateways". There is extensive documentation available for them. That documentation is a bit dated in that it doesn't incorporate the new $system.external API's but still mostly accurate. Obtaining a gateway connection to an External Language Server by simply invoking $system.external.getGateway(<gatewayname>) is just one way we've simplified that interface.

There are default ELS's defined for Java, Python, DotNet, and R. Each is supported not only by the getGateway() that accepts the name of the ELS but also by get<Language>Gateway() functions for Java, DotNet, Python and R that return gateway connections to the default language servers.

In the 2021.1 preview, you can execute $system.external.Help() to get a description of the External Language Server interface.

How are you purging this data? SQL? Or by calling a class method such as %DeleteExtent?

Just to clarify - inheritance considers the primary super class hierarchy. %Persistent must be the first class some where in the primary super class hierarchy. Sample.Employee extends Sample.Person which extends %Library.Persistent.

If there is no storage definition then a storage definition whose type is %Storage.Persistent is created when compiling the class. This is done before the storage definition is compiled (every class member is compiled by the class compiler). Then, when the storage definition is compiled and if the type is %Storage.Persistent then the %Storage.Persistent.STORAGECOMPILERCLASS will generate a full storage definition for the current class definition.

What does that mean? Well - if this is the first time this storage definition has been compiled then it is most likely empty, other than the type class. The STORAGECOMPILERCLASS will generate a complete storage definition. But even if the storage definition is already defined, the STORAGECOMPILERCLASS still reviews that definition. If there are any changes detected to the class definition then the storage definition is updated to reflect those changes.

That means that the user can manually edit the storage definition at any time and the storage compiler will simply make sure that the definition is "complete" - no missing properties, no missing indexes, and so on.

Bottom line is that the user is free to make any desired changes, including deleting the storage definition completely.

Keep in mind that some changes can make the compiled class incompatible with existing data.

Class utility.StatementColumns Extends %SQL.CustomQuery
{

Parameter SQLNAME As String = "statement_columns";

Property columns As %Collection.ListOfObj [ Internal, Private ];

Property columnPtr As %String [ Internal, Private ];

Property atEnd As %Boolean [ Internal, Private ];

Property columnType As %String;

Property colName As %String(MAXLEN = 255);

Property ODBCType As %Integer;

Property precision As %Integer;

Property scale As %Integer;

Property isNullable As %Boolean;

Property label As %String(MAXLEN = 255);

Property tableName As %String;

Property schemaName As %String;

Property qualifier As %String;

Property isAutoIncrement As %Boolean;

Property isCaseSensitive As %Boolean;

Property isCurrency As %Boolean;

Property isReadOnly As %Boolean;

Property isRowVersion As %Boolean;

Property isUnique As %Boolean;

Property isAliased As %Boolean;

Property isExpression As %Boolean;

Property isHidden As %Boolean;

Property isIdentity As %Boolean;

Property isKeyColumn As %Boolean;

Property isRowId As %Boolean;

Property isList As %Boolean;

Property property As %Dictionary.CompiledProperty;

/// The objects type class
Property typeClass As %Dictionary.CompiledClass;

Property clientType As %Integer;

Method %OpenCursor(statement As %String) [ Private ]
{
    set ..columns = $system.SQL.Prepare(statement).%Metadata.columns
    set ..atEnd = 0
    set ..columnPtr = ""
}

Method %FetchCursor() As %Library.Integer
{
    set response = 0
    if '..atEnd {
        set next = ..columnPtr
        set column = ..columns.GetNext(.next)
        if next '= "" {
            set response = 1
            set ..columnPtr = next
            do ..mapColumnToRow(column)
        } else {
            set ..atEnd = 1
            set ..columnPtr = ""
            do ..clearRow()
        }
    }
    return response
}

Method mapColumnToRow(column As %SQL.StatementColumn)
{
    set ..columnType = "SQLRESULTCOL"
    set ..colName = column.colName
    set ..ODBCType = column.ODBCType
    set ..precision = column.precision
    set ..scale = column.scale
    set ..isNullable = column.isNullable
    set ..label = column.label
    set ..tableName = column.tableName
    set ..schemaName = column.schemaName
    set ..qualifier = column.qualifier
    set ..isAutoIncrement = column.isAutoIncrement
    set ..isCaseSensitive = column.isCaseSensitive
    set ..isCurrency = column.isCurrency
    set ..isReadOnly = column.isReadOnly
    set ..isRowVersion = column.isRowVersion
    set ..isUnique = column.isUnique
    set ..isAliased = column.isAliased
    set ..isExpression = column.isExpression
    set ..isHidden = column.isHidden
    set ..isIdentity = column.isIdentity
    set ..isKeyColumn = column.isKeyColumn
    set ..isRowId = column.isRowId
    set ..isList = column.isList
}

Method clearRow()
{
    set ..columnType = ""
    set ..colName = ""
    set ..ODBCType = ""
    set ..precision = ""
    set ..scale = ""
    set ..isNullable = ""
    set ..label = ""
    set ..tableName = ""
    set ..schemaName = ""
    set ..qualifier = ""
    set ..isAutoIncrement = ""
    set ..isCaseSensitive = ""
    set ..isCurrency = ""
    set ..isReadOnly = ""
    set ..isRowVersion = ""
    set ..isUnique = ""
    set ..isAliased = ""
    set ..isExpression = ""
    set ..isHidden = ""
    set ..isIdentity = ""
    set ..isKeyColumn = ""
    set ..isRowId = ""
    set ..isList = ""
}

}

Robert - what do you think? Should I just post the class text (single class, simple) or try to put it on GitHub?

I agree with Tim but I'll take it one step further. Foreign keys are much more useful than relationships. After all, a relationship is simply a foreign key that maintains references to instances of the related class in memory. The projection of a relationship to SQL is simply as a foreign key. The set of related objects is simply populated using an SQL query. The problem with relationships is that they are extremely sticky and that can cause large numbers of objects to be inadvertently swizzled into memory. With foreign keys you have no in-memory model. That means with a foreign key you have to manage desired swizzling. Some view that as a problem, I view it as an advantage.

Another advantage of using foreign keys is that you can define multiple foreign keys using the same key component properties. No need to define a direct reference.

Creative minds might come up with a calculated property whose value is derived from the foreign key components, adding property methods to manipulate the related object/objects. This calculated property could be the direct reference. Perhaps transient would be better as a transient property also has instance memory allocated for it.