Runtime Environment Overview

Before you begin troubleshooting, debugging, or patching the FWD runtime environment, and before you are able to support or extend your converted application, a high level description of the fundamental services of the FWD runtime environment is in order. This chapter explores, at a high level, FWD's general purpose, application server functionality and describes the Progress compatibility layer of the FWD runtime environment.

The Forward conversion process transforms a Progress 4GL application into a pure Java equivalent. The results of the automated conversion process are:

  • one or more jar files with the compiled, pure Java equivalent of the application;
  • a runtime directory containing the configuration necessary for the application to operate;
  • a Relational Database Management System (RDBMS) loaded with the database schema and all necessary application data.

At its core, the FWD runtime environment is a general purpose, secure, distributed application platform. Built upon this foundation is a layer of Java infrastructure that provides the identical behavior and function as the equivalent Progress 4GL language constructs. In total, this compatible runtime is necessary to enable the resulting jar file(s) to run identically to the original application.

General Purpose Runtime

Some of the core FWD runtime modules are not specific to the Progress 4GL. Instead, they provide a general purpose platform for secure distributed computing. The most important of these modules are the Distributed Application Protocol, the Directory Service, and the Security Manager.

Distributed Application Protocol (DAP)

The DAP is designed as a message passing system where messages are exchanged by nodes arranged into a logical network.

The network is comprised of leaf nodes and routing nodes. Leaf nodes are end-points in the network. They may only have a direct connection to a routing node. Routing nodes may connect to any type of node and are a superset of leaf node function with the additional ability to forward or route messages between two nodes which have no direct connection. The logical network can be arbitrarily complex by connecting routing nodes together into mesh, star, tree or other topologies. Nothing in the protocol has any knowledge or dependency upon the topology of the logical network. That is left as a runtime choice.

For the network to exist, a session must be established between two nodes (leaf ↔ router or router ↔ router). As more nodes establish sessions, the network grows. Messages may be passed between any two nodes that have an active session with the same logical network. A message may be directly sent (if the two nodes are directly connected) or indirectly sent (if no direct connection exists between the nodes).

Messages can contain a graph of arbitrarily complex objects as a payload. The interface is used to enable the transmission and reception of messages over a network connection.

Each end of a session has a message queue that is used to send and receive messages. Once a message is enqueued, if the caller does not require synchronous processing, the caller continues immediately without waiting for transmission. If the caller requires the semantics of a synchronous call, then that calling thread will block while the message is sent to the other side, processed there and the result is returned. Even though the protocol is based on messages and queues (an inherently asynchronous concept), at the high level interface to the caller, the mechanism naturally provides for both synchronous and asynchronous forms of processing.

The communications transport for a session is provided by Transaction Layer Security (TLS), which is based on the de-facto SSL 3.0 standard. This is a reliable connection-oriented (TCP-based) protocol which uses encryptions and digital certificates to ensure the privacy and integrity of all network operations.

Over this lower level network infrastructure is built a secure remote object protocol that allows two Java applications to make method calls synchronously (e.g. remote procedure call) with minimal effort. This can be thought of as a simpler, faster and more secure form of Remote Method Invocation (RMI) as it is similar in function.

The remote object technology provides a local proxy object that implements one or more interfaces. A reference to this object can be obtained easily and calls to its methods are remoted to the object server. The proxy translates method calls into messages which are sent over the DAP to the peer node on the other side of the session. The calling thread will block until a matching reply message is received.

On the peer node, a server object must have been registered. Messages from the remote proxy object are read from the inbound queue by the dispatcher. This dispatcher matches the message with its destination object and method. The method is invoked and when complete the result (or any thrown exception) is packaged as a message and sent back to the requesting node. At that point the waiting thread is unblocked and dequeues the reply message, returning it to the remote proxy. The remote proxy unpackages the returned data (or thrown exception) and returns it (or throws it) to the caller.

All access to exported entry points (and registered server objects) is controlled by a DAP-specific resource plug-in in the security manager.

Systems of arbitrary complexity can be built easily using this remote object implementation. Clients, servers, peer-to-peer systems, batch processes, web agents and more are all possible.

Directory Service

Complex networked systems require configuration data. Such data sets are often large and require a complex structure for efficient organization. One flexible approach is to structure configuration data in a tree. In addition, this configuration data needs to be shared throughout the logical network. To accommodate these requirements, a centralized directory service has been created.

A directory is a hierarchy of typed objects. All the directory objects are structured to form a tree. These directory objects have a type (called a class) which determines what information can be stored in an instance of the type. Named data values stored in a directory object are called attributes. Attributes can be any of the following primitive data types:

  • integer
  • double
  • boolean
  • string
  • byte array
  • bit field (contains arbitrary bit patterns with a specified maximum number of bits in total)
  • bit selector (a mutually exclusive bit field which can only have a single bit set at any given moment)
  • date
  • time

Using these types, directory objects of arbitrary complexity can be designed. These objects can have any number of attributes (data of one of the primitive types) and any number of child objects which can themselves be arbitrarily complex.

Every directory object is a node in the directory tree and as such, has an ID. Every node can be identified by this ID string which describes how to find the object in the tree from its root. For example:




The path is made of links separated by the forward slash ('/') character like in file system directories. Every link names a node in the subtree of its parent. Sibling objects all have unique names, although the same names can be reused at different levels of the tree.

The directory service provides a general purpose interface which supplies functions such as reading, writing, enumerating and searching for directory objects. Applications are written to this interface.

For every call to the directory service, the access rights of the caller are checked to ensure that the caller is allowed to execute the requested operation. This access control function is implemented with a security manager resource plug-in. If the operation is allowed, then the request is passed on to the instance of the remapper.

The remapper is a plug-in which implements a standardized low-level interface (for the directory service) and remaps these primitive operations to the proper semantics of some specific back-end data source. At this time, remappers exist for Lightweight Directory Access Protocol (LDAP) Servers (via JNDI) and for an XML-based directory. Other directory back-ends can be added over time due to the pluggable design of the directory service.

In this manner, a general purpose and standardized interface is provided to applications while the back-end directory technology can be varied. The plug-in architecture allows the choice of directory back-end to be made at installation time rather than at development time. This means that one does not need to hard code the application to a specific directory back-end, in any way.

The integration with the security manager enables a highly flexible security policy. This allows granular control over the users/groups (security subjects) and the operations that those subjects are allowed to do on specific portions of the directory tree. For example, a specific group may have rights to read from certain parts of the directory and may be allowed to write to a specific directory object, while another group (or user) may have completely different rights to those same locations. Rights can be assigned down to the specific directory object or can be set at any parent object (which allows a policy to be set for the sub-tree of objects contained under that parent object). The following operations can be secured:

  • enumerate
  • create
  • delete
  • add (determines if a child node can be added)
  • read
  • write
  • no access (special negative right that disables all access no matter what other operations are enabled)

This secure directory service is accessible over the DAP such that its interface is exported as remote objects. This allows easy, remote access to centralized directory data.

It is important to note that any kind of structured data can be stored in the directory, but generally the directory is used for often-read but seldom-changed data such as configuration data. The directory back-end can be a general purpose database (if one writes a back-end plug-in that provides such a feature) but generally the external data source for the directory is an environment that is optimized for read access (e.g. LDAP servers are usually optimized in this manner). As such, this is not meant to be a general purpose database but rather a special purpose data store.

Of the configuration data stored in the directory, the majority of that data is usually dedicated to security purposes. For example, the directory contains all account data, access control lists (ACLs) and other security configuration.

Security Manager

The security manager is a subsystem which enables strong application-level security. Application-level security is a means of protecting application data and logic from unauthorized access or modification in a multi-user environment. Note that as a Java subsystem, this code cannot provide operating system (OS) level security. Strong OS-level security must still be provided outside of the security manager.

Servers provide shared resources that can be accessed by multiple users. Using the DAP, multi-threading is used to provide a scalable server environment. This means that the server will listen on predefined ports for incoming TLS connections from remote processes. Each incoming session must be authenticated by the security manager before it can be allowed to operate. Authentication is the process of verifying that an entity is whom they claim to be. In this manner, the security manager is made aware of every new subject in the system and that security context (the subject's identity as known in the security configuration and which is established by an authentication process) is associated with the new session (if authentication succeeds).

Security subjects are entities in the system which access resources. Resources represent data or points in application logic that have to provide controllable access. The following subjects are possible:

  • user (an interactive end-user)
  • process (an agent program or non-interactive/batch process)
  • group (a list of other subjects)

Clients are programs that connect to the server and which represent a specific subject in the system. When a client attempts to establish a session, it must provide authentication data that establishes the represented subject's identity. For most interactive clients, this would be some kind of logon dialog that allows the user to specify a userid and password. A pluggable architecture for this authentication process is provided by the security manager. This allows the client and server sides of the authentication process to be augmented by custom logic, without any changes to the security manager code. This enables a wide range of possible authentication solutions including the use of biometrics (e.g. fingerprint readers) as well as multi-factor authentication (e.g. something you are + something you have + something you know). Client programs that are non-interactive or batch processes will usually take advantage of the certificate-based authentication modes that are provided by the security manager. This allows a subject's identity to be associated with a known certificate issued by a known certificate authority. The client that supplies this certificate during authentication can be authenticated without any interactive processing (if this is enabled by the security manager's configuration for that subject's account). There is even a provision for anonymous connections to the server. Anonymous subjects are also associated with a security context and may have rights assigned. This richness in authentication capabilities allows for highly distributed systems of arbitrary complexity.

Once authenticated, any request a client submits to the server will run on a thread which is assigned the security context that identifies the associated subject. This context is intrinsic to each thread and can only be accessed within the security manager. Thus it cannot be changed or even inspected by any other code on the server. This allows security decisions to be made based on the subject associated with each request.

For any resource which is protected, a resource plug-in is loaded into the security manager. This plug-in provides an custom external interface to request specific security decisions for specific instances of that resource. The plug-in also provides the resource-specific backing logic to make the decision.

Resource instances are differentiated with a name. For example, if the resource is a "file system object" then the resource name would be a fully qualified path and file name to a specific file system object.

Resource plug-ins make security decisions in cooperation with the security manager. The security manager provides services to identify all access control lists (ACLs) that are assigned to the security context (the internal representation of a subject's identity) for the thread requesting the security decision. ACLs describe how security subjects relate to resources in terms of rights. This means that given a subject (security context) and a specific resource name, a list of assigned ACLs can be produced by the security manager. The resource plug-in then compares the requested rights (the operation or access that the subject needs) with the rights specified in each ACL until it determines that access should be allowed or denied.

The rights that are possible for a given resource are completely defined by the resource plug-in. The security manager has no understanding of specific rights but rather treats them as opaque data. This allows the resource plug-in to define custom data structures of arbitrary complexity to represent the rights and the security manager can still associate these rights with a subject and resource instance in an ACL. This enables a very granular and flexible security model which is customized to each resource's needs.

While the resource plug-in provides the mechanism to make security decisions, the request for a security decision must actually come from the application code that manages the resource. This application code provides a service to multiple subjects and when a request is made that must be controlled or limited, that service code must make the appropriate request to the resource plug-in to get a security decision. What the code does with that decision is not controlled by the resource plug-in or the security manager. Thus, to properly secure any given resource, all service layers by which that resource can be accessed must make the proper requests for security decisions at the proper point in its processing AND it must properly honor the security decision.

Service layers may use security decisions for the purposes of simple boolean decisions (allow/deny access) or for the purposes of filtering. Filtering allows for a range of access from completely allowed to partially allowed to completely denied. For example, data in a database table might be filtered using a security manager resource plug-in by specifying which subjects have access to which records in the database table. Then the service layer might filter a query result set down based on record-level security decisions.

Users can be grouped for the purpose of easy access rights management. A group is a collection of users that share a set of ACLs. Users may participate in multiple groups. The security context for such users has references to all relevant groups.

All of the security configuration is stored in the directory. This includes account data (users, groups, processes), authentication plug-in configuration, resource plug-in configuration, ACLs, rights and all other security configuration. The directory service is a separate component of the runtime which provides the security manager with access to this data storage.

The directory, security manager and the remote object protocol are all integrated into a single working system such that authentication, authorization, access control, auditing and other critical functions are handled consistently and comprehensively.

Progress 4GL Compatibility Infrastructure

Using the general purpose runtime infrastructure as a base, a server environment has been created which hosts applications that have been converted from Forward. Built on top of the generic components is a Progress 4GL compatibility layer. This layer provides a set of common services for handling a very large percentage of the functionality available in the Progress 4GL language. The complete server environment is referred to as the "Forward Server" or by the short name "FWD Server".

A client environment ("FWD Client") likewise has been created which uses the general purpose runtime as a base. Using this client a session can be established with the FWD Server and users can logon and interact with the converted application in the same manner as the original Progress 4GL version. This client implements the majority of the user interface processing for the Progress 4GL compatibility environment and in this role it is referred to as the thin client. Since the client is designed for both interactive and batch usage, batch or utility programs that are non-interactive can be run using the same FWD Client software.

Once a session is started (the client process has been launched and the user or process has been authenticated), then there will be a thread on the server that has its security context defined by the associated user account's identity. This thread will execute the business logic in the server process, with no access to any resources, rights or data of any other user account. The FWD Server process runs all connected users in the same Java Virtual Machine (JVM) but isolates each thread's access using the security manager.

Both the client and the server are simply Java processes and it is possible to write a custom application to connect and utilize the server's resources. It is likewise possible to add other hand-written programs to the server to export additional functionality.

The original application's custom security implementation is converted into a common and standardized security model during the conversion process. This approach leverages the security manager implementation. Likewise, all of the application's configuration including security relevant data is migrated into the centralized secure directory. Usage of this data is mapped to the directory as needed.


The converted application runs entirely on the FWD Server, no generated code ever runs on the FWD Client. The generated code can be categorized into three groupings:

  • business logic
  • data model objects (DMOs)
  • frame definitions

These groupings together form the converted application code which is hosted inside a compatibility environment that provides fidelity with the behavior and semantics of the Progress 4GL environment.

All converted code is dependent upon the base language support. This ranges from the most elemental support for Progress data types (e.g. logical or character) to the most complex block properties. Expression support, built-in functions, control flow and many other basic services are all provided by the base language support.

The converted business logic is where the majority of the original Progress 4GL programs reside. This is where variables and other resources like streams are defined, where the block structure of the program exists and where all control flow is managed. All top-level application entry points (a Progress 4GL procedure which is run as the "startup procedure") are exposed as exported entry points via the distributed application protocol.

After a FWD Client successfully establishes a session (this includes proper authentication), it calls one of these exported entry points to start application execution. That call does not return until the application exits.

The converted data model objects define the structure, validation and other features of the database access. The business logic uses buffers and obtains DMOs for these buffers via queries or by creating a new record. Once a DMO is available its properties can be read or written via getter/setter methods. The business logic will use and modify the DMOs in the same manner as the original Progress 4GL logic did.

The DMO implementation is dependent upon a layer of persistence services that implements the Progress 4GL compatibility features for database support. This layer is tightly integrated with Hibernate, an open-source Object-Relational Mapping (ORM) technology. Hibernate is the backing infrastructure that translates DMOs into records of a table and vice versa. Hibernate uses Java Database Connectivity (JDBC) for its relational database access. In areas where different databases support variations in SQL syntax, Hibernate adds its own "dialect" support, which largely hides the differences between different relational database implementations. The use of JDBC and Hibernate does provide a high degree of database independence, with syntax support for 30+ databases. At the time of this writing, however, only PostgreSQL is fully supported by FWD. In addition to supporting the varying syntax dialects of individual database implementations, FWD requires that a database has the ability to execute user-defined functions implemented in Java at the database server, and a handful of other features.

As part of the conversion process, all static elements of the user interface (UI) are separated from the business logic. Instead, a single explicit frame definition is written for each unique frame. These frame definitions specify the widgets (name and type), layout, frame position, formatting, frame options (e.g. title, down iterations) and widget options (e.g. label, data type). These frame definitions are encoded as Java classes.

Since Progress 4GL is a single threaded "fat client" environment, 4GL business logic generally has synchronous UI dependencies written deeply into the control flow of the program. In fact, in many cases the transaction processing and error handling are very sensitive to the placement of these UI language statements. In the converted Java application, while the static frame definitions have been completely removed from the business logic, the business logic's dependencies upon synchronous UI interaction remain as part of the control flow of a program. In this sense, the business logic can be thought of (loosely) as the controller in a Model-View-Controller (MVC) architecture. The frame definitions are the views and the DMOs are the model. For example, the business logic may need to display a report on the screen so it may have a query in a loop which obtains a DMO and then fields from that DMO are written into the widgets of the frame and the frame is told to display.

The frame definition is a Java class on the server which is used by the business logic to interact with the logical concept of a frame. As an example, the business logic might call a setter method to update the data in a given widget. Likewise the frame object implements an extensive list of methods to provide access to behavior expected in the Progress 4GL. While the frame objects provide a server-side interface, the actual UI processing such as layout, rendering and event handling is all handled on the thin client. To achieve this separation, the frame's definition is converted (at runtime) into a configuration and this is sent down to the client to be instantiated however the particular client (e.g. TTY, GUI, web...) must handle it. Likewise, the data associated with a frame's widgets (a screen buffer) is transferred to and from the thin client as needed to keep the state synchronized. This is possible since the Progress 4GL is single threaded. This means that any time the business logic executes a UI language statement, that logic blocks until the client is done.

A Progress 4GL compatible UI driver provides the server-side UI functionality and handles calling the UI primitive operations (that are exported from the thin client via the DAP) as required to maintain compatible behavior. This same component provides a number of exported interfaces which are called recursively by the thin client in order to handle certain features such as field level validation and triggers. Both of these call-backs must run on the server BUT they also must run nested inside already active UI processing. For example, a Progress 4GL WAIT-FOR language statement does not return when a trigger executes. Rather, the trigger is a call-back executed nested within the WAIT-FOR processing. All of the complexity of the Progress 4GL event processing model is supported including nested WAIT-FOR processing that can be arbitrarily deep.

For the most current description of the Progress 4GL features which are supported (as well as those yet to be supported), please see the Forward FAQ on the FWD website (

Thin Client

The user (or possibly a batch process) connects to this environment by running a FWD Client. The core function of this process is called the "thin client". The client process is “thin” from the perspective that it does nothing except connect to the server, authenticate and then invoke an entry point (to business logic) on the server side. The business logic may invoke user interface code (if that is required in the original application) and in such cases, frame definitions are loaded and sent down to the client node. The client does not require access to the converted application code nor does it require direct access to the database or directory. The server drives all client activities using a set of user interface “primitives“. This allows the client code to be completely generic (not application-specific) as well as reasonably thin. All real processing occurs on the server, though any interactive client does cooperate with the server in processing things such as triggers or field level validation expressions.

As part of the session setup process (especially for interactive clients), a customizable authentication process is integrated into the thin client. If the client authentication plug-in is enabled, an application-specific class (as configured in the directory) is downloaded dynamically to the client over the secure session. The client executing this plug-in can provide a logon dialog and/or use any other programmatic facility such as biometric readers to obtain authentication data. This data is sent to the server and if authentication succeeds, the session is established.

The thin client has been designed with the intention that it only provides a presentation layer for the application. Unlike the Progress client, no business logic or data access of ANY kind is executed on the thin client.

Multiple different client types are enabled via this approach, while the server's user interface logic is identical for (and agnostic to) each client type. This is enabled by implementing an abstract interface of UI primitive operations and a data driven approach to defining resources such as frames. This approach will hold completely for both TTY (character user interface or CHUI) and GUI client types. For both of these types, there is a very large amount of common behavior and features which lends itself well to abstraction.

Other UI types may or may not be able to leverage much of this common code, though it is likely that there will be overlap. At the time of this writing, the web client does not yet exist. The CHUI is quite complete and the common portions of the GUI client are well implemented/tested. The GUI-specific layout and drawing is not yet available.

When the server sends a frame definition down to the thin client, this configuration is used to instantiate all the backing frame and widget resources needed to support the specified features. Subsequent UI primitive operations on a frame will include screen buffers or other state needed to execute. In response to these primitive UI operation requests by the server, the thin client will implement the backing function using common code where possible. This common code then uses type-specific low-level operations to handle drawing, layout and certain other functions such as keyboard reading.

For the CHUI client, these low-level operations are implemented using an open source Java technology called CHARVA. This environment in turn drives the terminal using the NCURSES interface via a JNI library (

When the GUI client is implemented, it is expected that Swing will be used as the basis for the low-level operations, however any other GUI technology (such as the Eclipse Foundation's SWT) framework would also suffice.

A small other area where the thin client makes use of an external native component that it invokes via JNI is for child process creation, that was mentioned in chapter FWD Project Source Code Tour. Progress 4GL has multiple language statements which create child processes: some of them do so for the purposes of creating input, input-output or output streams which the 4GL code can read from and/or write to the child process (e.g. INPUT THROUGH, INPUT-OUTPUT THROUGH etc). Other language statements allow the interactive invocation of a child process, using a shell-like command line (e.g. OS-COMMND or UNIX). All of these mechanisms turn out to have behavior which cannot be duplicated using the weak process launching capabilities of Java, so to be fully compatible child process launching had to be handled in native code.

This code results into a shared library, which needs to be made available to the client at runtime in case it needs to use features related to process creation. A convenient way of doing this is to list its containing folder in the the client JVM's java.library.path system property. By default the library is placed by the build process in p2j/build/lib, the same place where resides.

For the most current description of the Progress 4GL features which are supported (as well as those yet to be supported), please see the Forward FAQ on the FWD website (