Flow-Based Programming - Chap. XXI
General Framework for Interactive Applications


This chapter has been excerpted from the book "Flow-Based Programming: A New Approach to Application Development" (van Nostrand Reinhold, 1994), by J.Paul Morrison.

A second edition (2010) is now available from CreateSpace eStore and Amazon.com.

The 2nd edition is also available in e-book format from Kindle (Kindle format) and Lulu (epub format).

To find out more about FBP, click on FBP.                               

For definitions of FBP terms, see Glossary.                                                                                       

This chapter talks mainly about our experience, pre-1994, using the FBP implementation called DFDM and IBM's ISPF.  More recently, we built an e-business application using the Java implementation of FBP [now called JavaFBP].  This was also essentially a loop-shaped network, but running across multiple servers (other chapters in my book talk about the ease of distributing FBP applications across physical networks).  Communication between the user and the servers was handled by IBM's MQSeries transporting XML messages. The application also needed to communicate with multiple "back-ends", which it did using either MQSeries or CORBA.

Material from book starts here:

In this chapter I am going to describe a general framework for interactive applications, showing a general structure and some component types which could help in the design of such applications.

We will start by reproducing Figure 19.3, which shows an IP substream travelling from application logic to a screen manager process and back again, and showing how it can be fleshed out to produce a very general design for interactive applications. You will remember the following diagram:

where SM  is a Screen Manager
      ST  starts the network as a whole
      the dotted line represents logic processes

Figure 21.1

You will remember that in IMS we had to split the process marked SM into SI and SO (Screen Input and Screen Output respectively), and change the function of the "return connection". However, for now, we will work with the above diagram, bearing in mind that it is very easy to convert it into one which will run in the IMS/DC environment.

In what follows I will describe a system we built using a single generalized Screen Manager component (which I will refer to as ISM1 - ISPF Screen Manager 1) which used IBM's ISPF both to write to and read from a terminal, but the concepts are extremely general and can be applied to other screen management software.

Although some systems allow a screen to be generated without using a program, it is simpler to assume that every application starts by putting up a "What do you want to do?" type screen. So assume that ST causes SM to output a menu screen. SM will have to have a place where the user's answer can be stored, so we can assume that ST sends out a substream consisting of at least three IPs: open bracket, "request" IP, zero or more data IPs and close bracket. The brackets are needed so we can have a variable number of data IPs in the substream. The request IP will have, among other data, the name of the screen to be displayed. This substream then arrives at ISM1, which puts up a menu; the user enters a choice; the substream goes through the processing logic (which may change the contents of the IPs, or even add or remove IPs from the substream); and eventually we get back to ISM1 which puts up a new menu.

Now let's look a bit more closely at ISM1. This component accepted an input substream, put any variable data into position on the screen using the data descriptors associated with the data, and waited for action on the part of the user. When that occurred, the modified data was placed back into the right places in the data IPs, and the substream was then sent on to the next process downstream. ISPF identifies fields on the screen by name, and ISM1 used the field names from the descriptor to determine where to put each variable field.

In addition to this substream, referred to as the "fixed substream", ISM1 also accepted an additional, optional substream, called the "repeating substream". The mental image supported was that the screen has a fixed part, normally describing one or a small number of individual entities, and an optional list. Thus we could show a person's family on the screen: his or her personal information, the spouse's information (a separate IP), these providing fairly complete information, and zero or more children, showing just name, age and gender, say. If the user wanted more information on a child, he or she could select the child, and get a full screen devoted to that child, which might have further lists, e.g. education. One of the really neat things about being able to use IPs in this way is that both the list of children and the full screen describing a single child can be driven by the same IP - we just decide how much information we are going to show from that IP. By the way, since each screen was built using two substreams, we bracketed them together so that ISM1 would think of them as a unit - so ISM1 was using a substream of substreams.

Because field names are not unique in the repeating part of the screen, we could not use ISPF field names to control this part of the display, so we used a run-time table describing which fields from each IP went where in the repeating section. This had some interesting capabilities - ISM1 allowed you to specify more than 1 line per repeating IP, and the developer could also specify whether a "select" column (simulating the 1-byte column ISPF provides for selecting one or more items from a list) was required or not.

ISM1 also used the dynamic attributes which we talked about in Chapter 11 to keep track of which fields had been modified, and which were null. As I mentioned in that chapter, ISM1 also provided a special display for fields which had been "tagged" with error codes, and would let the user step through these errors using a reserved function key. ISM1 actually would not allow the user to go on to the next screen until all these "tags" had been removed one way or another! There has been lots of debate about whether this is a good idea or whether systems should be more forgiving! However, the important thing to remember is I am talking about the design of a single component - this in no way affects or is affected by the basic architecture of FBP.

So far, ISM1's abilities might seem about what you would need if you were "black boxing" a display function. However, it also provided another capability, which dramatically simplified the logic in the other components of the application: we have discussed this in Chapter 11 under the title of "representations". As I said in that chapter, representations mainly come into play when you need to present data to humans, or port it across systems.

In a prototype of an interactive application using straight ISPF I found three PL/I fields had to be defined for every numeric field on the screen:

  • the field in a computational format

  • a zoned decimal field (e.g. 000001234)

  • a character field for input in case the user wanted to modify the field

When we converted this prototype to use ISM1, the number of fields we had to declare in the HLL portion of the application dropped by 2/3! We also discovered a number of additional bonuses:

  • you could send an IP with an attached descriptor to ISM1 and it would automatically be displayed in the desired format

  • the user could enter the data in free-form, but you could be sure that it wouldn't get into the system unless it was a valid representation

  • you could implement a standard input convention for your whole shop - e.g. require that the field be clear apart from the incoming data (some screen managers allow you to leave junk at the end of a field following the data just entered)

  • you could send an IP to ISM1 for interactive handling, or you could send it to a file writer and you didn't need to make any changes to your data IPs. The effect of this was enormously improved testing and regression testing, because you could test a lot of your logic in batch.

On one project in IBM Canada, this last technique was used very effectively by my colleague, Philip Ewing. Later in this chapter I will share with you what he has written about that project.

We have now sketched out a screen management component ISM1, which accepts one or two substreams as input, and outputs them again after the user has responded. If you are working in the IMS/DC environment, it wouldn't be all that hard to split these functions and link them together using persistent storage.

Now let's look at the first figure in this chapter. We need to fill in the logic between SM and the application logic. To do this, the first step is to interpret the user's action. Restricting ourselves for simplicity to ISPF and 3270-type terminals, the user may decide to:

  • modify any data field, including Select fields as a special case

  • enter a command in the command area

  • hit a function key

  • hit a Program Attention key (this will lose modified data)

  • hit Attention

  • position the screen cursor to a particular field

These will of course often be combined, e.g. putting an M [for "maximum"] in the command line and hitting PF8 ["down"] causes a jump to the bottom of the data in ISPF.

All these actions have to be encoded so that downstream processes can decide what is the appropriate response. If we add in more modern devices and interfaces, obviously there are still more variations, e.g. monitoring key-strokes and mouse movements in real time, but it seems that we will still have the cycle (or maybe many concurrent ones) of display - user action - interpret user response - program action - display.

In the ISPF world, and also IMS/DC, function keys are usually treated as commands, so one of the standard outputs of our Screen Manager will be a "command". These may be the very frequent ones like UP, DOWN, END and HELP, which are almost universal, or more application-specific ones. It turns out that these commands are convenient bases for the decision about what to do next. Always remember that each of the components described here can be used independently of any other. Now, in Chapter 7 we described DFDM's dynamic subnets - subnets which were linked as separate load modules and were loaded in dynamically and given control by a special component called the Subnet Manager. This will provide a convenient way of subdividing and managing our application. The Subnet Manager is driven by IPs containing dynamic subnet names, so we need a component which will take the output of the Screen Manager and generate the subnet names for the Subnet Manager. Let's call this the User Response Analyzer (URA).

The URA component's job is to look up in a table patterns consisting of screen + action, screen only or action only, and decide what to do about them. As we said, since it sits upstream of the Subnet Manager, its main job is to select subnet names to be sent to the Subnet Manager, but you might decide to have it bypass the Subnet Manager, and send its input IPs directly to the Screen Manager. In this case, you could have it decide screen names. You could also have it do both.

You will notice that we haven't said where this table should be held: it could be compiled into a load module, stored as a flat file, or held in a data base. Perhaps a file would be appropriate during development, and a load module in production. You will perhaps notice our predilection for tables - this is one of the most important ways of achieving portable code (remember Bucky Pope's class codes, alluded to in an earlier chapter).

The URA table might therefore look something like this:

  Old Screen         User Action      Subnet           New Screen
A                   CHOICE1         SUBNET1          B
B                   END             A
A                   HELP            HELP_FOR_A
HELP_FOR_A          END             A  

Figure 21.2

Obviously this table is very easy to modify - in fact, if you add a comment capability (an asterisk in col. 1 means ignore this line), it really becomes self-explanatory.

The last component I am going to describe is the List Manager, another general component. Its fundamental metaphor was sets of lists which persisted in storage, organized by "levels" - thus employees might be on one level, their children, departments worked in and courses taken might be three different lists at the next level. It could accept commands to do various things with these lists and levels, such as "create a new level", "insert a list at the current level", "jump to the next lower level", "pop up one level", "output a list (non-destructively)", "delete a level", and so on. Although (because?) this component was very powerful, it took the most work to manage its input and output. It was very interesting for another reason also - the List Manager perhaps most closely resembled an OO "object", in that it had an internal state, being constantly modified by incoming commands (messages) with or without accompanying data. Its structure seemed to match our perception of what was going on in the prototypical interactive application - i.e. the user would display an employee, then ask to go down one level to find his or her children, pop back to the previous level, and so on. Because it was a single looper process, we could just manage these lists by working with IP pointers - we didn't have to pay the overhead of chaining or unchaining IPs. Also, it provided a focal point, in case we needed to store really big lists, where lists could overflow to disk. We also expected that, when we implemented this design on IMS, it would be very easy to dump all our lists to disk at the end of a transaction, and retrieve them when they were needed again.

In hindsight, the problems we ran into with the List Manager were probably to be expected, but they came as somewhat of a surprise to us! I believe we were still thinking of interactive applications as sequential, so the command-driven, single store made sense. However, it was so convenient to stash things away in the List Manager's storage that we had more and more processes sticking stuff in there and taking it out. The more complex our networks became, the harder it became to control the exact sequence in which the commands arrived at the List Manager. What we had done, of course, was to implement a somewhat more complex array of pigeon-holes, and the non-destructive read-out which seemed so attractive at first caused the same problems FBP was trying to avoid! Strange sequencing problems started to show up - lists would get attached to the wrong level, lists would show up on two different levels, and so on. In turn, the sequence of the command IPs had to be controlled more tightly, introducing still more complexity. In hindsight, I believe we would have been better off using tree structures flowing between processes, rather than complex data structures within a process. Alternatively, a List Manager should only be fed by a single process, and this is the way I have shown it in the next diagram. Lastly, I believe that the underlying metaphor may not have been quite correct. For instance, suppose the user is stepping through an employee's employment history and decides to start looking at her courses. Should this be made another level? Or are all these lists at the same level? A better metaphor might have been to be able to pop up new windows as new lists are requested. It's also useful to be able to open multiple windows on the same list (but you have to be careful about updates!).

We can now show the final picture. Remember that this is only a skeleton - you can add additional processes to the diagram, and extend it in other ways also. And remember also that the List Manager, although shown in the diagram, is not the only way to manage storage of data.

where SM  is a Screen Manager
      ST  starts the network as a whole
      URA is the User Response Analyzer
      SUBN is the Subnet Manager
      LM  is the List Manager  

Figure 21.3

What we have described here is the structure we called the DOEM, pronounced "dome", (DFDM On-line Environment Manager), still fondly remembered by some of the people who worked on it! It was at the same time a skeleton structure, a set of components and an approach to designing interactive applications. This is reuse at a higher level than the level we have been mostly talking about up until now, and from that point of view a precursor of the way interactive systems will be built in the future. While the DOEM was a very powerful set of concepts, some of its components were more satisfactory than others in terms of their encapsulation of useful function and the simplicity of the underlying mental image. In some ways, the DOEM fell into the pitfall I have warned about elsewhere in the book - we tried to make it very general, based on our ideas of what a DOEM should provide, without frequent consultations with real users. Or we may have been talking to the "wrong" users. We never did build it for the IMS/DC platform, although we basically knew how to go about it. As it turned out, we didn't need that implementation anyway, for the reasons I am about to relate. This story is salutary, so I am going to tell it in some detail, as a cautionary tale for those embarking on developing reusable code.

Most of the time we were working on the DOEM, we were supporting two projects - let's call them A and B. The intent was to provide team A with an IMS/DC version of the DOEM, and team B with a CMS version. This seemed reasonable because a number of the components could be shared, and, although the CMS version was certainly simpler (single Screen Manager module, etc.), we understood pretty much how to build the DOEM on IMS/DC. However, the two teams' approaches to working with us were very different. The A team tended to be demanding and critical, frequently asking for specialized modifications of components or new facilities just for their own application, while B was more willing to work with us and to stay within the facilities that were already available or in plan. Both projects had the potential to be very important products, for different reasons, and both groups felt that they were getting benefit from DFDM, but both of them required quite a bit of our time, both to provide general support and to code and test the reusable components being supplied for the two environments.

Our development team was a small one and, under the circumstances, was getting stretched very thin trying to support both projects! Finally, management decided that we could only support one of these projects, and, after much soul-searching, they picked B. We started working intensively with B to make sure that the CMS DOEM worked well with their product, and as the two started to come together, we all realized that this had been a good decision. This product is now a successful product in its own right in the Latin American market.

The A team were told that they could continue to use DFDM, but not the DOEM, and that we could no longer afford to give them special support. We really expected them to decide to drop the use of DFDM altogether, and while this would have been disappointing, we felt that this would be a pragmatic decision on their part. However, at this point, a very strange thing happened: faced with the possibility of losing the use of this productivity tool and having to redesign and rewrite a lot of their code, the A team turned right around and started to solve their own problems using basic DFDM! Instead of having us build complex generalized components, they found simpler ways of doing what they needed, and the result was a less complex, more maintainable system. Their product is also now a success, and is saving the company considerable amounts of money.

Actually, an additional project using the DOEM appeared suddenly on the scene one day, rather to our surprise! It seemed that a bright young contractor had been given the job of building a small interactive system, and had built it in a matter of a few weeks, using the DOEM, without telling any of the DOEM development team! We were very conscious that our documentation was nowhere near adequate at that time, but he said he had no trouble understanding and using it! Of course, he is very bright, but how often does something like that happen using conventional programming tools?

Since fairy tales usually have morals, let me propose the following: "Sometimes it is better to redesign a squeaky wheel than just put more oil on it".

The Screen Manager, ISM1 (actually an earlier version of it) was also used by itself, before we even thought of the DOEM, on an earlier project within IBM Canada, and this project became very successful, not least because Philip Ewing was excited by the concept of FBP (he still is!), and was discovering neat new uses for it all the time. As you may have gathered, ISM1 was a very powerful component, and all by itself considerably simplified the development of interactive applications. Its development predated the rest of the DOEM by several years, so we had used it for several small projects. Here is what Philip has written about our experience on this project (called BLSB).

DFDM was selected for use on the BLSB project because of the significant productivity improvements that were anticipated. The development team was not disappointed. Significant savings were realized in the following ways:
  1. We were able to prototype more easily, beginning with a simple screen display, and adding functions one by one until the user was satisfied. The full function prototype could be modified to add a new edit or data-base lookup in a matter of hours, without disrupting the existing code.
  2. Testing was made simpler because we were able to unplug the online screens and feed in test SCREEN REQUEST ENTITIES [abbreviated to SREs - these correspond to the "request IPs" referred to above] from files, and save the returned SREs into separate files based on type of error. In this way all of the application function in the online system could be tested in batch.
  3. Building on the experience gained in the function testing, the legacy data was converted to the new database format by feeding in the old data in SRE format (simulating re-keying all of the previous 3 years of data through the new system). The errors were saved in separate files based on the ERROR CODE that the application put into the SRE before returning it to the screen. Each file was known to contain only one type of error. In three iterations through this process we were able to convert and load 64,000 history records with only 12 records needing to be re-keyed manually. In addition to not having to write a separate conversion program, we were also assured that all of the data that was now in the database had passed all of the rigorous editing that had been built into the new application logic.
  4. A great deal of effort in the design stage was saved because we could decompose functions to very granular levels before implementing. This meant that less thought needed to be put into the way different functions might affect each other, because different functions were now completely decoupled.
  5. The "off the shelf" screen display function alone saved about 700 lines of application coding to handle ISPF panel displays. We did know ISPF before starting this project, but would not have needed to, since all of the ISPF specific code was in a DFDM-supplied function.
"Less than 24% of the functions needed to be coded by the project, the rest came off the shelf. Furthermore, of the ones that we did have to code, the most complex was about 100 lines of code."

A comment made to me recently about the BLSB project: "We allowed 3 weeks for testing, but it worked the first time."!

Another project which was very interesting was a system we built to do project resourcing, called PRORES, designed by A. Confalonieri, and built by myself, using a Screen Manager similar to ISM1 and a User Response Analyzer, running on CMS. This Screen Manager was also driven by IPs with descriptors, but generated and accepted 3270 data streams (extended data stream), rather than using ISPF. It used a WYSIWYG representation of the screen, and was the heart of the prototyping tool which I have mentioned several times elsewhere in this book.

The logic for PRORES was all written in REXX, and, considering that PRORES had to do a very large number of date calculations for each screen, its performance was surprisingly good. The basic idea was that, for each project that you were working on, you just entered a number of person-months and PRORES would generate all the dates and staffing requirements for the standard 5 phases of a project (Requirements, External Design, Internal Design, Development, Implementation), using formulae based on the standard "Volkswagen" shape or "snail curve" that most projects follow. You could also specify that a project should be "flat", instead of standard. If you constrained the end date to be earlier or later, you would get a more humped or flattened staffing curve. If you didn't specify the start date, it would use the date of the day you ran it on. You could also request a graphical display of a department or division's projects, and it would use PGF, GDDM's Presentation Graphics Facility) to show all the projects in Stacked Bar format on a single chart across time. The Stacked Bar format meant that the project loadings were displayed cumulatively, so the top edge of the diagram showed the total staffing curve for the whole department or division. Management could then flatten and lower the overall curve by shifting projects around, stretching them out or compressing them, or moving projects between departments. Suppose you had two projects PROJ1 and PROJ2, both with the characteristic snail curve:

Figure 21.4

Now getting PGF to superimpose them gives the following kind of picture:

Figure 21.5

The outer "envelope" then shows the total cumulative loading for the two projects. With a relatively large number of projects, you can adjust things so that the top line is flat most of the time. "Flat projects" (projects which had a constant loading over their whole lifetime) could be used to handle things like vacations, education, overhead, etc. All dates were constrained to business days, and once they were all calculated, individual project dates could be modified as desired.

Technically, this project was interesting because of the languages and software involved. It was also a decision assist type application. You would get a screen just full of generated dates, and then, depending on which dates were changed, it would do intelligent things with them. This meant that it was very important for the Screen Manager to report on which fields had been modified. All the calculations were done in REXX. Date displays were handled by means of date input and output routines written in Assembler, driven by the Screen Manager using descriptors, so REXX only saw dates in canonical form (number of days from a reference date). If you pick your reference date correctly, then day of the week is just "date modulo 7", and you can make Sunday 0, Monday 1, and so forth up to 6 (Saturday). This system was used intensively over a short period to help reorganize a portion of our division.

I am going to mention briefly another use I made of the Screen Manager because, although it was not a complete project, I found it very suggestive of the shape of user workbenches to come. Most of the application development on IBM hosts is done today (1993) using ISPF/PDF on TSO or CMS EXECs. Within this kind of workbench, developers use quite a wide range of "languages": HLLs, Assembler, JCL, DBDs, PSBs, MFS, FBP networks (hopefully!), 4GLs, and of course documentation in his or her own national language. All of these are held in different data sets, and have different, although standard, processing applied to them. PDF follows the "action/object" paradigm: decide the action, then select the object. Having to choose the action first means that you always have to know what language the thing you are working on was written in. Also, in PDF you pick the same EDIT for everything, but then usually have to go to a completely different menu to process the text you have entered, and you always have to reenter the object's name, even if you were working on it a few seconds ago! Native CMS is a little different since it is command-oriented, but here you have to remember the command name to do the desired processing. Of course both lists support lists with optional "wildcards", but it is still hard to move a single object through a series of phases (like edit, compile, run test, etc.).

I figured that it would be nice if everything a developer was working on could be treated as an object of a particular type, with a unique name. The developer could just select the object she wanted to work on, and the system would know what language it was written in, and display an action bar showing what actions could be applied to it. So the interface would prompt you for a component name (with optional "wildcards"), or you could ask for all components of a given type, and you could just click on an entry in the action bar, without having to worry about choosing an inappropriate action for the object's type. Make the whole thing table-driven, and you have a very powerful, friendly system for application development - I know, because I built one for the CMS environment! All I had to do was select the object, and an appropriate action bar would come up, which would let me select from a list of CMS EXECs (e.g. EDIT, COMPILE and the most important one of all: DESCRIBE). If the object types are user-modifiable, you can be more specific, i.e. "Assembler source" could be split into "programs" and "macros", or you could have types like "screen", which will generate MFS or BMS, plus declares for the message layouts. You could drive syntax-sensitive editors for different languages, or for objects of type "diagram", you could make the EDIT option call a picture editor. I also felt that each action should go as far as possible, i.e. if you decide to COMPILE an Assembler source program, what you really want to do is translate it from human format to machine format - there is no particular point in making ASSEMBLE one action and then LINKEDIT another. The COMPILE action could also automatically update control tables for use by tools like MAKE or symbolic debuggers.

The reason I said this prototype was suggestive is that, if you can build this kind of development environment using FBP, and we know that FBP also lends itself to building compilers and text processing software, this conjures up the very appealing image of a totally user-modifiable development environment, built out of communicating standard components. This wouldn't be just a set of tools - this would let developers continuously expand and improve the workbench itself!

Last, but not least, in this rather varied set of examples, there was a project, which might be called an Electronic Information Booth, to provide a visitor to our building with information such as how to find people, information about the building (layout, statistics, etc.), promotions during the month, a "trading post", and the cafeteria menu for the week. Everything was to be highly graphical and menu-driven. I prototyped this using the same screen manager I described above and a User Response Analyzer to implement the paths between the screens. As I mentioned above, this screen manager had a fairly complete graphical specification facility based on polygons, so it was well suited to developing a lot of pictures in a hurry! Since I already had the Screen Manager and User Response Analyzer, it was really just a matter of working with our very talented resident artist, Bob White, to develop the pictures. As it was around Thanksgiving, we decorated the Thanksgiving menu with a rather nice stylized corn-cob. During the Christmas season, we put the Christmas menu on its own screen with a little Christmas tree at the top! Later on, it was decided to implement this application using PCs, but our prototype certainly played a significant role in convincing management of the validity of the idea.