Is there an obvious way to do this in python?

[H J van Rooyen]

Hi,

I want to write a small system that is transaction based.

I want to split the GUI front end data entry away from the file handling and
record keeping.

Now it seems almost trivially easy using the sockets module to communicate
between machines on the same LAN, so that I want to do the record keeping on one
machine.

I want to keep the "server" machine as simple as possible - just doing record
keeping on a stimulus response basis - I would prefer it to do one thing at a
time to completion because this style of operation, though limited in
performance, keeps a lot of hassles out of life - a transaction has either
completed, or it has not - recovery scenarios are relatively easy...

Up to this point, I don't have a problem - my toy system can create a dummy
transaction, and I can echo it from the "server" machine, with more than one
"user" machine running - so I think it is feasible to have several tens of "data
entry terminal" systems running, served by one not very strong machine.

Now what I would really like to do is to differentiate between the 'User"
machines, so that some can do a full range of transactions, and others a limited
range.

And I would like to make this flexible, so that it becomes easy to introduce new
transactions, without having to run around updating the code in all the user
machines, with the concomitant version number hassles.

And I would like to do the whole thing in python - so my question is this - is
it possible to do the equivalent of dynamic linking? - i.e. if I keep a list of
what a user is allowed to do - can I somehow send him just the bits he needs to
do the job, without having to change the static code on his machine? - it seems
to me that the eval() thingy could possibly do this for me, by sending it data
that makes it do import statements followed by calls to whatever... - will this
work, or is there a better way?

Or has all this been done already? - and no I don't want a web server and php
and browsers and Java and html or xml... - I want to write something that works
simply and reliably - its just short message accounting type data...

- Hendrik

[H J van Rooyen]

"Yu-Xi Lim" <yu**@ece.gatec h.eduwrote:
| H J van Rooyen wrote:
| |
| |And I would like to make this flexible, so that it becomes easy to
introduce
| new
| |transactions, without having to run around updating the code in all the
user
| |machines, with the concomitant version number hassles.
| |
| |Then you want a web front end.
| >
| This seems to me to assume that the server does all the work
|
| Depends on what you mean by "all the work."

What I mean by this is that the server does stuff that I think belongs on the
client -
like getting involved in the nitty gritty of what the client should display -
I want the client to be smart enough to do the assembly of the elements of a
transaction
by itself, going back to the server for data only when its needed - remember
this is essentially an
accounting type data entry package - most of the stuff is typed in as text, when
processing documents from the outside, while the locally produced docs like
invoices would be generated by the system, to a large extent by making choices
amongst alternatives known to the server -

so I see the client interacting with the server quite a lot, eventually to be
able do things like auto completion of things like stock codes and descriptions,
customer details, etc. - but I don't want every keystroke flying over the LAN
and
being handled by the server...

In a sense I want to build an architecture that assembles a record from a mix of
user input and user choices out of alternatives (which unfortunately would have
to be kept on the server) and that then ships this to the server to process, and
to keep track of - such a record would be either accepted or rejected by the
server, and it would be the responsibility of the client to ensure that the
transaction is completed - like in a banking system, I envisage ways for the
client to 'poll' the server to get the state of the last transaction, to make
this possible.

|
| Operations such as filtering results are best done at the server unless
| your have extremely high-bandwidth connections so that each client can
| examine the entire data set and perform the operations themselves (with
| minimal time of course, since your other clients may be waiting on that
| transaction).
|
| Transactions, too, will have to be supported by the server, or else you
| may be left with partial transactions if a client gets disconnected
| somehow as well as the need to implement complex locking systems yourself.
|
| As for the other conditions, such as privilege and access control, I
| think you'd find that centrally managed is the most manageable in the
| long run.
|
| You won't regret making it server-centric. The experts have already done
| the optimisations and have ways of getting around the possible
| bottleneck of having a single server perform most of the operations.
|
| |Yes, it's called a web frontend for a SQL DBMS. There's no shortage of
| |Python frameworks to do this kind of things.
| |
| >
| I kind of wanted to avoid the web based stuff - the application data is so
small
| and trivial, in a sense.
|
| You'd find that the python frameworks, especially those modeled after
| Ruby on Rails, make creating trivial applications, such as the front-end
| you describe, trivial. Go take a look at Django and TurboGears and the
| others. Some have videos demonstrating stuff like how to make a
| blog/wiki/other-database-app in 5 minutes. Most come with a built-in
| webserver, though generally those aren't tested or guaranteed for
| high-load environments.

I get the feeling I am drinking out of a fire hose... I will look.

|
| All you need to add is a DB. Most recommend postgresql, and I'd
| recommend that too, to provide the features you are looking for. Avoid
| the lightweight DB systems such as Gadfly, sqlite, MS Jet/Access since
| those don't have the necessary features.
postgres seems the way to go - if there is anything that is coming across clear,
it is this.

Have you any ideas about the "code change on the fly" requirement? or is it a
complete no no?

thanks - Hendrik

[H J van Rooyen]

"Bruno Desthuilliers" <on***@xiludom. growrote:
|H J van Rooyen wrote:
|"Bruno Desthuilliers" <bd************ *****@free.quel quepart.frwrote :
|>
|>
8<-----------------------------------------------
||You'll at least need bits of SQL (but SQLAlchemy may hide away most of
||it) and HTML (but there are some python packages that knows how to build
||HTML from declarative Python code).
||
|>
|that is good news - which packages?

http://divmod.org/trac/wiki/DivmodNevow
http://divmod.org/trac/wiki/DivmodNevow/Athena
http://starship.python.net/crew/frie...html/main.html
http://aspn.activestate.com/ASPN/Coo.../Recipe/366000
http://dustman.net/andy/python/HyperText/
http://pyhtmloo.sourceforge.net/

Thanks for the references I will try to check them all out

8<--------------------------------------------------
|If my original post was unclear I am sorry - the point I want answered, if
|possible, is how to make the client code effectively updateable on the fly -
|because the answer to this will influence the whole design of the rest of the
|system...
|
|This is something I have been thinking about... IMHO what you want is
|not to "update client code on the fly", but to make the client mostly a
|kind of interpreter for what the server sends in. That is, the client
|code itself doesn't contain any application logic, it gets it from the
|server and execute it. This can certainly be done with Pyro.
|
|Now while this may be an interesting project, I'm not really sure it's
|worth the effort when we already have HTTP, HTML and AJAX...

You may be right and it might not be worth the trouble - but what you mention
above is closer to the sort of thing I have in mind - it is essentially using
python to create a script language, and moving scripts around - but hey - python
is already a script language...

so if Pyro is for 'moving the scripts around' - Then that is what I must look at
very hard...

- thanks - Hendrik

[Bruno Desthuilliers]

H J van Rooyen wrote:

"Bruno Desthuilliers" <on***@xiludom. growrote:

(snip)

|If my original post was unclear I am sorry - the point I want answered, if
|possible, is how to make the client code effectively updateable on the fly -
|because the answer to this will influence the whole design of the rest of the
|system...
|
|This is something I have been thinking about... IMHO what you want is
|not to "update client code on the fly", but to make the client mostly a
|kind of interpreter for what the server sends in. That is, the client
|code itself doesn't contain any application logic, it gets it from the
|server and execute it. This can certainly be done with Pyro.
|
|Now while this may be an interesting project, I'm not really sure it's
|worth the effort when we already have HTTP, HTML and AJAX...

You may be right and it might not be worth the trouble - but what you mention
above is closer to the sort of thing I have in mind - it is essentially using
python to create a script language, and moving scripts around - but hey - python
is already a script language...

Yes, but it's not (alas) supported by browsers...

so if Pyro is for 'moving the scripts around' - Then that is what I must look at
very hard...

It's not for "moving the scripts around", it's for remote objects - kind
of like Java's RMI, but, well, much more pythonic !-). Now the point is
that Python being very powerful when it comes to dynamism and
introspection, it should be possible to have a common client that
basically just knows how to connect to the application server (using
pyro). Once connected, the client asks the server for a set of objects
(forms, menus etc) and the corresponding data. These objects then use
the same mechanism to interact with the server. It's basically similar
to the interaction between a browser and a web app - in that the client
is potentially able to run any application sent by the server -, but
with much more specialized client and app server and another protocol -
and no other language than Python.
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'o****@xiludom. gro'.split('@')])"

[H J van Rooyen]

"Nick Vatamaniuc" <va******@gmail .comwrote:
|HJ,
|
|As far as GUI language/library goes:
|
|Some people suggested HTML, but I think HTML is a very awkward way to
|create a good looking dynamic GUI that is _both_ easy to use and fast
|and easy to design (which is what you would want probably). Just to
|have a nice user editable table for example, you would have to jump
|through hoops (using Javascript, DOM, CSS etc), while you could do it
|much easier with PyGTK and wxPython, especially if you use a gui
|designer like Glade or wxGlade. Even Tkinter beats HTML as far as
|building GUIs in Python goes. I believe this might change in the future
|with the adaption of SVG but that will take a while... That said, if
|your interface can "get by" with just buttons, text boxes, and text
|areas HTML will be the best choice.
|

At the moment my toy system just uses Tkinter- Buttons, Labels, Entry boxes and
Listboxes (which I populate from a dict) - and it has logic in it to do only one
dummy transaction, which I just ship to the server machine where nothing
happens - it is just echoed back to the client which prints it on stdout - If I
stay with this it will be a most uninteresting GUI - but that is not the point
at issue now...

|
|As far as "updating-on-the-fly" goes:
|
|For the client to get the code on the fly you will have to implement
|some sort of a downloader in the first place that when the user logs
|in, it downloads the GUI code from the server and runs it. So if you
|update the code the day before the next day they will get a different
|interface, or if a new user/machine type is created you just have the
|new code ready on the server and it will automatically be downloaded
|and run, is that right?
|

This is broadly what I had in mind, yes - but sort of down to a transaction
level - this user does invoicing, this one enters cheques, this one does credit
notes, and their supervisor can do all three, and in a different department its
different because the jobs are different, but the invoicing GUI module is the
same for wherever its used...

|Here is then how I see your use case:
|1) You would define your business rules in your database, you will have
|usernames, user types, access rights, data tables, columns types,
|relations, views, etc...

Yes no matter how you do it, you need to keep a per user or per machine set of
what can be done. - in banking terms - a sort of Terminal Management System...

My thinking is simpler than this, because you are already thinking in "data base
speak" - now dont get me wrong - I realise that I will have to use a database -
but for me to start thinking in terms of views and stuff is kind of premature
when I dont even have a clear picture in my head of the kind of data the said
management system should keep, as I am not sure of what can, and cannot be done.

|2) Then each user type will have a specific interface GUI code kept on
|the server (perhaps as a zipped binary GUI.zip in a database column
|called ClientSpecificG UI).

I would like to split this down further - see above - so for each user there is
a sort of *pointer* to each of the kinds of transactions she can do, and these
are shipped separately and *linked* at the client side...

|3) The client starts your Application.py which is the same across all
|clients. They will enter their username/password. The Application.py
|then sends those to the server to log into the DB.

*nods*

|4) After successful login, the Application.py performs a SELECT query
|to download the zipped GUI.py file.
|5) GUI.py is unzipped and executed to start the GUI. The Application.py
|code will pass the DB connection object to the GUI.py, so the GUI can
|continue to talk with the database. GUI.py runs and does its magic, in
|the meantime Application.py waits for GUI.py to finished and then both
|exit.

|Is that what you had in mind?

Very close - I have not even thought this far - I did not think of building a
GUI for each user, I thought of building it for each transaction - kind of a
series of things like my toy - and then I got stuck on the "linking the separate
transaction guis into an app on the fly" bit, which is why I started the
thread - I really want to know if it is possible to do this sort of thing in
Python, and so far Bruno has come up with Pyro, while everybody else (including
Bruno) is beating me over the head with HTML

Now part of the reason I would like to go the transaction type route instead of
the "per user" route is robustness and maintainability , and the ability it
would give me to introduce new transaction types easily - as I see it if say an
invoice's GUI code is stable I would never have to touch it again even if I
combine it with anything else, as it would have been designed from the start to
combine with others of it's own ilk, under a kind of communications controller
that is standard...
|NOTE: This means that the client will need to have all the required
|libraries at just the right versions. Imagine that your user decides to
|upgrade to Python 3000 because it sounds cooler than plain old Python
|2.4 ;) , but then they won't realize that it will break your code and
|they might not be able to run your application anymore. So you would
|have to know at least roughly how much control over the whole client
|machine you will have. Will they all be regular desktops that users
|will use day to day and then once in a while launch your application
|then close it, or will these all be dedicated terminals like an ATM?
|The two are very different. You can assume complete control of all the
|OS environment in a dedicated terminal but not in the case of a user
|desktop.

True - have not even considered this - this would, I imagine, vary from site to
site, and depend more on local IT policy - this is probably the strongest
argument to date against going this route, as these machines would not be as
tightly controlled as an ATM...

but then - If I want to use Python in the client at all, I would have to somehow
come to terms with this - its more the normal sort of version control that would
have to be done - after all if a user's machine is an XT running DOS - then its
going to have to be upgraded before it can be used as a terminal...

So this is more an argument against the use of Python on the client and I don't
like that...

|Hope this helps,
|Nick Vatamaniuc

8<----------------------------------------------------

Yes it does, Thanks. I feel we are getting closer to the point where I can
decide if what I am thinking of is practical or a pipe dream - it sounded so
simple - make a series of guis for a series of transactions, ship them to the
client, where there is a simple top level thingy that swallows them and ties
them all together and handles the comms to the server... I mean the language is
called Python... :-)

- Hendrik

[Bruno Desthuilliers]

H J van Rooyen wrote:
(snip)

I would like to split this down further - see above - so for each user there is
a sort of *pointer* to each of the kinds of transactions she can do, and these
are shipped separately and *linked* at the client side...

(snip)

>
Now part of the reason I would like to go the transaction type route instead of
the "per user" route is robustness and maintainability ,

Nothing prevents you from managing rights with a user -allowed
transactions mapping...

BTW, note that "transactio n" has a very definite meaning in DBMS jargon,
which is somewhat different from what you use this term for. This may
become a source of confusion...

>
|NOTE: This means that the client will need to have all the required
|libraries at just the right versions. Imagine that your user decides to
|upgrade to Python 3000 because it sounds cooler than plain old Python
|2.4 ;) , but then they won't realize that it will break your code and
|they might not be able to run your application anymore. So you would
|have to know at least roughly how much control over the whole client
|machine you will have. Will they all be regular desktops that users
|will use day to day and then once in a while launch your application
|then close it, or will these all be dedicated terminals like an ATM?
|The two are very different. You can assume complete control of all the
|OS environment in a dedicated terminal but not in the case of a user
|desktop.

True - have not even considered this - this would, I imagine, vary from site to
site, and depend more on local IT policy - this is probably the strongest
argument to date against going this route, as these machines would not be as
tightly controlled as an ATM...

but then - If I want to use Python in the client at all, I would have to somehow
come to terms with this - its more the normal sort of version control that would
have to be done - after all if a user's machine is an XT running DOS - then its
going to have to be upgraded before it can be used as a terminal...

So this is more an argument against the use of Python on the client and I don't
like that...

Well, this is an argument against any kind of fat client whatever the
language, and one of the key reason for the growing demand for web
applications - reducing deployment problems to the minimum...
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'o****@xiludom. gro'.split('@')])"

[Bryan Olson]

H J van Rooyen wrote:
[...]

This is broadly what I had in mind, yes - but sort of down to

a transaction

level - this user does invoicing, this one enters cheques,

this one does credit

notes, and their supervisor can do all three, and in a

different department its

different because the jobs are different, but the invoicing

GUI module is the

same for wherever its used...

Pretty much any business has to deal with those things. You are
right about the need for transactions and authorization, and
that you don't want to have to manually update all the clients
whenever your modify the applications. Everyone has those
problems, and consequently there's a large body of experience
and conventional best practice. The highly redundant advice
you've been getting is recommending that you to do those things
that way most successful modern systems do them.

No question what you describe calls for a DBMS. Further, this
sounds like a job for a "three tier" architecture, what Dennis
L. Bieber explained as [db-server] <-[app-server] <-[client].
In the old days a three-tier system called for a large
data-processing staff; today one guy can set up a simple one in
an afternoon.

In a three-tier system, either the database, the server
application can enforce authorization policy. In order for the
database to enforce authorization, the application creates one
database connection for each client user, and logs in as the
user. That works well for most accounting-type systems, where
all the users are known in advance. In systems such as web
shopping carts, the application typically has its own user ID,
and creates all database connections as itself. The app, not the
database, is then responsible for keeping users from reading and
altering each other's data.

Trying to enforce authorization at the client is generally a
mistake. The user has too much control over his local machine.
He is not limited to running just the code you provide.
Don't get too hung up on choice of DBMS, but stick with portable
standard SQL. I bet you'll find your potential customers want it
to run against the database they already have. The particular
tables your system needs will become part of their larger
schema.
--
--Bryan

[Nick Vatamaniuc]

Hendrik,

---snip---
Now part of the reason I would like to go the transaction type route
instead of the "per user" route is robustness and maintainability , and
the ability it would give me to introduce new transaction types easily
- as I see it if say an invoice's GUI code is stable I would never have
to touch it again even if I combine it with anything else, as it would
have been designed from the start to combine with others of it's own
ilk, under a kind of communications controller that is standard...
---snip---

It could be possible to separate your GUI into various modules. So you
could have for example:
BillingGUI.py
InvoicingGUI.py
QueryGUI.py
and so on.
Then your central control application (Application.py ) would get the
set of module names allowed for that client to run during login. So
after the user logs in, the next thing would be to "SELECT ..." all
the GUI modules from the database to be downloaded. The client won't
know which ones are there, only the server.

So the set of possible GUIs will be sent over to the client (preferable
in a zipped form). They are unzipped in some temporary folder (here you
can do some caching to only download if the set changed).

Then it would be imporant for you to create some nameing rule or some
interface so the Application.py will know what GUI modules look like.
For example you could have the pattern [Capitalizedname]GUI.py be a
GUI module. The Application.py will then inspect the folder, and create
a button and label for each possible GUI module and present that to the
user. When the user clicks on the Button, the Application.py window
gets hidden and the [Modulename]GUI.py module is executed. When done,
the Application.py will be shown again and the user can either continue
with another module they are allowed to use or quit.

How does that sound?

Also, you mentioned that you won't have more than a couple of simple
fill-in forms and with drop-down options and so on. HTML then might be
the way to go. If that's all there will ever be just try to do HTML
(see Cherrypy, Turbogears and others....). If your GUI will be more
complicated in the future, just stick with what you know (Tkinter for
example).

Good luck,
Nick Vatamaniuc
H J van Rooyen wrote:

"Nick Vatamaniuc" <va******@gmail .comwrote:
|HJ,
|
|As far as GUI language/library goes:
|
|Some people suggested HTML, but I think HTML is a very awkward way to
|create a good looking dynamic GUI that is _both_ easy to use and fast
|and easy to design (which is what you would want probably). Just to
|have a nice user editable table for example, you would have to jump
|through hoops (using Javascript, DOM, CSS etc), while you could do it
|much easier with PyGTK and wxPython, especially if you use a gui
|designer like Glade or wxGlade. Even Tkinter beats HTML as far as
|building GUIs in Python goes. I believe this might change in the future
|with the adaption of SVG but that will take a while... That said, if
|your interface can "get by" with just buttons, text boxes, and text
|areas HTML will be the best choice.
|

At the moment my toy system just uses Tkinter- Buttons, Labels, Entry boxes and
Listboxes (which I populate from a dict) - and it has logic in it to do only one
dummy transaction, which I just ship to the server machine where nothing
happens - it is just echoed back to the client which prints it on stdout - If I
stay with this it will be a most uninteresting GUI - but that is not the point
at issue now...

|
|As far as "updating-on-the-fly" goes:
|
|For the client to get the code on the fly you will have to implement
|some sort of a downloader in the first place that when the user logs
|in, it downloads the GUI code from the server and runs it. So if you
|update the code the day before the next day they will get a different
|interface, or if a new user/machine type is created you just have the
|new code ready on the server and it will automatically be downloaded
|and run, is that right?
|

This is broadly what I had in mind, yes - but sort of down to a transaction
level - this user does invoicing, this one enters cheques, this one does credit
notes, and their supervisor can do all three, and in a different department its
different because the jobs are different, but the invoicing GUI module is the
same for wherever its used...

|Here is then how I see your use case:
|1) You would define your business rules in your database, you will have
|usernames, user types, access rights, data tables, columns types,
|relations, views, etc...

Yes no matter how you do it, you need to keep a per user or per machine set of
what can be done. - in banking terms - a sort of Terminal Management System...

My thinking is simpler than this, because you are already thinking in "data base
speak" - now dont get me wrong - I realise that I will have to use a database -
but for me to start thinking in terms of views and stuff is kind of premature
when I dont even have a clear picture in my head of the kind of data the said
management system should keep, as I am not sure of what can, and cannot be done.

|2) Then each user type will have a specific interface GUI code kept on
|the server (perhaps as a zipped binary GUI.zip in a database column
|called ClientSpecificG UI).

I would like to split this down further - see above - so for each user there is
a sort of *pointer* to each of the kinds of transactions she can do, and these
are shipped separately and *linked* at the client side...

|3) The client starts your Application.py which is the same across all
|clients. They will enter their username/password. The Application.py
|then sends those to the server to log into the DB.

*nods*

|4) After successful login, the Application.py performs a SELECT query
|to download the zipped GUI.py file.
|5) GUI.py is unzipped and executed to start the GUI. The Application.py
|code will pass the DB connection object to the GUI.py, so the GUI can
|continue to talk with the database. GUI.py runs and does its magic, in
|the meantime Application.py waits for GUI.py to finished and then both
|exit.

|Is that what you had in mind?

Very close - I have not even thought this far - I did not think of building a
GUI for each user, I thought of building it for each transaction - kind of a
series of things like my toy - and then I got stuck on the "linking the separate
transaction guis into an app on the fly" bit, which is why I started the
thread - I really want to know if it is possible to do this sort of thing in
Python, and so far Bruno has come up with Pyro, while everybody else (including
Bruno) is beating me over the head with HTML

Now part of the reason I would like to go the transaction type route instead of
the "per user" route is robustness and maintainability , and the ability it
would give me to introduce new transaction types easily - as I see it if say an
invoice's GUI code is stable I would never have to touch it again even if I
combine it with anything else, as it would have been designed from the start to
combine with others of it's own ilk, under a kind of communications controller
that is standard...
|NOTE: This means that the client will need to have all the required
|libraries at just the right versions. Imagine that your user decides to
|upgrade to Python 3000 because it sounds cooler than plain old Python
|2.4 ;) , but then they won't realize that it will break your code and
|they might not be able to run your application anymore. So you would
|have to know at least roughly how much control over the whole client
|machine you will have. Will they all be regular desktops that users
|will use day to day and then once in a while launch your application
|then close it, or will these all be dedicated terminals like an ATM?
|The two are very different. You can assume complete control of all the
|OS environment in a dedicated terminal but not in the case of a user
|desktop.

True - have not even considered this - this would, I imagine, vary from site to
site, and depend more on local IT policy - this is probably the strongest
argument to date against going this route, as these machines would not be as
tightly controlled as an ATM...

but then - If I want to use Python in the client at all, I would have to somehow
come to terms with this - its more the normal sort of version control that would
have to be done - after all if a user's machine is an XT running DOS - then its
going to have to be upgraded before it can be used as a terminal...

So this is more an argument against the use of Python on the client and I don't
like that...

|Hope this helps,
|Nick Vatamaniuc

8<----------------------------------------------------

Yes it does, Thanks. I feel we are getting closer to the point where I can
decide if what I am thinking of is practical or a pipe dream - it sounded so
simple - make a series of guis for a series of transactions, ship them to the
client, where there is a simple top level thingy that swallows them and ties
them all together and handles the comms to the server... I mean the language is
called Python... :-)

- Hendrik

[H J van Rooyen]

"Bruno Desthuilliers" <on***@xiludom. gro>wrote:
| H J van Rooyen wrote:
| "Bruno Desthuilliers" <on***@xiludom. growrote:
| (snip)
| |If my original post was unclear I am sorry - the point I want answered,
if
| |possible, is how to make the client code effectively updateable on the
fly -
| |because the answer to this will influence the whole design of the rest of
the
| |system...
| |
| |This is something I have been thinking about... IMHO what you want is
| |not to "update client code on the fly", but to make the client mostly a
| |kind of interpreter for what the server sends in. That is, the client
| |code itself doesn't contain any application logic, it gets it from the
| |server and execute it. This can certainly be done with Pyro.
| |
| |Now while this may be an interesting project, I'm not really sure it's
| |worth the effort when we already have HTTP, HTML and AJAX...
| >
| You may be right and it might not be worth the trouble - but what you
mention
| above is closer to the sort of thing I have in mind - it is essentially
using
| python to create a script language, and moving scripts around - but hey -
python
| is already a script language...
|
| Yes, but it's not (alas) supported by browsers...
|
| so if Pyro is for 'moving the scripts around' - Then that is what I must
look at
| very hard...
|
| It's not for "moving the scripts around", it's for remote objects - kind
| of like Java's RMI, but, well, much more pythonic !-). Now the point is
| that Python being very powerful when it comes to dynamism and
| introspection, it should be possible to have a common client that
| basically just knows how to connect to the application server (using
| pyro). Once connected, the client asks the server for a set of objects
| (forms, menus etc) and the corresponding data. These objects then use
| the same mechanism to interact with the server. It's basically similar
| to the interaction between a browser and a web app - in that the client
| is potentially able to run any application sent by the server -, but
| with much more specialized client and app server and another protocol -
| and no other language than Python.
|

This is getting more and more interesting - its not the simple minded mechanism
I had in mind, but it will achieve the same thing, and it exists already - and I
can imagine it to be a very powerful mechanism, if I understand you correctly -
I am going to have to cry "time out!" to go and do some reading...

Thank you. - Hendrik

[H J van Rooyen]

"Dennis Lee Bieber" <wl*****@ix.net com.comwrote:
| On Thu, 3 Aug 2006 09:17:41 +0200, "H J van Rooyen"
| <ma**@microcorp .co.zadeclaimed the following in comp.lang.pytho n:
|
| Can I not use the ssl module for encrypting the connections? - Please also
| understand that the system is aimed at small to medium companies, in
ouse -
| From my perspective the only valid reason to use a database would be for
the
| ease of reporting - the files are not large - and the speed of a dict lookup
in
| python is hard to beat for normal transaction processing...
| >
| You might want to read the "Kode Vicious" column in a recent issue
| of Queue (probably last months issue -- it's been in my carry-bag for a
| few weeks).
|
| For an "in house" effort, encrypting the LAN traffic is probably not
| the most meaningful focus. Securing the data /storage/ is more important
| -- why secure the LAN traffic if someone can walk off with a backup of
| unsecured database. And who'd want to even spend time with a LAN sniffer
| on unencrypted traffic if that same backup is available for filching.
| >

This makes sense - message is - lock your server room...

| NO! the last thing on my mind - want a dynamic process similar to banking
| terminals - see my response to Simon please
| >
| ? ATMs? Or internal clerk consoles?

Semantics - sorry - neither - thinking of credit card terminals - in which area
I have dabbled in a bit...

These things have horrendously complex terminal management systems and elaborate
mechanisms to download all manner of parameters to change their behaviour... -
most have their internal state controlled by the server, sometimes formally,
sometimes just via param download...
| Pretty much everything is already in the terminal software -- what
| the operator has access to, and sees, is dependent upon the privileges
| defined for their "account". No "dynamic" loading of code (for security,
| I'd not even permit remote updates -- I'd require a floppy or CD from
| inside the secure box to change operating software; as soon as you
| permit updates to be pushed from outside you expose the risk of a
| cracker pushing a customized code set).
|

I was not aiming for this paranoid level of security - when I said "secure" or
"reliable" I was just looking for something that would work and that I could
trust not to fall over all the time...

On the small boxes, the loading of software mechanism varies from manufacturer
to manufacturer - some allow, others disallow the download and activation of new
apps - likewise for the update of existing ones - but most banks don't use these
mechanisms, even if they are available - they all tend to bring the device in to
a trusted facility. I don't know one of them that actually use the mechanism on
a per transaction basis, because it would be, amongst other things, too slow -
but most of the systems can force the terminal into a reload of at least its set
of parameters the next time it logs in - and this mechanism is used by for
instance AMEX to vary the messages displayed on their terminals - not quite
"code" update - but the user can't tell the difference.

I used the example because this sort of facility is the kind of thing I want to
find out if Python could do, in a more dynamic way than what the terminals that
can use it, actually use it - little thinking that I was sowing confusion -
sorry...

If you are familiar with the sort of Terminal Management Systems I am talking
about, that control the behaviour of the remote box via parameter download -
then what I want find out is how to do that in Python, but with the added
proviso that I must also be able to download "pieces" of the application - where
"download" in this sense is just to a PC in the next office on the local LAN -
So far the Pyro package that Bruno has steered me towards sounds the most
promising way of doing this - but I still haven't been able to "research" it ...

Thanks for the input and sorry about the confusion.

- Hendrik

[H J van Rooyen]

"Dennis Lee Bieber" <wl*****@ix.net com.comwrote:
| On Thu, 3 Aug 2006 14:05:19 +0200, "H J van Rooyen"
| <ma**@microcorp .co.zadeclaimed the following in comp.lang.pytho n:
|
| What I mean by this is that the server does stuff that I think belongs on
the
| client -
| like getting involved in the nitty gritty of what the client should
display -
| I want the client to be smart enough to do the assembly of the elements of a
| transaction
| by itself, going back to the server for data only when its needed - remember
|
| One thing to consider: Where is the separation between the database
| and the client. I believe most textbooks these days tend recommend:
|
| [db-server] <-[app-server] <-[client]
|
| (db-server and app-server can be the same hardware; the idea is that
| clients do not have direct access to the database system, and hence the
| back-end can be changed out without affecting any client... also,
| clients don't need to handle database errors, etc.)

Agreed - I would also need this application server to do the server end of my
"terminal management system"

|
| so I see the client interacting with the server quite a lot, eventually to
be
| able do things like auto completion of things like stock codes and
descriptions,
| customer details, etc. - but I don't want every keystroke flying over the
LAN
| and
| being handled by the server...
| >
| And where did you see the client obtaining the "completion data" --
| direct access to some other database tables or did you intend to
| download /all/ possible data.

no just a framework of more or less static stuff like document types, name and
address data, and account names and codes that is not subject to continous
change... *grin* I would have to apply *some* skull sweat to the problem...

|
| Typical web-based applications may have a minimal bit of data
| validation running on the client (JavaScript ... things like making sure
| /something/ has been entered into required fields, but not verifying
| that it makes sense), and only when the user clicks on a submit is
| everything sent to the application server, which then generates the
| needed SQL from the data fields for submittal to the database server.
|
| transaction is completed - like in a banking system, I envisage ways for the
| client to 'poll' the server to get the state of the last transaction, to
make
| this possible.
| >
| Bad choice... Upon submittal to the server, there should be a
| mandatory "good/bad" return code...
Here I take umbrage - not bad, good choice - let me elucidate - the return code
you are talking about is the normal thing, and it is the dead wrong way to
operate if that is all you do - what I am talking about is the failure
scenario - the way to make any transactionally based system robust is to have a
"GetTranRes ult" transaction that is routinely used before starting a new
transaction, to see if the previous one has completed properly - that way, the
client can either - continue with the new transaction, or - resubmit the old one
to try to get it to work or fail, or thirdly - advise the user that the previous
transaction has failed - it is, when you have thought about the flows and all
the possible ways in which they can fail - truly the only way to operate
reliably.

The above is the simple minded way to use such a facility -

You could also argue that the time to use this "polling" transaction is after
submitting the transaction, but if your return code is to be trusted, its not
needed - if the flows complete normally, its not needed - it is just needed for
built in recovery, and it works like a charm if both the server and the client
try to keep state over power failures and other disastrous events like losing
comms halfway through - especially for multi flow transactions that must update
different things on the server...

And it does no harm to ask the server the result of your last transaction when
you come up after power failure - you can then set the screens and stuff up to
the point where the server knows about them and have the user just curse a bit,
instead of a lot...

And most importantly, the data is safe (unless the database is corrupted, in
which case the likelyhood is high that yer screwed in any case) - and on this
note - if you have the reversed transaction too - i.e. a kind of
"GetPreviousLog gedTransactionR esult" from the server to the client, you can make
recovery less painful, even in the case of database corruption...

Bad choice indeed! *snorts*

;-)

- Hendrik