Sharing a common memory area in Delphi between PCs

Question

I have a Delphi 2006 app that gathers data and displays it as a summary of many channels, one channel per row on a TDrawGrid. I have the same app running on various other PCs on the network, but these other PC's are slaves - they don't gather data but merely provide a remote display of the summary.

At present, the slaves just show a mimic of the summary display screen on the master, and this is implemented via broadcasts by way of mailslots from the master.

I want to implement this in a different way, to reduce the load on the master, and provide the slaves with a bit more flexibility and independence on how they interpret the data. Also, I am having issues with mailslot broadcast of the data across subnets.

Can I use some shared memory scheme to lay the data down to a memory-mapped file where the slaves can have access from anywhere (over the web, even)? We are talking about a memory size of 100k bytes max, say, updated by the master at around once per second, probably in a thread, to keep the master foreground task responsive.

Answer 1

The simplest way is using a file on a share that the master writes to and the slaves only read. Some kind of synchronization might be necessary, if you want to prevent "dirty reads". On the other hand, it might not matter, depending on the kind of data you want to display.

Using a simple file has the advantage that it does not require any additional software (e.g. a daetabase server or some middleware) following the KISS principle. But of course it is far from sexy ;-) and does not use the correct buzzword technology.

Answer 2

You could use a database such as DBISAM, Firebird, etc.. With DBISAM, I've used a trick of reading the first 8 bytes of the database file which seems to be a header. If it changes, I know that the data in the table has changed, otherwise it hasn't. You can utilize this in the client if you use a polling loop, or if you want to continue to use the mailslots as a notification method. i.e. poll the file every 10 seconds or upon mailslot notification, whichever comes first.

Answer 3

We use MSMQ for something similar.

Answer 4

Shared memory won't work over web (unless you run VPN) and it doesn't work well over network in general (views can be desynchronized and you can't synchronize them across network).

I can see several solutions to your task:

Option 1. Use message-oriented middleware (MOM), such as MSMQ, kbmMW, our MsgConnect to broadcast notifications which include only changes in your data. This way the clients won't need to poll the server additionally for data snapshot. All MOM solutions use TCP connections for operations and this is more reliable than mailslots.

Option 2. Use some client-server DBMS, probably the one which supports notifications to clients (I am not an expert in DMBS so I can't tell you the names).

Answer 5

What's wrong with using TCP/IP? You can use Indy (ships with Delphi already) or ICS to have your main (master) app respond to IP requests (eg., HTTP or ICMP or whatever suits your needs for data) with a thread or two, and have the "slave" apps just request the data via the IP address of the master on a specific port. This would work on an intranet or via the internet transparently.