We need to send transaction logs from multiple networked embedded
devices to a centralized database located on a server.
The server may be not available, so the devices need to cache locally
the transactions.

We are currently keeping a local log of the transactions, which we
close every n minutes and send to the server via ftp. This way, we are
vulnerable only to the loss of the last n minutes of work.
It is important that we don't lose any transactions, since they are
worth money.

I'm planning on updating the mechanism using a better local storage,
like a sqlite database, and some kind of standard middleware.

I looked at Message Oriented Middlewares, XMPP and AMQP, but I still
don't see a clear winner...

I understand that the closest scenario is that of multiple mail
servers routing mail to a centralized address, but that would be
overkill.

Any advice?

Thank you