Timo Sirainen <tss@iki.fi> writes:
The big problem is what the protocol should be. Use some existing RPC protocol? It should be something extensible so that a plugin in imap process can talk to a plugin in storage process, without the base processes knowing anything about the details (e.g. imap-quota plugin asking quota usage from storage's quota plugin). In any case the client side API should be asynchronous. That can make it annoyingly difficult to use though. Wonder if I could switch to erlang or something for the imap/pop3 processes :)
I realise I'm coming a little late to this conversation. Various people have suggested various RPC mechanisms. Might I humbly suggest also looking at 9P (or 9P2000) the Plan9 filesystem protocol. [1]
The general design philosophy in Plan9 is not to use an increasing number of application specific RPC mechanisms but instead to implement everything in a synthetic filesystem tree. The 9P protocol handles the basic operations required to navigate the filesystem tree and read/write to the files within. The semantics of "RPC" are then relegated to the specification for the layout of the filesystem and meaning assigned to reading or writing to files within that tree. This allows for extensibility (and incidently discoverability) - new features or "plugins" appear as a new file or directory hierarchy in the presented filesystem layout.
As a concrete example the so called upas/fs [2] presents a view of a mailbox independent of the actual storage mechanism used for the data contained in the mailbox. From the manpage
The mailbox itself becomes a directory under /mail/fs. Each message in
the mailbox becomes a numbered directory in the mailbox directory, and
each attachment becomes a numbered directory in the message
directory. Since an attachment may itself be a mail message, this
structure can recurse ad nauseam.See [2] for more details. I'm not suggesting this layout would be exactly right for the Dovecot storage protocol but it may provide a starting point.
It's probably worth noting the irony, given this is a maillist about Dovecot, in that this approach almost obsoletes the need for an IMAP protocol in the first place (the upas/fs style layout as documented doesn't really provide sufficient support for server side search for example).
Footnotes: [1] http://plan9.bell-labs.com/magic/man2html/5/0intro [2] http://plan9.bell-labs.com/magic/man2html/4/upasfs