On Aug 29, 2008, at 4:38 PM, David Blevins wrote:
Been looking into how to do service discovery between client/server
and server/server.
We've got some code in the client to do failover on a server list
but by default it really isn't wired up. It'd be nice to get
something in so even if we don't support full clustered replication,
at the very least we could support a bunch of servers that "work
together" in a stateless fashion.
Looking at adding a new "multicast" server service that just
advertises the URIs of the other networks services available in the
system. We'd probably want it off by default (maybe), but then a
client could just sort of boot without being pointed to a specific
server address and theoretically find a server to talk to.
More progress on the multicast discovery with grouping and failover.
On the area of tracking and communicating a new server list for the
cluster I added a ClusterMetaData which is similar to what the
ServerMetaData was aiming to be. The issue with the ServerMetaData is
that it's tracked on a per-ejb-proxy basis and any updates to the list
of servers in the cluster are only reflected in the proxy immediately
used. All other proxies will still hold onto the outdated list.
Second, not all request types could be clustered and have failover,
essentially only ejb requests could failover, jndi and authentication
could not. Now the ClusterMetaData version associated with the
ServerMetaData is sent to the server *before* the main request and
then the server can send back a new list regardless of which type of
request it is.
On the failover side, I essentially had to rewrite all of the
synchronization related to connection management and shutting down the
server. The code simply was not written so that it could be stopped
cleanly from a client's perspective. It's now capable of a graceful
shutdown which is of course critical for failover. Active requests
will be finished, new requests will be refused, inactive connections
with the client will be cleanly and immediately closed on the server
side. There is a bit of a trick in this regard as even if one side
perfectly and cleanly closes it may not be visible to the other side
for quite a while. Still hacking on that. Chatted with Hiram about
it a bit and it doesn't really sound like there is a "cure", only
things you can do to overcome the symptoms.
-David
