James Strachan wrote:
On 19 Dec 2005, at 14:54, Jules Gosnell wrote:
James Strachan wrote:
On 19 Dec 2005, at 14:14, Jules Gosnell wrote:
James Strachan wrote:
On 19 Dec 2005, at 11:53, Jules Gosnell wrote:
, whether there is other suitable Geronimo or ASF-licensed
code available, or whether we will need to write our own
WADI- autodiscovery classes. The important thing is to impose
as few dependencies on the client as possible. The client side
code should literally be a few lines. Clients using clusters
should not suddenly find themselves sucking down e.g. the
whole of activemq, just to do a once off autodiscovery. Early
versions of WADI had its own autodiscovery code. If we need
them, they could be resuscitated.
There's no reason why you can't do a simple implementation of
ActiveCluster which doesn't use ActiveMQ - its just a simple API.
Sure - but I'm talking about the EJB-client side - where we just
want to throw across as thin a line as possible, in order to haul
a decent strength cable back. An EJB client would not need the
ActiveCluster API (I'm not thinking in terms of making EJB
clients fully fledged cluster members), but simply a way of
locating the cluster and requesting a membership snapshot of it.
Thats exactly what the ActiveCluster API is for :). Though by all
means come up with another API if you can think of a better way of
doing it.
This could be done by just broadcasting a query packet at a well
known multicast address and waiting for the first well-formed
response.
Sure - an *implementation* of ActiveCluster API could do exactly that.
???
well, maybe I'm thinking of the wrong piece of activecluster then ?
any piece of code could broadcast a packet... which piece of
activecluster's API are you suggesting here ?
You said...
but simply a way of locating the cluster and requesting a
membership snapshot of it.
The entire purpose of the ActiveCluster APi is to locate members in
clusters & be informed when they go down. There's a zillion ways of
implementing this - AC just tries to be a common, simple API we can
share.
Sure
we really are talking about just a remoting proxy which needs to
find, but not 'join' a cluster.
AC can be used purely to discover nodes and choose which one to use
as a server. The local in-JVM node does not actually have to actually
advertise itself to members of the cluster if it doesn't wish to -
thats an implementation detail. (e.g. if you only want servers to be
discoverable but keep clients invisible).
OK - I see - but I'm not sure that I am drawn to this approach :-)
Using the EJB client and server side example - you may want to know
which clients are connected to what servers so that you can load
balance; so even though you may not consider clients as 'members of
the server cluster' being able to know who is connected to who is
quite useful. EJB clients could expose statistics that can be used to
help choose the right server a new node should join.
understood.
But whatever - if you don't wanna use the AC API its no big deal; use
whatever abstraction you see fit - I just wanted to explain the point
of AC; for discovering nodes in a cluster and being able to choose
which one to communicate with; it can be implemented any way you like
(simple multicast stuff, using HTTP, LDAP stuff etc).
Its not that I don't want to use AC client side - its that I don't see
clients as being members of the cluster in any way - passive or active.
but, I guess this opens up an interesting topic...
I'm thinking in terms of relatively large numbers of clients to servers.
I wasn't thinking in terms of clients receiving any notifications about
changes in cluster membership or state location.
This is probably because WADI comes from a background in which the
client is completely dumb, but also because a server may support many
thousands of clients. I see the overhead of trying to pessimistically
keep these clients in sync with developments within the cluster as
outweighing the benefit. The architecture that I have in mind is more
optimistic. It waits for something exceptional to happen - e.g. some
state migrating as its node shuts down followed by a client for that
state trying to make a call to it - then pays a small but exceptional
cost in terms of a fail-over (which may become an autodiscovery in the
even more exceptional case that all nodes in the cluster have been
cycled between successive calls from the client).
Thus, in the usecase that I expect, I think that the optimistic approach
will be more efficient than the more pessimistic. However you should
expect the unexpected - right :-)
I guess that this way of thinking has prevented me from seeing clients
as cluster members.... when perhaps they might be seen that way ?
Perhaps we could look at how AC impls auto discover cluster members,
then abstract that out into another interface (Client) which Node could
inherit/make use of ?
Perhaps we could even have different Client implementations - Optimistic
as I envisage and Pessimistic with targeted backchat coming from the
cluster about relevant membership and state location changes (although I
guess this means running some sort of server within the client stub?).
I guess this means that the Optimistic stub is a Client and the
Pessimistic stub a Node (which listens but does not join the Cluster and
isa Client also).
How does that sound ?
Jules
Maybe it'd help if we separated AC into the 'messaging' part and the
'discovery' part?
James
-------
http://radio.weblogs.com/0112098/
--
"Open Source is a self-assembling organism. You dangle a piece of
string into a super-saturated solution and a whole operating-system
crystallises out around it."
/**********************************
* Jules Gosnell
* Partner
* Core Developers Network (Europe)
*
* www.coredevelopers.net
*
* Open Source Training & Support.
**********************************/
