Guillaume,Thank you for the clarifications! This kind of clustering support is yet another good reason for people to migrate to ServiceMix 4, very nice work indeed!
Regards, Gert Guillaume Nodet wrote:
2009/2/10 Gert Vanthienen <[email protected]>:Guillaume, From a user perspective, registering the endpoint through an OSGi bundle should be fine. For the JBI packaging, we should take care that the implementation doesn't tie the components' build to ServiceMix 4 or we run into the same problem we used to have with ServiceMix 3. We should probably also take care of ServiceMix 3 users by adding a warning or something to the log if they configure that flag because it won't do anything there, right? Given that these ClusterRegistrations are in the OSGi service registry, I suppose we can also add some console commands to enable/disable clustering at runtime.You're referring to adding a property on the endpoint I suppose ? If we add that on the endpoint exporter (which is smx4 specific, we would not have to take care about smx3 users). Implementing a set of commands is a really nice idea.Once we have this done, we can look into making this cluster engine more intelligent, e.g. by automatically detecting exchanges to be sent to the engine as you described and perhaps by adding a communication mechanism for ServiceMix 4 instances to share information about endpoints (and thus allow them to turn on/off clustering if the endpoint is available on another node automatically).Yes, we should be able to add more intelligence later.Just for clarity: this feature is meant a replacement for the JCA/JMS Flow in ServiceMix 3. Would this solution solve all of our concerns with those flows or are there still situations where a set of JMS endpoints on both ServiceMix instances would be better than using the built-in clustering solution?Yes, I think so. The cluster engine is more scalable, more efficient, and the fact that the user has to configure on explicitely on a given endpoint put a slight burden on the user at the benefit of not needing to send all exchanges in the NMR through the cluster. The engine has also some kind of throughput control where it can controls the maximum number of exchanges sent into the NMR at a given time to not overload it. I can't really think of some use case where it would be better to have explicit endpoints, unless you really need to have a really fine grained control over the jms messages sent: for example one could want some jms messages to be persistent and others not. This is currently not possible to configure that, though we should be able to enhance the ClusterRegistration later to hold such informations.Regards, Gert Guillaume Nodet wrote:On Mon, Feb 9, 2009 at 13:31, Jean-Baptiste Onofré <[email protected]> wrote:Hi Guillaume, If I have right understood, the ClusterRegistration in the registry provides a filter. After that, the user needs to define if the endpoint is "member" of the cluster or not. It means that, in a cluster, we can have : - standalone endpoints (dealing only with the local SMX instance) - clustered endpoints (dealing with all SMX cluster members). Does it mean that this kind of endpoints are federate in all cluster members (the endpoint is present in all cluster member) ?The ClusterRegistration provides a filter that will match an endpoint set. If the endpoint that created and sent the exchange matches, the exchanges will be re-routed. In such case, the cluster engine will send a JMS message, that will be consumed by another cluster engine. The consumption is based on JMS selectors, so it will only consume messages that will be transformed into a JBI exchange, which target endpoint exists locally. So you can achieve both load-balancing and remoting. Let's take a real example: HTTP consumer -> EIP routing slip -> (xslt transformation , HTTP provider, xslt transformation) This route defines a transformation on the request, send it to another web service, and transform the response back. If you configure the HTTP consumer to be "clustered", then, it will look somewhat like: HTTP -> JMS -> EIP If you deploy this whole application on two instances (assuming the underlying JMS brokers are connected together), an http request coming to the first smx instance could be processed (eip and the remaining of the flow) by another instance (load-balancing / some failover). You could also imagine deploying the HTTP consumer only on one instance, and the remaining of the application only on a second instance (remoting).If I compare with application servers clustering (with the EJBs session replication, entity turns, etc), when you setup the application server in cluster mode, your application works in cluster mode (you can't choose if it's cluster compliant or not). Using JBoss Groups, you can register applications in cluster or not. Maybe it can be interesting to let the user to choose which endpoints is cluster one or not with something like, defining a ClusterRegistration service that store the cluster endpoints set. Like this, we can manage OSGi and JBI packaging, implicit cluster and manual cluster. But the user needs to define clustered endpoint.Not sure if we can really compare with app servers clustering. If you want true clustering, you need to make sure the whole state can failed over onto another node. Unfortunately, most of the components maintain some state. For example if an endpoint receives an exchange and create a copy of it to send it to another endpoint, wait for the response and then copy it as the response of the original exchange, this original exchange if smx crashes unless it is stored somewhere. The servicemix-eip component has a configurable storage that is used to store all such data. It defaults to an simple map without persistence, but you can configure if to use a database if you want. However, this downgrades performances a lot. Usually, the best way is to put a single cluster endpoint for a whole flow at the beginning of this flow. In our example above, clustering the HTTP consumer should be somewhat sufficient: if the something wrong happens when processing the eip, xslt transformations or the call to the other web service, the transaction can be rolled back and retried on another smx instance. However, if the http client is waiting for a response and the first smx instance (hosting the http consumer) crashes, there's really nothing we can do ... Anyway, an interesting option would be to have a more complex filter that automatically detects exchanges that need to be clustered if for a given flow, no exchange have gone through the cluster engine yet. It would automatically cluster all consumer endpoints, but not subsequent exchanges.It's only a quick think about SMX cluster. The topic is very interesting. Regards JB On Mon 09/02/09 09:28, "Guillaume Nodet" [email protected] wrote:I've commited my ongoing work about servicemix 4 clustering at https://svn.apache.org/repos/asf/servicemix/smx4/nmr/trunk/jbi/ cluster/It's not in the build yet and I committed it for discussion purpose.This work has two goals: * provide some persistence in the JBI layer * provide some transparent remoting between JBI endpoints The way I've began implementing that is to use an ExchangeListener in the NMR to re-route exchanges to a cluster endpoint (I guess it should be renamed to something like "cluster engine" to avoid being confused with "clustered endpoints"). The org.apache.servicemix.cluster.requestor package dervies from the spring message listener container and implements a jms layer which is able to provide request / response in an asynchronous way. I've experimented different things, and the one i've been focusing latelyis to use a single JMS queue and selectors. Let me explain a bit.The JMS flow in servicemix 3 was using lots of different destinations (one per container + one per endpoint + one per service qname + one perinterface qname). The problem with such a design is that a jmsconsumercan easily consume only from one destination (unless we use somespecificactivemq features). Another problem is that if not using activemq,setting up lots of JMS destinations can be really tedious. The use of asingle destination leads to fewer consumers, at the expense of usingjmsselectors. Previsouly, i've tried to use two queues (one forrequestsand another one for responses) but there's no real benefits in doingthisimho.The other thing i've been focusing on is to make sure that processing ajms message does not block a thread, and yet be able to use jms or xatransactions. This is not so easy. For example the spring jms listenercontainers do use a thread for consuming the jms message and processit,expecting the processing to happen synchronously. However, inservicemixsynchronous processing is a bad idea, as if it involves sending anhttprequest and waiting for a response, this means blocking a thread fornothing.For scalability, we need to not block threads if possible. But springmessage listener containers only support synchronous processing, so I'vehacked two new containers, one being JMS compliant, and another onespecificto ActiveMQ which is much more performant. It uses aMessageAvailableListenerto be notified when consumers have messages to be processed instead ofwastingthreads to poll actively for messages.Anyway, both containers can support client ack, jms local transactions orxa transactions in asynchronous mode.I haven't really worked on how to register such endpoints (from the user point of view). At the moment, we need to register a ClusterRegistration in the OSGiregistry. Such registrations contains a filter that will be used todecide if a new active / consumer exchange should be re-routed to the cluster engine or not.The most simple filter would be a filter that checks the source endpointandwill thus cluster all exchanges outgoing from a given endpoint.As for how to register such objects, one way would be to put that on the endpoint exporter that is used in smx4 to register jbi endpoints with the OSGi packaging,but this would not work with JBI packaging (such registrations would haveto bedeployed in a separate osgi bundle). I was also thinking about adding asimpleboolean property on all endpoints, something like clustered="true".Sorry for the long rant, but I should have sent this email way earlier ...Feedback welcome.-- Cheers, Guillaume Nodet ------------------------ Blog: http://gnodet.blogspot.com/------------------------Open Source SOA http://fusesource.com
