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Re: [ecf-dev] remote services, load balancing
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Hi Wim,
I suppose that you could use service ranking to achieve load balancing,
but that would have some serious disadvantages. What I mean by that is
that using service ranking for 'poor man's' load balancing would mean that:
1) The consumer would have to call getServiceReferences on their own to
get/use a new service reference (with the current highest ranking).
That adds a good deal of complexity on the client side (managing the
service reference yourself, instead of having ds or servicetracker do it).
2) With such a scheme the 'load balancing' would only be done at the
service level. It's often convenient to have load balancing be done at
the *method* level (for the service interface)...and that's one of the
advantages of what's described here:
http://wiki.eclipse.org/Load_Balancing_Remote_Services
A single proxy, when used by a client (transparently...i.e. the client
can just call methods on the client proxy), the *individual method
calls* are load balanced among n implementers of that service. Thus the
client doesn't have to get a new service reference, and can just call
the methods on that service reference multiple times...and those
individual method request/responses will be load balanced among the n
servers.
So I would say that yes, you can do poor man's load balancing as you
described using service ranking, but it makes things fairly complex for
the client...and doesn't provide a high degree of resolution...i.e. it's
desirable to be able to load balance a single service/service reference
(as the ECF load balancing does)...for simplicity and flexibility.
Scott
Wim Jongman wrote:
Is it possible for a service to change its SERVICE_RANKING after a
service request is received by changing its ServiceRegistration object
and changing this back again after the request has been fulfilled? To
have some poor mans load balancing?
as per OSGi core: 5.2.7 Locating Services
client:
Service = getServiceReferences("heavy.used.service");
Service.doWork();
as per OSGi core: *5.2.3 Registering Services*
server:
doWork(){
myServiceRegistration.setProperty(SERVICE_RANKING, LOW);
workworkwork
myServiceRegistration.setProperty(SERVICE_RANKING, HIGH);
http://www.osgi.org/javadoc/r2/org/osgi/framework/Constants.html#SERVICE_RANKING
Regards,
Wim
On Mon, Apr 26, 2010 at 5:08 PM, Scott Lewis <slewis@xxxxxxxxxxxxx
<mailto:slewis@xxxxxxxxxxxxx>> wrote:
Hi Greg,
Greg House wrote:
Hello,
I am moving the discussion
(http://www.eclipse.org/forums/index.php?t=rview&goto=528121&th=166590#msg_528121
<http://www.eclipse.org/forums/index.php?t=rview&goto=528121&th=166590#msg_528121>
<http://www.eclipse.org/forums/index.php?t=rview&goto=528121&th=166590#msg_528121
<http://www.eclipse.org/forums/index.php?t=rview&goto=528121&th=166590#msg_528121>>)
to the mailing list as requested by Scott.
Sorry if this e-mail is too long. I will explain the use-case
I am trying to implement.
Consider two or more OSGi platforms. Each platform exports
(remote) services that the other platforms can use.
At any time a new platform can appear (or disappear) and the
other platforms should be able to see and use the new services
without any additional
configuration or restart.
Because some of the services are expensive and time-consuming
operations I want to be able to deploy them on
several machines and the requests to these services to be
load-balanced. In addition, I want to be transparent for a
consumer
if a local or a remote service is used. I don't want in my
code to import any OSGi packages.
For the platform and remote services implementations I am
using Equinox and ECF.
Now I know that if we remove the load-balancing requirement
all this can be implemented with (remote) declarative services.
Moreover, the distribution and discovery providers can be
independently selected.
However, the OSGI specification does not say anything about
load-balancing, so one has to use some “nonstandard” approach and
stick to specific distribution and discovery providers (and
external components) – is my understanding correct?
Yes.
The ECF has a solution to load-balance remote-services with
JMS/ActiveMQ.
For my use-cases it is OK to use ECF ActiveMQ provider and
maintain separate ActiveMQ installations. However, is it
possible the
usage of services to be transparent for the consumers? The
service providers define the properties (local, remote,
load-balanced)
of their implementation and the consumers just use the services?
Yes, this is correct. The load-balanced service usage can be
transparent for the consumers.
I tried to convert the
org.eclipse.ecf.examples.loadbalancing.* examples to
declarative services and to remove all ECF references from the
code
but with no success.
What went wrong? I don't immediately see a reason why this
wouldn't work...for the loadbalancing consumer/client.
The conversion of
org.eclipse.ecf.examples.loadbalancing.server to declarative
services is straightforward.
About org.eclipse.ecf.examples.loadbalancing.servicehost I
created the following OSFI-INF/component.xml:
<?xml version="1.0" encoding="UTF-8"?>
<implementation class="null" />
<property name="service.exported.interfaces" type="String"
value="*"/>
<property name="service.exported.configs" type="String"
value="ecf.jms.activemq.tcp.manager.lb.svchost"/>
<property name="org.eclipse.ecf.containerFactoryArgs">
tcp://192.168.1.1:61616/exampleQueue
<http://192.168.1.1:61616/exampleQueue>
tcp://192.168.1.1:61616/exampleTopic
<http://192.168.1.1:61616/exampleTopic>
</property>
<property name="ecf.rsvc.proxy" type="String" value="true"/>
<service>
<provide
interface="org.eclipse.ecf.examples.loadbalancing.IDataProcessor"/>
</service>
<reference cardinality="1..1"
interface="org.eclipse.ecf.core.IContainerFactory"
name="IContainerFactory" policy="static"/>
</scr:component>
Should I use the service.exported.interfaces property? Is
<implementation class="null" /> correct?
I would have to check the ds spec, but no, I don't think that
'null' is allowable for the implementation class.
The main problem I have is with
org.eclipse.ecf.examples.loadbalancing.consumer.
I created the following OSGI-INF/component.xml file:
<?xml version="1.0" encoding="UTF-8"?>
<implementation
class="org.eclipse.ecf.internal.examples.loadbalancing.consumer.DataProcessorConsumerApplicationDS"/>
<reference bind="doTests" cardinality="0..n"
interface="org.eclipse.ecf.examples.loadbalancing.IDataProcessor"
name="IDataProcessor" policy="dynamic"/>
</scr:component>
Is this somehow related to DefaultProxyContainerFinder and
should I specify “ecf.jms.activemq.tcp.client” in the config file?
I first have to ask whether you are using the latest from HEAD or
ECF 3.2. The reason I have to ask is that *since* 3.2 release the
policy for handling automatic creation of containers in
DefaultProxyContainerFinder has changed as per this bug:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=303979
Now (in HEAD/3.3/Helios), the default is to automatically create a
container instance when discovered.
*BUT*...even if you are using HEAD....I think that there could be
a bug in the ActiveMQ/JMS provider. Specifically, when containers
are discovered, and the DefaultProxyContainerFinder is consulted
to find a suitable container, part of what the
DefaultProxyContainerFinder does (using the auto create option
that was changed in 303979) is that it gets the meta-data from the
discovery and looks for a *compatible* container type. By
'looks', I mean that it calls this method on container instantiatiors:
org.eclipse.ecf.core.provider.IRemoteServiceContainerInstantiator.getImportedConfigs(ContainerTypeDescription,
String[])
And if the provider on the consumer responds with a non-null
String array, then it has a match.
But it is quite possible that the JMS/ActiveMQ provider
implementation of the getImportedConfigs method does not match
with the load balancing container types (as these are 'special'),
returns null, and the client container is never created/found by
the DefaultProxyContainerFinder on the consumer. Does this fit
with what you are seeing? BTW...what discovery mechanism are you
using? And do you have evidence that it is working properly on
your net?
I will be happy to look into this later today/Monday and fix it if
necessary, but would it be possible to get your declarative
services versions of the load balancing example so that I can
test? Would you possibly be willing to contribute these projects
as ECF examples?
How is "cardinality" handled in the case of load-balanced service?
The cardinality is handled as with a 'normal' declarative
service...i.e. the cardinality will determine whether the
component is activated if the given reference is present (1...n)
or not (0...n).
I would appreciate any help or directions how to proceed.
I think the bug with the JMS/ActiveMQ provider I described above
*may* be the issue. If you can help me diagnose and test we
should get it going in short order.
More important, is my use-case achievable with Equinox and ECF?
Yes, it is.
Scott
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