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From Bryan Thompson <br...@systap.com>
Subject Re: Don't let Jini Standards become an impediment to development
Date Wed, 09 Sep 2015 20:56:47 GMT
So can we capture this information about snapshots in the service interface
javadoc?  For example,

LookupCache::

ServiceItem <https://river.apache.org/doc/api/net/jini/core/lookup/ServiceItem.html>[]
lookup(ServiceItemFilter
<https://river.apache.org/doc/api/net/jini/lookup/ServiceItemFilter.html>
filter,
                   int maxMatches)

ServiceItem[] array whose elements each satisfy the filter, and that were
previously discovered to be registered with one or more lookup services in
the managed set. An empty array will be returned if noServiceItem is found
that matches the criteria or if the cache is empty.

We can state that the returned ServiceItem[] contains a snapshot of the
discovered ServiceItems, that changes to the returned array do not effect
the LookupCache, etc.

Or

JoinManager

public JoinManager(Object
<http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Object.html?is-external=true>
serviceProxy,
           Entry
<https://river.apache.org/doc/api/net/jini/core/entry/Entry.html>[]
attrSets,
           ServiceID
<https://river.apache.org/doc/api/net/jini/core/lookup/ServiceID.html>
serviceID,
           DiscoveryManagement
<https://river.apache.org/doc/api/net/jini/discovery/DiscoveryManagement.html>
discoveryMgr,
           LeaseRenewalManager
<https://river.apache.org/doc/api/net/jini/lease/LeaseRenewalManager.html>
leaseMgr)
            throws IOException
<http://java.sun.com/j2se/1.4.2/docs/api/java/io/IOException.html?is-external=true>


attrSets - array of Entry consisting of the attribute sets with which to
register the service

The documentation for attrSets could state that a snapshot of the
attributes will be made by the JoinManager.

I freely admit that there could be a lot of places where such clarification
might be added.

This suggests an obvious tradeoff between applying what is basically the
same change to a bunch of javadoc annotations (and verifying in each case
that this is true of the implementation, or simply assuming that it is true
of the contract for that implementation?) and adding a package.html or wiki
page that discusses the contract for Entry and Entry[] and captures the
outputs of this discussion. And then point people to that contract.

I would personally opt for the latter. Maybe as part of a FAQ dealing (in
this case) with River and concurrency.

Thanks,
Bryan



----
Bryan Thompson
Chief Scientist & Founder
SYSTAP, LLC
4501 Tower Road
Greensboro, NC 27410
bryan@systap.com
http://blazegraph.com
http://blog.bigdata.com <http://bigdata.com>
http://mapgraph.io

Blazegraph™ <http://www.blazegraph.com/> is our ultra high-performance
graph database that supports both RDF/SPARQL and Tinkerpop/Blueprints
APIs.  Blazegraph is now available with GPU acceleration using our disruptive
technology to accelerate data-parallel graph analytics and graph query.

CONFIDENTIALITY NOTICE:  This email and its contents and attachments are
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On Wed, Sep 9, 2015 at 4:19 PM, Greg Trasuk <trasukg@stratuscom.com> wrote:

>
> > On Sep 9, 2015, at 10:20 AM, Bryan Thompson <bryan@systap.com> wrote:
> >
> > I have found this to be a very interesting conversation.  My own bias is,
> > we are talking about 4.0. Let's focus on releasing 3.0 ;-)
> >
>
> Don’t get me started :-)
>
> > I completely understand Peter's concerns about reasoning about visibility
> > for concurrency.
> >
> > I understand from Greg that the visibility comes from the network
> barrier.
> >
> > How does this play out when changes are published into either a service
> or
> > a client cache for the attributes used to publish or discover a proxy?
> > This would seem to be the place where there is an opportunity for
> > concurrency issues.
> >
> > Looking at
> > http://river.apache.org/doc/api/net/jini/core/lookup/ServiceItem.html, I
> > see that the ServiceItem is three fields:
> >
> > Entry <http://river.apache.org/doc/api/net/jini/core/entry/Entry.html
> >[]*attributeSets
> > <
> http://river.apache.org/doc/api/net/jini/core/lookup/ServiceItem.html#attributeSets
> >*
> > Attribute sets.
> > Object
> > <
> http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Object.html?is-external=true
> >*service
> > <
> http://river.apache.org/doc/api/net/jini/core/lookup/ServiceItem.html#service
> >*
> > A service object.
> > ServiceID
> > <http://river.apache.org/doc/api/net/jini/core/lookup/ServiceID.html
> >*serviceID
> > <
> http://river.apache.org/doc/api/net/jini/core/lookup/ServiceItem.html#serviceID
> >*
> > A service ID, or null if registering for the first time.
> > It seems reasonable if the attributeSets field's value is replaced.  But
> > there would be a visibility concern about this.
> >
> > It's been a while since I wrote service cache logic.  Is the visibility
> > guarantee here provided by an interface providing notice of attributeSet
> > changes?
> >
>
> Interesting question.  Let’s clarify it by asking “What exactly are we
> doing with the ServiceItem instance”.  Also, note that ServiceItem is not
> itself an Entry, it just holds an array of Entry objects.
>
> Anyway, you mention service cache logic, so let’s look at
> ServiceDiscoveryManger and LookupCache:
>
> In the simple case, I call 'lookup(ServiceTemplate tmpl, int minMatches,
> int maxMatches, ServiceItemFilter filter, long waitDur)’.  I get back an
> array of ServiceItem instances.  The call is over.  I don’t expect those
> instances to be updated magically when a service updates its registrations
> in Reggie.  Once I get the ServiceItems from SDM, they’re mine.  I can pass
> them elsewhere or alter their values as I like.  If I were to take the
> Entry objects out and pass them to a different SDM call, I’d expect the SDM
> to take a snapshot of their values at the instant I make the call.  In no
> case do I expect ServiceItem to be a ‘live’ object.
>
> More complex case, I ask SDM to create a LookupCache by calling
> 'createLookupCache(ServiceTemplate tmpl, ServiceItemFilter filter,
> ServiceDiscoveryListener listener)’…
>
> Again, I expect SDM to snap a copy of my ServiceTemplate Entries when I
> make the call.  I get back a LookupCache.  I call 'lookup(ServiceItemFilter
> filter, int maxMatches)’ to get an array of ServiceItems that match my
> original ServiceTemplate.  Again, they’re not live objects; they correspond
> to a snapshot at some instant of time.  I don’t even really care about when
> it is - if I don’t find a service I can use, I need to wait for a while and
> try again anyway.  Who cares!  Even if I do find a good-looking service, it
> might fail by the time I get around to calling it, so I’d have to deal with
> that failure, by waiting a while and doing another lookup.
>
> If the LookupCache figures it has something to tell me, it will fire a
> ServiceDiscovery event, with the pre- and post-event ServiceItem values.  I
> would expect those values to not be live objects either.  Personally, I
> wouldn’t get too hung op on guarantees of receiving events either.  How can
> SDM possibly guarantee that I get changes from Reggie, when Reggie is on
> the other end of a network connection?  There could always be a temporary
> failure that causes me to miss an event or two (I mean I suppose you could
> come up with some elaborate “reliable event” mechanism, but why bother in
> this case?  In the end, if you can’t find a service, you wait for awhile
> and do another lookup anyway).  The conservative approach is to go do a
> full lookup if I ever need to know the latest picture of Reggie’s state.
>
> So I’m not quite sure what the alleged visibility issue is.  When SDM or
> LookupCache calls me, I’d expect the objects to be fully constructed.  If
> I’m passing the ServiceItems off to another thread, I really should be
> doing that inside an appropriately synchronized block, so there’s a proper
> happens-before boundary.
>
> Is there possibility for wackiness inside ServiceDiscoveryManager?  Yes, I
> would think so.  But that’s not my problem as a user!  If I were going to
> rewrite ServiceDiscoveryManager, I might be tempted to create objects that
> are immutable, and adopt copy-on-write semantics, but those would be purely
> implementation decisions, not API decisions.  I have yet to see how the
> Entry specification is affected by any concurrency issues.  Certainly, one
> might want to take a look at org.apache.river.reggie.EntryRep, but Entry
> objects (i.e. the public API) are fine as they are.
>
> Cheers,
>
> Greg Trasuk
>
>
> > Thanks,
> > Bryan
> >
> >
> > ----
> > Bryan Thompson
> > Chief Scientist & Founder
> > SYSTAP, LLC
> > 4501 Tower Road
> > Greensboro, NC 27410
> > bryan@systap.com
> > http://blazegraph.com
> > http://blog.bigdata.com <http://bigdata.com>
> > http://mapgraph.io
> >
> > Blazegraph™ <http://www.blazegraph.com/> is our ultra high-performance
> > graph database that supports both RDF/SPARQL and Tinkerpop/Blueprints
> > APIs.  Blazegraph is now available with GPU acceleration using our
> disruptive
> > technology to accelerate data-parallel graph analytics and graph query.
> >
> > CONFIDENTIALITY NOTICE:  This email and its contents and attachments are
> > for the sole use of the intended recipient(s) and are confidential or
> > proprietary to SYSTAP. Any unauthorized review, use, disclosure,
> > dissemination or copying of this email or its contents or attachments is
> > prohibited. If you have received this communication in error, please
> notify
> > the sender by reply email and permanently delete all copies of the email
> > and its contents and attachments.
> >
> > On Wed, Sep 9, 2015 at 9:00 AM, Greg Trasuk <trasukg@stratuscom.com>
> wrote:
> >
> >>
> >>> On Sep 9, 2015, at 8:05 AM, Peter <jini@zeus.net.au> wrote:
> >>>
> >>> It's worth noting that volatile is not excluded by the Entry
> >> specification.
> >>>
> >>> This doesn't provide any performance benefit whatsoever, but it does
> >> provide visibility guarantees between threads.  There is no atomicity…
> >>>
> >>> Greg, what are your thoughts?
> >>
> >>
> >> Entries are not intended for communication between threads.  They’re
> >> intended for communication between memory spaces across a network
> >> boundary.  Any performance difference with volatile (or even final
> fields)
> >> would be entirely masked by transit time on the network (1).
> >>
> >> You seem to want to change the Entry specification to reflect concerns
> >> about in-process concurrency, and I think that just fundamentally
> breaks a
> >> useful abstraction, an abstraction that is defined in the core of the
> >> public interface to River.
> >>
> >> I’m trying to come at this question from the point of view of a system
> >> architect or distributed system implementer, i.e. a “user” of River,
> not a
> >> River implementer.  From that perspective, what is it that you see as
> >> “broken” about the Entry spec (
> >> http://river.apache.org/doc/specs/html/entry-spec.html)?  How does
> >> requiring Entry fields to be final make it easier for me to build a
> >> distributed system?  Could we get opinions from Dennis?  Bryan?  Other
> >> users?  Their downstream users?
> >>
> >> Cheers,
> >>
> >> Greg
> >>
> >> (1) - I have to state my usual caveat here - reasoning a priori about
> >> performance in a complex system is a fool’s errand.  You need to have a
> >> model system and use case that you can analyze and experiment on.  But
> I do
> >> know we can run a whole lot of processor cycles in the time it takes
> >> 1400-or-so bytes to move across the room.
> >>
> >>>
> >>> Regards,
> >>>
> >>> Peter.
> >>>
> >>> On 9/09/2015 7:00 PM, Peter wrote:
> >>>> Greg,
> >>>>
> >>>> Thanks for putting the time & effort to look up these links.
> >>>>
> >>>> Primitive fields are an interesting case, I guess they had to be
> >> disallowed, because the serial form of an int is different from an
> Integer,
> >> at least for java Serialization.
> >>>>
> >>>> The performance trade off I mentioned relates to the time it takes for
> >> reflection to set each field in an Entry and when defensive copying is
> >> used, instead of pass by reference.
> >>>>
> >>>> ServiceDiscoveryManager is a good example of complexity, even after 15
> >> years, it still contains bugs because its design is too complex (even
> I'm
> >> not 100% confident I've fixed them all).
> >>>>
> >>>> Everything should be made as simple as possible, but not simpler.
> >>>>
> >>>> Ken Arnold made Entry's simple for this reason, but in those days, the
> >> jvm still had issues with its memory model, final fields didn't have the
> >> thread safety guarantee they have now and most computers only had one
> cpu.
> >>>>
> >>>>
> >>
> https://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html#finalWrong
> >>>>
> >>>> Allowing Entry fields to be final, doesn't significantly increase
> >> complexity, in fact, in can assist in reducing complexity in concurrent
> >> code.
> >>>>
> >>>> I'm still convinced of the benefits of allowing final fields in
> Entry's
> >> :)
> >>>>
> >>>> How about a compromise?
> >>>>
> >>>> Leave existing Entry implementations as is, but allow final fields in
> >> new Entry's?
> >>>>
> >>>> Then in River code, we treat Entry's as immutable, regardes of whether
> >> they are or not?
> >>>>
> >>>> This way, existing code won't break from final modifiers, but new code
> >> can benefit significantly.
> >>>>
> >>>> I'm not aware of any Entry using final fields at present.
> >>>>
> >>>> Regards,
> >>>>
> >>>> Peter.
> >>>>
> >>>>
> >>>>
> >>>> On 9/09/2015 3:55 PM, Greg Trasuk wrote:
> >>>>>> On Sep 9, 2015, at 12:30 AM, Peter<jini@zeus.net.au>  wrote:
> >>>>>>
> >>>>>> Thanks Greg,
> >>>>>>
> >>>>>> Was it a case of; because we can't set final fields (well not
> without
> >> a Permission anyway), that they shouldn't be included in Entry
> serialized
> >> state, because then we can't deserialize them?
> >>>>>>
> >>>>> No, it’s just the definition of “what’s in the schema”.  A class in
> >> the normal sense is a template for objects that are going to be
> >> instantiated and used as objects.  A class that implements Entry is not
> >> really doing the same thing.  It’s really a schema for a set of values
> that
> >> can be stored or used in a matching operation.  Nothing more, nothing
> >> less.  Dynamic operations in matching are a recipe for undefined
> behaviour
> >> (e.g. changing a value that’s used as a key in a hash map), so the Entry
> >> spec says “don’t do that”, by requiring that the matchable fields exist
> as
> >> fields, not calculated properties or anything else.  Using getter or
> setter
> >> methods would  provide an abstraction from the storage, but an Entry is
> >> fundamentally a storage structure, so it outlaws the abstraction. (Ken
> >> Arnold explained this nicely in http://www.artima.com/intv/sway2.html).
> >>>>>
> >>>>> Also, a null value in a field is a “wildcard”, hence the statement
> >> that the fields are serialized individually, so field-by-field
> comparisons
> >> are possible.  Notice that if you look at the usage, the class that
> >> implements Entry is never directly serialized - both Reggie and
> Outrigger
> >> create a representation object that has an array of serialized fields.
> >> They are treating the Entry class as a schema definition.  The Entry
> >> instance itself is like a record formatted according to that schema.
> >>>>>
> >>>>> Now, even though we’re not really going to use it as a traditional
> >> class, certain features of classes might still be useful.  Like, it
> might
> >> be useful to have constants for frequently-used values.  So you can
> still
> >> have static or non-static final fields (I can’t think of a good reason
> to
> >> have non-static final fields in an Entry class, but I suppose there
> might
> >> be a use case).  But any matching or storage operation only applies to
> >> fields defined in the “schema” - by definition, the public, non-static,
> >> non-final fields.
> >>>>>
> >>>>>> I've done my best to fix the existing implementations, so hopefully
> >> they won't need further fixes, however, the fixes were very difficult
> and
> >> these implementations very difficult to reason about, because there is
> so
> >> much mutable state.  In ServiceDiscoveryManager, a thread holds a lock
> >> while waiting for the result of a remote call, there was no solution I
> >> could find to remove this lock.
> >>>>>>
> >>>>>> To quote Keith Edwards "The Special Semantics of Attributes":
> >>>>>>
> >>>>>>  "All the methods of the object are ignored for purposes of
> >>>>>>  searching, as are "special" data fields: static, transient,
> >>>>>>  non-public, or final fields.  Likewise all fields that are
> primitive
> >>>>>>  types (such as ints and booleans) are ignored; only references to
> >>>>>>  other objects within an attribute are considered for searching.”
> >>>>>>
> >>>>> I’ll confess, it’s always struck me as odd that primitive fields are
> >> ignored, and it’s certainly bit me once or twice.  But I understand that
> >> it’s designed so that all the “matchable” fields are objects, so can be
> >> stored and compared in a uniform way (typically as MarshalledObjects).
> >> Since Java5 added auto-boxing/unboxing, there’s no real coding overhead
> for
> >> the developer.
> >>>>>
> >>>>>> So our choices are (for River 4.0):
> >>>>>>
> >>>>>> 1. Break backward compatibility and increase scalability,
> performance
> >>>>>>    and reduce bugs, by not ignoring final fields in Entry's, but
> >>>>>>    instead mandating them.
> >>>>> No.  That’s something else.  That’s not an Entry.  Such an object
> >> might be useful for implementations, but that’s not an Entry.
> >>>>>
> >>>>> An Entry is basically a local data structure that I can modify
> >> freely.   The value is crystallized when I send the Entry somewhere.
> Like
> >> to a JavaSpace.   If I’m using an Entry to store things locally where I
> >> have concurrent access to the entry fields, that’s my problem, not the
> >> Entry’s.
> >>>>>
> >>>>>> 2. Or continue full compatibility and live with lower performance,
> >>>>>>    less scalability and harder to debug code.
> >>>>>>
> >>>>> If that tradeoff is true, you’re misusing or misunderstanding
> Entries.
> >>>>>
> >>>>> Not to mention, if you speed up Lookups, so what?  They don’t happen
> >> that often - only on startup of a client or failure of a service, in
> most
> >> cases outside of the test framework.  JavaSpaces are a different issue,
> but
> >> again, without knowing the use case of the JavaSpace (messaging,
> >> persistence, read-mostly, write-mostly, who knows?), it’s hard to do any
> >> reasoning about performance.
> >>>>>
> >>>>> Java’s default serialization mechanism is slow?  Well, I don’t know
> >> that for sure for a given use case, but OK, in both the Registrar and
> the
> >> JavaSpaces case, the client receives a proxy.  The current proxy
> >> implementations use JERI, but that’s an implementation detail.  The
> proxy
> >> is free to implement a different serialization or communication
> mechanism.
> >> Don’t like reflection?  I guess you could use ASM or BCEL to create a
> >> marshaller object for each Entry type that a given JavaSpace
> implementation
> >> wants to support.  You could probably even generate them dynamically.
> For
> >> that matter, you could precompile Google Protocol Buffers for the most
> >> frequent Entry types.  Is it worth the effort?  I don’t know, but it’s
> >> technically feasible.  Jini’s dynamic code approach guarantees that it’s
> >> possible.
> >>>>>
> >>>>>> I think there's plenty of time for implementations to prepare for
> >> River 4.0, if we start talking about it now.
> >>>>>>
> >>>>>> Regards,
> >>>>>>
> >>>>>> Peter.
> >>>>>>
> >>>>>> How are these for code comments (from ServiceDiscoveryManager)?
> >>>>> “Those who enjoy laws or sausages shouldn’t see either being made” -
> >> misquoting Bismarck or Saxe.  Would you rather the original developer
> >> didn’t document areas that she thinks might eventually cause trouble?
> >>>>>
> >>>>> What can I tell you?  Concurrency is hard.  Distributed programming
> is
> >> hard.  Hate to say it, but in a distributed scenario, it might not be
> >> realistic to expect deterministic behaviour out of a system.  Jini is
> all
> >> about embracing failure and indeterminacy. (Ken Arnold again -
> >> http://www.artima.com/intv/distrib.html)
> >>>>>
> >>>>>>               // Don't like the fact that we're calling foreign code
> >> while
> >>>>>>               // holding an object lock, however holding this lock
> >> doesn't
> >>>>>>               // provide an opportunity for DOS as the lock only
> >> relates to a specific
> >>>>>>               // ServiceRegistrar and doesn't interact with client
> >> code.
> >>>>>>               matches = proxy.lookup(tmpl, Integer.MAX_VALUE);
> >>>>>>
> >>>>>>  /* The cache must be created inside the listener sync block,
> >>>>>>
> >>>>>>            * otherwise a race condition can occur. This is because
> >> the
> >>>>>>            * creation of a cache results in event registration which
> >>>>>>            * will ultimately result in the invocation of the
> >> serviceAdded()
> >>>>>>            * method in the cache's listener, and the interruption of
> >> any
> >>>>>>            * objects waiting on the cache's listener. If the
> >> notifications
> >>>>>>            * happen to occur before commencing the wait on the
> >> listener
> >>>>>>            * object (see below), then the wait will never be
> >> interrupted
> >>>>>>            * because the interrupts were sent before the wait()
> >> method
> >>>>>>            * was invoked. Synchronizing on the listener and the
> >> listener's
> >>>>>>            * serviceAdded() method, and creating the cache only
> >> after the
> >>>>>>            * lock has been acquired, together will prevent this
> >> situation
> >>>>>>            * since event registration cannot occur until the cache
> is
> >>>>>>            * created, and the lock that allows entry into the
> >> serviceAdded()
> >>>>>>            * method (which is invoked once the events do arrive) is
> >> not
> >>>>>>            * released until the wait() method is invoked .
> >>>>>>            */
> >>>>>>
> >>>>>>       /**
> >>>>>>        * With respect to a given service (referenced by the
> parameter
> >>>>>>        * newItem), if either an event has been received from the
> >> given lookup
> >>>>>>        * service (referenced by the proxy parameter), or a snapshot
> >> of the
> >>>>>>        * given lookup service's state has been retrieved, this
> method
> >>>>>>        * determines whether the service's attributes have changed,
> >> or whether
> >>>>>>        * a new version of the service has been registered. After the
> >>>>>>        * appropriate determination has been made, this method
> >> applies the
> >>>>>>        * filter associated with the current cache and sends the
> >> appropriate
> >>>>>>        * local ServiceDiscoveryEvent(s).
> >>>>>>        *
> >>>>>>        * This method is called under the following conditions: -
> >> when a new
> >>>>>>        * lookup service is discovered, this method will be called
> >> for each
> >>>>>>        * previously discovered service - when a gap in the events
> >> from a
> >>>>>>        * previously discovered lookup service is discovered, this
> >> method will
> >>>>>>        * be called for each previously discovered service - when a
> >> MATCH_MATCH
> >>>>>>        * event is received, this method will be called for each
> >> previously
> >>>>>>        * discovered service - when a NOMATCH_MATCH event is
> >> received, this
> >>>>>>        * method will be called for each previously discovered
> >> service Note
> >>>>>>        * that this method is never called when a MATCH_NOMATCH event
> >> is
> >>>>>>        * received; such an event is always handled by the
> >> handleMatchNoMatch
> >>>>>>        * method.
> >>>>>>        *
> >>>>>>        * When this method is called, it may send one of the
> >> following events
> >>>>>>        * or combination of events: - a service changed event - a
> >> service
> >>>>>>        * removed event followed by a service added event - a service
> >> removed
> >>>>>>        * event
> >>>>>>        *
> >>>>>>        * A service removed event is sent when the service either
> >> fails the
> >>>>>>        * filter, or the filter produces an indefinite result; in
> >> which case,
> >>>>>>        * the service is also discarded.
> >>>>>>        *
> >>>>>>        * A service changed event is sent when the service passes the
> >> filter,
> >>>>>>        * and it is determined that the service's attributes have
> >> changed. In
> >>>>>>        * this case, the old and new service proxies are treated as
> >> the same if
> >>>>>>        * one of the following conditions is met: - this method was
> >> called
> >>>>>>        * because of the receipt of a MATCH_MATCH event - the old and
> >> new
> >>>>>>        * service proxies are byte-wise fully equal (Note that the
> >> lookup
> >>>>>>        * service specification guarantees that the proxies are the
> >> same when a
> >>>>>>        * MATCH_MATCH event is received.)
> >>>>>>        *
> >>>>>>        * A service removed event followed by a service added event
> >> is sent
> >>>>>>        * when the service passes the filter, and the conditions for
> >> which a
> >>>>>>        * service changed event would be considered are not met; that
> >> is, this
> >>>>>>        * method was not called because of the receipt of a
> >> MATCH_MATCH event;
> >>>>>>        * or the old and new service proxies are not byte-wise fully
> >> equal.
> >>>>>>        *
> >>>>>>        * The if-else-block contained in this method implements the
> >> logic just
> >>>>>>        * described. The parameter matchMatchEvent reflects the
> >> pertinent event
> >>>>>>        * state that causes this method to be called. That is, either
> >> a
> >>>>>>        * MATCH_MATCH event was received, or it wasn't, (and if it
> >> wasn't, then
> >>>>>>        * a full byte-wise comparison is performed to determine
> >> whether the
> >>>>>>        * proxies are still the same).
> >>>>>>        *
> >>>>>>        * To understand when the 'else' part of the if-else-block is
> >> executed,
> >>>>>>        * consider the following conditions: - there is more than one
> >> lookup
> >>>>>>        * service with which the service registers (ex. LUS-0 and
> >> LUS-1) -
> >>>>>>        * after the service registers with LUS-0, a NOMATCH_MATCH
> >> event is
> >>>>>>        * received and handled (so the service is now known to the
> >> cache) -
> >>>>>>        * before the service registers with LUS-1, the service is
> >> replaced with
> >>>>>>        * a new version - the NOMATCH_MATCH event resulting from the
> >> service's
> >>>>>>        * registration with LUS-1 is received BEFORE receiving the
> >>>>>>        * MATCH_NOMATCH/NOMATCH_MATCH event sequence that will
> >> ultimately
> >>>>>>        * result from the re-registration of that new version with
> >> LUS-0 When
> >>>>>>        * the above conditions occur, the NOMATCH_MATCH event that
> >> resulted
> >>>>>>        * from the service's registration with LUS-1 will cause this
> >> method to
> >>>>>>        * be invoked and the proxies to be fully compared (because
> >> the event
> >>>>>>        * was not a MATCH_MATCH event); and since the old service
> >> proxy and the
> >>>>>>        * new service proxy will not be fully equal, the else part of
> >> the
> >>>>>>        * if-else-block will be executed.
> >>>>>>        *
> >>>>>>        * This method applies the filter only after the above
> >> comparisons and
> >>>>>>        * determinations have been completed.
> >>>>>>        */
> >>>>>>
> >>>>>> On 9/09/2015 1:40 PM, Greg Trasuk wrote:
> >>>>>>>> On Sep 8, 2015, at 10:40 PM, Peter<jini@zeus.net.au>   wrote:
> >>>>>>>>
> >>>>>>>> On 8/09/2015 11:26 PM, Greg Trasuk wrote:
> >>>>>>>>> That’s the current state.  Changing (e.g. by enforcing a builder
> >> pattern or something) would add unneeded complexity for the user if you
> ask
> >> me.
> >>>>>>>> Yes, I agree, a constructor is suitable, keep it simple.
> >>>>>>>>
> >>>>>>>>> In fact, the fact that the Entry fields are non-final is the way
> >> Jini knows it’s a field and not a constant.  i.e. in the AbstractEntry
> >> docs, it specifically says "The entry fields of an Entry are its public,
> >> non-primitive, non-static, non-transient, non-final fields."
> >>>>>>>> There's no code that inspects the field and checks whether it's
> >> final, not within River that I'm aware of, feel free to show me where if
> >> I'm wrong :).
> >>>>>>> com.sun.jini.outrigger.EntryRep.  Look for ‘Modifier.FINAL’ in the
> >> ‘usableField(…)’ method.  There are 12 other uses of ‘FINAL’ in River
> >> 2.2.2.  Not to mention any external product (Blitz, Rio, who knows what)
> >> that are designed around the Entry specification.
> >>>>>>>
> >>>>>>> The EntryRep you posted below is com.sun.jini.reggie.EntryRep.
> >> You’ll notice that it calls com.sun.jini.reggie.ClassMapper, which also
> >> filters final fields out of the comparisons and mappings.
> >>>>>>>
> >>>>>>>> The reason these fields are non final, is so they can be set with
> >> reflection, this is a detail of the Entry specification we should look
> at
> >> changing.
> >>>>>>>>
> >>>>>>> The reason they’re non-final is so that services like Reggie and
> >> Outrigger know what constitutes an Entry field that they might need to
> >> match.  Entry fields are used to form the templates for matching
> entries.
> >>>>>>>
> >>>>>>>> Entry's are not subject to the usual serialization rules.  All
> >> fields in an Entry in superclass to subclass order are stored in an
> >> EntryRep (appended).
> >>>>>>>> At present these are set after construction using reflection,
> >> however a constructor that accepts an array parameter, will allow the
> child
> >> most class to pass that array up through all constructors to reconstruct
> >> the Entry without using reflection (magnitudes faster performance wise).
> >>>>>>>>
> >>>>>>>> Eliminating the use of reflection during deserialization will
> >> increase performance, and immutability increases scalability.
> >>>>>>>>
> >>>>>>>>> Where do we use entries?  Typically in doing Service
> registrations
> >> and lookups.  Most users can understand that the Entry is going to be
> >> serialized and used remotely.
> >>>>>>>> Clients of Javaspaces and utility classes too, like JoinManager
> and
> >> ServiceDiscoveryManager.
> >>>>>>>>> Add to documentation?  Sure.  Change the API?  Maybe add an
> >> optional builder-style object.  (e.g. create a StatusBuilder that acts
> as a
> >> bean and generates Status entries).  But even so, it seems like a small
> >> enough part of using Jini to not bother much with.
> >>>>>>>> Due to the way the current Entry spec works, you can't add a field
> >> to an Entry without breaking compatibility with subclassses.
> >>>>>>>>
> >>>>>>>> So the proposed change would be a new public constructor, that
> >> accepts an array, containing field object values in the order that
> fields
> >> occur and that all fields be final.
> >>>>>>>>
> >>>>>>>> I'd also propose including a warning that once published the order
> >> and number of fields in an Entry should not be changed if it can be
> >> extended, otherwise if there's a chance that additional fields might
> need
> >> to be appended at a later date, a reccommendation that the class be made
> >> final, so it can't be subclassed.
> >>>>>>>>
> >>>>>>>> So in other words, the following changes will break an Entry:
> >>>>>>>>
> >>>>>>>> 1. A change in the class heirarchy.
> >>>>>>>> 2. A change in the order or number or type of fields.
> >>>>>>>>
> >>>>>>>> An Entry is best thought of an interface definition for a defined
> >> group of objects in serial form.
> >>>>>>>>
> >>>>>>>>> Another way to think about it is that Entries don’t mean much
> >> until you send them somewhere.  So you need to make sure they’re all
> setup
> >> and everything they refer to is stable when you send them.  To me,
> that’s a
> >> user-understanding item that’s difficult to enforce through code.
> >>>>>>>> But because Entry fields are mutable and unsynchronized, and used
> >> in utility classes such as ServiceDiscoveryManager, it becomes
> difficult to
> >> manage when Entry's are passed around between threads.   To submit an
> Entry
> >> to a service, it is passed to a thread pool where it becomes a
> serialized
> >> method invocation.
> >>>>>>>>
> >>>>>>>> To ensure the changes made by one thread are visible to another,
> >> the Entry must be published safely, otherwise the changes can't be
> >> guaranteed visible between threads.  There is no synchronization on
> Entry
> >> field access, so this makes it very difficult to reason about an Entry,
> one
> >> strategy I've adopted is defensive copying, however all this does, is
> >> guarantee that any modifications made by client code, doesn't affect a
> copy
> >> in ServiceDiscoveryManager (for example), there is a memory usage cost,
> >> because internal copies can't be shared with client ServiceItemFilter’s
> >>>>>>> Typical usage of a ServiceDiscoveryManager, for instance, would be
> >> if I want to perform a lookup I’ll create a ServiceTemplate that
> contains
> >> the Entries I want matched.  If I want to create a LookupCache, I’ll
> pass
> >> in a ServiceTemplate to the createLookupCache method.
> >>>>>>>
> >>>>>>> If you have a separate thread that messes around with those entries
> >> while the lookup is in process, or while the LookupCache is active,
> that’s
> >> a client error - you deserve what you get.
> >>>>>>>
> >>>>>>> If ServiceDiscoveryManager does it wrong, we need to fix
> >> ServiceDiscoveryManager.
> >>>>>>>
> >>>>>>>> .
> >>>>>>>>
> >>>>>>>> Reasoning about shared state becomes much simpler if Entry's are
> >> immutable and the upside is scalability and performance improves,
> >> especially for clients of lookup services deserializing a lot of Entry
> >> objects.
> >>>>>>>>
> >>>>>>> Entries aren’t intended to encapsulate shared state in any widely
> >> concurrent way.  They’re intended to be created on a thread, and then
> sent
> >> over the network.  If there is shared state inside a given service,
> that’s
> >> the concern of the service implementation.  Does Outrigger handle its
> >> concurrency correctly?  I don’t know- I’ve never looked.  If it doesn’t
> >> then we need to fix Outrigger, not change the Entry specification.
> >>>>>>>
> >>>>>>> It’s good that we’re having these discussions, so we can make sure
> >> that the real-world usages get taken into account before we go altering
> the
> >> specifications.
> >>>>>>>
> >>>>>>> Cheers,
> >>>>>>>
> >>>>>>> Greg Trasuk.
> >>>>>>>
> >>>>>>>
> >>>>>>>> /**
> >>>>>>>> * An EntryRep contains the fields of an Entry packaged up for
> >>>>>>>> * transmission between client-side proxies and the registrar
> server.
> >>>>>>>> * Instances are never visible to clients, they are private to the
> >>>>>>>> * communication between the proxies and the server.
> >>>>>>>> *<p>
> >>>>>>>> * This class only has a bare minimum of methods, to minimize
> >>>>>>>> * the amount of code downloaded into clients.
> >>>>>>>> *
> >>>>>>>> * @author Sun Microsystems, Inc.
> >>>>>>>> *
> >>>>>>>> */
> >>>>>>>> class EntryRep implements Serializable, Cloneable {
> >>>>>>>>
> >>>>>>>>   private static final long serialVersionUID = 2L;
> >>>>>>>>
> >>>>>>>>   /**
> >>>>>>>>    * The Class of the Entry converted to EntryClass.
> >>>>>>>>    *
> >>>>>>>>    * @serial
> >>>>>>>>    */
> >>>>>>>>   public EntryClass eclass;
> >>>>>>>>   /**
> >>>>>>>>    * The codebase of the entry class.
> >>>>>>>>    *
> >>>>>>>>    * @serial
> >>>>>>>>    */
> >>>>>>>>   public String codebase;
> >>>>>>>>   /**
> >>>>>>>>    * The public fields of the Entry, each converted as necessary
> to
> >>>>>>>>    * a MarshalledWrapper (or left as is if of known java.lang
> >> immutable
> >>>>>>>>    * type).  The fields are in super- to subclass order.
> >>>>>>>>    *
> >>>>>>>>    * @serial
> >>>>>>>>    */
> >>>>>>>>   public Object[] fields;
> >>>>>>>>
> >>>>>>>>   /**
> >>>>>>>>    * Converts an Entry to an EntryRep.  Any exception that results
> >>>>>>>>    * is bundled up into a MarshalException.
> >>>>>>>>    */
> >>>>>>>>   public EntryRep(Entry entry) throws RemoteException {
> >>>>>>>>   EntryClassBase ecb =
> >> ClassMapper.toEntryClassBase(entry.getClass());
> >>>>>>>>   eclass = ecb.eclass;
> >>>>>>>>   codebase = ecb.codebase;
> >>>>>>>>   try {
> >>>>>>>>       EntryField[] efields =
> >> ClassMapper.getFields(entry.getClass());
> >>>>>>>>       fields = new Object[efields.length];
> >>>>>>>>       for (int i = efields.length; --i>= 0; ) {
> >>>>>>>>       EntryField f = efields[i];
> >>>>>>>>       Object val = f.field.get(entry);
> >>>>>>>>       if (f.marshal&&   val != null)
> >>>>>>>>           val = new MarshalledWrapper(val);
> >>>>>>>>       fields[i] = val;
> >>>>>>>>       }
> >>>>>>>>   } catch (IOException e) {
> >>>>>>>>       throw new MarshalException("error marshalling arguments",
> e);
> >>>>>>>>   } catch (IllegalAccessException e) {
> >>>>>>>>       throw new MarshalException("error marshalling arguments",
> e);
> >>>>>>>>   }
> >>>>>>>>   }
> >>>>>>>>
> >>>>>>>>   /**
> >>>>>>>>    * Convert back to an Entry.  If the Entry cannot be
> constructed,
> >>>>>>>>    * null is returned.  If a field cannot be unmarshalled, it is
> >> set
> >>>>>>>>    * to null.
> >>>>>>>>    */
> >>>>>>>>   public Entry get() {
> >>>>>>>>   try {
> >>>>>>>>       Class clazz = eclass.toClass(codebase);
> >>>>>>>>       EntryField[] efields = ClassMapper.getFields(clazz);
> >>>>>>>>       Entry entry = (Entry)clazz.newInstance();
> >>>>>>>>       for (int i = efields.length; --i>= 0; ) {
> >>>>>>>>       Object val = fields[i];
> >>>>>>>>       EntryField f = efields[i];
> >>>>>>>>       Field rf = f.field;
> >>>>>>>>       try {
> >>>>>>>>           if (f.marshal&&   val != null)
> >>>>>>>>           val = ((MarshalledWrapper) val).get();
> >>>>>>>>           rf.set(entry, val);
> >>>>>>>>       } catch (Throwable e) {
> >>>>>>>>           if (e instanceof IllegalArgumentException) {
> >>>>>>>>           // fix 4872566: work around empty exception message
> >>>>>>>>           String msg = "unable to assign " +
> >>>>>>>>               ((val != null) ?
> >>>>>>>>               "value of type " + val.getClass().getName() :
> >>>>>>>>               "null") +
> >>>>>>>>               " to field " + rf.getDeclaringClass().getName() +
> >>>>>>>>               "." + rf.getName() + " of type " +
> >>>>>>>>               rf.getType().getName();
> >>>>>>>>           e = new ClassCastException(msg).initCause(e);
> >>>>>>>>           }
> >>>>>>>>           RegistrarProxy.handleException(e);
> >>>>>>>>       }
> >>>>>>>>       }
> >>>>>>>>       return entry;
> >>>>>>>>   } catch (Throwable e) {
> >>>>>>>>       RegistrarProxy.handleException(e);
> >>>>>>>>   }
> >>>>>>>>   return null;
> >>>>>>>>   }
> >>>>>>>>
> >>>>>>>>   /**
> >>>>>>>>    * We don't need this in the client or the server, but since we
> >>>>>>>>    * redefine equals we provide a minimal hashCode that works.
> >>>>>>>>    */
> >>>>>>>>   public int hashCode() {
> >>>>>>>>   return eclass.hashCode();
> >>>>>>>>   }
> >>>>>>>>
> >>>>>>>>   /**
> >>>>>>>>    * EntryReps are equal if they have the same class and the
> fields
> >>>>>>>>    * are pairwise equal.  This is really only needed in the
> server,
> >>>>>>>>    * but it's very convenient to have here.
> >>>>>>>>    */
> >>>>>>>>   public boolean equals(Object obj) {
> >>>>>>>>   if (obj instanceof EntryRep) {
> >>>>>>>>       EntryRep entry = (EntryRep)obj;
> >>>>>>>>       if (!eclass.equals(entry.eclass) ||
> >>>>>>>>       fields.length != entry.fields.length)
> >>>>>>>>       return false;
> >>>>>>>>       for (int i = fields.length; --i>= 0; ) {
> >>>>>>>>       if ((fields[i] == null&&   entry.fields[i] != null) ||
> >>>>>>>>           (fields[i] != null&&
> >> !fields[i].equals(entry.fields[i])))
> >>>>>>>>           return false;
> >>>>>>>>       }
> >>>>>>>>       return true;
> >>>>>>>>   }
> >>>>>>>>   return false;
> >>>>>>>>   }
> >>>>>>>>
> >>>>>>>>   /**
> >>>>>>>>    * Deep clone (which just means cloning the fields array too).
> >>>>>>>>    * This is really only needed in the server, but it's very
> >>>>>>>>    * convenient to have here.
> >>>>>>>>    */
> >>>>>>>>   public Object clone() {
> >>>>>>>>   try {
> >>>>>>>>       EntryRep entry = (EntryRep)super.clone();
> >>>>>>>>       entry.fields = (Object[])entry.fields.clone();
> >>>>>>>>       return entry;
> >>>>>>>>   } catch (CloneNotSupportedException e) {
> >>>>>>>>       throw new InternalError();
> >>>>>>>>   }
> >>>>>>>>   }
> >>>>>>>>
> >>>>>>>>   /**
> >>>>>>>>    * Converts an array of Entry to an array of EntryRep.  If
> >> needCodebase
> >>>>>>>>    * is false, then the codebase of every EntryRep will be null.
> >>>>>>>>    */
> >>>>>>>>   public static EntryRep[] toEntryRep(Entry[] entries, boolean
> >> needCodebase)
> >>>>>>>>   throws RemoteException
> >>>>>>>>   {
> >>>>>>>>   EntryRep[] reps = null;
> >>>>>>>>   if (entries != null) {
> >>>>>>>>       reps = new EntryRep[entries.length];
> >>>>>>>>       for (int i = entries.length; --i>= 0; ) {
> >>>>>>>>       if (entries[i] != null) {
> >>>>>>>>           reps[i] = new EntryRep(entries[i]);
> >>>>>>>>           if (!needCodebase)
> >>>>>>>>           reps[i].codebase = null;
> >>>>>>>>       }
> >>>>>>>>       }
> >>>>>>>>   }
> >>>>>>>>   return reps;
> >>>>>>>>   }
> >>>>>>>>
> >>>>>>>>   /** Converts an array of EntryRep to an array of Entry. */
> >>>>>>>>   public static Entry[] toEntry(EntryRep[] reps) {
> >>>>>>>>   Entry[] entries = null;
> >>>>>>>>   if (reps != null) {
> >>>>>>>>       entries = new Entry[reps.length];
> >>>>>>>>       for (int i = reps.length; --i>= 0; ) {
> >>>>>>>>       entries[i] = reps[i].get();
> >>>>>>>>       }
> >>>>>>>>   }
> >>>>>>>>   return entries;
> >>>>>>>>   }
> >>>>>>>> }
> >>>>>>>>
> >>>>>
> >>>>
> >>>>
> >>>
> >>
> >>
>
>
>

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