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From Greg Trasuk <tras...@stratuscom.com>
Subject Re: Don't let Jini Standards become an impediment to development
Date Wed, 09 Sep 2015 05:55:31 GMT

> 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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