Just thought I'd throw in my 2 cents: Don't forget that if you do not call defaultReadObject() on the ObjectInputStream during deserialisation, any additional fields added later will break serial compatibility.
The Serialization builder pattern allows you to substitute and migrate classes and multiple serial forms can coexist. It does this by separating the serial form from implementation. It solves the serialization problem for long lived objects in a distributed system. For an extreme example see the reference collections library included with river, many collections share the same serial form which can be coexist with a new serial form, class implementations can replaced during deserialisation in future releases. It's also available as a project on sourceforge called custardapple. See the river wiki page on serialization for more info. Cheers, Peter. ----- Original message ----- > Can't tell from discussion so far but if serialVersionUID wasn't > hardwired, I'm thinking likelihood of compatibility becomes quite > limited? > > On 4 February 2013 22:10, Dennis Reedy <[email protected]> wrote: > > I'm actually not sure if Dawid has to actually do anything here. The entries > > have been written into the space and have as their annotation a URL that has > > been provided by the entries defining classloader. In this case the entry is > > annotated using Rio's artifact URL scheme. > > > > If the change(s) are compatible changes (compatible serialization wise), > > then > > a client can come along at a time later, take the matched entry (as needed > > dynamically load classes from the annotated codebase), create a new entry > > and > > write that back into the space. The new entry will have as it's annotation > > the > > new artifact. > > > > If the changes are not compatible, then IMO, the change should be > > implemented > > using a new class (either name or in a new package). Take the old entry, > > create the new, write it back to the space. > > > > HTH > > > > Dennis > > > > On Feb 4, 2013, at 1249PM, Dan Creswell wrote: > > > > > On 4 February 2013 17:32, Dawid Loubser <[email protected]> wrote: > > > > Thanks Gerard, > > > > > > > > That does sound reasonable, but wouldn't I effectively lose the unique > > > > individual codebase annotations of each entry? I have various unrelated > > > > services that interact in often-complex ways. Consider the following: > > > > > > > > * In foo-api, I have an entry called FooEvent > > > > * In my space-based timer api, I have an entry called PublishLater, and > > > > a particular instance of PublishLater contains an instance of FooEvent, > > > > and a timestamp that says when to publish the nested entry. > > > > > > > > The timer service (and the timer-api) has no knowledge of foo-api. There > > > > would be no generic way to write that PublishLater entry to XML, and > > > > parse it again, making sure that the nested FooEvent has the correct > > > > codebase (which will be distinct from the codebase of the higher-level > > > > Entry). I have many such occurrences of entries generically containing > > > > other entries, and the codebase has to remain intact for each. > > > > > > > > I think I will (as Dan suggested( have to write a Java-based migration > > > > tool, that (using reflection) reconstructs each Entry, taking care to, > > > > at each level, retain the proper codebase, with only the changes > > > > required for the migration. Because I'm using Rio's maven-based class > > > > loading, I know that where a codebase URL was "artifact:foo:bar-api:1.0" > > > > I can now reconstruct it, replacing it with "artifact:foo:bar-api:1.1". > > > > > > > > This will be very interesting indeed, and I need to do it ASAP :-( A > > > > production deployment depends on this. After reading the Entry spec, it > > > > seems that only at each top-level field of an Entry can each object have > > > > a different codebase, right? (and not at lower levels within those > > > > objects). If so, that'll make things a lot easier. > > > > > > I think it would be possible for something below top-level field to > > > have its own codebase but that would be extremely rare (too ugly to > > > work with). > > > > > > More importantly I don't think you need to be that generic as I > > > suspect that your codebase probably does obey the "top-level field" > > > rule you mention. You could check that somewhat by doing a JavaSpace05 > > > contents and dumping out class and associated classloader plus > > > codebase if present for each entry top to bottom (in fact you could > > > store it all up in a couple of hashtables and then dump it out which'd > > > save you reading through piles of duplicates). > > > > > > > > > > > Anybody have any experience doing this the "hard" (with Java > > > > classloading) way? > > > > > > Anyone who's implemented a JavaSpace at least ;) > > > > > > Seriously, if you need some advice or whatever, punt a request up here.... > > > > > > > > > > > Dawid > > > > > > > > > > > > On Mon, 2013-02-04 at 06:56 -0800, Gerard Fulton wrote: > > > > > One easy option may be to write a simple client using your old code to > > > > > serialize the entries in the space to XML on disk. Then launch your > > > > > new > > > > > application and put entries into the space instance. > > > > > > > > > > > > > > > On Mon, Feb 4, 2013 at 3:34 AM, Dawid Loubser <[email protected]> > > > > > wrote: > > > > > > > > > > > Thanks for the quick response, Dan! > > > > > > > > > > > > I want to understand the classloading a bit better. Let me explain > > > > > > to > > > > > > you how I *think* it works. Also, for reference, I'm using the rio > > > > > > project, that has a special classloader that understands URLs in the > > > > > > form "artifact:foo:bar:1.0" and which loads classes from Maven > > > > > > artifacts, but I think it's conceptually the same as any other URL > > > > > > scheme etc. > > > > > > > > > > > > * When an Entry it written to space, it's turned into a > > > > > > MarshalledInstance. This is annotated with the codebase (a > > > > > > collection > > > > > > of URLs). Immediate question: Is there only one codebase at the > > > > > > top-level of the entry, or does every object in the graph have (or > > > > > > can > > > > > > have) its own codebase? > > > > > > > > > > > > * When a worker takes/reads an entry (which might contain things > > > > > > that > > > > > > both are on the worker's classpath, and perhaps lower-level content > > > > > > that is not (i.e. specialisations that it does not have to > > > > > > understand), how does the space proxy know what to do? I imagine it > > > > > > uses the thread context class loader, but then how does it > > > > > > deserialise > > > > > > the objects that is not on that classpath (using the codebase > > > > > > annotation of the MarshalledInstance, I imagine) whilst not > > > > > > colliding > > > > > > with the classes already available to the worker? Using some sort of > > > > > > parent/child delegation? > > > > > > > > > > > > I've got a very tricky ClassCastException problem I'm trying to > > > > > > debug, > > > > > > where it's clearly the same class loaded by two classloaders, and > > > > > > thus > > > > > > the field cannot be assigned. I don't know how to get "in there" and > > > > > > solve the problem, it seems I can only respond to the > > > > > > UnusableEntryException, get the partial entry, and lose the rest? > > > > > > > > > > > > thanks so much, > > > > > > Dawid > > > > > > > > > > > > > > > > > > On Mon, 2013-02-04 at 11:17 +0000, Dan Creswell wrote: > > > > > > > On 4 February 2013 11:10, Dawid Loubser <[email protected]> > > > > > > > wrote: > > > > > > > > Hi all, > > > > > > > > > > > > > > > > I have a bunch of entries in a JavaSpace (representing > > > > > > > > long-running > > > > > > > > process state, i.e. they exist for days or weeks), and these > > > > > > > > contain some objects that were generated from XML (using JAXB). > > > > > > > > That vocabulary has evolved (additions only) but now, of course, > > > > > > > > the computed SerialVersionUIDs will be different. When I > > > > > > > > redeploy > > > > > > > > my workers that have been built against the new API, they will > > > > > > > > surely fail when reading the old entries. > > > > > > > > > > > > > > > > Any strategies as to how I can migrate the data in the space? > > > > > > > > I'm > > > > > > > > running a persistent outrigger (snaplogstore). I was thinking > > > > > > > > of, > > > > > > > > in a worker with an 'old' classpath, draining the space, and > > > > > > > > storing those entries in some non-java representation on disk, > > > > > > > > and > > > > > > > > then in a worker with the 'new' classpath, reading those entries > > > > > > > > and re-populating the space. > > > > > > > > > > > > > > Slightly more complicated but it's possible to have one worker do > > > > > > > all > > > > > > > this with some classloader magic. You basically load old and new > > > > > > > definitions into separate classloaders with the old version being > > > > > > > directly on the classpath, the other dynamically loaded from > > > > > > > something not on the classpath. > > > > > > > > > > > > > > Then you can take the old easily and use reflection magic to > > > > > > > populate > > > > > > > a new and write it. > > > > > > > > > > > > > > One other challenge is that most JavaSpace implementations don't > > > > > > > like > > > > > > > mixed schemas do probably you're better to create a second space, > > > > > > > write the migrated ones into that and then turn off the old one > > > > > > > (or > > > > > > > copy back to the old once you've cleared it down/re-built it). > > > > > > > > > > > > > > > > > > > > > > > Migrating data in a space is surely something that must have > > > > > > > > caused > > > > > > > > problems for somebody before, and I'd love to tackle this > > > > > > > > problem > > > > > > > > drawing on some experience of others. > > > > > > > > > > > > > > > > regards, > > > > > > > > Dawid > > > > > > > > > > > > > > > > > > > > > > > > > >
