O conteudo do presente relat orio e de unica resp onsabilidade do s autor es
The contents of this rep ort are the sole resp onsibility of the author s
An Overview of MOLDS A Meta Ob ject
Library for Distributed Systems
Alexandre Oliva Luiz Eduardo Buzato
Relat orio T ecnico IC
Abril de
An Overview of MOLDS
A Meta Ob ject Library for Distributed Systems
Alexandre Oliva Luiz Eduardo Buzato
oliva dcc unicamp br buzato dcc unicamp br
Lab orat orio de Sistemas Distribu dos
Instituto de Computa c ao
Universidade Estadual de Campinas
April
Abstract
This pap er presents a library of meta ob jects suitable for developing distributed systems The re exive
architecture of Guaran a makes it p ossible for these meta ob jects to b e easily combined in order to form
complex dynamically recon gurable meta level b ehavior We brie y describ e the implementation of
Guaran a on Java Then we explain how several meta level services such as p ersistence distribution
replication and atomicity can b e implemented in a transparent and exible way
Resumo
Este artigo apresenta uma biblioteca de meta ob jetos adequada para o desenvolvimento de sistemas
distribu dos A arquitetura re exiva Guaran a torna p oss vel que esses meta ob jetos sejam facilmente
combinados a m de desemp enhar comp ortamento de meta n vel complexo e recon gur avel dinamica
TM
mente Descreve se sucintamente a implementa c ao de Guaran a em Java Em seguida explica se
como v arios servi cos de meta n vel como p ersist encia distribui c ao replica c ao e atomicidade p o dem ser
implementados de forma transparente e ex vel
Keywords Re ection Distributed Ob jects Persistence Replication Atomic Actions
Java is a trademark of Sun Microsystems Inc
Intro duction A meta ob ject may reply with a result for the
op eration an alternate op eration to b e p erformed
Computational re ection henceforth just re
instead of the one requested by the base level or a
ection has proven to b e a useful feature for build
request for the original op eration to b e p erformed
ing distributed systems in a transparent way
Unless it provides a result itself it may ask to b e
presented the result of the op eration after it is p er
Guaran a is a re exive architecture that aims
formed or even to b e able to mo dify this result
at reuse of re exive solutions It provides sim
Any meta ob ject is an instance of the class
ple mechanisms for combining multiple meta ob jects
MetaObject that is a Java class just like any other
into the meta level con guration of a single ob ject
Hence its instances may b e made re exive to o by as
These meta ob jects may implement meta level re
so ciating them with other meta ob jects which leads
quirements such as distribution p ersistence repli
to the so called p otentially in nite tower of meta
cation atomicity etc
ob jects This class may b e sp ecialized sub
This pap er is organized as follows In Section
classed which leads to one of the key concepts intro
we brie y describ e our implementation of the re
duced by Guaran a a sub class of MetaObject called
exive architecture of Guaran a on top of a mo di
Comp oser
ed freely available Java Virtual Machine Then we
A comp oser is a meta ob ject that delegates op er
intro duce MOLDS a library of meta ob jects that
ations and results to other meta ob jects Comp osers
provide essential features for developing reliable dis
may delegate to meta ob jects sequentially or concur
tributed systems Finally in Section we summarize
rently or following whatever p olicy ts the needs of
the b ene ts of designing a library such as MOLDS in
a develop er A sample comp oser is provided that
a re exive architecture like Guaran a and describ e
delegates op erations to the elements of an array of
the current state of development of b oth Guaran a
meta ob jects and delegates results to the same meta
and MOLDS
ob jects in the reverse order
Some of the delegatee meta ob jects of a com
p oser can b e comp osers themselves which leads to
The Java based implementa
a hierarchical organization of the meta ob jects di
rectly or indirectly asso ciated with a base level ob
tion of Guaran a
ject This organization called the ob ject s meta
con guration is orthogonal to the tower of meta
Java is a simple yet p owerful ob ject oriented
ob jects each meta ob ject may have its own indep en
language Java classes are compiled into high level
dent meta con guration Furthermore a meta ob ject
ob ject oriented cross platform byteco des that can b e
may b elong to the meta con guration of more than
executed on Java Virtual Machines JVM Since the
one ob ject
sp eci cation of the JVM is op en anyone can im
Asso ciating an ob ject with its primary meta
plement it so Java has b ecome available on several
ob ject is an op eration provided by the kernel of
di erent hardware platforms and freely mo di able
Guaran a In fact this op eration is so general that
and redistributable source co de for some of these im
it allows any meta ob ject in a meta con guration to
plementations is available at no cost The re exive
b e replaced with another Any recon guration must
architecture of Guaran a was implemented on top of
b e approved by the previous meta con guration if
one of these platforms
the base level ob ject was not re exive its class is in
In Guaran a every Java ob ject may b e directly
formed ab out the recon guration request and may
asso ciated with zero or one meta ob ject called the
prevent it
ob ject s primary meta ob ject An ob ject that is as
so ciated with a meta ob ject will b e called a re ex Ob jects created by re exive ob jects have their
2
ive ob ject Every op eration targeted to a re exive meta con gurations determined by their creator s
ob ject is intercepted rei ed represented as a meta meta con guration Furthermore the meta
level ob ject and presented to the ob ject s primary con guration of the class of the new ob ject is noti ed
meta ob ject b efore the ob ject s constructor is invoked so that it
By op eration we mean metho ds and constructors invo cations monitor synchronized enter and exit op erations eld reads
and writes Since Java arrays are ob jects to o array elements reads and writes as well as array length reads are considered
op erations to o
may try to mo dify the meta con guration of its new Persistence
instance The kernel of Guaran a makes it p ossi
ble to create pseudo ob jects that are uninitialized in
A p ersistent ob ject is one whose lifetime spans
stances of a given class These ob jects may b e turned
the application that created it The state of p er
into real ob jects by invoking a constructor or initial
sistent ob jects can b e stored in les databases or
izing its elds but it may remain a pseudo ob ject
long running pro cesses An ob ject can b e made p er
and b e used for example as a proxy of an ob ject
sistent by simply adding a p ersistence meta ob ject to
in a separate address spaces The meta con guration
its meta con guration
of the class the pseudo ob ject b elongs to is also no
A p ersistence meta ob ject may b e implemented
ti ed so it may mo dify the pseudo ob ject s meta
using two di erent approaches i it may intercept
con guration or even prevent the creation of the
all eld up date op erations and up date the p ersis
pseudo ob ject
tent storage accordingly p ossibly in background or
This noti cation is done by using another op er
ii it may up date the p ersistent storage only when the
ation provided by the kernel of Guaran a any in
p ersistent ob ject is no longer used by the running ap
stance of a class that implements the interface Mes
plication
sage can b e broadcasted to p ossibly all comp onent
Whatever the choice every ob ject must b e given
meta ob jects of a meta con guration of an ob ject
a unique identi er that can b e used for maintain
Such an op eration is necessary b ecause for security
ing references from one p ersistent ob ject to another
reasons we made it is imp ossible to obtain a refer
as well as for reincarnating an ob ject from p ersistent
ence to the primary meta ob ject of an ob ject Fur
storage into a running application This unique iden
thermore we b elieve this helps maintaining a clear
ti er might b e maintained by the p ersistence meta
separation of concerns b etween the base and the meta
ob ject itself however a unique identi er may b e use
level just like encapsulation encourages go o d ob ject
ful for other purp oses so we recommend the creation
oriented design
of a separate identi cation meta ob ject
Guaran a provides an interface that allows arbi
Whenever an ob ject typ e eld of a p ersistent ob
trary Op eration ob jects to b e created in the meta
ject is assigned to the referred to ob ject must also
level even op erations that would violate encapsu
b e made p ersistent otherwise it will not b e p ossible
lation can b e created and p erformed by using this
to recreate the complete state of the referring ob ject
interface However for the sake of security such
afterwards This can b e accomplished by probing the
op erations must b e created using Op erationFactory
meta con guration of this ob ject with a broadcast
ob jects that are given to meta ob jects whenever
message If no p ersistence nor identi cation meta
they are asso ciated with an ob ject This ensures
ob ject exists in the ob ject s meta con guration the
that only comp onent meta ob jects of an ob ject s
ob ject must recon gured so as to b ecome p ersistent
meta con guration and meta level ob jects trusted by
or the eld assignment must b e denied by throwing
them can obtain priviledged access to this ob ject
an exception
Comp osers may distribute restricted op eration facto
In order to reincarnate a p ersistent ob ject there
ries to meta ob jects they delegate to
are two p ossible approaches i the p ersistence meta
library may provide a metho d that can b e called from
the base level that reincarnates an ob ject given its
unique identi cation or ii a base level re exive con
Reusable Meta Ob jects for
tainer that represents the p ersistent storage may b e
Distributed Systems
used to reincarnate p ersistent ob jects transparently
An ob ject is reincarnated by creating a pseudo
The meta level proto col of Guaran a was designed ob ject whose elds are lled in from the p ersistent
in a way that makes it p ossible to create meta storage Reincarnation of referred ob jects can b e
ob jects that implement sp eci c meta level b ehav done on demand as they are accessed from the base
iors and to easily comp ose them into complex meta level Even elds might b e reincarnated individu
con gurations In this section we delineate how some ally The implementation of such meta ob ject would
meta level services for distributed computing can b e b e much more complicated but it may pay o if the
implemented in Guaran a ob ject s state is large enough
Replication ies to remote replica s meta ob jects in every address
space so that they can communicate This is not an
Ob ject replication may b e used in order to in
overkill since group communication proto cols usu
crease availability and fault tolerance of an ob ject If
ally require every memb er of the group to know every
one replica fails others may keep the ob ject running
other memb er
There is a very simple way of implementing repli
cation with Guaran a Every replica executes meth
Caching
o ds and reads elds without exchanging information
with other replicas Field mo di cations however are
Having to send every single op eration targeted to a
broadcasted to all replicas in a totally ordered
remote ob ject through the network may cause seri
way so that all replicas p erform eld writes in the
ous negative impact on the p erformance of an appli
same sequence Synchronization op erations must b e
cation On the other hand replicating an ob ject may
sub ject to the same total order
intro duce to o much overhead for an ob ject that is
Other replication mechanisms may broadcast
frequently up dated
metho d invo cations and even eld read op erations to
An intermediate solution may b e achieved by
multiple replicas then run an election algorithm to
caching the contents of elds of an ob ject in proxy
select a result However this intro duces some prob
ob jects These elds could b e up dated p erio dically
lems that are hard to solve For example when one
or when synchronization op erations take place Up
replicated ob ject interacts with another the interac
date op erations in the proxy ob ject might not need
tion must o ccur as if the ob jects were not replicated
to b e immediately forwarded to the actual ob ject or
at all So when one replicated ob ject invokes an
replicas This is somewhat dep endent on the
other all the individual invo cations must b e identi
requirements of the application but it may prove to
ed as replicas of a single invo cation and the op era
b e very useful in certain situations
tion must b e p erformed only once on each replica of
A caching meta ob ject can b e easily implemented
the invoked ob ject
as a comp oser that selectively delegates op erations to
a distribution meta ob ject
Distribution
Implementing transparent interaction b etween ob
Migration
jects lo cated in separate virtual machines was made
Ob jects such as mobile agents may have to move
easy by the intro duction of pseudo ob jects An ap
from one address space to another This may b e
proach similar to that taken for p ersistence may b e
achieved by creating a replica of the moving ob ject in
used to lo cate remote ob jects There are di erences
the target address space then removing the replica
i instead of lo cating ob jects in a database they will
from the source address space
b e searched for in a distributed name server which
However this may b e to o costly a way to migrate
might b e viewed as a database after all and ii
an ob ject Another p otentially faster approach is to
instead of reincarnating the ob ject a proxy of the
have a meta ob ject that stops delivering op erations
remote ob ject is created as a pseudo ob ject
to the ob ject as so on as it decides the ob ject must mi
Whenever an op eration is requested to the proxy
grate Then it marshalls the complete internal state
its distribution meta ob ject marshalls the op eration
of the ob ject and sends it to a remote meta ob ject
and sends a message through a network channel to
that is going to b ecome a memb er of the migrated ob
a meta ob ject lo cated in the actual target ob ject s
ject s meta con guration It creates a pseudo ob ject
address space This meta ob ject just creates an op
and lls in its elds with the marshalled image of the
eration equivalent to the requested one and delivers it
ob ject At this p oint the original ob ject will have b e
for meta level interception As so on as a result for the
come a proxy ob ject that simply forwards op erations
op eration is available it is marshalled and returned
to the migrated ob ject until the proxy is garbage col
to the proxy s meta ob ject which unmarshalls the
lected
result and returns it as the result of the op eration
This facility may b e used as a basis for having If an ob ject migrates many times an op eration
distributed replicas Instead of implementing inter may have to ow through several proxies b efore it
meta ob ject communication in the replication meta reaches the actual ob ject In this case it may b e
ob jects themselves now we just have to keep prox useful to have an algorithm that notices whether an
Atomicity ob ject migrated any further and sets up a short cut
to the most recently known lo cation of the ob ject
Atomic actions involve three prop erties i seri
from then on
alizability that ensures that the execution of concur
We should note that there is some overlap of the
rent atomic actions is equivalent to at least one serial
migration and the p ersistence functionalities Af
execution ii atomicity that is either all its e ects
ter all a p ersistent ob ject may b e implemented by
b ecome visible or none do and iii p ermanence of
migrating it to and from a long running server pro
e ect
cess On the other hand migration could b e easily
The last prop erty requires ob jects involved in an
implemented by storing the mobile ob ject in p ersis
atomic action to b e kept in stable storage so that
tent storage then reincarnating it in another address
even if one of the hosts running a distributed atomic
space
action fails its e ects are p ermanent
Instead of implementing one mechanism on top
of another we b elieve the correct approach is to fac
The atomicity prop erty requires a global co ordi
tor out the common functionality required by b oth
nation of all ob jects involved in an atomic action If
mechanisms and implement the di erences as sp e
the atomic action is committed all ob jects involved
cializations Caching meta ob jects may also share
must have its states made p ersistent if it ab orts all
functionality with these two mechanisms
ob jects must b e reverted to the states previous to the
b eginning of the atomic action
The serializability prop erty requires some kind of
Accounting
concurrency control on op erations There are op
Meta ob jects for accounting can b e easily asso ciated
timistic and p essimistic p olicies Pessimistic algo
with arbitrary ob jects One may count the invo ca
rithms rely on lo cking for ensuring serializable exe
tions of a particular metho d or up dates of a eld
cutions optimistic ones let separate atomic actions
or even complex multi ob ject patterns of interaction
op erate on separate copies of ob jects and check for
Classes can b e con gured so that all instances are
serializability at commit time
given appropriate accounting meta ob jects
Monitoring
Atomic actions may b e totally controlled at the
base level for example by providig a class Atomi
In addition to the ability of maintaining information
cAction that takes an instance of the Java standard
ab out base level ob jects it may b e useful for meta
class Runnable as its constructor argument The
ob jects to interact with base level ob jects from the
metho d run of this argument is then executed inside
meta level
the atomic action If it terminates successfully the
It is p ossible to interconnect otherwise indep en
transaction is committed if it throws an exception
dent ob jects through meta ob jects This can b e used
the atomic action may ab ort
to implement the MVC pattern connecting a
Concurrency control may take place transpar
mo del ob ject with its views transparently the con
ently at the meta level using whatever selected p ol
trol can b e totally implemented in the meta level
icy However if it is a p essimistic one it should b e
We might also use meta ob jects that implement
p ossible to pre declare lo cks for example from b oth
Statecharts to mo del and control the b ehavior
the base and the meta level
of base level ob jects Transitions in the Statechart
could b e triggered by the interception of op erations
Instead of explicitly creating and managing
or results there may b e additional conditions for
atomic actions in the base level certain ob jects may
the transition to take place involving the state of
b e con gured as atomic ones so that every op
the base level ob ject as well as internal meta ob ject
eration on that ob ject is p erformed inside an inde
state
p endent atomic action It may b e useful for meta
Monitoring multiple distributed ob jects may re level control of atomic actions to b e able to sp ecify
quire the construction of consistent global snap that a particular op eration should b e p erformed in
shots Algorithms for obtaining consis side a given atomic action as a nested atomic action
tent global snapshots can b e implemented completely or sharing data with other threads running the same
in the meta level transaction
Conclusion References
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Both Guaran a and MOLDS are free software re
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This work is partially supp orted by FAPESP L E Buzato H K Liesenb erg C M F Rubira
Funda c ao de Amparo a Pesquisa do Estado de R Anido and M B F de Toledo Uma ar
S ao Paulo grant for Alexandre Oliva quitetura de software para o desenvolvimento
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