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Department of Computer Science Technical
Reports
Department of Computer Science
1993
Ensuring Relaxed Atomicity for Flexible Transactions in Ensuring Relaxed Atomicity for Flexible Transactions in
Multidatabase Systems Multidatabase Systems
Aidong Zhang
Marian H. Nodine
Omran Bukhres
Report Number:
93-069
Zhang, Aidong; Nodine, Marian H.; and Bukhres, Omran, "Ensuring Relaxed Atomicity for Flexible
Transactions in Multidatabase Systems" (1993).
Department of Computer Science Technical Reports.
Paper 1082.
https://docs.lib.purdue.edu/cstech/1082
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ENSURING RELAXED ATOMICITY
FOR
FLEXIBLE TRANSACfIONS IN
MULTIDATABASE SYSTEMS
Aidong Zhang
Marian H. Nodine
Omran Bukhres
CSD-TR-93-069
November 1993
Ensuring
Relaxed
Atomicity
for
Flexible
Transactions
in
Multidatabase
Systems
Aidong Zhang·
Marian H. Nodine*
Omran
Bukhres*
-Department
of
Computer
Sciences
Purdue
University
West Lafayette,
IN
47907 USA
{azbang, bukhres}@cs.purdue.edu
"'Department of
Computer
Sciences
Brown University
Providence,
RI
02912 USA
mhn@cs.brown.edu
Abstract
Global tra.nsaction management
and
the
preservation
of
local autonomy are conflicting re-
quirements
in
the design
of
multidatabase
transaction management systems. A flexible trans-
action model for
the
specification
of
global transactions makes it possible to construct robust
global transactions while still preserving local autonomy.
This
paper
presents an approach
that
preserves semi-atomicity. a weaker form of atomicity applicable to flexible transactions
which
span several local database systems
that
maintain serializability and recoverability.
We
first
offer
a fundamental characterization of the flexible transaction model and then precisely define
semi-atomicity
for
flexible transactions.
We
then investigate the principles underlying the ap-
proach to
ROOble
tra.nsaction management which ensures this property. Finally,
we
construct a
class offlexible transactions
which
can
be
executed
in
the presence offailures using our proposed
commit protocol. The results demonstrate
that
the flexible transaction model substantially en-
hances the scope ofglobal transaction management beyond
that
offered
by
the traditional global
transaction model.
1
Introduction
A
multidatabase
system
(MDBS) is a collection of
autonomous
local
databases
(LDBSs)
that
can
be
accessed as a single unit.
There
are
two types
of
transactions
in a
multidatabase.
A local
transaction,
which accesses a local
database
only,
is
submitted
directly to a local
database
system.
A global
transaction,
in
contrast,
may
access several local
databases.
Such a global
transaction
is
submitted
to a global
transaction
manager
(GTM)
superimposed
upon
a
set
of
local
autonomous
1
database
systems, where
it
is parsed
into
a series
of
global
subtransactions
to
be
submitted
to
the
local
database
systems.
While atomicity
in
traditional
distributed
databases
can
be
ensured using well-known proto-
cols [4],
multidatabase
systems cannot use these protocols directly because
of
the
stronger
autonomy
requirements
of
the
component local.
databases
[3,
21}.
Of
particular
concern
is
the
fact
that
multi-
databases
cannot
assume
that
all
participating
local
databases
support
a visible prepare-to-commit
state
for global
subtransactions,
in which
they
have
not
yet been
committed
but
are
guaranteed
the
ability
to
commit.
In
such situations, a local
database
system
that
participates
in a multi-
database
environment
may
unilaterally
abort
a global subtransa.ction
without
agreement from
the
global. level
(termed
a
local
unilateral abort). As a result,
it
becomes difficult
to
ensure
that
a single
logical. commit action
of
the
subtransactions
of
a global
transaction
will
be
consistently carried
out
at
multiple local sites.
In
addition, even when local
database
systems do provide such
support,
the
potential
blocking
and
long delays caused by prepare-to-commit
states
would severely degrade
local execution autonomy.
A flurry
of
research activity has been devoted to
the
problems
of
enhancing
transaction
man-
agement by using extended
transaction
models.
l
In
particular, some flexible
transaction
models
proposed for
the
MDBS
environment, such as Flex Transactions
[8]
and S-transaction
[22],
increase
the
failure resiliency
of
global transactions by allowing alternative
subtransactions
to
be
executed
when a local
database
fails
or
a
subtransaction
aborts.
This
flexibility allows a global.
transaction
to adhere
to
a weaker form
of
atomicity, which we
term
semi-atomicity, while still maintaining
its correct execution in
the
multidatabase.
Semi-atomicity allows a global
transaction
to
commit
even
if
some
subtransactions
abort,
provided
that
their
alternative
subtransactions
commit.
The
following example is illustrative:
Example
1 Consider the following
global
transaction. A user
of
an Automatic Teller Machine
(ATM)
wishes to withdraw $50 from his savings account al in bank b
l
and to obtain the money in
cash from the
ATM.
If
the
ATM
fails
to
disburse the cash, he will instead transfer the $50 to his
checking account
a2
in bank b
2
. With Flex Transactions, this is represented
by
the
follOWing
set
of
subtransactions:
tl:
Withdraw $50 from savings account
al
in bank b
l
.
t2: Disburse $50 from the
ATM.
ta: Transfer $50 to checking account
a2
in bank 6
2
.
In this
global
transaction, either
{tl'
t
2
}
or
{h,
t
a
}
is acceptable, with
{tl'
t2} prefern;d.
Ift
2
fails,
ta may replace
t2.
The entire
global
transaction thus may not have to
be
aborted, even
if
t
2
fails,
but
if
both
t
2
and fa fail, then
it
must abort. 0
1 Many such modelB have
appeared
in
[6,
7].
2
Since
the
flexible transaction model was proposed, much research has been devoted
to
its ap-
plication [13, 2, 1, 12].
Most
of
this work has assumed
the
availability
of
visible prepare-to-commit
states
in local
database
systems.
In
such a scenario,
the
preservation
of
the
semi-atomicity of
flexible transactions is relatively straightforward.
1.1
Proposed
Research
In
this
paper,
we offer a precise definition
of
the
fundamental model
and
of
the
semi-atomicity
property
of
flexible transactions.
We
present
an
approach which preserves semi-atomicity in an
MDBS environment
in which
the
local
database
systems
are
requ.ired only
to
ensure serializability
and
recoverability
[4].
In
the
proposed formulation, a flexible
transaction
is defined as a set
of
subtransactions
upon
which a set
of
partial orders is specified. Each partial
order
provides one
alternative for
the
successful execution of
the
flexible transaction. This methodology differs from
previous approaches in
that
no
specific application semantics
are
involved. Therefore, a theoret-
ical basis for flexible
transaction
management can
be
built. We
then
classify
the
set
of flexible
transactions
that
can
be
executed in an error-prone MDBS environment. As
the
compensation
and
retry
approaches
[15)
are
unified and employed as flexible
transaction
failure recovery techniques,
local prepare-to-commit
states
are
no longer required.
We
demonstrate
that
the
flexible transaction
model substantially enhances
the
scope
of
global transaction management beyond
that
offered by
the
traditional
global transaction model.
1.2
Related
Research
In
order
to
handle local unila.teral
aborts,
approaches using forward recovery (redo
and
retry)
and
backward recovery (compensation) have been proposed in
the
literature. These approaches seek to
ensure
the
semantic atomicity
[9]
of
global transactions in MDBSs. When a subtransa.ction of a
global
transaction
aborts,
the
GTM
may
either re-execute
it
until commitment
or
undo
the
effects
of
the
committed
subtransactions
of
the
global transaction.
The
strategies characterizing these
approaches can
be
classified by
the
relative timing
of
the commitment
of
subtransactions in
the
local
databases
with respect
to
the
global transaction
commit/abort
decision [16]. [23,5] enforce a
global decision on
the
subtransactions by redoing or retrying
them
as necessary. [19,14,18] commit
subtransactions locally before a global decision is
made
and
rely
on
compensation when a global
transaction is aborted. [15,
17]
combine these two approaches.
With
the
forward approach, all
subtransa.ctions
must
be redoable or retriable, while
the
backward approach requires
that
all sub-
transactions
must
be
compensatable.
With
the
combined approach, only one subtransaction
of
each
global
transaction
can be neither retriable
nor
compensatable,
and
the
rest of its subtransactions
must
be
either retriable
or
compensatable. Consequently,
the
ability
to
specify global transactions
becomes severely limited when
the
traditional global transaction model is employed in MDBSs.
3