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Concurrency Control in Distributed Database Management System


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There are so many algorithms of concurrency control that has
proposed for distributed database system even the complex and large algorithms,
but as we know that a distributed database systems are becoming popular and a
commercial reality, performance tradeoffs for distributed concurrency control
are still not well recognize. In this research paper I tried to focus on some
major and important issues after studying four representative algorithms. 1)
Wound-Wait. 2) Distributed 2PL. 3) Basic Timestamp ordering and a Distributed
optimistic algorithm. 4) Using a detailed model of a distributed DBMS.

Keywords: Cohort Process (Ci), Master Process (M)


1 Introduction:

From the past years, Distributed Databases have taken
attention in the database research community. Data distribution and replication
offer opportunities for improving performance through parallel query execution
and load balancing as well as increasing the availability of data. In fact,
these opportunities have played a major role in motivating the design of the
current generation of database machines (e.g., 1, 2). This paper addresses
some of the important performance issues related to these algorithms.

Most of the distributed concurrency control algorithms come
into one of three basic classes: locking algorithms 3,4,5,6,7, Timestamp
algorithms, 8,9,1, and optimistic (or certification) algorithms 10,11,12,
13. Many proposed algorithms reviewed 14 and describe how additional
algorithms may be synthesized by combining basic mechanisms from the locking
and timestamp classes 14.

Given the many proposed distributed concurrency control
algorithms, a number of researchers have undertaken studies of their performance.
For example, the behavior of various distributed locking algorithms was
investigated in 15, 4, 16, 17. Where algorithms with varying degrees of
centralization of locking and approaches to deadlock handling have been studied
and compared with one another. Several distributed Timestamp based algorithms
were examined in 18. A qualitative study addressing performance issues for a
number of distributed locking and timestamp algorithms was presented in 14.
The performance of locking was compared with that of basic timestamp ordering
in 19, with basic and multi-version timestamp ordering in 20. While the
distributed concurrency control performance studies to date have been useful
but a number of important questions remain unanswered.

These include:

1. How do the
performance characteristics of the various basic algorithm classes compare
under alternative assumptions about the nature of the database, the workload,
and the computational environment?

2. How does the
distributed nature of transactions affect the behavior of the various classes
of concurrency control algorithms?

3. How much of a
performance penalty must be incur for synchronization and updates when data is
replicated for availability or query performance reasons?

We examine four concurrency control algorithms in this
study, including two locking algorithms, a timestamp algorithm and an
optimistic algorithm. The algorithms considered span a wide range of
characteristics in terms of how conflicts are detected and resolved. Section 2
describes our choice of concurrency control algorithms. The structure and
characteristics of our model are described in Section 3 and section 4 describes
the testing of algorithms. Finally, Section 5 summarizes the main conclusions
of this study and raises questions that we plan to address in the future.


For this study, we have chosen to examine four algorithms
that we consider to be representative of the basic design space for distributed
concurrency control 138 International Journal of Computer Science &
Communication (IJCSC) mechanisms. We summarize the salient aspects of these
four algorithms in this section. In order to do this, we must first explain the
structure that we have assumed for distributed transactions.

2.1. The Structure of
Distributed Transactions
Figure 1 shows a general distributed transaction in terms of the processes
involved in its execution. Each transaction has a master process (M) that runs
at its site of origination. The master process in turn sets up a collection of
Cohort processes (Ci) to perform the actual processing involved in running the
transaction. Since virtually all query processing strategies for distributed
database systems involve accessing data at the site(s) where it resides, rather
than accessing it remotely. There is at least one such cohort for each site
where data is accessed by the transaction. In general, data may be replicated,
in which case each cohort that update any data items is assumed to have one or
more update (Uij) processes associated with it at other sites. In particular, a
cohort will have an update process at each remote site that stores a copy of
the data items that it updates. It communicates with its update processes for
concurrency control purposes, and it also sends them copies of the relevant
updates during the first phase of the commit protocol described below.

The centralized two-phase commit protocol will be used in
conjunction with each of the concurrency control algorithms examined. The protocol
works as follows 5:

Fig 1: Distributed Transaction Structure

When a cohort finishes executing its Portion of a query, its
sends an “execution complete” message to the master. When the master has
received such a message from each cohort, it will initiate the commit protocol
by sending “prepare to commit” messages to all sites. Assuming that a cohort
wishes to commit, it sends a “prepared” message back to the master, and the
master will send “commit” messages to each cohort after receiving prepared
messages from all cohorts. The protocol ends with the master receiving “committed”
messages from each of the cohorts. If any cohort is unable to commit, it will
return a “cannot commit” message instead of a “prepared” message in the first
phase, causing the master to send “abort” instead of “commit” messages in the
second phase of the protocol. When replica update processes are present, the
commit protocol becomes a nested two-phase commit protocol. Messages flow
between the master and the cohorts, and the cohorts in turn interact with their
updaters. That is, each cohort sends “prepare to commit” messages to its
updaters after receiving such a message from the master, and it gathers the
responses from its updaters before sending a “prepared” message back to the
master; phase two of the protocol is similarly modified.

2.2. Distributed
Two-Phase Locking (2PL)

The first algorithm is the distributed “read any, write all”
two-phase locking algorithm described in 15. Transactions set read locks on
items that they read, and they convert their read locks to write locks on items
that need to be updated. To read an item, it suffices to set a read lock on any
copy of the item, so the local copy is locked; to update an item, write locks
are required on all copies. Write locks are obtained as the transaction
executes, with the transaction blocking on a write request until all of the
copies of the item to be updated have been successfully locked. All locks are
held until the transaction has successfully committed or aborted.

Deadlock is a possibility, Local deadlocks are checked for
any time a transaction blocks, and are resolved when necessary by restarting
the transaction with the most recent initial startup time among those involved
in the deadlock cycle. (A cohort is restarted by aborting it locally and
sending an “abort” message to its master, which in turn notifies all of the
processes involved in the transaction.) Global deadlock detection is handled by
a “Snoop” process, which periodically requests waits-for information from all
sites and then checks for and resolves any global deadlocks (using the same
victim selection criteria as for local deadlocks). We do not associate the
“Snoop” responsibility with any particular site. Instead, each site takes a
turn being the “Snoop” site and then hands this task over to the next site. The
“Snoop” responsibility thus rotates among the sites in a round-robin fashion,
ensuring that no one site will become a bottleneck due to global deadlock
detection costs.

2.3. Wound-Wait (WW)

The second algorithm is the distributed wound-wait locking
algorithm, again with the “read any, write all” rule. It differs from 2PL in
its handling of the deadlock problem: Rather than maintaining waits-for
information and then checking for local and global deadlocks, deadlocks are
prevented via the use of timestamps. Each transaction is numbered according to
its initial startup time, and younger transactions are prevented from making
older ones wait. If an older transaction requests a lock, and if the request
would lead to the older transaction waiting for a younger transaction, the An
Approach for Concurrency Control in Distributed Database System 139 younger
transaction is “wounded” – it is restarted unless it is already in the second
phase of its commit protocol (in which case the “wound” is not fatal, and is
simply ignored). Younger transactions can wait for older transactions so that
the possibility of deadlocks is eliminated.

2.4. Basic Timestamp
Ordering (BTO)

The third algorithm is the basic timestamp ordering
algorithm of 9, 14. Like wound-wait, it employs transaction startup
timestamps, but it uses them differently. Rather than using a locking approach,
BTO associates timestamps with all recently accessed data items and requires
that conflicting data accesses by transactions be performed in timestamp order.
Transactions that attempt to perform out-of-order accesses are restarted. When
a read request is received for an item, it is permitted if the timestamp of the
requester exceeds the item’s write timestamp. When a write request is received,
it is permitted if the requester’s timestamp exceeds the read timestamp of the
item; in the event that the timestamp of the requester is less than the write
timestamp of the item, the update is simply ignored (by the Thomas write rule

For replicated data, the “read any, write all” approach is
used, so a read request may be sent to any copy while a write request must be
sent to (and approved by) all copies. Integration of the algorithm with
twophase commit is accomplished as follows: Writers keep their updates in a
private workspace until commit time.

2.5. Distributed
Certification (OPT)

The fourth algorithm is the distributed, timestamp-based,
optimistic concurrency control algorithm from 13, which operates by
exchanging certification information during the commit protocol. For each data
item, a read timestamp and a write timestamp are maintained. Transactions may
read and update data items freely, storing any updates into a local workspace
until commit time. For each read, the transaction must remember the version
identifier (i.e., write timestamp) associated with the item when it was read.
Then, when all of the transaction’s cohorts have completed their work, and have
reported back to the master, the transaction is assigned a globally unique
timestamp. This timestamp is sent to each cohort in the “prepare to commit”
message, and it is used to locally certify all of its reads and writes as
follows: A read request is certified if (i) the version that was read is still
the current version of the item, and (ii) no write with a newer timestamp has
already been locally certified. A write request is certified if (i) no later
reads have been certified and subsequently committed, and (ii) no later reads
have been locally certified already.



Figure 2 shows the general structure of the model. Each site
in the model has four components: a source, which generates transactions and
also maintains transactionlevel performance information for the site, a
transaction manager, which models the execution behavior of transactions, a
concurrency control manager, which implements the details of a particular
concurrency control algorithm, and a resource manager, which models the CPU and
I/O resources of the site. In addition to these per-site components, the model
also has a network manager, which models the behavior of the communications
network. Figure 3 presents a slightly more detailed view of these components
and their key interactions.

Fig 2: Distributed DBMS Model Structure Fig




3: A Closer Look at the Model


3.1. The Transaction Manager Each transaction in the
workload will have a master process, a number of cohorts, and possibly a number
of 140 International Journal of Computer Science & Communication (IJCSC)
updaters. As described earlier, the master resides at the site where the
transaction was submitted. Each cohort makes a sequence of read and writes
requests to one or more files that are stored at its site; a transaction has
one cohort at each site where it needs to access data. Cohorts communicate with
their updaters when remote write access permission is needed for replicated
data, and the updaters then make the required write requests for local copies
of the data on behalf of their cohorts. A transaction can execute in either a
sequential or parallel fashion, depending on the execution pattern of the
transaction class.

3.2. The Resource

The resource manager can be viewed as a model of the
operating system for a site; it manages the physical resources of the site,
including its CPU and its disks. The resource manager provides CPU and I/O
service to the transaction manager and concurrency control manager, and it also
provides message sending services (which involve using the CPU resource). The
transaction manager uses CPU and I/O resources for reading and writing disk
pages, and it also sends messages. The concurrency control manager uses the CPU
resource for processing requests, and it too sends messages.

3.3. The Network

The network manager encapsulates the model of the
communications network. Our network model is currently quite simplistic, acting
just as a switch for routing messages from site to site. The characteristics of
the network are isolated in this module; it would be a simple matter to replace
our current model with a more sophisticated one in the future.

3.4. The Concurrency
Control Manager

 The concurrency
control manager captures the semantics of a given concurrency control
algorithm, and it is the only module that must be changed from algorithm to
algorithm. As was illustrated in Figure 3, it is responsible for handling
concurrency control requests made by the transaction manager, including read
and write access requests, requests to get permission to commit a transaction,
and several types of master and cohort management requests to initialize and
terminate master and cohort processes.



Suppose two transactions with the same time-stamp are
submitted from the client 1 &2. The cohort processes (T11 & T12) of
transaction T1 and T21 and T22 of transaction T2 are executing on server 1
& 3 and server 1 & 4 respectively.


In the first case,
where the cohort processes (T11& T21) of transaction T1 and T2 updating the
same data content Q1.

Two phase locking
Protocol (2PL) will ensure that T11 and T21 update the data content in
sequential manner. Both the cohorts at first make the request to lock the data
content Q1. Database Management System will grant the lock either to the T11 or
to the T21 depending on their arrival at server.

Wound-Wait (WW)
protocol will uses to ensure the consistency of the database. If the TS (T11)
RTS (Q1) and the suppose Q1 is presently locked by the T21 then T21 will
wounded to wait and Q1 will be allocated to the T11. When T11 will finish the execution
then Q1 will given to the T21 for further execution.

Basic Time-Stamp
Ordering (BTO) is uses to ensure that whether all the cohorts reading and
updating the correct value version of the contents and ensuring the consistency
of the database or not. If T11 have locked the data content and updating the
data content then BTO protocol will ensure T11 is allowed to update only if
TS(T11)>RTS(Q1) otherwise the updates will be ignored.

If T11 reading the data contents then it will ensure that if
TS (T11)>WTS (Q1) then only T11 is permitted otherwise simply ignored.

Certification (OPT) protocol is uses to ensure the correct version of data
contents read and update by the transactions. This protocol uses the read
certification where it ensure that the read version of the data content is
still current version and no write with newer time-stamp has been locally or
globally certified. In the case of write request certification it ensures that
no later read have been certified and subsequently commit and no later read
have been locally or globally already certified.

In the Second case,
where cohort processes (T12 & T22) of transactions T1 and T2 are updating
the data content Q2 on server3 &4 respectively.

Two Phase locking
Protocol (2PL) will ensure that data content Q2 should be updated in
sequential manner. T12 and T22 will make request to lock the Q2 data content.
If the TS (T12) RTS(Q2)
otherwise the updates will be ignored. If T12 reading the data contents then it
will ensure that if TS (T12)>WTS (Q2) then only T12 is permitted otherwise
simply ignored. Distribution Certification (OPT) protocol will use the read and
write certification to maintain the consistency of the data content.

In this testing, we find that in all the cases our algorithm
is maintaining the consistency of the data contents an also avoiding and
resolving the deadlock.



In this paper we have tried to get rid of on distributed
concurrency control performance tradeoffs by studying the performance of four
representative algorithms – Distributed 2PL, wound-wait, basic timestamp
ordering, and a distributed optimistic algorithm – using a common performance
framework. We examined the performance of these algorithms under various
degrees of contention, data replication, and workload “Distributions.”

The combination of these results suggests that “optimistic
locking,” where transactions lock remote copies of data only as they enter into
the commit protocol (at the risk of end-of-transaction deadlocks), may actually
be the best performer in replicated databases where messages are costly, We
plan to investigate this assumption in the future.




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