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International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 
CONSISTENCY OF DATA REPLICATION 
PROTOCOLS IN DATABASE SYSTEMS: A 
REVIEW 
Alireza Souri1, Saeid Pashazadeh*2 and Ahmad Habibizad Navin3 
1, 3Department of Computer Engineering, Tabriz Branch, Islamic Azad University, 
Tabriz, Iran. 
2* Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran. 
Abstract: 
In this paper, a review for consistency of data replication protocols has been investigated. A brief 
deliberation about consistency models in data replication is shown. Also we debate on propagation 
techniques such as eager and lazy propagation. Differences of replication protocols from consistency view 
point are studied. Also the advantages and disadvantages of the replication protocols are shown. We 
advent into essential technical details and positive comparisons, in order to determine their respective 
contributions as well as restrictions are made. Finally, some literature research strategies in replication 
and consistency techniques are reviewed. 
Keywords: 
Database system, consistency, data replication, update propagation. 
1. Introduction 
Consistency of Replication models is essential to abstract away execution particulars, and to 
classify the functionality of a given system. Also a consistency model is a method for come to a 
joint considerate of each other’s rights and responsibilities. 
Database system attracts lots of consideration. A large-scale database storage system [1,2] is 
among the fundamental conveniences in the cloud, unstructured peer-to-peer (P2P) networks [3], 
grid environment [4] or in similar systems. The system with large-scale database system typically 
assigns computing replicas near their input data[5]. A good data management develops very 
important conditions in such a scenario. Data in a distributed database system [6] is replicated for 
increasing reliability, availability and performance. There are two mechanisms for locations of 
data replicas such as static and dynamic replicated system[7,8], which regulates replica locations 
based on session information of requests [9]. 
DOI : 10.5121/ijit.2014.3402 19
International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 
On the one hand, in these consistency models, its performance suitability for data replication 
architecture are not specified exactly. On the other hand, a consistency model dos not guarantees 
the high performance and high scalability for a data replication mechanism [10]. 
Consistency, accessibility, scalability, security, fault tolerant and performance [11] are areas for 
system implementation[12]. High accessibility and performance are basics for such a system with 
large-scale distributed database system. We have to make a tradeoff between consistency and 
replication. There are dissimilar levels of weak and strong consistency. A distributed database 
system may deliver levels of consistency weaker than one-copy-serializability but stronger than 
eventual consistency. Also there are levels of consistency such as data-centric and client-centric 
models for a data replication mechanism. So the recognizing usage of consistency models in each 
data replication mechanism is necessary. In Section 2, we introduce consistency models in client 
view and server view, adapted from the theory of database concurrency control. Then, we depict 
on consistency protocols in Section 3. With these discussions, we can represent a comparison 
among eventual consistency and client-centric consistency models. Properties of their 
implementations can also be deduced accordingly. In section 4, we discuss replication models and 
propagation techniques. We show all of the replication protocols according to the update 
propagation and replication mechanisms in Section 5. Section 6 shows a classified review for 
replication and consistency techniques in some research strategies. Section 7 is the conclusion 
and describes future work finally. 
20 
2. Consistency models 
In this section, a series of consistency models are considered. We discuss about differences of 
consistency models. Variant methods to categorizing the consistency models can be originated 
from [13] and [14].One of the important properties of a system design is consistency model. This 
property can typically offered in relations of a state that can be true or false for different 
implementations. Consistency models are referred to as the contracts between process and data 
for ensuring correctness of the system. Consistency models are presented through a number of 
consistency criteria to be satisfied by assessments of operations [15].For standard consistency 
conditions of the ACID properties [16], there exists some methods for consistency guarantee. In 
ACID consistency method, database is in a consistent state when a transaction is finished. In the 
client level there are four component: 
• DS is a storage system. 
• PA is the process operation for each read or write by DS. 
• PB is sovereign of process PA that performs each read and write operation from the DS. 
• PC is sovereign of process PA that performs each read and write operation from the DS. 
• 
In the client level consistency, it is important that how and when an observer is occurred. The PA, 
PB and PC processes see updates with a data item in the storage system. There are two 
consistency types such as Data-Centric consistency and Client-Centric consistency[17]. 
In Data-Centric consistency there are: 
• Strict consistency. All of the A, B and C send back the result of update value when the 
update procedure is completed.
International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 
• Sequential consistency. The level of sequential consistency is lower than strict 
consistency. Each read and write operation is performed by all replicas on their data item 
x sequentially. Also each discrete procedure operations execute the identified order. 
• Causal consistency. This consistency is weaker than strict and sequential consistency[18]. 
If transaction T1 is influenced or caused on an earlier transaction T2, each replica should 
be first see T2, and then see T1. 
• FIFO consistency. FIFO consistency is relaxed to implement because it is being 
guaranteed two or more writes from a single source must arrive in the order issued. 
Basically, this means that with FIFO consistency, all writes generated by different 
processes are concurrent. 
21 
In Client-Centric consistency models there are: 
• Eventual consistency. This model guarantees that if a updates are complete to the item 
eventually [19], then all accesses on this data item send back the previous updated value 
[20]. 
• Monotonic Reads. In this model if an operation reads the data item x, always each 
following read operation on data item x send back same value x or a more recent value. 
• Monotonic Writes. In this model if an operation writes on the data item x, always each 
following write operation on data item x comes after related write operation on the data 
item x. 
• Read-your-Write. The result of a write operation on the data item x always will be 
realized by a following read operation on x by the same value. 
• Write follow read. In this model the effect of a write operation on a data item x following 
a previous read operation on data item x by the same value that is guaranteed to take 
place on the same or a more recent value of x that was read. 
3. Consistency Protocols 
In this section, we describe the consistency protocols according to[21]. A consistency protocol 
explains as an implementation of a specific consistency model. We track the group of our 
conversation on consistency models[22]. 
3.1 Primary Replica Based Protocol 
In this protocol, All write operations to a data item x is attended by one specific replica that called 
primary replica. This primary replica [23] is accountable for updating other replicas, the client 
just cooperates by this primary replica[16]. 
Two requirements should be happen for this generous of protocol [24]: 
• All read and write operations for updating a data item x should spread and be executed all 
replicas at some time. 
• These operations should be executed in the same order.
International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 
22 
3.2 Replicated Write Protocol: 
In this protocol, each write operations are sent to each replica to update procedure. There are two 
types for replicated write protocols. 
3.2.1. Active Replication: 
In active replication, each replica contains a concomitant procedure that transports out the update 
operations. Unlike other protocols, update operations are normally propagated through the write 
operation. This propagation causes the operation is sent to each replica. Also there is required a 
total order for all write operations that each replica execute the same order of write commands 
[25]. 
3.2.2. Quorum Based: 
This protocol specifies that the clients obtain the authorization of several servers before any 
reading or writing a replicated data item x [26]. For example, the write operations only want to be 
executed on fragment of all replicas before return to the client. It use elections to avoid write-read 
conflict and write-write conflict [27]: 
• R is the number of replicas of each data item. 
• Rr is number of replicas that a client should contacts by them for reading a value. 
• Rw is number of replicas that a client should contacts by them for writing a value. 
• For preventing the Write-Write and Write-Read conflicts, Rr + Rw > R and Rw + 
Rw > R should be satisfied. 
4. Update propagation strategies 
Update propagation can be measured in two methods [28] 
• The update operations are applied to all replicas as part of the unique contract. 
• Each replica is updated by the originating transaction. Update operations send to other 
replicas asynchronously as a discrete transaction for each node [29]. 
There are two update propagation methods: Eager techniques and lazy techniques. Normally the 
eager protocols are identified as read-one/write-any (ROWA) protocols. First, they have not 
transactional inconsistencies. Second, an update transaction can read a local copy of the data item 
x and be sure that a refresh value is read. Consequently, there is no essential to executing a remote 
read. Finally, the variations to replicas are completed atomically. When we use to a 2PC 
execution, the update speed is restricted and it cause that the response time performance of the 
update transaction is low. When one of the copies is inaccessible, the update transaction cannot 
terminate meanwhile all the copies updated essentially. Lazy protocol is used to new mechanisms 
for guaranteeing strong mutual consistency [30]. These mechanisms may be bright to endure 
some inconsistency between the replicas for better performance[31].
International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 
In a distributed database system the requests permit to access data from local and remote 
databases [32]. Distributed methods spread on the update procedure to the local copy where the 
update transaction creates, then the updates are broadcasted to the other replica. If distributed 
techniques are attached by eager propagation approaches, then the distributed concurrency control 
approaches can sufficiently report the concurrent updates problem [13]. Table 1 is shown the 
comparisons of update propagation and propagation techniques [33]. 
23 
Table 1. Comparisons of update propagation techniques 
Consistency Updating Performance Failure 
Eager 
update 
Strong 
consistency 
Up-to-date with 
High response time 
Not transactional 
inconsistency, 
Changes are 
atomic 
Restricted update 
speed, transaction 
crash and 
Lower availability 
Lazy update Weak 
consistency 
Out-of-date problem 
and Low response 
time 
Not fault tolerant, 
good 
response time 
Dirty read problem, 
Data inconsistency 
and transaction 
inversion 
Centralized 
techniques 
- 
Up-to-date with 
Update without 
synchronization 
Appropriate for 
few master sites 
High overload and 
bottleneck 
problems 
Distributed 
techniques 
- 
Up-to-date with 
Concurrency control 
methods 
Highest system 
availability 
Management 
problem, Copies 
need to be 
synchronized 
5. Replication Protocols 
[13] presented a categorization for replicas data protocols. This is important that When one of the 
update propagation mechanisms such as eager or lazy incomes and who should complete updates 
mechanism such as primary copy or update-everywhere. In eager propagation mechanism, the 
propagation of updates is contained by the restrictions of a transaction. The client does not 
receive the notification of commit message up to necessary duplicates have been updated in the 
system. In the Lazy mechanism, the update procedure of a local copy is committed. the update 
propagation be accomplished [34]. There is an expensive way for providing response time and 
message overhead in consistency of eager mechanism. An optimization for prevent from these 
problems is using Lazy replication approach. However, the update procedure is executed 
separately, therefore inconsistency conditions might occur. [35]. When the updates are broadcast 
to replicas in eager or lazy mechanism, two architectures are needed for updates such as 
centralized and distributed. Table 2 shows the four replication mechanisms such as eager 
distributed and eager centralized for eager mechanism, lazy distributed and lazy centralized for 
lazy mechanism [36].
International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 
24 
Table 2. Update propagation vs. propagation techniques 
Centralized Distributed 
Eager 
Eager Primary Copy 
Eager One Master by Restricted Transparency Update Everywhere 
One Master with Full Transparency 
Lazy 
Lazy Primary Copy 
Single Master with Limited Lazy Update Everywhere 
Transparency 
5.1 Eager Centralized Protocol 
In eager centralized protocol, there is a site as a master that navigates the read and writes 
operations on a data item (x). This protocol guarantees strong consistency techniques for update 
propagation. In update procedure, all updates are applied to a logical data item (x) by using the 
perspective of the update transaction. This applying is committed by using the 2 Phase Commit 
protocol. So, when the update procedure is completed in its transaction, all copies return the 
similar values to the updated data items. The result of this mechanism is one-serializability-replication[ 
37]. The categories of eager centralized include eager primary copy, single master by 
restricted transparency and single master by full transparency. In eager primary copy, any data 
item (xi) has a master. One replica specified as the primary copy. In this case, there is no single 
master for controlling serializability condition. In the single master by restricted transparency, all 
of the updates have been sent to the specified master directly. For a read operation, a read lock 
occurred on data item x and the read operation is executed. The result of the operation is returned 
to the client. Also for a write operation, a write lock occurred on data item x and this operation is 
executed. The result of write operation is returned to the client. In the single master by full 
transparency, the replica coordination level has been performed by a router. The router sends the 
entire read and writes operations to the master directly. The master executes each operation and 
returns the result of execution to the client. 
5.2 Eager Distributed Protocol 
In eager distributed protocol, first the update applied to the local replica, then the update 
procedure is propagated to other replicas. The eager update everywhere is a type of eager 
distributed protocol. 
5.3 Lazy Centralized Protocol 
Lazy centralized protocol is like to eager centralized protocol. In this protocol, first the updates 
are applied to a master and then propagated to the clients. The significant alteration is that the 
propagation procedure does not take place via the update process. However, after the 
commitment of transactions, if a client executes a read operation (x) on its local copy, it may read 
a non-refresh data, then data item x may have been updated at the master, nevertheless the update 
may not have been propagated to the clients yet. The categories of lazy centralized include lazy 
primary copy and single master by restricted transparency. In lazy primary copy, each read and 
write operation sends to a master. All of the updating results have been send back to the client. In
International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 
single master by restricted transparency, the update procedure is executed to the master directly. 
When one update has committed, the new transaction is sent to the clients. 
25 
5.4 Lazy Distributed Protocol 
In Lazy distributed protocol, the update transactions can execute on each replica. Also these 
updates are propagated to the other replicas lazily. Lazy update everywhere is a type of lazy 
distributed protocol. In this type, each read and write operation are performed on the local copy 
and the update transactions commit locally. Comparison of replication protocols about 
consistency conditions is shown in Table 3. 
Table 3. Comparison of replication protocols 
Replication strategies Advantages Disadvantages 
Eager Centralized 
The coordination do not 
needs for Update 
transactions, there is no 
inconsistencies 
Extensive response time, 
Local copies are can only be 
Read, Only useful with few 
updates 
Lazy Centralized 
The coordination do not 
needs for Update 
transactions, there is 
diminutive response times 
Inconsistencies, Local copies 
are not refresh 
Eager Distributed No inconsistencies 
Updates need to be 
coordinated, Long response 
times 
Lazy Distributed 
Shortest response times, No 
centralized coordination 
Inconsistencies, Updates can 
be lost 
6. Comparison of Consistency and replication classification 
In this section, some popular and applicable research strategies of replication and consistency 
techniques in database systems are discussed. However, Amjad, et al. [38] presented a survey for 
dynamic replication strategies in data grid. But, they just considered replication protocols without 
consistency models. We discuss the consistency models and replication methods in each research 
approach. 
In a distributed system for providing and handling extremely available service via no single point 
of failure, Lakshman and Malik [39] proposed a quorum-based protocol. This system replicates 
data by using in replicated-write group. Also they present three quorum values for guarantee 
eventual consistency model. [40] presented the design of a highly available key-value storage 
system (Dynamo) which is supports eventual consistency model via quorum-based protocol hat it 
allows for better availability in presence of failure. A new dimension of different cloud providers 
(MetaStorage) based on quorum strategy was presented by [41]. They proposed a new 
consistency model based on static approach. Dingding, et al. [42] proposed a new I/O model to 
achieve a good tread-off between scalability and consistency problems. Their model based on 
static replication and guarantee eventual consistency model. A new model based on generic
International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 
broadcast was proposed by Pedone and Schiper [43] that support causal consistency model. Also 
Aguilera, et al. [44] considered the problem of generic broadcast in asynchronous systems with 
crashes and presented a new thrifty generic broadcast based on dynamic replication approach that 
support causal consistency model. Sousa, et al. [45] proposed a technique for native clocks and 
the constancy of network suspensions to decrease the faults in the ordering of cautious deliveries 
in wide area networks. They present their model based on static replication approach that 
guarantee strong consistency model. An algorithm that handles replication efficiently in active 
replication was presented by [46]. Their algorithm is based on static replication approach that 
focused on strong consistency model. They could not manage their algorithm when the rollback 
problem is occurred. Xinfeng [47] presented a middleware for using a timestamp-based protocol 
to maintain the replica consistency. Their algorithm is based on static replication approach that 
focused on strong consistency model for improving scalability problem.A static distributed data 
replication mechanism of cloud in Google file system was proposed through[48]. They 
considered some features when creating conclusions on replicas of data: 1-insertion the new 
replicas on mass servers by choosing lower-average disk space consumption, 2-limiting the sum 
of replica establishments on each mass server and 3- spreading replicas of a mass crossways 
stand. Their algorithm is based on static replication approach that support eventual consistency 
model. 
Wenhao, et al. [49] proposed a novel cost-effective dynamic data replication strategy named CIR 
in cloud data centers. They applied an incremental replication approach to minimizing the number 
of replicas while meeting the reliability condition in order to facilitate the cost-effective data 
replication management goal. Their approach could reduce the data storage cost substantially, 
especially when the data are only stored for a short duration or have a lower reliability 
requirement. Also their strategy is based on dynamic replication approach that support causal 
consistency model. 
Qingsong, et al. [50] proposed a dynamic distributed cloud data replication algorithm CDRM to 
capture the relationship between availability and replica number. They focused on dynamic 
replication approach that supports a causal consistency model. Ranganathan and Foster [51] 
presented six different replication strategies for three different access patterns: Best Client, 
Cascading Replication, No Replication or Caching, Plain Caching, Caching plus Cascading 
Replication, and Fast Spread. They guarantee the reduction of access latency and bandwidth 
consumption based on dynamic replication approach. A centralized data replication algorithm 
(CDRA) for Grid sites was presented by [52]. Their algorithm reduced the total file access time 
with the consideration of limited storage space of Grid sites. Choi and Youn [53] proposed a 
dynamic hybrid protocol (DHP) which effectively combines the grid and tree structure. This 
protocol can detect read-write conflict and write-write collision for consistency maintaining. 
Their protocol is based on dynamic replication approach that supports an eventual consistency 
model. 
An evolutionary algorithm to find the optimal replication strategy was proposed by [54]. They 
optimized reliability, latency and storage of the system. Because they considered static replication 
approach, their protocol did not take total data center energy cost as the primary optimization 
target. Lloret, et al. [55] presented a protocol for exchanging information, data, services, 
computing and storage resources between all interconnected clouds. Their protocol is based on 
26
International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 
static replication approach that guarantees an eventual consistency model.Table 4 summarizes the 
discussed research strategies and introduces their advantages and disadvantages. 
27 
Table 4. A collection of research strategies on consistency and replication 
Articl 
e 
Main idea 
Consistenc 
y method 
Replicatio 
n scheme 
Advantages Disadvantages 
[39] 
Presenting a distributed 
system for handling and 
providing highly available 
service by no single point 
of failure. 
Eventual Dynamic 
Providing good 
scalability and 
supports dynamic 
control over data 
layout and format. 
The main 
consistency model 
is restricted to 
eventual 
consistency. 
[40] 
presenting the design of a 
highly available key-value 
storage system (Dynamo) 
Eventual Dynamic 
providing a novel 
interface for 
developers to using 
the large e-commerce 
operations 
The response time 
for replicas not 
considered 
[41] 
Presenting a new 
dimension of different 
cloud providers 
(MetaStorage) based on 
quorum strategy 
Eventual Static 
MetaStorage has a 
highly available and 
scalable distributed 
hash table for control 
consistency-latency 
The strategy can 
only guarantee 
single consistency 
model 
[42] 
proposing a new I/O 
model to reach a good 
tread-off between 
Scalability and 
Consistency 
Eventual Static 
this new model has 
many advantages 
over the conventional 
asynchronous-synchronous 
model 
Limiting 
consistency 
maintenance to 
eventual 
consistency model 
[43] 
Ordering the delivery of 
messages only if needed, 
based on the semantics of 
the messages. 
Causal Static 
Showing better 
scalability via 
optimizing the 
atomic broadcast 
protocol with relaxed 
causal consistency 
Static consistency 
model, semantic of 
data is difficult to 
identify without 
knowing the 
environment. 
[44] 
considering the problem of 
generic broadcast in 
asynchronous systems 
with crashes 
Causal Dynamic 
By defining a 
parsimonious 
approach for the set 
of messages in 
generic broad- cast 
ensures can have 
optimal scalability 
The availability 
has not considered 
and the number of 
replicas are not 
shown 
[45] 
Proposing a technique for 
local clocks and the 
stability of network delays 
to reduce the mistakes in 
the ordering of tentative 
deliveries in wide area 
networks 
Strong Static 
Improves scalability 
based on the 
assumption that data 
conflict is rarely 
occurring. 
Fixed consistency 
model, expensive 
cost process 
[46] 
Presenting an algorithm 
that handles replication 
efficiently in active 
replication 
Strong Static Improves scalability 
Fixed consistency 
model 
[47] 
Presenting a middleware 
for using a timestamp-based 
protocol to maintain 
the replica consistency 
Strong Static Improves scalability 
Fixed consistency 
model, expensive 
roll back process.
International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 
28 
[48] 
propose a static distributed 
data replication 
mechanism in cloud 
Eventual Static 
insertion the new 
replicas on mass 
servers by choosing 
lower-average disk 
space consumption, 
Fixed replica 
number is used for 
all files which may 
not be the best 
solution for data. 
[49] 
proposing a novel cost-effective 
dynamic data 
replication strategy named 
CIR in cloud data centers 
Causal Dynamic 
applies an 
incremental 
replication approach 
to minimize the 
number of replicas 
and it can reduce the 
data storage cost 
substantially 
their approach is 
only based on the 
reliability 
parameters and 
pricing model of 
Amazon S3 which 
makes it is not 
suitable for Google 
cluster 
[50] 
Proposing a dynamic 
distributed cloud data 
replication algorithm 
CDRM to capture the 
relationship between 
availability and replica 
number. 
Causal Dynamic 
maintains the 
minimum replica 
number for 
a given availability 
requirement, 
Improves scalability 
The scalability 
approach is not 
proposed 
[51] 
presenting six different 
replication strategies 
for three different access 
patterns 
Eventual Dynamic 
Reduction in access 
latency and 
bandwidth 
consumption. 
The fixed 
consistency model 
and limited 
number of replica 
[52] 
presenting a centralized 
data replication algorithm 
(CDRA) and designing a 
distributed caching 
algorithm 
wherein Grid sites 
Eventual Dynamic 
reduce the total file 
access time with the 
consideration 
of limited storage 
space of Grid sites 
The limitation of 
the algorithm is 
that it considers 
only the access 
cost. 
[53] 
proposing a dynamic 
hybrid protocol (DHP) 
which effectively 
combines the grid 
and tree structure 
Eventual Dynamic 
The protocol can 
detect read/write 
conflict and 
write/write collision 
for consistency 
maintaining. 
The grid and tree 
structure can only 
support read-one/ 
write-all 
mechanism but 
hybrid protocol 
can have read-all/ 
write-all 
[54] 
Presenting an evolutionary 
algorithm to find the 
optimal replication 
strategy 
Eventual Static 
optimize latency, 
storage and 
reliability of the 
system 
This algorithm 
cannot take total 
data center energy 
cost as the primary 
optimization 
target. Also it 
doesn’t take into 
account the load 
balancing of the 
replicas. 
[55] 
Presenting a protocol for 
exchanging information, 
data, services, computing 
and storage resources 
between all interconnected 
clouds 
Eventual Static 
highly scalable and 
load balancing 
approaches 
The resource cost 
is not considered 
in replicas
International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 
Table 5 displays a summarized form of structures of all research strategies studied in above. 
These structures include availability, scalability, reliability, response time, bandwidth, load 
balancing, number of replicas and storage cost. 
29 
Table 5. The popular factors of replication and consistency techniques 
Article Availability Scalability Reliability 
Response 
time 
Bandwidth 
consumption 
Load 
balancing 
Optimal 
number 
of 
replicas 
Storage 
cost 
[39]         
[40]         
[41]         
[42]         
[43]         
[44]         
[45]         
[46]         
[47]         
[48]         
[49]         
[50]         
[51]         
[52]         
[53]         
[54]         
[55]         
7. Conclusion 
This paper presents a review for data replication protocols in the database systems. Also it 
discusses consistency models of replication mechanisms in different update propagations. By 
comparing propagation approaches we can use to type of consistency methods for implementing 
various data replication mechanisms By notice to comparison of replication protocols, a 
consistent replication protocol have important issue in managing and implementing database 
systems. In future work, we discuss efficient factors of consistency protocols in distributed 
databases that extended in distributed database systems. 
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Consistency of data replication

  • 1. International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 CONSISTENCY OF DATA REPLICATION PROTOCOLS IN DATABASE SYSTEMS: A REVIEW Alireza Souri1, Saeid Pashazadeh*2 and Ahmad Habibizad Navin3 1, 3Department of Computer Engineering, Tabriz Branch, Islamic Azad University, Tabriz, Iran. 2* Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran. Abstract: In this paper, a review for consistency of data replication protocols has been investigated. A brief deliberation about consistency models in data replication is shown. Also we debate on propagation techniques such as eager and lazy propagation. Differences of replication protocols from consistency view point are studied. Also the advantages and disadvantages of the replication protocols are shown. We advent into essential technical details and positive comparisons, in order to determine their respective contributions as well as restrictions are made. Finally, some literature research strategies in replication and consistency techniques are reviewed. Keywords: Database system, consistency, data replication, update propagation. 1. Introduction Consistency of Replication models is essential to abstract away execution particulars, and to classify the functionality of a given system. Also a consistency model is a method for come to a joint considerate of each other’s rights and responsibilities. Database system attracts lots of consideration. A large-scale database storage system [1,2] is among the fundamental conveniences in the cloud, unstructured peer-to-peer (P2P) networks [3], grid environment [4] or in similar systems. The system with large-scale database system typically assigns computing replicas near their input data[5]. A good data management develops very important conditions in such a scenario. Data in a distributed database system [6] is replicated for increasing reliability, availability and performance. There are two mechanisms for locations of data replicas such as static and dynamic replicated system[7,8], which regulates replica locations based on session information of requests [9]. DOI : 10.5121/ijit.2014.3402 19
  • 2. International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 On the one hand, in these consistency models, its performance suitability for data replication architecture are not specified exactly. On the other hand, a consistency model dos not guarantees the high performance and high scalability for a data replication mechanism [10]. Consistency, accessibility, scalability, security, fault tolerant and performance [11] are areas for system implementation[12]. High accessibility and performance are basics for such a system with large-scale distributed database system. We have to make a tradeoff between consistency and replication. There are dissimilar levels of weak and strong consistency. A distributed database system may deliver levels of consistency weaker than one-copy-serializability but stronger than eventual consistency. Also there are levels of consistency such as data-centric and client-centric models for a data replication mechanism. So the recognizing usage of consistency models in each data replication mechanism is necessary. In Section 2, we introduce consistency models in client view and server view, adapted from the theory of database concurrency control. Then, we depict on consistency protocols in Section 3. With these discussions, we can represent a comparison among eventual consistency and client-centric consistency models. Properties of their implementations can also be deduced accordingly. In section 4, we discuss replication models and propagation techniques. We show all of the replication protocols according to the update propagation and replication mechanisms in Section 5. Section 6 shows a classified review for replication and consistency techniques in some research strategies. Section 7 is the conclusion and describes future work finally. 20 2. Consistency models In this section, a series of consistency models are considered. We discuss about differences of consistency models. Variant methods to categorizing the consistency models can be originated from [13] and [14].One of the important properties of a system design is consistency model. This property can typically offered in relations of a state that can be true or false for different implementations. Consistency models are referred to as the contracts between process and data for ensuring correctness of the system. Consistency models are presented through a number of consistency criteria to be satisfied by assessments of operations [15].For standard consistency conditions of the ACID properties [16], there exists some methods for consistency guarantee. In ACID consistency method, database is in a consistent state when a transaction is finished. In the client level there are four component: • DS is a storage system. • PA is the process operation for each read or write by DS. • PB is sovereign of process PA that performs each read and write operation from the DS. • PC is sovereign of process PA that performs each read and write operation from the DS. • In the client level consistency, it is important that how and when an observer is occurred. The PA, PB and PC processes see updates with a data item in the storage system. There are two consistency types such as Data-Centric consistency and Client-Centric consistency[17]. In Data-Centric consistency there are: • Strict consistency. All of the A, B and C send back the result of update value when the update procedure is completed.
  • 3. International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 • Sequential consistency. The level of sequential consistency is lower than strict consistency. Each read and write operation is performed by all replicas on their data item x sequentially. Also each discrete procedure operations execute the identified order. • Causal consistency. This consistency is weaker than strict and sequential consistency[18]. If transaction T1 is influenced or caused on an earlier transaction T2, each replica should be first see T2, and then see T1. • FIFO consistency. FIFO consistency is relaxed to implement because it is being guaranteed two or more writes from a single source must arrive in the order issued. Basically, this means that with FIFO consistency, all writes generated by different processes are concurrent. 21 In Client-Centric consistency models there are: • Eventual consistency. This model guarantees that if a updates are complete to the item eventually [19], then all accesses on this data item send back the previous updated value [20]. • Monotonic Reads. In this model if an operation reads the data item x, always each following read operation on data item x send back same value x or a more recent value. • Monotonic Writes. In this model if an operation writes on the data item x, always each following write operation on data item x comes after related write operation on the data item x. • Read-your-Write. The result of a write operation on the data item x always will be realized by a following read operation on x by the same value. • Write follow read. In this model the effect of a write operation on a data item x following a previous read operation on data item x by the same value that is guaranteed to take place on the same or a more recent value of x that was read. 3. Consistency Protocols In this section, we describe the consistency protocols according to[21]. A consistency protocol explains as an implementation of a specific consistency model. We track the group of our conversation on consistency models[22]. 3.1 Primary Replica Based Protocol In this protocol, All write operations to a data item x is attended by one specific replica that called primary replica. This primary replica [23] is accountable for updating other replicas, the client just cooperates by this primary replica[16]. Two requirements should be happen for this generous of protocol [24]: • All read and write operations for updating a data item x should spread and be executed all replicas at some time. • These operations should be executed in the same order.
  • 4. International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 22 3.2 Replicated Write Protocol: In this protocol, each write operations are sent to each replica to update procedure. There are two types for replicated write protocols. 3.2.1. Active Replication: In active replication, each replica contains a concomitant procedure that transports out the update operations. Unlike other protocols, update operations are normally propagated through the write operation. This propagation causes the operation is sent to each replica. Also there is required a total order for all write operations that each replica execute the same order of write commands [25]. 3.2.2. Quorum Based: This protocol specifies that the clients obtain the authorization of several servers before any reading or writing a replicated data item x [26]. For example, the write operations only want to be executed on fragment of all replicas before return to the client. It use elections to avoid write-read conflict and write-write conflict [27]: • R is the number of replicas of each data item. • Rr is number of replicas that a client should contacts by them for reading a value. • Rw is number of replicas that a client should contacts by them for writing a value. • For preventing the Write-Write and Write-Read conflicts, Rr + Rw > R and Rw + Rw > R should be satisfied. 4. Update propagation strategies Update propagation can be measured in two methods [28] • The update operations are applied to all replicas as part of the unique contract. • Each replica is updated by the originating transaction. Update operations send to other replicas asynchronously as a discrete transaction for each node [29]. There are two update propagation methods: Eager techniques and lazy techniques. Normally the eager protocols are identified as read-one/write-any (ROWA) protocols. First, they have not transactional inconsistencies. Second, an update transaction can read a local copy of the data item x and be sure that a refresh value is read. Consequently, there is no essential to executing a remote read. Finally, the variations to replicas are completed atomically. When we use to a 2PC execution, the update speed is restricted and it cause that the response time performance of the update transaction is low. When one of the copies is inaccessible, the update transaction cannot terminate meanwhile all the copies updated essentially. Lazy protocol is used to new mechanisms for guaranteeing strong mutual consistency [30]. These mechanisms may be bright to endure some inconsistency between the replicas for better performance[31].
  • 5. International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 In a distributed database system the requests permit to access data from local and remote databases [32]. Distributed methods spread on the update procedure to the local copy where the update transaction creates, then the updates are broadcasted to the other replica. If distributed techniques are attached by eager propagation approaches, then the distributed concurrency control approaches can sufficiently report the concurrent updates problem [13]. Table 1 is shown the comparisons of update propagation and propagation techniques [33]. 23 Table 1. Comparisons of update propagation techniques Consistency Updating Performance Failure Eager update Strong consistency Up-to-date with High response time Not transactional inconsistency, Changes are atomic Restricted update speed, transaction crash and Lower availability Lazy update Weak consistency Out-of-date problem and Low response time Not fault tolerant, good response time Dirty read problem, Data inconsistency and transaction inversion Centralized techniques - Up-to-date with Update without synchronization Appropriate for few master sites High overload and bottleneck problems Distributed techniques - Up-to-date with Concurrency control methods Highest system availability Management problem, Copies need to be synchronized 5. Replication Protocols [13] presented a categorization for replicas data protocols. This is important that When one of the update propagation mechanisms such as eager or lazy incomes and who should complete updates mechanism such as primary copy or update-everywhere. In eager propagation mechanism, the propagation of updates is contained by the restrictions of a transaction. The client does not receive the notification of commit message up to necessary duplicates have been updated in the system. In the Lazy mechanism, the update procedure of a local copy is committed. the update propagation be accomplished [34]. There is an expensive way for providing response time and message overhead in consistency of eager mechanism. An optimization for prevent from these problems is using Lazy replication approach. However, the update procedure is executed separately, therefore inconsistency conditions might occur. [35]. When the updates are broadcast to replicas in eager or lazy mechanism, two architectures are needed for updates such as centralized and distributed. Table 2 shows the four replication mechanisms such as eager distributed and eager centralized for eager mechanism, lazy distributed and lazy centralized for lazy mechanism [36].
  • 6. International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 24 Table 2. Update propagation vs. propagation techniques Centralized Distributed Eager Eager Primary Copy Eager One Master by Restricted Transparency Update Everywhere One Master with Full Transparency Lazy Lazy Primary Copy Single Master with Limited Lazy Update Everywhere Transparency 5.1 Eager Centralized Protocol In eager centralized protocol, there is a site as a master that navigates the read and writes operations on a data item (x). This protocol guarantees strong consistency techniques for update propagation. In update procedure, all updates are applied to a logical data item (x) by using the perspective of the update transaction. This applying is committed by using the 2 Phase Commit protocol. So, when the update procedure is completed in its transaction, all copies return the similar values to the updated data items. The result of this mechanism is one-serializability-replication[ 37]. The categories of eager centralized include eager primary copy, single master by restricted transparency and single master by full transparency. In eager primary copy, any data item (xi) has a master. One replica specified as the primary copy. In this case, there is no single master for controlling serializability condition. In the single master by restricted transparency, all of the updates have been sent to the specified master directly. For a read operation, a read lock occurred on data item x and the read operation is executed. The result of the operation is returned to the client. Also for a write operation, a write lock occurred on data item x and this operation is executed. The result of write operation is returned to the client. In the single master by full transparency, the replica coordination level has been performed by a router. The router sends the entire read and writes operations to the master directly. The master executes each operation and returns the result of execution to the client. 5.2 Eager Distributed Protocol In eager distributed protocol, first the update applied to the local replica, then the update procedure is propagated to other replicas. The eager update everywhere is a type of eager distributed protocol. 5.3 Lazy Centralized Protocol Lazy centralized protocol is like to eager centralized protocol. In this protocol, first the updates are applied to a master and then propagated to the clients. The significant alteration is that the propagation procedure does not take place via the update process. However, after the commitment of transactions, if a client executes a read operation (x) on its local copy, it may read a non-refresh data, then data item x may have been updated at the master, nevertheless the update may not have been propagated to the clients yet. The categories of lazy centralized include lazy primary copy and single master by restricted transparency. In lazy primary copy, each read and write operation sends to a master. All of the updating results have been send back to the client. In
  • 7. International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 single master by restricted transparency, the update procedure is executed to the master directly. When one update has committed, the new transaction is sent to the clients. 25 5.4 Lazy Distributed Protocol In Lazy distributed protocol, the update transactions can execute on each replica. Also these updates are propagated to the other replicas lazily. Lazy update everywhere is a type of lazy distributed protocol. In this type, each read and write operation are performed on the local copy and the update transactions commit locally. Comparison of replication protocols about consistency conditions is shown in Table 3. Table 3. Comparison of replication protocols Replication strategies Advantages Disadvantages Eager Centralized The coordination do not needs for Update transactions, there is no inconsistencies Extensive response time, Local copies are can only be Read, Only useful with few updates Lazy Centralized The coordination do not needs for Update transactions, there is diminutive response times Inconsistencies, Local copies are not refresh Eager Distributed No inconsistencies Updates need to be coordinated, Long response times Lazy Distributed Shortest response times, No centralized coordination Inconsistencies, Updates can be lost 6. Comparison of Consistency and replication classification In this section, some popular and applicable research strategies of replication and consistency techniques in database systems are discussed. However, Amjad, et al. [38] presented a survey for dynamic replication strategies in data grid. But, they just considered replication protocols without consistency models. We discuss the consistency models and replication methods in each research approach. In a distributed system for providing and handling extremely available service via no single point of failure, Lakshman and Malik [39] proposed a quorum-based protocol. This system replicates data by using in replicated-write group. Also they present three quorum values for guarantee eventual consistency model. [40] presented the design of a highly available key-value storage system (Dynamo) which is supports eventual consistency model via quorum-based protocol hat it allows for better availability in presence of failure. A new dimension of different cloud providers (MetaStorage) based on quorum strategy was presented by [41]. They proposed a new consistency model based on static approach. Dingding, et al. [42] proposed a new I/O model to achieve a good tread-off between scalability and consistency problems. Their model based on static replication and guarantee eventual consistency model. A new model based on generic
  • 8. International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 broadcast was proposed by Pedone and Schiper [43] that support causal consistency model. Also Aguilera, et al. [44] considered the problem of generic broadcast in asynchronous systems with crashes and presented a new thrifty generic broadcast based on dynamic replication approach that support causal consistency model. Sousa, et al. [45] proposed a technique for native clocks and the constancy of network suspensions to decrease the faults in the ordering of cautious deliveries in wide area networks. They present their model based on static replication approach that guarantee strong consistency model. An algorithm that handles replication efficiently in active replication was presented by [46]. Their algorithm is based on static replication approach that focused on strong consistency model. They could not manage their algorithm when the rollback problem is occurred. Xinfeng [47] presented a middleware for using a timestamp-based protocol to maintain the replica consistency. Their algorithm is based on static replication approach that focused on strong consistency model for improving scalability problem.A static distributed data replication mechanism of cloud in Google file system was proposed through[48]. They considered some features when creating conclusions on replicas of data: 1-insertion the new replicas on mass servers by choosing lower-average disk space consumption, 2-limiting the sum of replica establishments on each mass server and 3- spreading replicas of a mass crossways stand. Their algorithm is based on static replication approach that support eventual consistency model. Wenhao, et al. [49] proposed a novel cost-effective dynamic data replication strategy named CIR in cloud data centers. They applied an incremental replication approach to minimizing the number of replicas while meeting the reliability condition in order to facilitate the cost-effective data replication management goal. Their approach could reduce the data storage cost substantially, especially when the data are only stored for a short duration or have a lower reliability requirement. Also their strategy is based on dynamic replication approach that support causal consistency model. Qingsong, et al. [50] proposed a dynamic distributed cloud data replication algorithm CDRM to capture the relationship between availability and replica number. They focused on dynamic replication approach that supports a causal consistency model. Ranganathan and Foster [51] presented six different replication strategies for three different access patterns: Best Client, Cascading Replication, No Replication or Caching, Plain Caching, Caching plus Cascading Replication, and Fast Spread. They guarantee the reduction of access latency and bandwidth consumption based on dynamic replication approach. A centralized data replication algorithm (CDRA) for Grid sites was presented by [52]. Their algorithm reduced the total file access time with the consideration of limited storage space of Grid sites. Choi and Youn [53] proposed a dynamic hybrid protocol (DHP) which effectively combines the grid and tree structure. This protocol can detect read-write conflict and write-write collision for consistency maintaining. Their protocol is based on dynamic replication approach that supports an eventual consistency model. An evolutionary algorithm to find the optimal replication strategy was proposed by [54]. They optimized reliability, latency and storage of the system. Because they considered static replication approach, their protocol did not take total data center energy cost as the primary optimization target. Lloret, et al. [55] presented a protocol for exchanging information, data, services, computing and storage resources between all interconnected clouds. Their protocol is based on 26
  • 9. International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 static replication approach that guarantees an eventual consistency model.Table 4 summarizes the discussed research strategies and introduces their advantages and disadvantages. 27 Table 4. A collection of research strategies on consistency and replication Articl e Main idea Consistenc y method Replicatio n scheme Advantages Disadvantages [39] Presenting a distributed system for handling and providing highly available service by no single point of failure. Eventual Dynamic Providing good scalability and supports dynamic control over data layout and format. The main consistency model is restricted to eventual consistency. [40] presenting the design of a highly available key-value storage system (Dynamo) Eventual Dynamic providing a novel interface for developers to using the large e-commerce operations The response time for replicas not considered [41] Presenting a new dimension of different cloud providers (MetaStorage) based on quorum strategy Eventual Static MetaStorage has a highly available and scalable distributed hash table for control consistency-latency The strategy can only guarantee single consistency model [42] proposing a new I/O model to reach a good tread-off between Scalability and Consistency Eventual Static this new model has many advantages over the conventional asynchronous-synchronous model Limiting consistency maintenance to eventual consistency model [43] Ordering the delivery of messages only if needed, based on the semantics of the messages. Causal Static Showing better scalability via optimizing the atomic broadcast protocol with relaxed causal consistency Static consistency model, semantic of data is difficult to identify without knowing the environment. [44] considering the problem of generic broadcast in asynchronous systems with crashes Causal Dynamic By defining a parsimonious approach for the set of messages in generic broad- cast ensures can have optimal scalability The availability has not considered and the number of replicas are not shown [45] Proposing a technique for local clocks and the stability of network delays to reduce the mistakes in the ordering of tentative deliveries in wide area networks Strong Static Improves scalability based on the assumption that data conflict is rarely occurring. Fixed consistency model, expensive cost process [46] Presenting an algorithm that handles replication efficiently in active replication Strong Static Improves scalability Fixed consistency model [47] Presenting a middleware for using a timestamp-based protocol to maintain the replica consistency Strong Static Improves scalability Fixed consistency model, expensive roll back process.
  • 10. International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 28 [48] propose a static distributed data replication mechanism in cloud Eventual Static insertion the new replicas on mass servers by choosing lower-average disk space consumption, Fixed replica number is used for all files which may not be the best solution for data. [49] proposing a novel cost-effective dynamic data replication strategy named CIR in cloud data centers Causal Dynamic applies an incremental replication approach to minimize the number of replicas and it can reduce the data storage cost substantially their approach is only based on the reliability parameters and pricing model of Amazon S3 which makes it is not suitable for Google cluster [50] Proposing a dynamic distributed cloud data replication algorithm CDRM to capture the relationship between availability and replica number. Causal Dynamic maintains the minimum replica number for a given availability requirement, Improves scalability The scalability approach is not proposed [51] presenting six different replication strategies for three different access patterns Eventual Dynamic Reduction in access latency and bandwidth consumption. The fixed consistency model and limited number of replica [52] presenting a centralized data replication algorithm (CDRA) and designing a distributed caching algorithm wherein Grid sites Eventual Dynamic reduce the total file access time with the consideration of limited storage space of Grid sites The limitation of the algorithm is that it considers only the access cost. [53] proposing a dynamic hybrid protocol (DHP) which effectively combines the grid and tree structure Eventual Dynamic The protocol can detect read/write conflict and write/write collision for consistency maintaining. The grid and tree structure can only support read-one/ write-all mechanism but hybrid protocol can have read-all/ write-all [54] Presenting an evolutionary algorithm to find the optimal replication strategy Eventual Static optimize latency, storage and reliability of the system This algorithm cannot take total data center energy cost as the primary optimization target. Also it doesn’t take into account the load balancing of the replicas. [55] Presenting a protocol for exchanging information, data, services, computing and storage resources between all interconnected clouds Eventual Static highly scalable and load balancing approaches The resource cost is not considered in replicas
  • 11. International Journal on Information Theory (IJIT),Vol.3, No.4, October 2014 Table 5 displays a summarized form of structures of all research strategies studied in above. These structures include availability, scalability, reliability, response time, bandwidth, load balancing, number of replicas and storage cost. 29 Table 5. The popular factors of replication and consistency techniques Article Availability Scalability Reliability Response time Bandwidth consumption Load balancing Optimal number of replicas Storage cost [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] 7. Conclusion This paper presents a review for data replication protocols in the database systems. Also it discusses consistency models of replication mechanisms in different update propagations. By comparing propagation approaches we can use to type of consistency methods for implementing various data replication mechanisms By notice to comparison of replication protocols, a consistent replication protocol have important issue in managing and implementing database systems. In future work, we discuss efficient factors of consistency protocols in distributed databases that extended in distributed database systems. References [1] S. Çokpınar and T. . Gündem, Positive and negative association rule mining on XML data streams in database as a service concept, Expert Systems with Applications, vol. 39, pp. 7503-7511, 6/15/ 2012. [2] X. Wang, X. Zhou, and S. Wang, Summarizing Large-Scale Database Schema Using Community Detection, Journal of Computer Science and Technology, vol. 27, pp. 515-526, 2012/01/01 2012. [3] G. Gao, R. Li, K. Wen, and X. Gu, Proactive replication for rare objects in unstructured peer-to-peer networks, Journal of Network and Computer Applications, vol. 35, pp. 85-96, 1// 2012.
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