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Lei Liu, Dongqing Liu, Shuai Lü, Peng Zhang, "An Abstract Description Method of Map-Reduce-Merge Using Haskell", Mathematical Problems in Engineering, vol. 2013, Article ID 147593, 12 pages, 2013. https://doi.org/10.1155/2013/147593
An Abstract Description Method of Map-Reduce-Merge Using Haskell
Map-Reduce-Merge is an improved parallel programming model based on Map-Reduce in cloud computing environment. Through the new Merge module, Map-Reduce-Merge can support processing multiple related heterogeneous datasets more efficiently. In order to demonstrate the validity and effectiveness of this new model, we present a rigorous description for Map-Reduce-Merge model using Haskell. Firstly, we describe the basic program skeleton of Map-Reduce-Merge programming model. Secondly, an abstract description for the Merge module is presented by analyzing the structure and function of the Merge module with Haskell as the description tool. Thirdly, we evaluate the Map-Reduce-Merge model on the basis of our description. We capture the functional characteristics of the Map-Reduce-Merge model by our abstract description, which can provide theoretical basis for designing more efficient parallel programming model to process join operation.
Recently, lots of research works on improving Google’s Map-Reduce  model have been proposed to analyze large volumes of data [2–5]. One important type of data analysis is joining multiple datasets. There are increasing efforts for implementing join algorithms using Map-Reduce or the improved Map-Reduce programming model [6–11]. Map-Reduce-Merge is such an effort that can directly express join operation and implement several join algorithms by the new Merge module. In this new model, Map and Reduce modules are inherited from Map-Reduce model, so that existing Map-Reduce programs can run directly on this new framework without modifications. Not only join operator but also all the other relational operators can be modeled using various combinations of the three primitives: Map, Reduce, and Merge. Map-Reduce-Merge removes the burden of implementing join algorithms. The emergency of Map-Reduce-Merge shows a trend that parallel databases and Map-Reduce learn with each other and new data analysis eco-systems are developed [12, 13].
Many formal methods can be used to describe programming model [14–17]. Lämmel  first delivers a rigorous description of Map-Reduce programming model as well as its advancement called Sawzall . He uses typed functional programming Haskell as a tool to describe the fundamental characteristics underlying the Map-Reduce and Sawzall model. The description is made up of several Haskell functions. Our paper is based on his work. We will present an abstract description for Map-Reduce-Merge model, especially for the new added Merge module. This paper makes the following contributions. Firstly, we define the basic program skeleton of Map-Reduce-Merge to capture the abstraction for Map-Reduce-Merge computations. Secondly, we decompose the Merge module according to its structure and present the rigorous description called moduleMerge. Thirdly, we analyze the Map-Reduce-Merge programming model based on our abstract description with an example. Some implementation details (such as fault tolerance and task scheduling [20–25]) will be considered in the future.
Haskell is characterized by strong type inference and type checking . The recent paper  suggests that Haskell can be used as a tool to support executable specification. Using Haskell as a description tool can be beneficial for both programming model designers and users. For designers, they can know explicitly what will happen during the execution of a Map-Reduce-Merge job, which is good for them to analyze and evaluate this new model. For users, they can use the abstract description as an executable specification in software development to ensure the correctness and robustness of software.
This paper is organized as follows. Section 2 gives a brief introduction of Map-Reduce-Merge programming model and Haskell programming language. Section 3 defines the basic programming skeleton of Map-Reduce-Merge programming model. Section 4 designs the helper functions composing the Merge module. Section 5 defines the helper functions designed in Section 4. Section 6 shows an example and gives a brief comment. Section 7 concludes this paper.
In this section, we will briefly recall Map-Reduce-Merge programming model and some concepts in Haskell.
2.1. Map-Reduce-Merge Programming Model
The most important feature of Map-Reduce-Merge is that it adds to Map-Reduce a Merge phase, so that it can directly support join algorithms of different datasets. Figure 1 illustrates the data flow of the Map-Reduce-Merge framework. It consists of three phases, two independent Map-Reduce phases, and a Merge phase. At first, two datasets are processed by corresponding Map and Reduce phases. Then in the Merge phase, a merger can select data to be merged from those two reducer outputs according to different join algorithm. Notice that the reducer outputs are stored in local disks instead of distributed file system because the Reduce phase is not the last phase any more.
The main purpose of the Merge module is to implement join algorithms. The Merge module includes four components: Partition Selector, Processors, Configurable Iterators, and Merger as shown in Figure 2. Partition Selector is a selector that determines which data partitions produced by up-stream reducers should be retrieved and then merged. Configurable Iterators include two logical iterators which can implement different join algorithms, including sort-merge join, nested-loop join, and hash join. Processors include two processor functions which define the logic to process data from different datasets. Merger includes a merger function where users can implement data processing logic on data from two sources.
2.2. Haskell Programming Language
To avoid confusion, we will introduce the map function and the Map type in Haskell, as well as the map primitive in Map-Reduce programming model.
In Haskell, the map function is a higher-order function. It takes two parameters, the first is a function whose type is , and the second is a list whose type is . It returns a list whose type is . The formal definition of map is as in Algorithm 1.
We illustrate how to use map to add one to every element in a list. Here, the expression (+1) represents a function that adds one to a variable, which equals to the function . The example is shown as in Algorithm 2.
The type Map is a build-in type in Haskell. It is an efficient implementation of maps from keys to values. We can use the function toList to convert a Map to an association list and the function fromList to build a Map from an association list.
The signature of map primitive is (K1, V1) [(K2, V2)]. The map primitive in essence corresponds to the first parameter of the function map in Haskell . We define the signatures of map, reduce, and merge primitives in Table 1. The first digit after letter K/V is used to distinguish different keys/values, while the second digit is used to distinguish different datasets.
3. Definition of mapReduceMerge
In this section, we define the mapReduceMerge function to model the abstraction for Map-Reduce-Merge computations. A full Map-Reduce-Merge job includes two individual Map-Reduce phases and a Merge phase as shown in Figure 1. Hence, we take for granted that the mapReduceMerge function can be decomposed into three helper functions that represent these three phases, respectively. The type and definition for mapReduceMerge are shown as in Algorithm 3.
The mapReduceMerge function is defined in terms of function application in Haskell. The arguments lTable and rTable corresponding to the types [Map K11 V11] and [Map K12 V12] are the input data for a Map-Reduce-Merge job. They are first processed by the functions mapReduce 1 and mapReduce2, respectively, and then are merged by the function moduleMerge. These helper functions correspond to three phases in a Map-Reduce-Merge computation.
Here, function composition, which is denoted by Haskell infix operator (.), is used to compose three helper functions. A more detailed definition for mapReduce can be found in . In this paper, we mainly focus on describing the Merge module with the moduleMerge function. By analyzing mapReduceMerge and mapReduce, we can discover the type for moduleMerge with its definition to be defined later (Algorithm 5).
Now, we will explain the types used in the definition of mapReduceMerge. The type Map K1 V1 represents the input split type for moduleMap. Typically, every split corresponds to a map task. Hence, the list type [Map K1 V1] represents all the input data of a Map-Reduce job. Similarly, the type Map K2 V3 represents the output result of a reduce task, and the list type [Map K2 V3] represents all the output data of a Map-Reduce job. Our types are compatible with the types defined in .
In parallel databases, a table is divided into different splits in order to store in large clusters. The splits form the unit of distribution and load balancing. In this paper, we use Table to represent a table, Tablet to represent a split of a table, and Record to represent a row in a table. According to our discussion above, Table has a type of [Map K V], Tablet has a type of Map K V, and Record has a type of (K, V). It happens that a table in Google’s Bigtable  is a sparse, distributed, persistent multidimensional sorted Map.
4. Discovery of the Types in Merge
In this section, we design some helper functions for moduleMerge and discover the types of those functions. The moduleMerge function is defined to model the abstraction for the Merge module. According to Figure 2, the components of the Merge module can be divided into two parts, data transferring and data processing. First part includes Partition Selector, which can select and transfer the output of up-stream reducers. Second part is made up of Processors, Configurable Iterators, and Merger, which can merge two different datasets. Hence, we design two helper functions for moduleMerge, which are getPartitionPair and mergeTwoPartition. The getPartitionPair function selects the output Tablets from up-stream reducers and then delivers those Tablets to mergers. The mergeTwoPartition function takes two Tablets as input and returns a new Tablet as the merge result. The types of these two functions are shown as in Algorithm 6.
Here, [Map K21 V31] denotes the input type of moduleMerge. It coincides with the result type of mapReduce1, which reflects the fact that the output of mapReduce1 is one input for moduleMerge. The type (Map K21 V31, Map K22 V32) represents two Tablets to be merged. The type [(Map K21 V31, Map K22 V32)] represents all the Tablet pairs that is a merger, process and the type [[(Map K21 V31, Map K22 V32)]] represents all the Tablet pairs that is all the mergers process. Every merger has a serial number in the Map-Reduce-Merge programming model. We implicitly express this characteristic by the merger position in the list.
We decompose the mergeTwoPartition function into two helper functions called getPair and mergeTwoRecord. The getPair function chooses and emits all the Record pairs that will be processed by the mergeTwoRecord function, where a Record pair is merged into a new Record. In fact, mergeTwoPartition is implemented by executing mergeTwoRecord many times. The types of these two functions are shown as in Algorithm 7.
Here, the type KV1 is short for the type (K21, V31), KV2 is short for (K22, V32), and KV3 is short for (K23, V33). The type [(KV1, KV2)] represents all the possible Record pairs that need to be merged. The type Maybe is a built-in type in Haskell, which is defined as “data Maybe a = Just a | Nothing.” Only those Record pairs that satisfy the merge condition can be merged into a new Record whose type is Just KV3. The type (Map K21 V31, Int) represents a Tablet and its number, which corresponds to the reducer number in Map-Reduce-Merge.
The getPair function has to decide whether two Records need to be merged or not, so the iterationLogic function is designed to implement this work. The same thing happened in the getPartitionPair function, where partitionSelector is designed to judge whether two Tablets need to be merged. The types for iterationLogic and partitionSelector are shown as in Algorithm 8.
We now summarize the components of the Merge module and the designed functions, as well as the relationship between them. In the Merge module, Partition Selector selects the input data for mergers from reducers. We design the partitionSelector function to model it. The partitionSelector function is invoked by getPartitionPair to transfer data from reducers to mergers. Hence, this phase is similar to the shuffle phase in Map-Reduce. Configurable Iterator includes two iterators corresponding to two datasets. All join algorithms, such as sort-merge join, nested-loop join, and hash join, have their own processing logic to control the relative movements of two iterators. In our paper, we abstract it by the iterationLogic function according to the characteristics of Haskell. The iterationLogic function is called by getPair to generate all the wanted Record pairs. Merger that implements the user-defined logic is captured by the mergeTwoRecord function. Processors include two functions called leftProcessor and rightProcessor, which can implement hash join. For simplicity, we model it with the iterationLogic function. The relationship of our functions is summarized in Figure 3.
5. Definition of the Helper Functions in Merge
In this section, we describe how to implement the functions whose types have been determined in last section. After every function, some concepts of Haskell are explained when needed.
The mergeTwoRecord function is composed of two helper functions match and mergeResult. We use the match function to judge whether the join keys of two Records satisfy the given condition. If K21 and K22 satisfy a merge condition defined by the match function, these two Records will be processed by mergeResult where users can implement data processing logic. The definitions of match and mergeResult are related to concrete applications. In Section 6, we will illustrate how to use these two functions. The definition of mergeTwoRecord is as in Algorithm 9.
In the definition of mergeTwoRecord, the symbol “@” which is called as-pattern is a kind of pattern matching forms in Haskell. It denotes that KV1 can be replaced by (K21, V31) and KV2 can be replaced by (K22, V32). The symbol “$” denotes function application which can be substituted by parentheses. In this definition, “Just $ mergeResult KV1 KV2” is equal to “Just (mergeResult KV1 KV2).” This representation is much simpler than using parentheses, especially when nested parentheses are needed. The vertical pipe “ ” represents guards in Haskell. It is used to judge whether the parameters satisfy some conditions. In this definition, if K21 and K22 satisfy the condition defined in the match function, KV1 and KV2 will be processed by the mergeResult function and then wrapped by the type Maybe. Otherwise, mergeTwoRecord will return Nothing as the result.
We implement getPair in two steps as follows.
Step 1. Get all possible combinations of Records from lTablet and rTablet; the result is the Cartesian product of these two Tablets.
Step 2. Filter the undesired Record pairs with iterationLogic.
In Haskell, list comprehensions are a handy way to produce lists. Their concepts are similar to set comprehensions in mathematics. Here in Step 1, the list comprehension is used for building a list out of two lists. We could also do the same thing using set comprehensions, like this: . The filter function takes a predicate and a list and returns the list whose elements satisfy that predicate. In Step 2, those elements which satisfy the conditions defined in iterationLogic will remain in the list, while others will be removed from the list. The definition of getPair is as in Algorithm 10.
The mergeTwoPartition function is implemented in the following four steps.
Step 1. Get the list of pairs from lTablet and rTablet by invoking getPair.
Step 2. Process every pair from the list with mergeTwoRecord. As a result; the return type is [Maybe KV3].
Step 3. Remove the element equal to Nothing from the list with the function filter. Then change the element type from Maybe KV3 to KV3 with the map function.
Step 4. Change the list type to the type Map with the fromList function.
In the definition of the mergeTwoPartition function, the symbol “” denotes expression. For example, the expression “=Nothing” represents a function whose parameter is and function body is /=Nothing. Hence, this function decides whether is equal to Nothing. The definition of mergeTwoPartition is as in Algorithm 11.
We implement the getPartitionPair function in the following four steps.
Step 1. Use the zip function to identify each Tablet with a number. It simulates the situation that we use the reducer number to select the output data.
Step 2. Get all possible combinations of the elements from lTable and rTable, just like the Cartesian product of two Tables. In fact, we need all possible combinations of lTablets and rTabelts.
Step 3. Filter the useless combinations with the partitionSelector function.
Step 4. Remove the Num part from pair (Tablet, Num).
The definition of getPartitionPair is as in Algorithm 12.
We implement the moduleMerge function in the following three steps.
Step 1. Get all the combinations for Tablets from lTable and rTable with the getPartitionPair function.
Step 2. Merge two Tablets from different Tables with the mergeTwoPartition function. The outermost map application corresponds to the parallel merge tasks. The innermost map application corresponds to a merge task where the mergeTwoPartition function is executed.
Step 3. Concatenate all the partitions that produced by a merger with the foldl function.
In the definition of the moduleMerge function, we use some concepts about type Map and its operations. The function foldl has three parameters: a binary function, an initial value, and a Map type value. It returns a result that is the same type as the initial value. The union function combines two dictionaries into one dictionary. The value empty is an empty dictionary. The definition of moduleMerge is as in Algorithm 13.
5.6. iterationLogic and partitionSelector
The iterationLogic and partitionSelector functions are implemented according to different join algorithms as in Algorithm 14.
The iterationLogic function judges whether two Records satisfy the merge condition. As we can see in this definition, we assume that it is always true no matter what the input data is. This is correct when we want to implement nested-loop join. The definition will change to K21=K22 if sort-merge join is implemented.
The partitionSelector function selects those Tablets that need to be merged. Just like the iterationLogic function, the return value is true when we want to implement nested-loop join. We will change the definition to left==right when sort-merge join is needed, on condition that two Map-Reduce phases use the same partitioners.
As we can see in this section and last section, our method to describe the Merge module is “firstly design the types and then define the functions.” The types are designed from top to down, while the definitions are defined from bottom to up. At last, we get the abstract description of the Merge module by using Haskell as shown in Figure 4.
6. Case Study
In this section, an example of how to use our description is designed to demonstrate the proposed method. Then a brief analysis on two join algorithms that have been implemented in Map-Reduce-Merge is given, including nested-loop join and sort-merge join.
There are two tables in this example: Employee and Department. We use Table1 and to represent them, respectively. The primary keys of the tables are shown in bold in Figures 5 and 6. One possible query is to compute the employee final bonus that is the product of bonus in Table1 and bonus adjustment in . This query result is stored in . To accomplish this query, we take steps as follows.
Firstly, we divide the record in a table into two parts, corresponding to Key and Value in Map-Reduce, respectively, and assign every attribute with a Haskell type. In Employee, we chose the composition of emp-id and dept-id as Key and the others as the Value. As to Department, we chose dept-id as Key and the others as the Value. All the types of Key and Value emerging in the tables are defined as in Algorithm 15.
Notice that the Key we use in Map-Reduce is not the same as the key we use in databases. In our example, we use the same Key as the primary key in Table1, while in we chose not to. In the subsequent steps, we implement the nested-loop join with our description. The way to process sort-merge join is similar except that we use other definitions of iterationLogic and partitionSelector, which will be discussed later.
Secondly, we construct the input data by modifying the table to the form of list of Maps as follows. Table1 consists of three Tablets, and consists of two Tablets (see Algorithm 16).
Thirdly, we define the functions match and mergerResult. In the match function, we guarantee that the employee dept-id is equal to the department dept-id. Only those Record pairs that satisfy this condition can be merged. In the mergeResult function, we implement the product of bonus and bonus adjustment to get the final bonus. The types and definitions of these two functions are shown as in Algorithm 17.
Finally, we run the following command in Haskell compiler WinGHCi:
Prelude > moduleMerge lTable rTable.
The result is as follows. It corresponds to Table in Figure 5:
It shows that our description can implement these two join algorithms. The corresponding dataflow graphs are Figures 5 and 6. We set the result of the partitionSelector function to be true in order to ensure that every Tablet from Table1 can be merged with all tables in . On the other hand, we set the function body of partitionSelector to be left==right so as to map reducers and mergers in a one-to-one relationship. We use the same strategy to define the iterationLogic function. The main difference between sort-merge join and nested-loop join is that the input of sort-merge join has been sorted, while the input of nested-loop join has not been. In this example, we take advantage of the combining phase strategy in Map-Reduce-Merge framework to reduce the remote read between reducers and mergers. Since the size of Table1 is much bigger than the size of , we combine the mergers with the reducers into same workers.
As we can see there are two processors and two iterators in the Merge module. Hence, Map-Reduce-Merge can implement two-way join algorithms. If we want to implement multiway join algorithms, a join tree (or a Map-Reduce-Merge workflow) is needed. According to the data flow of Figures 5 and 6, we can find that using sort-merge join can decrease the remote reads than nested-loop join. In Map-Reduce-Merge, sort-merge join is more efficient than nested-loop join when processing equal join. When processing more complicated join, the nested-loop join algorithm is in need.
Map-Reduce-Merge is an improved work based on Google’s Map-Reduce programming model. It improves the ability to express and to process join operation among multiple heterogeneous datasets. At the same time, it increases the complexity to understand the execution flow of a job. This paper presents a rigorous description of Map-Reduce-Merge to abstract the fundamental functions for Map-Reduce-Merge using Haskell. Based on the abstract description, we analyze the characteristics of Map-Reduce-Merge programming model. On one hand, our work can help with an unambiguous understanding of Map-Reduce-Merge and provide strong theoretical basis for designing more efficient parallel programming model to process join operation. On the other hand, programmers can use our description as a specification in software development. Our result can ensure the correctness and robustness of the software with Haskell strong type checking and type inference.
Since this paper mainly concentrates on describing the dataflow in Map-Reduce-Merge, an important future direction is to introduce some control parameters into our description to improve its flexibility and usability. In addition, cost information can cooperate with our description to estimate the performance of a Map-Reduce-Merge job.
This work was supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China under Grant no. 20120061120059; the China Postdoctoral Science Foundation under Grant no. 2011M500612; the Key Program for Science and Technology Development of Jilin Province of China under Grant no. 20130206052GX; the National Natural Science Foundation of China under Grant nos. 61300049, 61070084, and 61100090; the Program for New Century Excellent Talents in University under Grant no. NCET-10-0436; the Science and Technology Development Program of Jilin Province of China under Grant no. 20101509; the Fundamental Research Funds for the Central Universities under Grant nos. 201103124, 201103133; and the “985 Project” of Jilin University of China.
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