Module 6: Related Topics

Introduction

Hadoop by itself allows you to store and process very large volumes of data. However, building a large-scale distributed system can require functionality not provided by this base. Several other tools and systems have been created to fill the gaps and deliver a more full-featured set of distributed systems engineering tools.

Goals for this Module:

  • Understand how distributed consensus systems can be used to bootstrap larger distributed systems.
  • Understand how to write queries in the Pig log-processing language

Outline

  1. Introduction
  2. Goals for this Module
  3. Outline
  4. ZooKeeper
    1. Motivation
    2. Data Storage in ZooKeeper
    3. ZooKeeper Applications
    4. Distributed Consensus
  5. Pig
    1. Motivation
    2. Pig Latin
    3. Pig Latin Data Types
    4. Loading Data Into Pig
    5. Pig Latin Operators
    6. Setting Up Pig
  6. References

ZooKeeper

Motivation

Suppose you are building a large-scale distributed system. Several different services need to be brought online and must discover one another. It is not guaranteed that each service will have a fixed master IP address. For example, it may be the case that you start the same service on 100 nodes, and they elect a master from among whichever of the 100 boots first. Each of these disparate services must communicate with each other. How do all the nodes of each service find the master IP address of each other service? How do all the nodes in a single service agree on which one of them becomes the master?

ZooKeeper is a service designed to handle all of these problems. ZooKeeper will allow you to store small amounts of information in a central location. It provides coordinated access to this information. Most importantly, it provides high-availability: the ZooKeeper service is intended to run on a set of several machines, which prevents the loss of individual nodes from bringing down the cluster. But these nodes communicate information in a careful way, ensuring that all nodes in the ZooKeeper cluster provide the same consistent answer for a query, regardless of which ZooKeeper server you contact.

Data Storage in ZooKeeper

Several ZooKeeper daemons can be started on different machines. Clients can connect to any daemon in the cluster; the clients will always see the same view of the ZooKeeper world regardless of which daemon they connect to. User data is stored in objects with a hierarchical addressing system similar to that used by a conventional file system. It has a root object named /, and additional nodes can be extended off of this in a tree-like fashion. Each node of the tree can both hold data (i.e., act like a file) and have child nodes (i.e., act like a directory). The amount of data that can be stored in an object is small: there is a hard cap of 1 MB. The reason for the cap is so that the entire data store can be stored in the RAM of the ZooKeeper machines. This allows requests to be dispatched with high throughput. Changes are written to disk to provide permanence, but read requests are entirely handled by the data cached in memory. This is usually not a major limitation; the data stored at a node is intended to be used as a pointer. For example, ZooKeeper may know about the filename in another conventional file system, which contains the authoritative configuration file for a distributed system. The distributed system components first contact ZooKeeper to get the definitive filename, and then fetch that file for the configuration.

ZooKeeper Applications

ZooKeeper can be used for a variety of distributed coordination tasks. In addition to leader election, system bootstrapping, and various types of locks (mutual exclusion, reader/writer, etc), other synchronization primitives such as barriers, producer/consumer queues, priority queues, and multi-phase commit operations can be encoded in ZooKeeper. The ZooKeeper tutorial and recipes pages describe how to implement these algorithms. ZooKeeper itself is implemented in Java, but provides APIs for both Java- and C-based programs.

ZooKeeper can also be used as a central message board for an application. Individual nodes of a distributed system can store their current operational status in ZooKeeper for easy central reporting. The ZooKeeper service can also be used to form sub-groups of nodes or other hierarchical arrangements within a distributed system.

As mentioned, data stored in ZooKeeper is accessed by manipulating the nodes in the data hierarchy. This is done in a manner similar to file system access. But ZooKeeper does not implement the POSIX file system API. On the other hand, it also adds a set of other primitives not ordinarily found in a file system. Nodes can be opened with a number of special flags. One such flag is "ephemeral," meaning that the node disappears when the client who opened it disconnects. Another such flag is "sequence," which means that ZooKeeper will append a sequential id number to the node name you are trying to create. These id numbers are handed out in order, and the same id number is not reused. ZooKeeper does not provide exclusive locks on nodes directly, but a lock can be created by careful use of the ephemeral and sequence flags. The ZooKeeper recipes wiki page describes how to implement global locks using these flags. It also describes protocols for implementing shared (reader-writer) and revocable locks.

Distributed Consensus

A reasonable question is how the ZooKeeper service can function across multiple nodes and remain synchronized. If distributed synchronization is why your services must use ZooKeeper, how does ZooKeeper itself bootstrap this capability?

ZooKeeper implements a distributed consensus protocol. ZooKeeper internally uses a leader election protocol such as Paxos to determine which node in the ZooKeeper service is the master. While clients connect to any node in the ZooKeeper service, these additional nodes will forward agreed-upon facts back to clients. Updates to the shared state require the intervention of the master. All updates to the shared state are ordered with timestamps. These timestamped updates are then disseminated to the nodes in the ZooKeeper service. When a majority of nodes acknowledge an update, it is said to be held by a quorum of the nodes. Any fact that a quorum has agreed upon may be returned to clients. Conversely, any updates that have not reached a quorum will not be returned to the clients. The timestamps are used to order the updates to elements of the data store. If multiple updates are made to the state of an individual node, the newest update is used.

The use of a quorum ensures that the service always returns consistent answers. Because a vote is effectively held before returning a response, any nodes which hold stale data will be outvoted by the nodes with more current information. This also makes ZooKeeper resilient to failure. Up to 49% of the ZooKeeper service nodes can shut down or become desynchronized before ZooKeeper loses its ability to authoritatively answer responses. So if 11 nodes run ZooKeeper, up to 5 of these may disconnect without incident. After more than half the nodes fail, ZooKeeper will refuse service until the machines are restored.

If the node of the ZooKeeper cluster which was elected leader fails, then a new leader election will be held and the cluster will continue to function.

The reason for electing a leader in such a system is to ensure that timestamps assigned to updates are only issued by a single authority. ZooKeeper is designed to reduce or eliminate possible race conditions in distributed applications.

One consequence of ZooKeeper's design is that it is intended to serve many more read requests than writes. A ZooKeeper cluster can handle hundreds or thousands of clients, issuing many tens of thousands of requests per second--if the majority of these requests (90--99%) are reads. Only a small fraction should be updates.

ZooKeeper Example

The following code excerpt shows how to use ZooKeeper to implement a "barrier." A barrier separates a process into two logical halves. Multiple machines running in coordination with one another will all perform the first half of the process. No machine can begin the second half of the process until everyone has completed the first half. The barrier sits between these processes. As nodes reach the barrier, they all wait until everyone has reached the barrier. Then all nodes are released to begin the second half. A distributed barrier implementation written for ZooKeeper follows:

Watcher watcher = new Watcher() {
public void process(WatchEvent event) {}
};

ZooKeeper zk = new ZooKeeper(hosts, 3000, watcher);

Object notifyObject = new Object();
String root;
int size;

Barrier(ZooKeeper zk, String name, int size) throws KeeperException, InterruptedException {
this.zk = zk;
this.root = name;
this.size = size;
// Make sure the barrier node exists
try {
zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE, 0);
} catch (NodeExistsException e) {}
}


b.enter()
/** work with everyone **/
b.leave()



/**
* Join barrier
* @return
* @throws KeeperException
* @throws InterruptedException */
boolean enter() throws KeeperException, InterruptedException {
zk.create(root + "/" + name, new byte[0], Ids.OPEN_ACL_UNSAFE, CreateFlags.EPHEMERAL);
while (true) {
synchronized (notifyObject) {
ArrayList<String> list = zk.getChildren(root, new Watcher() {
public void process(Event e) { notifyObject.notifyAll(); }
});

if (list.size() < size) {
notifyObject.wait();
} else {
return true;
}
}
}
}

/**
* Wait until all reach barrier
* @return
* @throws KeeperException
* @throws InterruptedException */
boolean leave() throws KeeperException, InterruptedException {
zk.delete(root + "/" + name, 0);
while (true) {
synchronized (notifyObject) {
ArrayList<String> list = zk.getChildren(root, new Watcher() {
public void process(Event e) { notifyObject.notifyAll(); }
});

if (list.size() > 0) {
notifyObject.wait();
} else {
return true;
}
}
}
}

Listing 6.1: ZooKeeper Barrier Example

Pig

Motivation

Pig is a platform for analyzing large data sets. Pig's language, Pig Latin, lets you specify a sequence of data transformations such as merging data sets, filtering them, and applying functions to records or groups of records. Users can create their own functions to do special-purpose processing.

Pig Latin programs execute in a distributed fashion on a cluster. Our current implementation compiles Pig Latin programs into Map/Reduce jobs, and executes them using Hadoop on Kryptonite.

Thur purpose of Pig is to answer queries over semi-structured data such as log files. Large volumes of data are in mostly-organized formats such as log files, which define a set of standard fields for each entry. While the MapReduce programming model on top of Hadoop provides a convenient mechanism for delivering a large volume of log-structured information to an analysis program, writing analyses of mostly-structured information involves writing a large amount of tedious processing code.

Pig is a high-level language for writing queries over this sort of data. A query planner then compiles queries written in this language (called "Pig Latin") into maps and reduces which are then executed on a Hadoop cluster.

Anything which could be written in Pig can also be implemented as straight Java-based Hadoop MapReduce. But while individual programmers could develop their own suite of data analysis functions such as join, order by, etc., this requires individual programmers to develop their own (non-standard) libraries, and test them. Pig provides a tested and supported suite of the most common data-aggregation functions. It also allows programmers to provide their own application-specific code for purposes of loading and saving data, as well as for performing more complicated record-by-record evaluations.

Pig Latin

The programming language used to write Pig queries is called Pig Latin.

The MapReduce programming model can be thought of as composed of three distinct phases:

  1. Process input records
  2. Form groups of related records
  3. Process groups into outputs

In MapReduce, the first two of these steps are handled by the mapper, and the third step is handled by the reducer. Pig Latin exposes explicit primitives that perform actions from each phase. These primitives can be composed and reordered. Furthermore, it includes additional primitives to handle operations such as filtering and joining data sets.

Pig Latin Data Types

Values in Pig Latin can be expressed by four basic data types:

  • An atom is any atomic value (e.g., "fish")
  • A tuple is a record of multiple values with fixed arity. e.g., ("dog", "sparky").
  • A data bag is a collection of an arbitrary number of values. e.g., {("dog", "sparky"), ("fish", "goldie")}. Data bags support a scan operation for iterating through their contents.
  • A data map is a collection with a lookup function translating keys to values. e.g., ["age" : 25]

All data types are fully nestable; bags may contain tuples, and maps may contain bags or other maps, etc. This differs from a traditional database model, where data must be normalized into lists of atoms. By allowing data types to be composed in this manner, Pig queries line up better to the conceptual model of the data held by the programmer. Data types may also be heterogeneous. For example, the fields of a tuple may each have different types; some may be atoms, others may be more tuples, etc. The values in a bag may hold different types, as may the values in data maps. These can vary from one record to the next in the bag. Data map keys must be atoms, for efficiency reasons.

Loading Data Into Pig

The first step in using Pig is to load data into a program. Pig provides a LOAD statement for this purpose. Its format is: result = LOAD 'filename' USING fn() AS (field1, field2, ...).

This statement returns a bag of values of all the data contained in the named file. Each record in the bag is a tuple, with the fields named by field1, field2, etc. The fn() is a user-provided function that reads in the data. Pig supports user-provided Java code throughout to handle the application-specific bits of parsing. Pig Latin itself is the "glue" that then holds these application-specific functions together, routing records and other data between them.

An example data loading command is:

queries = LOAD 'query_log.txt'
USING myLoad()
AS (userId, queryString, timestamp)

The user-defined functions to load data (e.g., myLoad()) do not need to be provided. A default function for loading data exists, which will parse tab-delimited records. If the programmer did not specify field names in the AS clause, they would be addressed by positional parameters: $0, $1, and so forth.

The default loader is called PigStorage(). This loader can read files containing character-delimited tuple records. These tuples must contain only atomic values; e.g., cat, turtle, fish. Other loaders are listed in the PigBuiltins page of the Pig wiki. PigStorage() takes as an argument the character to use to delimit fields. For example, to load a table of three tab-delimited fields, the following statement can be used:

data = LOAD 'tab_delim_data.txt' USING PigStorage('\t') AS (user, time, query)

A different argument could be passed to PigStorage() to read comma- or space-delimited fields.

Pig Latin Operators

Pig Latin provides a number of operators which filter, join, or otherwise organize data.

FOREACH: The FOREACH command operates on each element of a data bag. This is useful, for instance, for processing each input record in a bag returned by a LOAD statement.

FOREACH bagname GENERATE expression, expression...

This statement iterates over the contents of a bag. It applies the expressions on the right of the GENERATE keyword to the data provided by the current record emitted from the bag. The expressions may be, for example, the names of fields. So to extract the names of all users who accessed the site (based on the query_log.txt example shown above), we could write a query like:

FOREACH queries GENERATE userId;

In the FOREACH statement, each element of the bag is considered independently. There are no expressions which reference multiple elements being extracted from the bag's iterator at a time; this allows the statement to be processed in parallel using Hadoop MapReduce.

Expressions emitted by the GENERATE element are not limited to the names of fields; they can be fields (by name like userId or by position like $0), constants, algebraic operations, map lookups, conditional expressions, or FLATTEN expressions, described below.

Finally, these expressions may also call user-provided functions that are written in Java. These user-provided functions have access to the entire current record through a Pig library; in this way, Pig can be used as the heavy-lifting component to automate record-by-record mapping using an application-specific Java function to perform tricky parsing or evaluation logic. Pig also provides several of the most commonly-needed functions, such as COUNT, AVG, MIN, MAX, and SUM.

FLATTEN is an expression which will eliminate a level of nesting. Given a tuple which contains a bag, FLATTEN will emit several tuples each of which contains one record from the bag. For example, if we had a bag of records containing a person's name and a list of types of pets they own:

(Alice, { turtle, goldfish, cat })
(Bob, { dog, cat })

A FLATTEN command would eliminate the inner bags like so:

(Alice, turtle)
(Alice, goldfish)
(Alice, cat)
(Bob, dog)
(Bob, cat)

FILTER statements iterate over a bag and return a new bag containing all elements which pass a conditional expression, e.g.:

adults = FILTER people BY age > 21;

The COGROUP and JOIN operations perform similar functions: they unite related data elements from multiple data sets. The difference is that JOIN acts like the SQL JOIN statement, creating a flat set of output records containing the joined cross-product of the input records. The COGROUP operator, on the other hand, groups the elements by their common field and returns a set of records each containing two separate bags. The first bag is the records of the first data set with the common field, and the second bag is the records of the second data set containing the common field.

To illustrate the difference, suppose we had the flattened data set mapping people to their pets, and another flattened data set mapping people to their friends. We could create a "pets of friends" data set out of these like the following. Here are the input data sets:

pets: (owner, pet)
----------------------
(Alice, turtle)
(Alice, goldfish)
(Alice, cat)
(Bob, dog)
(Bob, cat)

friends: (friend1, friend2)
----------------------
(Cindy, Alice)
(Mark, Alice)
(Paul, Bob)

Here is what is returned by COGROUP:

COGROUP pets BY owner, friends BY friend2; returns:

( Alice, {(Alice, turtle), (Alice, goldfish), (Alice, cat)},
         {(Cindy, Alice), (Mark, Alice)} )
( Bob, {(Bob, dog), (Bob, cat)}, {(Paul, Bob)} )

Contrasted with the more familiar, non-hierarchical JOIN operator:

JOIN pets BY owner, friends BY friend2; returns:

(Alice, turtle, Cindy)
(Alice, turtle, Mark)
(Alice, goldfish, Cindy)
(Alice, goldfish, Mark)
(Alice, cat, Cindy)
(Alice, cat, Mark)
(Bob, dog, Paul)
(Bob, cat, Paul)

In general, COGROUP command supports grouping on as many data sets as are desired. Three or more data sets can be joined in this fashion. It is also possible to group up elements of only a single data set; this is supported through an alternate keyword, GROUP.

A GROUP ... BY statement will organize a bag of records into bags of related items based on the field identified as their common key field. e.g., the pets bag from the previous example could be grouped up with:

GROUP pets BY owner; returns:

( Alice, {(Alice, turtle), (Alice, goldfish), (Alice, cat)} )
( Bob, {(Bob, dog), (Bob, cat)} )

In this way, GROUP and FLATTEN are effectively inverses of one another.

More complicated statements can be realized as well: operations which expect a data set as input do not need to use an explicitly-named data set; they can use one generated "inline" with another FILTER, GROUP or other statement.

When the final data set has been created by a Pig Latin script, the output can be saved to a file with the STORE command, which follows the form:

STORE data set INTO 'filename' USING function()

The provided function specifies how to serialize the data to the file; if it is omitted, then a default serializer will write plain-text tab-delimited files.

A number of additional operators exist for the purposes of removing duplicate records, sorting records, etc. This paper explains the additional operators and expression syntaxes in greater detail.

Setting Up Pig

Pig is an Apache incubator project; it has not made official packaged releases, but the source code can be retrieved from their subversion repository.

The Pig Incubator home page contains instructions on retrieving the Pig sources and compiling them.

References

Pig Tutorial: Included in this package for user to get hands-on help

Olston, C., Reed, B., Srivastava, U., et al. Pig Latin: A Not-So-Foreign Language for Data Processing. In Proceedings of the ACM SIGMOD 2008 International Conference on Management of Data. Vancouver, Canada, June 2008.

Pig Homepage

Pig Incubator Site (contains source code and setup instructions)

ZooKeeper Guide - The ZooKeeper manual

ZooKeeper Homepage

ZooKeeper Wiki

 

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