SimpleDB versus RDBMS

Amazon SimpleDB Developer Guide

SimpleDB is a highly scalable, simple-to-use, and inexpensive database in the cloud from
Amazon Web Services. But in order to use SimpleDB, you really have to change your
mindset. This isn’t a traditional relational database; in fact it’s not relational at all. For
developers who have experience working with relational databases, this may lead to
misconceptions as to how SimpleDB works.

This practical book aims to address your preconceptions on how SimpleDB will work for
you. You will be led quickly through the differences between relational databases and
SimpleDB, and the implications of using SimpleDB. Throughout this book, there is an
emphasis on demonstrating key concepts with practical examples for Java, PHP, and
Python developers.

You will be introduced to this massively scalable schema less key-value data store: what
it is, how it works, and why it is such a game changer. You will then explore the basic
functionality offered by SimpleDB including querying, code samples, and a lot more.
This book will help you deploy services outside the Amazon cloud and access them from
any web host.

You will see how SimpleDB gives you the freedom to focus on application development.
As you work through this book you will be able to optimize the performance of your
applications using parallel operations, caching with memcache, asynchronous operations,
and more.

Gain in-depth understanding of Amazon SimpleDB with PHP, Java, and Python
examples, and run optimized database-backed applications on Amazon’s Web
Services cloud.

What This Book Covers

Chapter 1, Getting to Know SimpleDB, explores SimpleDB and the advantages of
utilizing it to build web-scale applications.

Chapter 2, Getting Started with SimpleDB, moves on to set up an AWS account, enable
SimpleDB service for the account, and install and set up libraries for Java, PHP, and
Python. It also illustrates several SimpleDB operations using these libraries.

Chapter 3, SimpleDB versus RDBMS, sheds light on the differences between SimpleDB
and a traditional RDBMS, as well as the pros and cons of using SimpleDB as the storage
engine in your application.

Chapter 4, The SimpleDB Data Model, takes a detailed look at the SimpleDB data model
and different methods for interacting with a domain, its items, and their attributes. It
further talks about the domain metadata and reviews the various constraints imposed by
SimpleDB on domains, items, and attributes.

Chapter 5, Data Types, discusses the techniques needed for storing different data types in
SimpleDB, and explores a technique for storing numbers, Boolean values, and dates. It
also teaches you about XML-restricted characters and encoding them using base64

Chapter 6, Querying, describes the Select syntax for retrieving results from SimpleDB,
and looks at the various operators and how to create predicates that allow you to get back
the information you need.

Chapter 7, Storing Data on S3, introduces you to Amazon S3 and its use for storing large
files. It practically modifies a sample domain to add additional metadata including a file
key that is again used for naming the MP3 file uploaded to S3. The example used in this
chapter shows you a simple way to store metadata on SimpleDB while storing associated
content that is in the form of binary files on Amazon S3.

Chapter 8, Tuning and Usage Costs, mainly covers the BoxUsage of different SimpleDB
queries and the usage costs, along with viewing the usage activity reports.

Chapter 9, Caching, explains memcached and Cache_Lite in detail and their use for
caching. It further explores a way you can use memcached with SimpleDB to avoid
making unnecessary requests to SimpleDB, that is, by using libraries in Java, PHP, and

Chapter 10, Parallel Processing, analyzes how to utilize multiple threads for running
parallel operations against SimpleDB in Java, PHP, and Python in order to speed up
processing times and taking advantage of the excellent support for concurrency in

SimpleDB versus RDBMS

We have all used a Relational Database Management System (RDBMS) at some
point in our careers. These relational databases are ubiquitous and are available
from a wide range of companies such as Oracle, Microsoft, IBM, and so on. These
databases have served us well for our application needs. However, there is a new
breed of applications coming to the forefront in the current Internet-driven and
socially networked economy. The new applications require large scaling to meet
demand peaks that can quickly reach massive levels. This is a scenario that is hard
to satisfy using a traditional relational database, as it is impossible to requisition
and provision the hardware and software resources that will be needed to service
the demand peaks. It is also non-trivial and difficult to scale a normal RDBMS to
hundreds or thousands of nodes. The overwhelming complexity of doing this makes
the RDBMS not viable for these kinds of applications. SimpleDB provides a great
alternative to an RDBMS and can provide a solution to all these problems. However,
in order to provide this solution, SimpleDB makes some choices and design decisions
that you need to understand in order to make an informed choice about the data
storage for your application domain.

In this chapter, we are going to discuss the differences between SimpleDB and a
traditional RDBMS, as well as the pros and cons of using SimpleDB as the storage
engine in your application.

No normalization

Normalization is a process of organizing data efficiently in a relational database
by eliminating redundant data, while at the same time ensuring that the data
dependencies make sense. SimpleDB data models do not conform to any of the
normalization forms, and tend to be completely de-normalized. The lack of need
for normalization in SimpleDB allows you a great deal of fl exibility with your
model, and enables you to use the power of multi-valued attributes in your data.

Let’s look at a simple example of a database starting with a basic spreadsheet
structure and then design it for an RDBMS and a SimpleDB. In this example,
we will create a simple contact database, with contact information as raw data.

The obvious issue is the repetition of the name data. The table is inefficient and
would require care to update to keep the name data in sync. To find a person by
his or her phone number is easy.

SELECT * FROM Contact_Info WHERE Phone_Num = ‘555-854-9885’

So let’s analyze the strengths and weaknesses of this database design.

The design is simple, but as the name data is repeated, it would require care to keep
the data in sync. Searching for phone numbers by name would be ugly if the names
got out of sync.

To improve the design, we can rationalize the data. One approach would be to create
multiple phone number fields such as the following. While this is a simple solution,
it does limit the phone numbers to three. Add e-mail and Twitter, and the table
becomes wider and wider.

Finding a person by a phone number is ugly.

SELECT * FROM Contact_Info WHERE Phone_Num_1 = ‘555-854-9885’
OR Phone_Num_2 = ‘555-854-9885’
OR Phone_Num_3 = ‘555-854-9885’

Now let’s analyze the strengths and weaknesses of this database design.

The design is simple, but the phone numbers are limited to three, and searching by
phone number involves three index searches.

Another approach would be to use a delimited list for the phone number as follows:

This approach has the advantage of no data repetition and is easy to maintain,
compact, and extendable, but the only way to find a record by the phone number
is with a substring search.

SELECT * FROM Contact_Info WHERE Phone_Nums LIKE %555-854-9885%

This type of SQL forces a complete table scan. Do this with a small table and no
one will notice, but try this on a large database with millions of records, and the
performance of the database will suffer.

A delimited field is good for data that is of one type and will only be retrieved.

The normalization for relational databases results in splitting up your data into
separate tables that are related to one another by keys. A join is an operation that
allows you to retrieve the data back easily across the multiple tables.

Let’s first normalize this data.

This is the Person_Info table:

And this is the Phone_Info table:

Now a join of the Person_Info table with the Phone_Info can retrieve the list of
phone numbers as well as the e-mail addresses. The table structure is clean and other
than the ID primary key, no data is duplicated. Provided Phone_Num is indexed,
retrieving a contact by the phone number is efficient.

SELECT First_Name, Last_Name, Phone_num, Person_Info.ID
FROM Person_Info JOIN Phone_Info
ON Person_Info.ID = Phone_Info.ID
WHERE Phone_Num = ‘555-854-9885’

So if we analyze the strengths and weaknesses of this database design, we get:

While this is an efficient relational model, there is no join command in SimpleDB.
Using two tables would force two selects to retrieve the complete contact
information. Let’s look at how this would be done using the SimpleDB principles.

No joins

SimpleDB does not support the concept of joins. Instead, SimpleDB provides you
with the ability to store multiple values for an attribute, thus avoiding the necessity
to perform a join to retrieve all the values.

In the SimpleDB table, each record is stored as an item with attribute/value pairs. The
difference here is that the Phone_Num field has multiple values. Unlike a delimited list
field, SimpleDB indexes all values enabling an efficient search each value.

SELECT * FROM Contact_Info WHERE Phone_Num = ‘555-854-9885’

This SELECT is very quick and efficient. It is even possible to use Phone_Num multiple
times such as follows:

SELECT * FROM Contact_Info WHERE Phone_Num = ‘555-854-9885’
OR Phone_Num = ‘555-748-7854’

Let’s analyze the strengths and weaknesses of this approach:

No schemas

There are no schemas anywhere in sight of SimpleDB. You don’t have to create
schemas, change schemas, migrate schemas to a new version, or maintain schemas.
This is yet another thing that is difficult for some people from a traditional relational
database world to grasp, but this fl exibility is one of the keys to the power of scaling
offered by SimpleDB. You can store any attribute-value data you like in any way you
want. If the requirements for your application should suddenly change and you need
to start storing data on a customer’s Twitter handle for instance, all you need to do is
store the data without worrying about any schema changes!

Let’s add an e-mail address to the database in the previous example. In the relational
database, it is necessary to either add e-mail to the phone table with a type of contact
field or add another field. Let’s add another table named Email_Info.

Person_Info table:

Phone_Info table:

Email_Info table:

Using a traditional relational database approach, we join the three tables to extract
the requested data in one call.

SELECT First_Name, Last_Name, Phone_num, Person_Info.ID, Email_Addr
FROM Person_Info JOIN Phone_Info JOIN Email_Info
ON Person_Info.ID = Phone_Info.ID
AND Person_Info.ID = Email_Info.ID
WHERE Phone_Num = ‘555-854-9885’

Now let’s analyze the strengths and weaknesses of this approach:

We ignored the issue of join versus left outer join, which is really what should be
used here unless all contacts have a phone number and e-mail address. The example
is just to illustrate that the Contact_Info schema must be modified.

Contact_Info domain:

The obvious question is why is Email_Addr not in its own column? In SimpleDB,
there is no concept of a column in a table. The spreadsheet view of the SimpleDB
data was done for ease of readability, not because it refl ects the data structure. The
only structure in SimpleDB consists of the item name and attribute/value pairs. The
proper representation of the SimpleDB data is:

Use the following query to fetch a contact item by the e-mail address:

SELECT * FROM Contact_Info WHERE Email_Addr = ‘john@def.ccc’

Let’s analyze the strengths and weaknesses of this approach:

Simpler SQL

Structured Query Language (SQL) is a standard language that is widely used for
accessing and manipulating the data stored in a relational database. SQL has evolved
over the years into a highly complex language that can do a vast variety of things to
your database. SimpleDB does not support the complete SQL language, but instead
it lets you perform your data retrieval using a much smaller and simpler subset of an
SQL-like query language. This simplifies the whole process of querying your data.
A big difference between the simpler SQL supported by SimpleDB and SQL is the
support for multi-valued SimpleDB attributes, which makes it super simple to query
your data and get back multiple values for an attribute.

The syntax of the SimpleDB SQL is summarized in this syntax:

select output_list
from domain_name
[where expression]
[limit limit]

We will go into detail on SimpleDB SQL in Chapter 6, Querying.

Only strings

SimpleDB uses a very simple data model, and all data is stored as an UTF-8 string.
This simplified textual data makes it easy for SimpleDB to automatically index your
data and give you the ability to retrieve the data very quickly. If you need to store
and retrieve other kinds of data types such as numbers and dates, you must encode
these data types into strings whose lexicographical ordering will be the same as your
intended ordering of the data. As SimpleDB does not have the concept of schemas
that enforce type correctness for your domains, it is the developer’s responsibility to
ensure the correct encoding of data before storage into SimpleDB.

Working only in strings impacts two aspects of using the database: queries and sorts.

Consider the following Sample_Qty table:

Now try and execute the following SQL statement:

SELECT * FROM Sample_Qty WHERE Quantity= ‘1’

This SQL statement will retrieve nothing—not even items 101 and 102.

Selecting all records sorted by Quantity will return the order 101, 102, 103, 105, 104.

Dates present an easier problem, as they can be stored in ISO 8601 format to enable
sorting as well as predictable searching. We will cover this in detail in Chapter 5,
Data Types

Eventual consistency

Simple DB can be thought of as a Write-Seldom-Read-Many model. Updates are
done to a central database, but reads can be done from many read-only database
slave servers.

SimpleDB keeps multiple copies of each domain. Whenever data is written or
updated within a domain, first a success status code is returned to your application,
and then all the different copies of the data are updated. The propagation of these
changes to all of the nodes at all the storage locations might take some time, but
eventually the data will become consistent across all the nodes.

SimpleDB provides this assurance only of eventual consistency for your data. This
means that the data you retrieve from SimpleDB at any particular time may be
slightly out of date. The main reason for this is that SimpleDB service is implemented
as a distributed system, and all of the information is stored across multiple physical
servers and potentially across multiple data centers in a completely redundant
manner. This ensures the large-scale ready accessibility and safety of your data,
but comes at the cost of a slight delay before any addition, alteration, or deletion
operations you perform on the data being propagated throughout the entire
distributed SimpleDB system. Your data will eventually be globally consistent, but
until it is consistent, the possibility of retrieving slightly outdated information from
SimpleDB exists.

Amazon has stated in the past that states of global consistency across all the nodes
will usually be achieved “within seconds”; however, please be aware that this
timeframe will depend to a great degree on the processing and the network load on
SimpleDB at the time that you make a change to your data. An intermediate caching
layer can quickly solve this consistency issue if data consistency is highly important
and essential to your application. The principle of eventual consistency is the hardest
to grasp, and it is the biggest difference between a RDBMS and SimpleDB. In order
to scale massively, this is a trade-off that needs to be made at design time for your
application. If you consider how often you will require immediate consistency within
your web applications, you might find that this trade-off is well worth the improved
scalability of your application.

Flash: February 24, 2010 — consistent read added

While eventual consistency is still the normal mode for SimpleDB, Amazon
announced several extensions for consistent read. When using a GetAttributes
or SELECT, the ConsistentRead = true can be selected, forcing a read of the most
current value. This tells SimpleDB to read the items from the master database rather
than from one of the slaves, guaranteeing the latest updates or deletes. This does
not mean you can use this on all reads and still get the extreme scaling. In Chapter 8,
Tuning and Usage Costs,
we will look at the cost of using consistent reads.

A conditional PUT or DELETE was also announced, which will execute a database
PUT or DELETE only if the consistent read of a specific attribute has a specific value
or does not exist. This is useful if concurrent controls or counters primitives. In later
chapters, we will look at the implications of these new features.


Relational databases are designed around the entities and the relationships between
the entities, and need a large investment in hardware and servers in order to
provide high scaling. SimpleDB provides a great alternative that is designed around
partitioning your data into independent chunks that are stored in a distributed
manner and can scale up massively. SimpleDB provides the automatic partitioning
and replication of your data, while at the same time guaranteeing fast access and
reliability for your data. You can let Amazon scale their platform as needed using
their extensive resources, while you enjoy the ability to easily scale up in response to
increased demand!

The best feature of SimpleDB scalability is that you only pay for usage, not for the
large cluster needed in anticipation of large usage.

Low maintenance

Maintaining a relational database and keeping it humming with indexing takes
effort, know-how, and technical and administrative resources. Applications are
not static but dynamic things, and change constantly along with additions of new
features. All of these updates can result in changes and modifications to the database
schema along with increased maintenance and tuning costs. SimpleDB is hosted
and maintained for you by Amazon. Your task is as simple as storing your data and
retrieving it when needed. The simplicity of structured data and lack of schemas
helps your application be more fl exible and adaptable to change, which is always
around the corner. SimpleDB ensures that your queries are optimized and retrieval
times are fast by indexing all your data automatically.

Advantages of the SimpleDB model

SimpleDB’s alternative approach for storing data can be advantageous for meeting
your application needs when compared to a traditional relational database. Here’s
the list of advantages:

  • Reduced maintenance as compared to a relational database

  • Automated indexing of your data for fast performance

  • Flexibility to modify or change your stored data without the need to
    worry about schemas

  • Failover for your data automatically being provided by Amazon

  • Replication for your data across multiple nodes also handled for you
    by Amazon

  • Ability to easily scale up in response to increased demand without
    worrying about running out of hardware or processing capacity

  • Simplified data storage and querying using a simple API

  • The lack of object-to-relational mapping that is common for an RDBMS
    allows your structured data to map more directly to your underlying
    application code and reduce the application development time

Disadvantages of the SimpleDB model

SimpleDB’s alternative approach also has some disadvantages compared to a
relational database for certain applications.

  • Those using applications that always need to ensure immediate consistency
    of data will find that SimpleDB’s eventual data consistency model may not
    suit their needs. The consistent read announcement does change this, but the
    eventual consistency model is still the basis of the extreme scalability.

  • Using SimpleDB as the data storage engine in your applications needs
    the development team to get used to different concepts over a simple,
    traditional RDBMS.

  • Because relationships are not explicitly defined at the schema level as in a
    relational database, you might need to enforce some data constraints within
    your application code.

  • If your application needs to store data other than strings, such as numbers
    and dates, additional effort will be required on your part to encode the
    strings before storing them in the SimpleDB format.

  • The ability to have multiple attributes for an item is a completely different
    way of storing data and has a learning curve attached to it for new users
    who are exposed to SimpleDB.


In this chapter, we discussed the differences between SimpleDB and the traditional
relational database systems in detail. In the next chapter, we are going to review the
data model used by SimpleDB.

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