Thèse de doctorat: Algorithmes de classification répartis sur le cloud

tel-00744768, version 1 - 23 Oct 2012
30 CHAPTER 2. PRESENTATION OF CLOUD COMPUTING
2.4.1 Relational storage and ACID properties
Relational databases are structured through the relations between data entities.
They are inspired by relational algebra and tuple calculus theories, while they do
not respect all of their constraints. Relational databases are well understood and
offer great guarantees for reliably storing and retrieving data in a robust manner.
In addition to these guarantees, the relational model proved to be well-suited to
express real-world business data interdependencies. RDBMS are therefore widely
used in business applications.
A Structured Query Language (SQL) is provided together with the RDBMS to
make the use of data easier. SQL is a declarative programming language (with
some procedural elements) intended to manage data queries. Declarative languages
express what computations should return rather than explicitly defining
how the computations should be done. In the case of relational storages, queries
are expressed in SQL to define requests to the desired data, while the underlying
RDBMS performs the actual operations to answer the requests. Listing 2.1 gives
an example of a SQL query.
Listing 2.1: a SQL query sample
SELECT T1 . v a r i a b l e 1 , T1 . v a r i a b l e 2
FROM Table1 T1 , Table2 T2
WHERE T1 . v a r i a b l e 1 = T2 . v a r i a b l e 3
ORDER BY T1 . name
A single “business” operation applied on data stored in a database is called a
transaction. A single transaction can affect several pieces of data at once. The
guarantees provided by RDBMS can be understood by observing how transactions
are applied on the database. The following paragraphs introduce the main
relational storage guarantees by describing how they are translated in terms of
transactions management.
– Consistency is a property related to the way data are modified during transactions.
There are multiple definitions of consistency, resulting in different
consistency levels and depending on whether the storage is distributed or not.
Initially, consistency was a property that guaranteed that some user-specified
invariants on data were never broken. For example, such an invariant could
consist in having the sum of some variables equal the sum of other variables.
With the rise of distributed storages, the definitions of consistency have been
multiplied, and consistent distributed storage (also called strongly consistent
storages) often refers to the fact that each update is applied to all the relevant