ACID

ACID Nedir?
Bir relational (ilişkisel) veri tabanı ACID özelliğini sağlamalı. Yani Atomicity (ya hep ya hiç), Consistency (Veri bütünlüğü yani integrity kurallarını ihlal etmeyen), Isolation ve Durability (Transaction bitince veri diske yazılır) özelliklerini sağlamalı.

ACID Kelimesi Nereden Geliyor

Designing Data Intensive Applications kitabındaki açıklama şöyle

It was coined in 1983 by Theo Härder and Andreas Reuter in an effort to establish precise terminology for fault-tolerance mechanisms in databases. However, in practice, one database’s implementation of ACID does not equal another’s implementation. For example, as we shall see, there is a lot of ambiguity around the meaning of isolation. The high-level idea is sound, but the devil is in the details. Today, when a system claims to be “ACID compliant,” it’s unclear what guarantees you can actually expect. ACID has unfortunately become mostly a marketing term.

BASE  Nedir

BASE Properties For Distributed Database Transactions yazısına taşıdım.

Atomicity Nedir (Abortability)

Designing Data Intensive Applications kitabındaki açıklama şöyle. Yani transaction abort edilirse hiç bir değişiklik kaydedilmez.

In general, atomic refers to something that cannot be broken down into smaller parts. The word means similar but subtly different things in different branches of computing. For example, in multi-threaded programming, if one thread executes an atomic operation, that means there is no way that another thread could see the half-finished result of the operation. The system can only be in the state it was before the operation or after the operation, not something in between. 

By contrast, in the context of ACID, atomicity is not about concurrency. It does not describe what happens if several processes try to access the same data at the same time, because that is covered under the letter I, for isolation (see “Isolation” on page 225).

Rather, ACID atomicity describes what happens if a client wants to make several writes, but a fault occurs after some of the writes have been processed—for example, a process crashes, a network connection is interrupted, a disk becomes full, or some integrity constraint is violated. If the writes are grouped together into an atomic transaction, and the transaction cannot be completed (committed) due to a fault, then the transaction is aborted and the database must discard or undo any writes it has made so far in that transaction.

Without atomicity, if an error occurs partway through making multiple changes, it’s difficult to know which changes have taken effect and which haven’t. The application could try again, but that risks making the same change twice, leading to duplicate or incorrect data. Atomicity simplifies this problem: if a transaction was aborted, the application can be sure that it didn’t change anything, so it can safely be retried.

The ability to abort a transaction on error and have all writes from that transaction discarded is the defining feature of ACID atomicity. Perhaps abortability would have been a better term than atomicity, but we will stick with atomicity since that’s the usual word.

ConsistencyNedir

Designing Data Intensive Applications kitabındaki açıklama şöyle.

The idea of ACID consistency is that you have certain statements about your data (invariants) that must always be true—for example, in an accounting system, credits and debits across all accounts must always be balanced. If a transaction starts with a database that is valid according to these invariants, and any writes during the transaction preserve the validity, then you can be sure that the invariants are always satisfied.

However, this idea of consistency depends on the application’s notion of invariants, and it’s the application’s responsibility to define its transactions correctly so that they preserve consistency. This is not something that the database can guarantee: if you write bad data that violates your invariants, the database can’t stop you. (Some specific kinds of invariants can be checked by the database, for example using foreign key constraints or uniqueness constraints. However, in general, the application defines what data is valid or invalid—the database only stores it.)

Atomicity, isolation, and durability are properties of the database, whereas consistency (in the ACID sense) is a property of the application. The application may rely on the database’s atomicity and isolation properties in order to achieve consistency, but it’s not up to the database alone. Thus, the letter C doesn’t really belong in ACID.

Isolation Nedir

ACID - Isolation yazısına taşıdım

Durability Nedir

Designing Data Intensive Applications kitabındaki açıklama şöyle.

The purpose of a database system is to provide a safe place where data can be stored without fear of losing it. Durability is the promise that once a transaction has committed successfully, any data it has written will not be forgotten, even if there is a hardware fault or the database crashes.

In a single-node database, durability typically means that the data has been written to nonvolatile storage such as a hard drive or SSD. It usually also involves a write-ahead log or similar (see “Making B-trees reliable” on page 82), which allows recovery in the event that the data structures on disk are corrupted. In a replicated database, durability may mean that the data has been successfully copied to some number of nodes. In order to provide a durability guarantee, a database must wait until these writes or replications are complete before reporting a transaction as successfully committed.

As discussed in “Reliability” on page 6, perfect durability does not exist: if all your hard disks and all your backups are destroyed at the same time, there’s obviously nothing your database can do to save you.