The ACID paradigm is a cornerstone of database management systems. With respect to data modifications, the paradigm demands that transactions must fulfill certain requirements and have to guarantee that they are satisfied not only during regular operations but also in all cases of minor and major problems like mutual-locking, connection-loss, server-down, disk-full, disk-crash, ... .
Please note especially that the requirements are defined at the transaction level. They are named:
As shown in the previous chapter every single write operation or a collection of write operations is embedded in a transaction. Read operations may also be part of a transaction.
All writing operations within a transaction create a single, undividable unity. Either all of them succeed or none. Writing operations to different tables are an example of such a situation. Another example is the decrease of one person's bank account and the associated increase of another bank account during a money transfer.
At the end of a transaction, the database is in a consistent state. All defined integrity rules like uniqueness, check constraints, foreign-key and primary-key definitions are fulfilled. Furthermore, all involved triggers have been successfully executed. It's possible that during the lifetime of a transaction those rules may be broken, e.g. the foreign key relationship of two nodes in a doubly-linked list.
In essence, a transaction transfers the database from one consistent state to another consistent state.
In many cases, transactions run in parallel. But the database system gives them the illusion that they act one after the other. Depending on the chosen isolation level, the exact behavior of competing read and write operations may differ. Nevertheless, PostgreSQL guarantees in all cases, that read operations never block write operations and write operations never block read operations.
Durability guarantees that after a successful termination (COMMIT) of a transaction, the carried-out changes keep in the database, even if a significant problem like a disk crash occurs. PostgreSQL implements this by saving the data changes not only in the data files but - redundant - also in Write-Ahead-Log (WAL) files. Therefore it is recommended that data and WAL files shall be stored on different disks.