In database systems, durability is the ACID property that guarantees that the effects of transactions that have been committed will survive permanently, even in cases of failures, including incidents and catastrophic events. For example, if a flight booking reports that a seat has successfully been booked, then the seat will remain booked even if the system crashes.
Formally, a database system ensures the durability property if it tolerates three types of failures: transaction, system, and media failures. These kinds of interruptions can be originated at the transaction level by data-entry errors, operator cancellation, timeout, or application-specific errors, like withdrawing money from a bank account with insufficient funds. Specifically, a reliability mechanism requires primitives that explicitly state the beginning, the end, and the rollback of transactions, or logging and locking. The reason behind the choice of having volatile storage, which is subject to this type of failure, and non-volatile storage, is found in the performance differences of the existing technologies that are used to implement these kinds of storage. However, the situation is likely to evolve as the popularity of non-volatile memories (NVM) technologies grows.
In systems that include non-volatile storage, durability can be achieved by keeping and flushing an immutable sequential log of the transactions to such non-volatile storage before acknowledging commitment. Thanks to their atomicity property, the transactions can be considered the unit of work in the recovery process that guarantees durability while exploiting the log. In particular, the logging mechanism is called write-ahead log (WAL) and allows durability by buffering changes to the disk before they are synchronized from the main memory. In this way, by reconstruction from the log file, all committed transactions are resilient to system-level failures, because they can be redone. Non-committed transactions, instead, are recoverable, since their operations are logged to non-volatile storage before they effectively modify the state of the database. In this way, the partially executed operations can be undone without affecting the state of the system. After that, those transactions that were incomplete can be redone. Therefore, the transaction log from non-volatile storage can be reprocessed to recreate the system state right before any later system-level failure. Logging is done as a combination of tracking data and operations (i.e. transactions) for performance reasons.
Media level
At media level, failure scenarios affect non-volatile storage, like hard disk drives, solid-state drives, and other types of storage hardware components. At this level, there is a strong bond between durability and system and data recovery, in the sense that the main goal is to preserve the data, not necessarily in online replicas, but also as offline copies.
Therefore, in case of media failure, the durability of transactions is guaranteed by the ability to reconstruct the state of the database from the log files stored in the stable memory, in any way it was implemented in the database system.
Distributed databases
In distributed transactions, ensuring durability requires additional mechanisms to preserve a consistent state sequence across all database nodes. This means, for example, that a single node may not be enough to decide to conclude a transaction by committing it. In fact, the resources used in that transaction may be on other nodes, where other transactions are occurring concurrently. Otherwise, in case of failure, if consistency could not be guaranteed, it would be impossible to acknowledge a safe state of the database for recovery. For this reason, all participating nodes must coordinate before a commit can be acknowledged. This is usually done by a two-phase commit protocol.
In addition, in distributed databases, even the protocols for logging and recovery shall address the issues of distributed environments, such as deadlocks, that could prevent the resilience and recoverability of transactions and, thus, durability.