Two Phase Commit Protocol

Two-Phase Locking 2PL

This locking protocol divides the execution phase of a transaction into three parts. In the first part, when the transaction starts executing, it seeks permission for the locks it requires. The second part is where the transaction acquires all the locks. As soon as the transaction releases its first lock, the third phase starts. In this phase, the transaction cannot demand any new locks; it only releases the acquired locks.

Two Phase Commit Protocol

Two-phase locking has two phases, one is growing, where all the locks are being acquired by the transaction; and the second phase is shrinking, where the locks held by the transaction are being released.

To claim an exclusive (write) lock, a transaction must first acquire a shared (read) lock and then upgrade it to an exclusive lock.

Strict Two-Phase Locking

The first phase of Strict-2PL is same as 2PL. After acquiring all the locks in the first phase, the transaction continues to execute normally. But in contrast to 2PL, Strict-2PL does not release a lock after using it. Strict-2PL holds all the locks until the commit point and releases all the locks at a time.

Two Phase Commit Protocol

Strict-2PL does not have cascading abort as 2PL does.

In transaction processing, databases, and computer networking, the two-phase commit protocol (2PC) is a type of atomic commitment protocol (ACP). It is a distributed algorithm that coordinates all the processes that participate in a distributed atomic transaction on whether to commit or abort (roll back) the transaction (it is a specialized type ofconsensus protocol). The protocol achieves its goal even in many cases of temporary system failure (involving either process, network node, communication, etc. failures), and is thus widely utilized. However, it is not resilient to all possible failure configurations, and in rare cases user (e.g., a system’s administrator) intervention is needed to remedy an outcome. To accommodate recovery from failure (automatic in most cases) the protocol’s participants use logging of the protocol’s states. Log records, which are typically slow to generate but survive failures, are used by the protocol’s recovery procedures. Many protocol variants exist that primarily differ in logging strategies and recovery mechanisms. Though usually intended to be used infrequently, recovery procedures compose a substantial portion of the protocol, due to many possible failure scenarios to be considered and supported by the protocol.

In a “normal execution” of any single distributed transaction, i.e., when no failure occurs, which is typically the most frequent situation, the protocol consists of two phases:

  1. The commit-request phase (or voting phase), in which a coordinator process attempts to prepare all the transaction’s participating processes (named participants, cohorts, or workers) to take the necessary steps for either committing or aborting the transaction and to vote, either “Yes”: commit (if the transaction participant’s local portion execution has ended properly), or “No”: abort (if a problem has been detected with the local portion), and
  2. The commit phase, in which, based on voting of the cohorts, the coordinator decides whether to commit (only if all have voted “Yes”) or abort the transaction (otherwise), and notifies the result to all the cohorts. The cohorts then follow with the needed actions (commit or abort) with their local transactional resources (also called recoverable resources; e.g., database data) and their respective portions in the transaction’s other output (if applicable).
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