QuantaDB uses a concurrency control protocol called DSSN.
In database systems and transaction processing, concurrency control (CC) schemes interleave read/write requests from multiple clients simultaneously, giving the illusion that each transaction has exclusive access to the data. Distributed concurrency control refers to the concurrency control of a database distributed over a communication network. Serializability ensures that a schedule for executing concurrent transactions is equivalent to one that executes the transactions serially in some order. It is considered to be the highest level of isolation between concurrent transactions. It assumes that all accesses to the database are done using read and write operations. A desirable goal of a distributed database is distributed serializability, which is the serializability of a schedule of concurrent transactions over a distributed database.
The most common distributed concurrency control schemes are two-phase locking (2PL), snapshot isolation (SI), Multi-Version Optimistic Concurrency Control (MVOCC), and Read Committed (RC). They are also common centralized concurrency control schemes. Each control scheme allows for more concurrency, i.e., more schedules of concurrent transactions to be permitted and hence higher transaction throughput, of a distributed database system. 2PL can achieve a serializable isolation level, but RC and SI cannot. To make non-serializable concurrency control schemes provide a serializable isolation level, a serialization certifier can be used. One example is serializable snapshot isolation (SSI). Wang, et. al., in "Efficiently Making (Almost) Any Concurrency Control Mechanism Serializable" The VDLB Journal -- The International Journal on Very Large Data Bases, Volume, 26, Issue 4, August 2017, introduced a certifier called the "Serial Safety Net" (SSN), a serialization certifier that can make otherwise non-serializable RC and SI concurrency control schemes achieve a serializable isolation level, meanwhile allowing more concurrency than the SSI scheme. Figure 1 illustrates that SSN would allow higher concurrency than 2PL and SSI.
[Adapted from Wang, et. al.]
Figure 1. SSN compared to other CC schemes.
SSN assumes a shared memory where the updates to each versions' timestamps are instantaneous. This assumption is not valid in a distributed environment and may lead to poor throughput in a distributed environment. Also, when transactions are contentious, SSN may abort many transactions, which limits the effective transaction rate. Thus, while Wang, et. al. present a fully parallel multi-threading, latch-free, and shared-memory implementation for a multi-version database management system on a single multi-processor server, Wang et al. do not address a fully distributed multi-version database management system.
DSSN is a fully distributed concurrency control scheme that makes SSN validation to be distributed. It can greatly improve the performance of ACID transactional database system, which includes multi-core databases, clustered databases, distributed databases, especially geo-distributed databases.