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Distributed Transactions in Practice

Distributed transactions, especially those implemented with two-phase commit, have a mixed reputation. On the one hand, they are seen as providing an important safety guarantee that would be hard to achieve otherwise; on the other hand, they are criticized for causing operational problems, killing performance, and promising more than they can deliver [81, 82, 83, 84]. Many cloud services choose not to implement distributed transactions due to the operational problems they engender [85, 86].

Some implementations of distributed transactions carry a heavy performance penalty —for example, distributed transactions in MySQL are reported to be over 10 times slower than single-node transactions [87], so it is not surprising when people advise against using them. Much of the performance cost inherent in two-phase commit is due to the additional disk forcing (fsync) that is required for crash recovery [88], and the additional network round-trips.

However, rather than dismissing distributed transactions outright, we should examine them in some more detail, because there are important lessons to be learned from them. To begin, we should be precise about what we mean by “distributed transactions.” Two quite different types of distributed transactions are often conflated:

Database-internal distributed transactions

Some distributed databases (i.e., databases that use replication and partitioning in their standard configuration) support internal transactions among the nodes of that database. For example, VoltDB and MySQL Cluster’s NDB storage engine have such internal transaction support. In this case, all the nodes participating in the transaction are running the same database software.

Heterogeneous distributed transactions

In a heterogeneous transaction, the participants are two or more different technologies: for example, two databases from different vendors, or even nondatabase systems such as message brokers. A distributed transaction across these systems must ensure atomic commit, even though the systems may be entirely different under the hood.

Database-internal transactions do not have to be compatible with any other system, so they can use any protocol and apply optimizations specific to that particular technology. For that reason, database-internal distributed transactions can often work quite well. On the other hand, transactions spanning heterogeneous technologies are a lot more challenging.

 
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