使用Java锁进行数据库并发

Question

I have the following scenario. I have two tables. One stores multi values that are counters for transactions. Through a java application the first table value is read, incremented and written to the second table, as well as the new value being written back to the first table. Obviously there is potential for this to go wrong as it's a multiple user system. My solution, in Java, to the issue is to provide Locks that have to, well should , be aquired before any action can be taken on either table. These Locks, ReentrantLocks, are static and there is one for each column in Table 1 as the values are completely independent of each other.

Is this a recommended approached?

Cheers.

Answer 1

No. Use implicit Database Locks ¹ for Database Concurrency. Relational databases support Transactions which are a vital part of ACID : use them.

Java-centric locks will not work cross-VM and as such will not help in multi-User/Server environments.

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¹ Databases are smart enough to acquire/release locks to ensure Consistency and Isolation and may even use "lock free" implementations such as MVCC. There are rare occasions when explicit database locks must be requested, but this is an advanced use-case.

Answer 2

Whilst agreeing with some of the sentiments of @pst's answer, I would say this depends slightly.

If the sequence of events is, and probably always will be, essentially "SQL oriented", then you may as well do the locking at the database level (and indeed, probably implicitly via the use of transactions).

However if there is, or you are planning to build in, significant data manipulation logic within your app tier (either generally or in the case of this specific operation), then locking at the app level may be more appropriate. (In reality, you will probably still run your SQL in transactions so that you're actually locking at both levels.)

I don't think the issue of multiple VMs is necessarily a compelling issue on its own for relying on DB-level locking. If you have multiple server apps accessing the database, you will in any case want to establish a well-defined protocol for which data is accessed concurrently under what circumstances. And in a system of moderate complexity, you will in any case want to build in a system of running periodic sanity checks on the data. (Even if your server apps are perfectly behaved 100% of the time, will back end tech support never ever ever have to run some miscellaneous SQL on the database outside your app...?)