HBase的事務(wù)性(Transaction)

昵稱10504424 2013-02-20

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What is Transaction

先摘錄一段Wiki關(guān)于Transaction（事務(wù)）的釋義：

A transaction comprises a unit of work performed within a database management system (or similar system) against a database, and treated in a coherent and reliable way independent of other transactions. Transactions in a database environment have two main purposes:
1. To provide reliable units of work that allow correct recovery from failures and keep a database consistent even in cases of system failure, when execution stops (completely or partially) and many operations upon a database remain uncompleted, with unclear status.

2. To provide isolation between programs accessing a database concurrently. If this isolation is not provided, the program's outcome are possibly erroneous.
A database transaction, by definition, must be atomic, consistent, isolated and durable. Database practitioners often refer to these properties of database transactions using the acronym ACID.
Transactions provide an "all-or-nothing" proposition, stating that each work-unit performed in a database must either complete in its entirety or have no effect whatsoever. Further, the system must isolate each transaction from other transactions, results must conform to existing constraints in the database, and transactions that complete successfully must get written to durable storage.

其中，all-or-nothing很明確的表達了事務(wù)的本質(zhì)：“要么全成功，要么什么都沒發(fā)生過”?？梢园阉醋饕粋€原子操作。

HBase Transaction

HBase的支持的事務(wù)很有限，0.94版本的新特性中有一條：

[HBASE-5229] - Provide basic building blocks for "multi-row" local transactions.

引用NoSQLFan博客中的解讀：

“0.94版本具備更完整的事務(wù)支持: 之前Hbase提供行級的事務(wù)，不過每次事務(wù)只能執(zhí)行一個寫操作，比如連續(xù)地執(zhí)行一系列Put，Delete操作，那么這些操作是單獨一個個的事務(wù)，其整體并不是原子性執(zhí)行的。而在0.94版本中，可以實現(xiàn)Put、Delete在同一個事務(wù)中一起原子性執(zhí)行。見提案 HBASE-3584 ?！?/SPAN>

具體怎么用呢？有下面這一段Sample：

[java] view plain copy print ?

//Add API for atomic row mutations to HBase (currently Put and Delete).
Client API would look like this:
Delete d = new Delete(ROW);
Put p = new Put(ROW);
//...
AtomicRowMutation arm = new AtomicRowMutation(ROW);
arm.add(p);
arm.add(d);
myHtable.atomicMutation(arm);

//Add API for atomic row mutations to HBase (currently Put and Delete).
Client API would look like this:
Delete d = new Delete(ROW);
Put p = new Put(ROW);
//...
AtomicRowMutation arm = new AtomicRowMutation(ROW);
arm.add(p);
arm.add(d);
myHtable.atomicMutation(arm);

可以看到，這里的事務(wù)僅僅是針對某一行的一系列Put/Delete操作。不同行、不同表間一坨操作是無法放在一個事務(wù)中的。