OpenERP concurrent transaction
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问题：
多线程进行进行数据库操作，主要调用cr进行直接操作的时候，抛出错误：InternalError: current transaction is aborted, commands ignored until end of transaction block.
啥意思呢，就是oe并发的transaction会被放弃。
步科大大对此问题进行了全面权威总结：
首先，看代码（sql_db.py）：
class Cursor(object):
"""Represents an open transaction to the PostgreSQL DB backend,
acting as a lightweight wrapper around psycopg2's
psycopg1cursorobjects.
Cursoris the object behind thecrvariable used all
over the OpenERP code.
.. rubric:: [color=red]Transaction Isolation[/color]
One very important property of database transactions is the
level of isolation between concurrent transactions.
The SQL standard defines four levels of transaction isolation,
ranging from the most strict Serializable level, to the least
strict Read Uncommitted level. These levels are defined in
terms of the phenomena that must not occur between concurrent
transactions, such as dirty read, etc.
In the context of a generic business data management software
such as OpenERP, [color=red]we need the best guarantees that no data
corruption can ever be cause by simply running multiple
transactions in parallel[/color]. Therefore, the preferred level would
be the serializable level, which ensures that a set of
transactions is guaranteed to produce the same effect as
running them one at a time in some order.
However, most database management systems implement a limited
serializable isolation in the form of
snapshot isolation <http://en.wikipedia.org/wiki/Snapshot_isolation>_,
providing most of the same advantages as True Serializability,
with a fraction of the performance cost.
With PostgreSQL up to version 9.0, this snapshot isolation was
the implementation of both theREPEATABLE READand
SERIALIZABLElevels of the SQL standard.
As of PostgreSQL 9.1, the previous snapshot isolation implementation
was kept forREPEATABLE READ, while a newSERIALIZABLE
level was introduced, providing some additional heuristics to
detect a concurrent update by parallel transactions, and forcing
one of them to rollback.
OpenERP implements its own level of locking protection
for transactions that are highly likely to provoke concurrent
updates, such as stock reservations or document sequences updates.
Therefore we mostly care about the properties of snapshot isolation,
but we don't really need additional heuristics to trigger transaction
rollbacks, as we are taking care of triggering instant rollbacks
ourselves when it matters (and we can save the additional performance
hit of these heuristics).
As a result of the above, we have selectedREPEATABLE READas
the default transaction isolation level for OpenERP cursors, as
it will be mapped to the desiredsnapshot isolationlevel for
all supported PostgreSQL version (8.3 - 9.x).
Note: up to psycopg2 v.2.4.2, psycopg2 itself remapped the repeatable
read level to serializable before sending it to the database, so it would
actually select the new serializable mode on PostgreSQL 9.1. Make
sure you use psycopg2 v2.4.2 or newer if you use PostgreSQL 9.1 and
the performance hit is a concern for you.
.. attribute:: cache
Cache dictionary with a "request" (-ish) lifecycle, only lives as
long as the cursor itself does and proactively cleared when the
cursor is closed.
This cache should only be used to store repeatable reads as it
ignores rollbacks and savepoints, it should not be used to store
any data which may be modified during the life of the cursor.
"""
2. 以上是cr应用的全部说明，可能部分朋友看不太懂。没关系，看下面的就好了，（by 步科总结）
1. OE使用了最高的数据库隔离级别 ISOLATION_LEVEL_REPEATABLE_READ,因此事物(transaction)不能并发。
2. 多线程的情况下，其实很难避免事物的并发。
3. 在一次请求通常被OE处理为一个transaction块(savepoint)
4. 但是autocommit可以让每条sql语句都是一个事物(transaction)。这样pg会尽快执行完。从而降低事物（transaction)并发的概率。
so,编程中注意：
1. OE数据库操作，尽量避免多线程。
2. 如果一定要用，则尽量少开些线程。不够的话，请加消息队列缓冲。
3. 使用autocommit
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谢谢！
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