作者
digoal
日期
2020-07-23
标签
PostgreSQL , 只读 , alter system
背景
PostgreSQL 14支持将数据库设置为只读模式.
应用场景:
迁移数据前、割接业务前, 防止数据变更. (但是增量迁移对用户更友好, 停机时间短)
只读角色可能应用场景更广, 例如给DBA使用或者开发人员个人使用的账号, 防止误操作.
```
Quick demo:
We have few active sessions, section 1 has performed some writes and stayed
in the
idle state for some time, in between in session 2 where superuser
successfully changed
system state in read-only via ALTER SYSTEM READ ONLY command which kills
session 1. Any other backend who is trying to run write transactions
thereafter will see
a read-only system error.
------------- SESSION 1 -------------
session_1=# BEGIN;
BEGIN
session_1=*# CREATE TABLE foo AS SELECT i FROM generate_series(1,5) i;
SELECT 5
------------- SESSION 2 -------------
session_2=# ALTER SYSTEM READ ONLY;
ALTER SYSTEM
------------- SESSION 1 -------------
session_1=*# COMMIT;
FATAL: system is now read only
HINT: Cannot continue a transaction if it has performed writes while
system is read only.
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Succeeded.
------------- SESSION 3 -------------
session_3=# CREATE TABLE foo_bar (i int);
ERROR: cannot execute CREATE TABLE in a read-only transaction
------------- SESSION 4 -------------
session_4=# CHECKPOINT;
ERROR: system is now read only
System can put back to read-write mode by "ALTER SYSTEM READ WRITE" :
------------- SESSION 2 -------------
session_2=# ALTER SYSTEM READ WRITE;
ALTER SYSTEM
------------- SESSION 3 -------------
session_3=# CREATE TABLE foo_bar (i int);
CREATE TABLE
------------- SESSION 4 -------------
session_4=# CHECKPOINT;
CHECKPOINT
```
参考
https://www.postgresql.org/message-id/flat/CAAJ_b97KZzdJsffwRK7w0XU5HnXkcgKgTR69t8cOZztsyXjkQw@mail.gmail.com
https://www.postgresql.org/docs/devel/sql-altersystem.html
PostgreSQL 许愿链接
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PostgreSQL 解决方案集合
德哥 / digoal's github - 公益是一辈子的事.
