总结一下对象存储元数据与文件之间的关系

[liqul]

Data model
metadata -> object
metadata: key + others + create_time + pointer

Lock based

insert：

1. lock the key, even if the key is not presented
2. read by the key
   2.1 if does not exist, insert
   2.2 if exists, update the metadata (including create_time), backup the old pointer
3. es as a special case, no lock but version control

delete:
= insert a tombstone with an empty pointer

update:
only update non-key fields

search:
since each key has only one entry, just find it

MVCC

insert:
insert with current timestamp as version. if two insertions with the same version, one fails

delete:
= insert a tombstone with an empty pointer

update:
= insert a new version with a new timestamp. and other unmodified fields the same with the latest version

search:
for each key, get the version with the latest timestamp

[liqul]

mysql:
unique key = key + version (1, 2, 3, ...)
read the last version, if does not exist, insert with version = 1; if exists, start transaction to insert the new version (last version + 1), remove the last version, and backup the old pointer to a special table.
isolation level = read committed

[liqul]

mysql can be viewed as an optimization of mvcc

[liqul]

完整讨论对象服务的思路