_id（在mongo）中的重复文件 [英] Duplicate documents on _id (in mongo)

问题描述

我有一个分层的mongo系列，超过一百五十万个文件。我使用_id列作为分片键，该列中的值是整数（而不是ObjectIds）。

我对这个集合做了很多写操作，使用Perl驱动程序（插入，更新，删除，保存）和mongoimport。

我的问题是，在某种程度上，我在同一个_id上有重复的文档。

我删除了重复的内容，但其他的仍然出现。

你有什么想法可以来自哪里，或者我应该开始看什么？
（另外，我试图复制一个较小的测试集合，但不会插入任何重复项，无论执行什么写操作）。

解决方案

这实际上并不是Perl驱动程序的问题，它与分片的特性有关。 MongoDB只能在创建时在单个分片上的文档中强制执行唯一性，因此默认索引不需要唯一性。

在 MongoDB：配置Sharding 文档特别提到：

• 当您分割集合时，必须指定分片键。如果集合中有数据，mongo将需要先创建索引（加快分块过程）;




• 您可以使用{unique：true}选项确保基础索引强制执行唯一性，只要唯一索引是分片键的前缀。




• 如果未使用unique：true选项，则分片键不具有是唯一的。

I have a sharded mongo collection, with over 1.5 mil documents. I use the _id column as a shard key, and the values in this column are integers (rather than ObjectIds).

I do a lot of write operations on this collection, using the Perl driver (insert, update, remove, save) and mongoimport.

My problem is that somehow, I have duplicate documents on the same _id. From what I've read, this shouldn't be possible.

I've removed the duplicates, but others still appear.

Do you have any ideas where could they come from, or what should I start looking at? (Also, I've tried to replicate this on a smaller, test collection, but no duplicates are inserted, no matter what write operation I perform).

解决方案

This actually isn't a problem with the Perl driver .. it is related to the characteristics of sharding. MongoDB is only able to enforce uniqueness among the documents located on a single shard at the time of creation, so the default index does not require uniqueness.

In the MongoDB: Configuring Sharding documentation there is specific mention that:

• When you shard a collection, you must specify the shard key. If there is data in the collection, mongo will require an index to be created upfront (it speeds up the chunking process); otherwise, an index will be automatically created for you.

• You can use the {unique: true} option to ensure that the underlying index enforces uniqueness so long as the unique index is a prefix of the shard key.

• If the "unique: true" option is not used, the shard key does not have to be unique.

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