Validity check (Default: basic validity check) : the validity check verifies that the data model follows the semantic rules for the model format.

validity check(默认值：basic validity check)：有效性检查，验证数据模型是否遵从模型格式的语义规则。

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By default, both Apache and IIS embed data in the ETag that dramatically reduces the odds of the validity test succeeding on web sites with multiple servers.

默认情况下，Apache和IIS都会把数据嵌入ETag中，这会显著减少多服务器间的文件验证冲突。

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