November 2021
Intermediate to advanced
410 pages
12h 1m
Chinese
Content preview from MongoDB权威指南（第3版）
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130
｜
第
6
章
]);
while (true) {
if ($iterator->valid()) {
$document = $iterator->current();
printf("Consumed document created at: %s\n", $document->createdAt);
}
$iterator->next();
}
这个游标会不断地对查询结果进行处理或是等待新的查询结果，直到游标超时或查询操作
被人为中止。
6.4
TTL
索引
上一节已经提到，固定集合对于其内容何时被覆盖拥有有限的控制。如果需要更加灵活的
过期机制，那么可以使用
TTL
索引
，它允许为每一个文档设置一个超时时间。当一个文档
达到其预设的过期时间之后就会被删除。这种类型的索引对于类似会话保存这样的缓存场
景非常有用。
可以在
createIndex
的第二个参数中指定
"expireAfterSeconds"
选项来创建
TTL
索引：
> // 超时时间为24小时
> db.sessions.createIndex({"lastUpdated" : 1}, {"expireAfterSeconds" : 60*60*24})
这会在
"lastUpdated"
字段上建立一个
TTL
索引。如果一个文档的
"lastUpdated"
字段存
在并且它的值是日期类型，那么当服务器端时间比文档的
"lastUpdated"
字段的时间晚
"expireAfterSeconds"
秒时，文档就会被删除。
为了防止活跃的会话被删除，可以在会话上有活动发生时将 ...
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