如何在cassandra做轮询？ [英] How to do polling in cassandra?

问题描述

我试图找到一种方法来查询一个cassandra数据库，但我是新的，我不知道如何。

I'm trying to find a way to do polling over a cassandra database, but I'm new at this and I don't know how.

说我有一个表用户像这样

Lets say I have a table "users" like this

-> users
    -> user_name
    -> gender
    -> state

，我想不断轮询，以便知道是否有新用户添加到表。

and I want to do polling constantly so I know if a new user was added to the table. How can I do that?

推荐答案

关系数据库中的标准方法将涉及执行SELECT， ID降序，所以最新的行总是首先返回，所以你可以看到是否匹配你的最后一个最新行并识别更改 - 在cassandra，这将不工作，因为没有WHERE子句，结果是有序的通过分区的令牌，这是（几乎肯定）随机。

The standard approach in a relational DB would involve doing a SELECT, ordering by some time-related ID descending, so that the newest row would always be returned first, so you could see if that matched your last 'newest row' and identify change - in cassandra, that won't work, because without a WHERE clause, the results are ordered by the partition's token, which is (almost certainly) random.

然后，解决方案是创建一个表有一个分区，划分。例如：

The solution, then, is to create a table that has a partition, where users are sorted within a given partition. For example:

CREATE TABLE user_buckets (
    bucket text,
    user_timestamp timeuuid,
    user_username text,
    PRIMARY KEY(bucket, user_timestamp)
) WITH CLUSTERING ORDER BY (user_timestamp DESC);

在这种情况下，你将写入users表和user_buckets表，是合理的（例如日期（YYYY） - 其中每个分区包含在该年中注册的所有用户，或日期（YYYYMMDD） - 其中每个分区包含在那天注册的所有用户），然后使用SELECT .. 。FROM user_buckets WHERE bucket =（current-bucket）AND user_timestamp>（您看到的最后一个时间戳）。

In this case, you would write into both the users table and the user_buckets table, with 'bucket' being something reasonable (such as date(YYYY) - where each partition contains all of the users registering in that year, or date(YYYYMMDD) - where each partition contains all of the users registering in that day), and then use SELECT ... FROM user_buckets WHERE bucket=(current-bucket) AND user_timestamp > (last timestamp you've seen).

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