January 2018
Beginner to intermediate
234 pages
7h 8m
Chinese
Content preview from Kafka权威指南
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42
｜
第
3
章
字段来代替它。
新的
schema
如下：
{"namespace": "customerManagement.avro",
"type": "record",
"name": "Customer",
"fields": [
{"name": "id", "type": "int"},
{"name": "name", "type": "string"},
{"name": "email", "type": ["null", "string"], "default": "null"}
]
}
更新到新版的
schema
后，旧记录仍然包含 faxNumber 字段，而新记录则包含 email 字段。
部分负责读取数据的应用程序进行了升级，那么它们是如何处理这些变化的呢？
在应用程序升级之前，它们会调用类似 getName()、getId() 和 getFaxNumber() 这样的方
法。如果碰到使用新
schema
构建的消息，getName() 和 getId() 方法仍然能够正常返回，
但 getFaxNumber() 方法会返回 null，因为消息里不包含传真号码。
在应用程序升级之后，getEmail() 方法取代了 getFaxNumber() 方法。如果碰到一个使用旧
schema
构建的消息，那么
getEmail() 方法会返回
null
，因为旧消息不包含邮件地址。
现在可以看出使用
Avro
的好处了：我们修改了消息的
schema
，但并没有更新所有负责读
取数据的应用程序，而这样仍然不会出现异常或阻断性错误，也不需要对现有数据进行大 ...
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