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在 Elasticsearch 中，slowlog（慢日志）是用来记录查询和索引操作的性能数据，帮助你诊断性能瓶颈。你可以为查询 (search slowlog) 和索引 (index slowlog) 配置慢日志。

数据准备

POST /products/_doc/1
{"product_name": "Apple iPhone 15","price": 999.99,"category": "Smartphones","rating": 4.5,"discount": 0.1,"availability": true,"stock": 50,"release_date": "2023-09-01"
}POST /products/_doc/2
{"product_name": "Samsung Galaxy S24","price": 899.99,"category": "Smartphones","rating": 4.3,"discount": 0.05,"availability": true,"stock": 30,"release_date": "2023-10-10"
}POST /products/_doc/3
{"product_name": "Dell XPS 13","price": 1199.99,"category": "Laptops","rating": 4.8,"discount": 0.15,"availability": true,"stock": 20,"release_date": "2023-08-20"
}POST /products/_doc/4
{"product_name": "Sony WH-1000XM5","price": 349.99,"category": "Headphones","rating": 4.7,"discount": 0.2,"availability": false,"stock": 0,"release_date": "2023-06-15"
}POST /products/_doc/5
{"product_name": "Apple MacBook Air","price": 999.00,"category": "Laptops","rating": 4.6,"discount": 0.1,"availability": true,"stock": 15,"release_date": "2023-11-01"
}

检索请求实现流程

其中，query阶段的核心步骤如下。

❑客户端发送请求到协调节点。
❑协调节点转发请求到索引的每个主或副本分片。
❑分片本地查询完成后，将结果添加到本地的优先队列。
❑每个分片将本地结果返回给协调节点，协调节点合并完成后，形成全局排序列表。

fetch阶段的核心步骤如下。

❑协调节点接收到客户端请求后，将GET请求（query阶段形成的全局排序列表结果数据）转发给相关节点。
❑接收到请求的节点向协调节点返回结果数据。
❑待全部结果数据都返回后，协调节点将结果返回给客户端。

修改现有索引的慢日志配置 (动态配置)

你也可以通过 Elasticsearch 的 动态设置 API 来为已存在的索引启用慢日志配置，而不需要重启 Elasticsearch。

示例：动态配置查询慢日志

PUT /products/_settings
{"settings": {"index.search.slowlog.threshold.query.warn": "10s",  "index.search.slowlog.threshold.query.info": "5s",   "index.search.slowlog.threshold.query.debug": "0s",  "index.search.slowlog.threshold.query.trace": "500ms", "index.search.slowlog.threshold.fetch.warn": "1s",   "index.search.slowlog.threshold.fetch.info": "500ms","index.search.slowlog.threshold.fetch.debug": "0s","index.indexing.slowlog.level": "debug"  }
}GET /products/_search

示例：动态配置索引慢日志

PUT /products/_settings
{"settings": {"index.indexing.slowlog.threshold.index.warn": "10s","index.indexing.slowlog.threshold.index.info": "5s","index.indexing.slowlog.threshold.index.debug": "0s","index.indexing.slowlog.level": "trace"}
}

总结

1. 启用查询慢日志：可以通过设置查询慢日志阈值来启用，通常记录执行时间较长的查询。
2. 启用索引慢日志：记录索引、删除等操作的耗时，帮助定位性能瓶颈。

使用慢日志能够帮助你监控和调优 Elasticsearch 的性能，发现并优化长时间运行的查询和索引操作。