Elasticsearch Query Phase

Elasticsearch query phase，内容来自 B 站中华石杉 Elasticsearch 顶尖高手系列课程核心知识篇，英文内容来自 Elasticsearch: The Definitive Guide [2.x]，内容似乎有些过时，但是我觉得底层原理应该大同小异，欢迎拍砖

Query Phase

During the initial query phase, the query is broadcast to a shard copy (a primary or replica shard) of every shard in the index. Each shard executes the search locally and builds a priority queue of matching documents.

1. 搜索请求发送到某一个 coordinate node，构构建一个 priority queue，长度以 paging 操作 from 和 size 为准，默认为 10

2. coordinate node 将请求转发到所有 shard，每个 shard 本地搜索，并构建一个本地的 priority queue

3. 各个 shard 将自己的 priority queue 返回给 coordinate node，并构建一个全局的 priority queue

coordinate node 根据 from 和 size 参数，构建一个 priority queue， 大小就是 from+size。

from = 0，size = 10，构建一个 0+10 大小的队列，

from = 10000，size = 10，构建一个 10000+10 大小的队列

coordinate node 将请求转发到这个 index 对应的所有 primary shard 和 replica shard 上。

接收到请求的每个 shard 其实都会构建一个 from+size 大小的本地的 priority queue。

当 from = 10000，size = 10，每个 shard 都会构建一个 10000+10 = 10010 大小的 priority queue。

每个 shard 将自己的 10010 条数据返回给 coordinate node，coordinate node 将所有 priority queue 进行 merge，merge 成一份 from+size 大小的 priority queue，全局排序后的 queue，放到自己的 queue 中。

然后 coordinate node 从自己的 priority queue 中，取出当前需要获取的那一页数据，比如从第 10000 条到 10010 条。

1. The client sends a search request to Node 3, which creates an empty proority queue of size from+size.

2. Node 3 forwards the search request to a primary or replica copy of every shard in the index. Each shard executes the query locally and adds the results into a local stored priority queue of size from+size.

3. Each shard returns the doc IDs and sort values of all the docs in its priority queue to the docs in its priority queue to the coordinating node, Node 3, which merges these values into its own priority queue to produce a globally sorted list of results.

When a search request is sent to a node, that node becomes the coordinating node. It is the job of this node to broadcast the search request to all involved shards, and  to gather their responses into a globally sorted result set that it can return to the client.

Each shard executes the query locally and builds a sorted priority queue of length from+size - in other words, enough results to satisfy the global search request all by itself. It returns a lightweight list of results to the coordinating node, which contains just the doc IDs and any values required for sorting, such as the _score.

The coordinating node merges these shard-level results into its own sorted priority queue, which represents the globally sorted result set.

replica shard 如何提升搜索吞吐量

一次请求要打到所有 shard 的一个 replica/primary 上去，如果每个 shard 都有多个 replica，那么同时并发过来的搜索请求可以同时打到其他的 replica 上去

search requests can be handled by a primary shard or by any of its replicas. This is how more replicas (when combined with more hardware) can increase search throughput. A coordinating node will round-robin through all shard copies on subsequent requests in order to spread the load.