milvus/docs/developer_guides/chap07_query_node.md

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## 8. Query Node
#### 8.1 Overview
#### 8.2 Collection Replica
$collectionReplica$ contains a in-memory local copy of persistent collections. In common cases, the system has multiple query nodes. Data of a collection will be distributed across all the available query nodes, and each query node's $collectionReplica$ will maintain its own share (only part of the collection).
Every replica tracks a value called tSafe which is the maximum timestamp that the replica is up-to-date.
###### 8.1.1 Collection
``` go
type Collection struct {
Name string
Id uint64
Fields map[string]FieldMeta
SegmentsId []uint64
cCollectionSchema C.CCollectionSchema
}
```
###### 8.1.2 Field Meta
```go
type FieldMeta struct {
Name string
Id uint64
IsPrimaryKey bool
TypeParams map[string]string
IndexParams map[string]string
}
```
###### 8.1.3 Segment
``` go
type Segment struct {
Id uint64
ParitionName string
CollectionId uint64
OpenTime Timestamp
CloseTime Timestamp
NumRows uint64
cSegment C.CSegmentBase
}
```
#### 8.3 Data Manipulation Service
```go
type manipulationService struct {
ctx context.Context
pulsarURL string
fg *flowgraph.TimeTickedFlowGraph
msgStream *msgstream.PulsarMsgStream
node *QueryNode
}
```