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Summary

This article focuses on mainly introduces the importance of handoff and load balance segment features, as well as proposing a scenario for functions, and proposes a solution to handoff and load balance segments segment without interrupting queriesthe query.

Motivation

The query node of milvus currently has the following issues:

1. In the load collection/partition process, query nodes not only load the sealed segments, but also watch dmChannels so that they can receive vectors inserted in real time, and if new vectors are continuously inserted after the load collection operation, the query nodes will create a large number of growing segments, which can only service query requests in a near-brutal manner. At the same time, data node may have flushed these vectors into persistent storage and created the corresponding vector index.
2. Query nodes may experience unbalanced sealed segments during dynamic expansionIn the process of dynamic expansion and contraction of query nodes, there may be an imbalance in the number of sealed segments on different nodes. This results in a relatively slow query for high-load query node, which will becomes the final bottleneck for queries.

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Query node logically manages the sealed segments and the growing segments in two parts, historical and streaming module. As shown in the following figure, G3 represents the growing segment 3, and S8 represents the sealed segment 8.

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However, there are several scenarios issues to consider be considered in the implementation:

1. What if some query node associated with the handoff or load balance process is suddenly down
2. If there is a sudden downtime on query coord, how to properly resume the handoff and load balance tasks after the restart
3. If the same sealed segment exists on different query nodes at the same time, whether it will have an impact on the final query results
4. Different query nodes may not process query messages at the same speed, and how to ensure that different query nodes have the same global sealed segmentIDs when processing the same query message. If not handled well, it may cause the query to time out

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Prerequisite: The cache meta of querycoord records which sealed segment, growing segmentsegments and growing segments are loaded on each query node, and which dmchannels are on each querynodewatched by the query node. These metas will be are stored in etcd synchronouslyat the same time.

1.First of all, querycoord automatically generates handoff tasks and load balance tasks and writes them into etcd, and then clears them from etcd until the task is completed, ensuring that querycoord can accurately restore the task after restarting.

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4.After query coord successfully loads sealed segments on querynode, update the sealed segments list of each querynode in cache meta. For example, after balancing sealed segment 8 from node 1 to node 2, the meta of node1 changes from {S6, S7,S8} to { S6, S7}, node2's meta is changed from {S5} to {S5, S8}. If the query node suddenly goes down, directly follow the meta records of the coord to recover the query node. While querycoord updates the meta, it broadcasts sends a change info of sealed segments to querychannel. The proto of sealed segments change info is as follows:

message SealedSegmentsChangeInfo {
 common.MsgBase base = 1;
 int64 online_nodeID = 2;
 repeated SegmentInfo online_segments = 3;
 int64 offline_nodeID = 4;
 repeated SegmentInfo offline_segments = 5;
}

5.Query coord updates querychannelInfo in meta, mainly updating two content, one is global sealed segments, for example, after handoff growing segment S3, global sealed segments change from {S5, S6, S7, S8} to {S3, S5, S6 , S7, S8}, the other is seek_position, that is, the position of the previous SealedSegmentsChangeInfo in the querychannel. Each query node will receive these two initialization parameters when the querychannel is watched. This ensures that when the specified query message is processed , The global sealed segments received by all querynodes are consistent.

message QueryChannelInfo {
 int64 collectionID = 1;
 string query_channelID = 2;
 string query_result_channelID = 3;
 repeated SegmentInfo global_sealed_segments = 4;
 internal.MsgPosition seek_position = 5;
}

6.After querynode consumes SealedSegmentsChangeInfo from querychannel, it will do the following three things:

• Update the list of gloabal sealed segments in the query collection according to online_segments and offline_segments in SealedSegmentsChangeInfo
• If the growing segment ID in streaming appears in the list of global sealed segments, clear the corresponding growing segments from streaming module
• If the current nodeID==offline_nodeID, clear the sealed segments in offline_segments from historical module

Test Plan

• Unit tests

• CI tests