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2023-02-21-tiflash-supports-stale-read.md

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TiFlash supports stale read

Introduction

This proposal aims to make TiFlash support Stale Read.

Motivation or Background

Currently TiKV has already supported stale read, but TiFlash does not. There are two benefits of TiFlash to support stale read:

  • Reduces the read/wait index duration during reading. Small queries can run faster.
  • Avoid calling read_index API of TiKV leaders, to reduce the impact of OLTP workloads.

Goals

  1. TiFlash supports stale read, including AS OF TIMESTAMP, TIDB_BOUNDED_STALENES, tidb_read_staleness, tidb_external_ts.
  2. Since TiFlash MPP may involve the computation among all TiFlash nodes, supporting using stale read to read TiFlash data nearby is not the goal of this RFC.

Design

  1. Check the safe ts before read_index, if the stale read requirements are met (safe ts > read tso), read_index can be skipped.
  2. The assumption of the current MPP framework is that start_ts is globally unique and increasing in most cases. But in stale read scenarios, users can specify the timestamp, so the start_ts of different MPP queries issued by TiDB may be repeated. So the MPP framework needs to be modified. Specifically, we need to use a globally unique query_id to replace start_ts as the identifier of MPP queries. The query_id consists of three parts of ids:

  • The local time of TiDB when the query is initiated: query_ts.

  • An id incremented in TiDB local memory: local_query_id.

  • ETCD guarantees a globally unique serverID for each TiDB server: server_id.

    And The MinTSOScheduler will schedule MPP queries in the order of query_ts in most cases.

  1. TiDB client-go supports to get and cache the min_safe_ts from TiFlash stores. client-go will query and cache min_safe_ts from all TiKV and TiFlash stores for all of their local regions periodically. When users use TIDB_BOUNDED_STALENES or tidb_read_staleness to specify the stale read time range, TiDB will get min_safe_ts from the client-go cache, and generate the start_ts of the query based on min_safe_ts.