Deploy the HTAP Storage Engine Tiflash for an Existing TiDB Cluster

This document describes how to add or remove the TiDB HTAP storage engine TiFlash for an existing TiDB cluster on Kubernetes. As a columnar storage extension of TiKV, TiFlash provides both good isolation level and strong consistency guarantee.

Note

If a TiDB cluster has not been deployed yet, instead of referring to this document, you can configure a TiDB cluster on Kubernetes with the TiFlash-related parameters, and then deploy the TiDB cluster.

Usage scenarios

This document is applicable to scenarios in which you already have a TiDB cluster and need to use TiDB HTAP capabilities by deploying TiFlash, such as the following:

Hybrid workload scenarios with online real-time analytic processing
Real-time stream processing scenarios
Data hub scenarios

Deploy TiFlash

If you need to deploy TiFlash for an existing TiDB cluster, do the following:

Note

If your server does not have an external network, you can download the required Docker image on the machine with an external network, upload the Docker image to your server, and then use docker load to install the Docker image on the server. For details, see deploy the TiDB cluster.

Edit the TidbCluster Custom Resource (CR):

kubectl edit tc ${cluster_name} -n ${namespace}

Add the TiFlash configuration as the following example:

spec:
tiflash:
    baseImage: pingcap/tiflash
    maxFailoverCount: 0
    replicas: 1
    storageClaims:
    - resources:
        requests:
          storage: 100Gi
      storageClassName: local-storage

TiFlash supports mounting multiple Persistent Volumes (PVs). If you want to configure multiple PVs for TiFlash, configure multiple resources in tiflash.storageClaims, each resources with a separate requests.storage and storageClassName. For example:
```
tiflash:
    baseImage: pingcap/tiflash
    maxFailoverCount: 0
    replicas: 1
    storageClaims:
    - resources:
        requests:
            storage: 100Gi
      storageClassName: local-storage
    - resources:
        requests:
            storage: 100Gi
      storageClassName: local-storage
```
Note
- When deploying TiFlash for the first time, it is recommended that you plan how many PVs are required and configure the number of resources items in storageClaims accordingly.
- Once the deployment of TiFlash is completed, if you need to mount additional PVs for TiFlash, updating storageClaims directly to add disks does not take effect. This is because TiDB Operator manages TiFlash by creating a StatefulSet, and the StatefulSet does not support modifying volumeClaimTemplates after being created.
Configure the relevant parameters of spec.tiflash.config in TidbCluster CR. For example:
```
spec:
  tiflash:
    config:
      config: |
        [flash]
          [flash.flash_cluster]
            log = "/data0/logs/flash_cluster_manager.log"
        [logger]
          count = 10
          level = "information"
          errorlog = "/data0/logs/error.log"
          log = "/data0/logs/server.log"
```
For more TiFlash parameters that can be configured, refer to TiFlash Configuration Documentation.
Note
For different TiFlash versions, note the following configuration differences:
- If TiFlash version <= v4.0.4, you need to set spec.tiflash.config.config.flash.service_addr to ${clusterName}-tiflash-POD_NUM.${clusterName}-tiflash-peer.${namespace}.svc:3930 in TidbCluster CR, where ${clusterName} and ${namespace} need to be replaced according to the real case.
- If TiFlash version >= v4.0.5, there is no need to manually configure spec.tiflash.config.config.flash.service_addr.
- If you upgrade from TiFlash v4.0.4 or an earlier version to TiFlash v4.0.5 or a later version, you need to delete the configuration of spec.tiflash.config.config.flash.service_addr from the TidbCluster CR.

Adding PVs to TiFlash

Once the deployment of TiFlash is completed, to add PVs for TiFlash, you need to update the storageClaims to add disks, and then manually delete the TiFlash StatefulSet. The following are the detailed steps.

Warnings:
Deleting the TiFlash StatefulSet makes the TiFlash cluster unavailable during the deletion and affects related business. You must be cautious about whether to do the following.

Edit the TidbCluster Custom Resource (CR).

kubectl edit tc ${cluster_name} -n ${namespace}

TiDB Operator automatically mounts PVs in the order of the items in the storageClaims list. If you need to add more resources items to TiFlash, make sure to append new items only to the end of the original items, and DO NOT modify the order of the original items. For example:

tiflash:
    baseImage: pingcap/tiflash
    maxFailoverCount: 0
    replicas: 1
    storageClaims:
    - resources:
        requests:
            storage: 100Gi
      storageClassName: local-storage
    - resources:
        requests:
            storage: 100Gi
      storageClassName: local-storage
    - resources: #newly added
        requests:  #newly added
            storage: 100Gi  #newly added
      storageClassName: local-storage  #newly added

Manually delete the TiFlash StatefulSet by running the following command. Then, wait for the TiDB Operator to recreate the TiFlash StatefulSet.
```
kubectl delete sts -n ${namespace} ${cluster_name}-tiflash
```

Remove TiFlash

If your TiDB cluster no longer needs the TiDB HTAP storage engine TiFlash, take the following steps to remove TiFlash:

Adjust the number of replicas of the tables replicated to the TiFlash cluster.
To completely remove TiFlash, you need to set the number of replicas of all tables replicated to the TiFlash to 0.
1. To connect to the TiDB service, refer to the steps in Access the TiDB Cluster on Kubernetes.
2. To adjust the number of replicas of the tables replicated to the TiFlash cluster, run the following command:
```
alter table <db_name>.<table_name> set tiflash replica 0;
```
Wait for the TiFlash replicas of the related tables to be deleted.
Connect to the TiDB service and run the following command. If you can not find the replication information of the related tables, it means that the replicas are deleted:
```
SELECT * FROM information_schema.tiflash_replica WHERE TABLE_SCHEMA = '<db_name>' and TABLE_NAME = '<table_name>';
```

To remove TiFlash Pods, run the following command to modify spec.tiflash.replicas to 0:

kubectl patch tidbcluster ${cluster_name} -n ${namespace} --type merge -p '{"spec":{"tiflash":{"replicas": 0}}}

Check the state of TiFlash Pods and TiFlash stores.

Run the following command to check whether you delete the TiFlash Pod successfully:
```
kubectl get pod -n ${namespace} -l app.kubernetes.io/component=tiflash,app.kubernetes.io/instance=${cluster_name}
```
If the output is empty, it means that you delete the Pod of the TiFlash cluster successfully.

To check whether the stores of the TiFlash are in the Tombstone state, run the following command:

kubectl get tidbcluster ${cluster_name} -n ${namespace} -o yaml

The value of the status.tiflash field in the output result is similar to the example below.

tiflash:
    ...
    tombstoneStores:
    "88":
        id: "88"
        ip: basic-tiflash-0.basic-tiflash-peer.default.svc
        lastHeartbeatTime: "2020-12-31T04:42:12Z"
        lastTransitionTime: null
        leaderCount: 0
        podName: basic-tiflash-0
        state: Tombstone
    "89":
        id: "89"
        ip: basic-tiflash-1.basic-tiflash-peer.default.svc
        lastHeartbeatTime: "2020-12-31T04:41:50Z"
        lastTransitionTime: null
        leaderCount: 0
        podName: basic-tiflash-1
        state: Tombstone

Only after you delete all Pods of the TiFlash cluster successfully and all the TiFlash stores have changed to the Tombstone state, can you perform the next operation.

Delete the TiFlash StatefulSet.
1. To modify the TidbCluster CR and delete the spec.tiflash field, run the following command:
```
kubectl patch tidbcluster ${cluster_name} -n ${namespace} --type json -p '[{"op":"remove", "path":"/spec/tiflash"}]'
```
2. To delete the TiFlash StatefulSet, run the following command:
```
kubectl delete statefulsets -n ${namespace} -l app.kubernetes.io/component=tiflash,app.kubernetes.io/instance=${cluster_name}
```
3. To check whether you delete the StatefulSet of the TiFlash cluster successfully, run the following command:
```
kubectl get sts -n ${namespace} -l app.kubernetes.io/component=tiflash,app.kubernetes.io/instance=${cluster_name}
```
  If the output is empty, it means that you delete the StatefulSet of the TiFlash cluster successfully.
(Optional) Delete PVC and PV.
If you confirm that you do not use the data in TiFlash, and you want to delete the data, you need to strictly follow the steps below to delete the data in TiFlash:
1. Delete the PVC object corresponding to the PV
```
kubectl delete pvc -n ${namespace} -l app.kubernetes.io/component=tiflash,app.kubernetes.io/instance=${cluster_name}
```
2. If the PV reclaim policy is Retain, the corresponding PV is still retained after you delete the PVC object. If you want to delete the PV, you can set the reclaim policy of the PV to Delete, and the PV can be deleted and recycled automatically.
```
kubectl patch pv ${pv_name} -p '{"spec":{"persistentVolumeReclaimPolicy":"Delete"}}'
```
  In the above command, ${pv_name} represents the PV name of the TiFlash cluster. You can check the PV name by running the following command:
```
kubectl get pv -l app.kubernetes.io/component=tiflash,app.kubernetes.io/instance=${cluster_name}
```

Deploy the HTAP Storage Engine Tiflash for an Existing TiDB Cluster

Usage scenarios

Deploy TiFlash

Adding PVs to TiFlash

Remove TiFlash

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