英文:
How does TDengine handle data replication and high availability in a distributed environment?
问题
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TDengine在分布式环境中提供了几种处理数据复制和高可用性的机制：
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复制：TDengine支持集群中节点之间的数据复制。当一个节点接收到数据时，它可以将数据复制到集群中的其他节点，以确保数据冗余和容错性。
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集群：TDengine支持集群，允许多个节点作为单个逻辑单元一起工作。集群通过将数据和处理分布到多个节点上来提供高可用性和容错性。
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数据备份和恢复：TDengine提供了备份和恢复机制，以确保在硬件或软件故障的情况下数据的可用性。备份可以定期进行并存储在单独的位置，并且可以在数据丢失的情况下进行恢复。
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负载均衡：TDengine支持负载均衡，它将数据和处理分布到多个节点上，以确保没有单个节点过载。负载均衡有助于提高性能并减少停机的风险。
在我们尝试和期望的方面，我们在具有多个节点的分布式环境中测试了TDengine的复制和集群功能。我们期望TDengine通过复制数据并分布处理到多个节点上来提供高可用性和容错性。我们的测试表明，在这方面TDengine表现良好，在分布式环境中提供可靠的数据复制和高可用性。
英文:
TDengine provides several mechanisms to handle data replication and high availability in a distributed environment:
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Replication: TDengine supports data replication between nodes in a cluster. When a node receives data, it can replicate the data to other nodes in the cluster to ensure data redundancy and fault tolerance.
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Clustering: TDengine supports clustering, which allows multiple nodes to work together as a single logical unit. Clustering provides high availability and fault tolerance by distributing data and processing across multiple nodes.
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Data backup and restore: TDengine provides backup and restore mechanisms to ensure data availability in case of hardware or software failures. Backups can be taken at regular intervals and stored in a separate location, and can be restored in case of data loss.
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Load balancing: TDengine supports load balancing, which distributes data and processing across multiple nodes to ensure that no single node is overloaded. Load balancing helps to improve performance and reduce the risk of downtime.
In terms of what we tried and what we were expecting, we tested TDengine's replication and clustering features in a distributed environment with multiple nodes. We expected that TDengine would provide high availability and fault tolerance by replicating data and distributing processing across multiple nodes. Our tests showed that TDengine performed well in this regard, providing reliable data replication and high availability in a distributed environment.
答案1
得分: 0
高可用性的 vnode 可以通过 TDengine 中的复制实现。
TDengine 集群可以有多个数据库。每个数据库都有与之关联的一些 vnode。可以为每个数据库配置不同数量的副本。创建数据库时，使用参数 replica 来指定副本的数量。replica 的默认值为 1。自然地，单个副本无法保证高可用性，因为如果一个节点宕机，数据服务将不可用。需要注意的是，集群中的 dnodes 数量不能低于任何数据库设置的副本数，否则创建表操作将失败，并显示错误消息 "more dnodes are needed"。下面的 SQL 语句用于创建名为 "demo" 的数据库，其中指定了 3 个副本。
CREATE DATABASE demo replica 3;
数据库中的数据分为多个分片，并存储在多个 vgroup 中。每个 vgroup 中的 vnode 数量由数据库设置的副本数确定。每个 vgroup 中的 vnode 存储完全相同的数据。为了实现高可用性，vgroup 中的 vnode 必须位于不同的 dnodes 上，这些 dnodes 位于不同的主机上。只要 vgroup 中超过半数的 vnode 处于在线状态，该 vgroup 就能提供数据访问。否则，该 vgroup 无法提供数据访问，无法读取或插入数据。
一个 dnode 中可能包含多个数据库的数据。当一个 dnode 宕机时，可能会影响多个数据库。尽管从理论上讲，如果 vgroups 中超过半数的 vnode 处于在线状态，集群将提供数据访问以进行读取或插入数据，但由于 vnode 和 dnode 之间可能存在复杂的映射关系，很难保证在超过半数的 dnodes 处于在线状态时集群将正常工作。
Mnode 的高可用性
每个 TDengine 集群由 mnode 管理，mnode 是 taosd 的一个模块。为了实现 mnode 的高可用性，可以在系统中配置多个 mnode。当从头开始启动 TDengine 集群时，只有一个 mnode，然后可以使用 create mnode 命令并启动相应的 dnode 来添加更多的 mnode。
SHOW MNODES;
上述命令可以显示所有 mnode 的终端和角色/状态（leader、follower、candidate、offline）。在集群中启动第一个 dnode 时，必须在该 dnode 中有一个 mnode。如果没有至少一个 mnode，集群将无法工作。
从 TDengine 3.0.0 开始，使用 RAFT 协议来保证高可用性，因此 mnode 的数量应为 1 或 3。
英文:
High availability of vnode can be achieved through replicas in TDengine.
A TDengine cluster can have multiple databases. Each database has a number of vnodes associated with it. A different number of replicas can be configured for each DB. When creating a database, the parameter replica is used to specify the number of replicas. The default value for replica is 1. Naturally, a single replica cannot guarantee high availability since if one node is down, the data service is unavailable. Note that the number of dnodes in the cluster must NOT be lower than the number of replicas set for any DB, otherwise the create table operation will fail with error "more dnodes are needed". The SQL statement below is used to create a database named "demo" with 3 replicas.
CREATE DATABASE demo replica 3;
The data in a DB is divided into multiple shards and stored in multiple vgroups. The number of vnodes in each vgroup is determined by the number of replicas set for the DB. The vnodes in each vgroup store exactly the same data. For the purpose of high availability, the vnodes in a vgroup must be located in different dnodes on different hosts. As long as over half of the vnodes in a vgroup are in an online state, the vgroup is able to provide data access. Otherwise the vgroup can't provide data access for reading or inserting data.
There may be data for multiple DBs in a dnode. When a dnode is down, multiple DBs may be affected. While in theory, the cluster will provide data access for reading or inserting data if over half the vnodes in vgroups are online, because of the possibly complex mapping between vnodes and dnodes, it is difficult to guarantee that the cluster will work properly if over half of the dnodes are online.
High Availability of Mnode
Each TDengine cluster is managed by mnode, which is a module of taosd. For the high availability of mnode, multiple mnodes can be configured in the system. When a TDengine cluster is started from scratch, there is only one mnode, then you can use command create mnode to and start corresponding dnode to add more mnodes.
SHOW MNODES;
The end point and role/status (leader, follower, candidate, offline) of all mnodes can be shown by the above command. When the first dnode is started in a cluster, there must be one mnode in this dnode. Without at least one mnode, the cluster cannot work.
From TDengine 3.0.0, RAFT protocol is used to guarantee the high availability, so the number of mnodes is should be 1 or 3.
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