Migration Mode Selection

Overview

Currently, RedisShake has three migration modes: PSync, RDB, and SCAN, corresponding to sync_reader, rdb_reader, and scan_reader respectively.

• For scenarios of recovering data from backups, use rdb_reader.
• For data migration scenarios, prefer sync_reader. Some cloud vendors don't provide PSync protocol support, in which case scan_reader can be chosen.
• For long-term data synchronization scenarios, RedisShake currently can't handle them because the PSync protocol is not reliable. When the replication connection is disconnected, RedisShake will not be able to reconnect to the source database. If availability requirements are not high, you can use scan_reader. If the write volume is not large and there are no large keys, scan_reader can also be considered.

Different modes have their pros and cons. Refer to each Reader section for more information.

Redis Cluster Architecture

When the source Redis is deployed in a cluster architecture, you can use sync_reader or scan_reader. Both have switches in their configuration items to enable cluster mode, which will automatically obtain all nodes in the cluster through the cluster nodes command and establish connections.

Redis Sentinel Architecture

When the source Redis is deployed in a sentinel architecture and RedisShake uses sync_reader to connect to the master, it will be treated as a slave by the master and may be elected as the new master by the sentinel.

To avoid this situation, you should choose a replica as the source.

Cloud Redis Services

Mainstream cloud vendors all provide Redis services, but there are several reasons that make using RedisShake on these services more complex:

1. Engine limitations. Some self-developed Redis-like databases don't support the PSync protocol.
2. Architecture limitations. Many cloud vendors support proxy mode, i.e., adding a Proxy component between the user and the Redis service. Due to the Proxy component, the PSync protocol can't be supported.
3. Security limitations. In native Redis, the PSync protocol typically triggers fork(2), leading to memory bloat and increased user request latency. In worse cases, it may even cause out of memory. Although there are solutions to mitigate these issues, not all cloud vendors have invested in this area.
4. Business strategies. Many users use RedisShake to migrate off the cloud or switch clouds, so some cloud vendors don't want users to use RedisShake, thus blocking the PSync protocol.

The following introduces some RedisShake usage schemes in special scenarios based on practical experience.

Alibaba Cloud Redis & Tair

Alibaba Cloud Redis and Tair both support the PSync protocol, and sync_reader is recommended. Users need to create an account with replication permissions (able to execute PSync commands). RedisShake uses this account for data synchronization. See Create and manage accounts for specific creation steps.

Exceptions:

1. Version 2.8 Redis instances don't support creating accounts with replication permissions. You need to upgrade to a major version.
2. Cluster architecture Redis and Tair instances don't support the PSync protocol under proxy mode.
3. Read-write separation architecture doesn't support the PSync protocol.

In scenarios where the PSync protocol is not supported, scan_reader can be used. Note that scan_reader will put significant pressure on the source database.

AWS ElastiCache

Prefer sync_reader. AWS ElastiCache doesn't enable the PSync protocol by default, but you can request to enable it by submitting a ticket. AWS will provide renamed PSync commands in the ticket, such as xhma21yfkssync and nmfu2bl5osync. These commands have the same effect as the psync command, just with different names. Users only need to modify the aws_psync configuration item in the RedisShake configuration file. For a single instance, write one pair of ip:port@cmd. For cluster instances, write all ip:port@cmd pairs, separated by commas.

When it's inconvenient to submit a ticket, you can use scan_reader. Note that scan_reader will put significant pressure on the source database.

AWS MemoryDB

AWS MemoryDB doesn't provide PSync permissions. You can use scan_reader and rdb_reader.