Redis persistence: introduction and differences between RDB and AOF

Redis is a non-relational database that is mainly based on memory storage, which makes Redis perform well in scenarios with high concurrency and high read and write speed requirements. However, because Redis stores data in memory, after the server crashes or restarts, the data previously stored in the memory will be cleared and the data will not be persisted, which may cause serious data loss problems. In order to solve this problem, Redis provides a persistence mechanism, mainly using two strategies: RDB and AOF. This article will introduce in detail the RDB and AOF persistence mechanisms of Redis, as well as their differences.

RDB persistence

RDB persistence mechanism is to store the data in the current memory of Redis into a disk file. This disk file can be a snapshot or regular storage. For multiple snapshots, Redis saves the snapshot files on disk so that the snapshot files can be used to restore the data in the server memory after the Redis server is restarted.

RDB Snapshot

When selecting the RDB persistence mode, Redis will write the data at the current time to the disk in the form of a snapshot. This process is compressed, Redis can write multiple snapshots, and the interval between each snapshot can be set through the Redis.conf configuration file.

In Redis.conf, we can find the following configuration information:

# 快照持久化相关配置设置
save 900 1  ## 900秒(15分钟)之内至少发生1次数据变更，持久化快照
save 300 10  ## 300秒(5分钟)之内至少发生10次数据变更，持久化快照
save 60 10000  ## 60秒之内至少发生10000次数据变更，持久化快照

The above configuration indicates that every 15 minutes in Redis, or appears in Redis 10 times or 10,000 times During data addition and deletion operations, Redis will automatically save the data in memory to disk.

RDB snapshot method can effectively avoid data loss after Redis crashes, and can also be used for data backup and other purposes. However, if Redis crashes or restarts and the data in the latest snapshot file does not exist or is incomplete, data will be lost, so it is recommended not to set the snapshot time too long.

RDB advantages and disadvantages

RDB advantages:

1. RDB persistence method is based on snapshots, which can effectively ensure the integrity and availability of data.
2. The file size of RDB persistence method is relatively small and easy to store.
3. RDB persistence method is suitable for large-scale data backup, migration and other scenarios.

Disadvantages of RDB:

1. RDB persistence method uses a single file to store data. Each write needs to overwrite the previous file, which is prone to data loss. Case.
2. The frequency of RDB persistence is determined by time and the frequency of data changes. If the time is too long or the frequency of data changes is low, data will be lost.

AOF persistence

AOF (Append Only File) persistence is to store the write command in the form of appending the write file. When the service is restarted, Redis will perform data recovery according to the commands stored in this file. The AOF persistence method can ensure the permanent storage of data. Even if Redis is down or restarted, the data can be recovered.

AOF file format

The AOF file format is a data log file that is appended to the file for each write operation. Each line of record in the log file stores a Redis command, which is a complete Redis transaction. This transaction will be appended at the end of the AOF log file.

In AOF persistence mode, Redis will write each new command to the file in the disk in the form of append writing. Therefore, the size of the AOF file is constantly growing. When the AOF file exceeds the preset size limit, Redis will automatically trigger the rewriting of the AOF file. This process will clean up the expired data in the database and convert it into a snapshot for storage. The purpose of AOF file rewriting is to compress the size of the AOF file, so as to avoid reducing the performance of Redis due to excessively large AOF files.

Advantages and disadvantages of AOF persistence

Advantages of AOF persistence:

1. The mechanism of AOF persistence is based on command records, which can be more accurate Instantly restore the history and timeline of data changes.
2. AOF files can be saved in multiple different files. This feature can avoid the problem of reduced read and write performance caused by a single file being too large.
3. The AOF persistence method is very suitable for log statistics and processing.

Disadvantages of AOF persistence:

1. AOF files are larger than snapshot files and therefore occupy more storage capacity.
2. The AOF persistence method requires more system resources because commands need to be recorded in files.
3. AOF file rewriting is a performance-consuming process, and this process will affect the read and write performance of Redis.

The difference between RDB and AOF

1. RDB is a full backup, while AOF is an incremental backup.
2. By default, Redis uses RDB persistence, and AOF persistence needs to be turned on manually.
3. In AOF persistence mode, Redis will record each write command operation and append a new write record at the end of the log. In RDB persistence mode, Redis will write a snapshot of the data in the current state in memory to disk.
4. The AOF persistence method is more secure and reliable, but the capacity of AOF files is relatively larger, while the capacity of RDB files is relatively smaller.

Conclusion

The persistence mechanism of Redis is designed to avoid data loss when Redis crashes or restarts. RDB persistence and AOF persistence are two strategies provided by Redis. Their core difference lies in the way of data recording and the capacity of the storage file. When using Redis for data processing, we can choose different persistence methods according to our actual needs, so that Redis can perform better in data storage and recovery.

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