Redis Tutorial (1): Introduction to Redis

1. Introduction:

In the past few years, NoSQL databases once became synonymous with high concurrency and massive data storage solutions, and corresponding products have also sprung up like mushrooms after a rain. However, there are only a handful of products that can stand out among many products, such as Redis, MongoDB, BerkeleyDB and CouchDB. Since each product has different characteristics, their application scenarios also have certain differences. Only a brief explanation is given below:

1). BerkeleyDB is an extremely popular open source embedded database. , can be used in storage engines in more situations. For example, BerkeleyDB was used as the storage engine of MySQL before being acquired by Oracle. It can be predicted that this product has excellent concurrency scalability, supports transactions and nested transactions, and stores massive data. and other important features, which have extremely high usable value in storing real-time data. However, it should be pointed out that the license of this product is GPL, which means that it is not free to use in all cases.

2). MongoDB is defined as an Oriented-Document database server. Unlike BerkeleyDB, this database can run independently and provide related data services like other relational database servers. From the official documentation of this product, we can learn that MongoDB is mainly suitable for high-concurrency forums or blog websites. The main characteristics of these websites are high concurrent visits, more reads and less writes, large amounts of data, simple logical relationships, and documents. data as the primary data source, etc. Like BerkeleyDB, the license of this product is GPL.

3). Redis, a typical NoSQL database server, compared with BerkeleyDB, it can run independently on its own server host as a service program. Many times, people just regard Redis as a Key/Value database server, but this is not the case. In the current version, in addition to Key/Value, Redis also supports data structures such as List, Hash, Set, and Ordered Set, so it The uses are also broader. Regarding this misunderstanding, the Redis official website has also provided corresponding clarification. Different from the above two products, the License of Redis is the Apache License, which is completely free for now.

4). memcached, data caching server. Why is an explanation of this product given here? Very simple, because I think it is most similar to Redis in terms of usage. After all, this is a technical blog series about Redis. In view of this, we will briefly compare these two products. First, let’s talk about the biggest difference between them. Memcached only provides a data caching service. Once the server goes down, all the data previously cached in the memory will disappear. Therefore, it can be seen that memcached does not provide any form of data persistence function. Redis provides such functionality. Furthermore, Redis provides richer data storage structures, such as Hash and Set. As for their similarities, there are two main points. One is that they are completely free, and the other is that the command forms they provide are very similar.
 
2. Advantages of Redis:

1). Compared with other NoSQL products, Redis is extremely easy to use, so for developers who have experience in using similar products, You can use Redis to build your own platform in a day or two, or even a few hours.
2). While solving many common problems, it also provides relevant solutions for some personalized problems, such as index engines, statistical rankings, message queue services, etc.

3. The main problems with Redis in the current version:

1). The official version does not provide support for the Windows platform. The released official version only supports Unix-like and MacOSX platforms. .
2). Cluster support is not provided, but according to the official website, this feature is expected to be added in version 2.6.
3). In the Publication/Subscription function, if the master goes down, the slave cannot be automatically promoted to the master.

4. Comparison with relational database:

In the current version (2.4.7) of Redis, it provides support for five different data types. Only one of them, the string type, can be regarded as a Key-Value structure, while other data types are There are application scenarios that apply to their respective characteristics, and we will explain the specific details in later blogs in this series.
Compared with relational databases, due to its relatively simple storage structure, Redis does not provide good support for complex logical relationships. However, in scenarios applicable to Redis, we can gain efficiency from this. significant improvement. Even so, Redis still provides us with some basic concepts that a database should have, such as: you can choose to open different databases in the same connection. However, the difference is that the databases in Redis are named by numbers. By default The database opened under is 0. If you plan to switch databases while the program is running, you can use the Redis select command to open other databases, such as select 1. If you want to switch back to the default database later, just execute select 0.
In terms of data storage, Redis follows the mainstream idea of ​​​​existing NoSQL databases, that is, Key is the unique identifier for data retrieval. We can simply understand it as the key of the index in the relational database, and Value is used as the data storage The main object, each Value has a Key associated with it, which is like the location where the physical data in the index is stored in the data table. In Redis, Value will be regarded as a binary byte stream used to store data in any format, such as Json, XML, and byte streams of serialized objects, etc., so we can also imagine it as a BLOB type field in RDB. It can be seen that when performing data query, we can only use Key as the condition of our query. Of course, we can also apply some techniques provided in Redis to use Value as the Key of other data. We will cover this knowledge in the following blogs. introduce.
 
5. How to persist memory data:

By default, Redis will refer to the number of data modifications in the current database, and after reaching a certain threshold, the snapshot of the database will be stored in On the disk, we can set the threshold through the configuration file. Normally, we can also set Redis to save regularly. For example, when more than 1,000 key data are modified, Redis will perform a data persistence operation every 60 seconds. The default setting is that if there are 9 or less data modifications, Redis will persist every 15 minutes.
It can be seen from the solution mentioned above that if this method is adopted, the runtime efficiency of Redis will be very efficient. Whenever a new data modification occurs, only the cached data in the memory will occur. Changes, and such changes will not be persisted to the disk immediately, thus avoiding disk IO in most modification operations. However, things often have two sides. In this method, we do gain efficiency, but we lose data reliability. If the server where Redis is located goes down before the memory snapshot is persisted to disk, all the modified data that has not been written to disk will be lost. In order to ensure high reliability of data, Redis also provides another data persistence mechanism-Append mode. If the Redis server is configured in this way, then whenever data modification occurs, it will be persisted to disk immediately.

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