The difference between redis and memcache in PHP

The difference between redis and memcache in PHP

Memcached and redis in PHP are very similar: they are both in-memory databases , the data is stored in the memory and directly accessed through tcp. The advantages are fast speed and high concurrency. The disadvantages are limited data types and weak query function. It is generally used as a cache.

Recommendation: "Redis Video Tutorial" "memcached Video Tutorial"

Then the questioner said that redis can do all memcached things, so why? Does anyone else use memcached? That's because the two of them are not completely interchangeable. They also have their own advantages and disadvantages:

Memcached

Memcached's advantages:

Memcached can take advantage of multi-core, and the throughput of a single instance is extremely high, which can reach hundreds of thousands of QPS (depending on the byte size of key and value and the performance of the server hardware. The peak QPS in daily environment is about 4- About 6w).

Suitable for maximum load capacity and effectively decompress the server.

Supports direct configuration as session handle.

There are relatively few pitfalls in configuration and maintenance.

Limitations of Memcached:

The data structure is very simple and single, and only supports simple key/value data structures, unlike Redis which can support rich data types.

Persistence is not possible, data cannot be backed up and can only be used for caching, and all data will be lost after restarting.

Data synchronization cannot be performed, and data in MC cannot be migrated to other MC instances.

Memcached memory allocation uses the Slab

Allocation mechanism to manage memory. Large differences in value size distribution will cause memory utilization to decrease, and cause problems such as kicking out even when utilization is low. Users need to pay attention to value design.

The memcached server does not natively support horizontal expansion. A cache distribution strategy must be written on the client to implement distributed caching. Since data synchronization cannot be performed, a single machine failure in the production environment may affect some business operations. .

Redis

Advantages of Redis:

Supports multiple data structures, such as string (string),

list (double linked list), dict (hash table), set (set), zset (sorted set), hyperloglog (cardinality estimation), etc.

Supports persistence operations, and can persist AOF and RDB data to disk, so as to perform data backup or data recovery operations, which is a better way to prevent data loss.

Supports data replication through Replication. Through the master-slave mechanism, data can be replicated synchronously in real time, and multi-level replication and incremental replication are supported. The master-slave mechanism is an important means for Redis to perform HA.

Single-threaded request, all commands are executed serially, and there is no need to consider data consistency issues in concurrent situations.

Supports pub/sub message subscription mechanism, which can be used for message subscription and notification.

Supports simple transaction requirements, but there are few usage scenarios in the industry and it is not mature.

Limitations of Redis:

Redis can only use a single thread, and its performance is limited by CPU performance. Therefore, a single instance of CPU can only reach a maximum of 5-6wQPS per second. (Depending on the data structure, data size and server hardware performance, the QPS peak in the daily environment is about 1-2w).

Supports simple transaction requirements, but there are few usage scenarios in the industry and it is not mature, which has both advantages and disadvantages.

Redis consumes more memory on the string type. You can use dict (hash table) to compress storage to reduce memory consumption.

Summary

In my opinion, Redis has the characteristics of a database in many aspects, or it is a database system, while Memcached is just a simple K/V cache.

And whether to use redis or memcached depends on the needs of the subject, because if it is just for caching, memcached is enough to meet most needs. The emergence of redis only provides a better choice. But it does not mean that redis can completely replace memcached. Again, it depends on your needs.

In terms of technology, redis is newer than memcached, but when we mature, we say that memcached should be better. Besides, the trend is now starting to switch to mongodb. Because of the database characteristics of redis, mongodb is better. .

Many companies still use memcached in the caching strategy, followed by redis, and finally mongodb. I found that there is no such thing. It still takes time for the latest technology to be applied by the company team, and the old technology is This strategy is still used by many company teams because the technology is mature and more stable. This is why memcached is mentioned more than redis.

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