The comparison between Redis and memcached

Memcached is a high-performance distributed memory object caching system for dynamic Web applications to mitigate database load. It provides a dynamic, database-driven site speed by caching data and objects in memory to reduce the number of times the database is read.
Memcached is based on a hashmap that stores key/value pairs.

Redis is a key-value storage system, similar to memcached. However, it supports storing more value types, including string (string), list (linked list), set (set), Zset (sorted set-ordered collection), and HASHS (hash type). These data types support Push/pop, Add/remove, and intersection-set and difference sets, and richer operations, and these operations are atomic. Based on this, Redis supports sorting in a variety of different ways. As with memcached, data is cached in memory to ensure efficiency. The difference is that Redis periodically writes the updated data to disk or writes the modified operation to the appended record file, and Master-slave (Master-Slave) synchronization is implemented on this basis.
Redis is a high-performance Key-value database. The emergence of Redis, to a large extent, compensates for the lack of memcached such key/value storage, in some cases can be a good complement to the relational database. It provides the python,ruby,erlang,php client, which is very convenient to use.

Here's a look at the difference between Redis and memcached.

1. Network IO Model

Memcached is a multi-threaded, non-blocking IO multiplexing network model, divided into the main thread and the worker sub-thread, listening thread listening network connection, after accepting the request, the connection description Word pipe to the worker thread, read/write IO, the network layer using the Libevent encapsulated event Library , multithreading model can play a multi-core role, but the introduction of the cache coherency and lock problem, for example, memcached most commonly used stats command, the actual memcached all operations to the global variable lock, count, etc., resulting in performance loss.

Redis uses a single-threaded IO multiplexing model, which encapsulates a simple Aeevent event processing framework that implements Epoll, Kqueue, and select, which can be used to maximize the speed advantage for purely IO operations. However, Redis also provides some simple computing functions, such as sorting, aggregation, etc., for these operations, the single-threaded model can actually seriously affect the overall throughput, CPU calculation process, the entire IO schedule is blocked.

2. Memory management aspects

Memcached uses a pre-allocated pool of memory to manage memory using slab and chunk of different sizes, item selects the appropriate chunk storage based on size, the way memory pools can save the cost of requesting/freeing memory, and can reduce memory fragmentation, But this approach also leads to a certain amount of wasted space, and when memory is still large, new data may be rejected for reference to Timyang's article: http://timyang.net/data/Memcached-lru-evictions/

Redis uses on-site memory storage to store data, and rarely uses free-list to optimize memory allocation, and there is a degree of memory fragmentation, and the Redis data store command parameters, which store the time-to-date information separately, and call them temporary data. Non-temporary data is never removed, even if there is not enough physical memory, so that swap will not eliminate any non-temporal data (but will attempt to eliminate some temporary data), which is more appropriate for Redis as storage instead of the cache.

3. Data consistency issues

Memcached provides a CAS command that guarantees consistency of the same data for multiple concurrent access operations. Redis does not provide CAS commands, and this is not guaranteed, but Redis provides the functionality of a transaction that guarantees the atomicity of a sequence of commands and is not interrupted by any action.

4. Storage methods and other aspects

Memcached basically only supports simple key-value storage, does not support enumeration, and does not support persistence and replication functions.

In addition to Key/value, Redis supports numerous data structures such as list,set,sorted Set,hash, which provides the keys

enumeration, but not on-line, if you need to enumerate online data, Redis provides tools to scan its dump files, enumerate all the data, and Redis also provides the functionality of persistence and replication.

5. Client support for different languages

Memcached and Redis have rich third-party clients to choose from for different language clients, but because memcached has been developing for a longer period of time, many of Memcached's clients are more mature and stable in terms of client support at present. and Redis because its protocol itself is more complex than memcached, plus the author constantly add new features, and so on, the corresponding third-party client tracking speed may not catch up, sometimes you may need to make some changes on the basis of third-party clients to better use.

According to the above comparisons it is not difficult to see that when we do not want the data to be kicked out, or need more data types other than Key/value, or need to use the landing function, using Redis is more appropriate than using memcached.

Some of the peripheral features of Redis

In addition to being stored as storage, Redis also provides some other functions, such as aggregation calculation, pubsub, scripting, etc., for such functions need to understand its implementation principle, clearly understand its limitations, can be used correctly, such as pubsub function, This is actually not supported by any persistence, the consumer connection between the flash or the reconnection between the message is all lost, and such as aggregation calculation and scripting and other features are limited by the Redis single-threaded model, it is impossible to achieve high throughput, need to use caution.

In general, the Redis author is a very diligent developer who can often see that the author is experimenting with a variety of new ideas and ideas, and that the functionality of these areas requires that we need to know more about them before using them.

Summarize:

The best way to use 1.Redis is to in-memory all data.

2.Redis more scenes are used as substitutes for memcached.

3. It is more appropriate to use Redis when more data type support is required other than key/value.

4. Using Redis is more appropriate when the stored data cannot be excluded.

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The comparison between Redis and memcached