redis 面试问题问答Top 10

1)什么是Redis?

　　English：Redis is an open source (BSD licensed), in-memory data structure store, used as a database, cache and message broker【1】.

　　中文Redis是一个开源的内存数据库，支持多种数据结构的存取，常用来作为数据库，缓存和消息队列。

2)解释一下Redis的特性?

English：It supports data structures such as strings, hashes, lists, sets, sorted sets with range queries, bitmaps, hyperloglogs, geospatial indexes with radius queries and streams. Redis has built-in replication, Lua scripting, LRU eviction, transactions and different levels of on-disk persistence, and provides high availability via Redis Sentinel and automatic partitioning with Redis Cluster.

Other features include:

　　中文：Redis支持多种数据结构：一方面，支持字符串，哈希表，数组，集合和支持范围查询的有序集合等基础数据结构；另一方面也支持位图，hyperloglog，带半径查找的地理信息定位和流式结构的高级数据结构。同时，它自带了复制，lua脚本，lru缓存清除，事务和不同等级的持久化方式，更通过哨兵方式提供了高可用，通过集群方式提供了自动分片。

　　主要特性：

　　　　事务

　　　　发布/订阅

Lua脚本

KEY有效期设置

LRU缓存清除

自动故障转移

3) Memcached和Redis的区别是什么?

Redis	Memcached
虽然同样都支持缓存，但Redis支持一些其他特性如持久化和复制 Redis 不支持CAS(检查和设置)。这对于维护缓存一致性很有用 Redis h具有较强的数据结构;它可以处理字符串、二进制安全字符串、二进制安全字符串列表、排序列表等。 Redis 支持键值最大为2GB Redis是单线程的	Memcached 只支持缓存。 Memcached支持CAS(检查和设置)。在Memcached中，您必须序列化对象或数组才能保存它们，要将它们读取回来，您必须对它们进行反序列化。 Memcached键值支持最大250byte长度 Memcached 是多线程的

4) Redis的优势是什么?

　　速度快

　　支持服务器端锁定

　　有很多客户端库【6】

　　具有命令级原子操作(tx操作)

　　应用广泛

5) Redis有哪些不足?

　　单线程

　　持久化开销大

6) Redis的键操作有哪些?　

    DEL

    DUMP

    EXISTS

    EXPIRE

    EXPIREAT

    KEYS

    MIGRATE

    MOVE

    OBJECT

    PERSIST

    PEXPIRE

    PEXPIREAT

    PTTL

    RANDOMKEY

    RENAME

    RENAMENX

    RESTORE

    SCAN

    SORT

    TOUCH

    TTL

    TYPE

    UNLINK

    WAIT

7) 谁在使用Redis?　

　　Twitter GitHub Weibo Pinterest Snapchat Craigslist Digg StackOverflow Flickr【2】

8) 使用Redis时需要注意什么?

1. key名设计

(1)【建议】: 可读性和可管理性

以业务名(或数据库名)为前缀(防止key冲突)，用冒号分隔，比如业务名:表名:id

ugc:video:1

(2)【建议】：简洁性

保证语义的前提下，控制key的长度，当key较多时，内存占用也不容忽视，例如：

user:{uid}:friends:messages:{mid}简化为u:{uid}:fr:m:{mid}。

(3)【强制】：不要包含特殊字符

反例：包含空格、换行、单双引号以及其他转义字符

2. value设计

(1)【强制】：拒绝bigkey(防止网卡流量、慢查询)

string类型控制在10KB以内，hash、list、set、zset元素个数不要超过5000。

反例：一个包含200万个元素的list。

非字符串的bigkey，不要使用del删除，使用hscan、sscan、zscan方式渐进式删除，同时要注意防止bigkey过期时间自动删除问题(例如一个200万的zset设置1小时过期，会触发del操作，造成阻塞，而且该操作不会不出现在慢查询中(latency可查))，查找方法和删除方法

(2)【推荐】：选择适合的数据类型。

例如：实体类型(要合理控制和使用数据结构内存编码优化配置,例如ziplist，但也要注意节省内存和性能之间的平衡)

反例：

set user:1:name tom

set user:1:age 19

set user:1:favor football

正例:

hmset user:1 name tom age 19 favor football

3.【推荐】：控制key的生命周期，redis不是垃圾桶。

建议使用expire设置过期时间(条件允许可以打散过期时间，防止集中过期)，不过期的数据重点关注idletime。

9) Redis的持久化方式和各种优缺点?

　　Redis持久化提供了4个选项

　　1）RDB(快照)方式【The RDB persistence performs point-in-time snapshots of your dataset at specified intervals】

　　2）AOF(命令式如mysql 的binlog)【the AOF persistence logs every write operation received by the server, that will be played again at server startup, reconstructing the original dataset. Commands are logged using the same format as the Redis protocol itself, in an append-only fashion. Redis is able to rewrite the log on background when it gets too big】

　　3）不持久化

　　4）AOF 和RDB混合方式

　　各种方式的优缺点

RDB advantages

RDB is a very compact single-file point-in-time representation of your Redis data. RDB files are perfect for backups. For instance you may want to archive your RDB files every hour for the latest 24 hours, and to save an RDB snapshot every day for 30 days. This allows you to easily restore different versions of the data set in case of disasters.

RDB is very good for disaster recovery, being a single compact file can be transferred to far data centers, or on Amazon S3 (possibly encrypted).

RDB maximizes Redis performances since the only work the Redis parent process needs to do in order to persist is forking a child that will do all the rest. The parent instance will never perform disk I/O or alike.

RDB allows faster restarts with big datasets compared to AOF.

RDB disadvantages

RDB is NOT good if you need to minimize the chance of data loss in case Redis stops working (for example after a power outage). You can configure different save points where an RDB is produced (for instance after at least five minutes and 100 writes against the data set, but you can have multiple save points). However you'll usually create an RDB snapshot every five minutes or more, so in case of Redis stopping working without a correct shutdown for any reason you should be prepared to lose the latest minutes of data.

RDB needs to fork() often in order to persist on disk using a child process. Fork() can be time consuming if the dataset is big, and may result in Redis to stop serving clients for some millisecond or even for one second if the dataset is very big and the CPU performance not great. AOF also needs to fork() but you can tune how often you want to rewrite your logs without any trade-off on durability.

AOF advantages

Using AOF Redis is much more durable: you can have different fsync policies: no fsync at all, fsync every second, fsync at every query. With the default policy of fsync every second write performances are still great (fsync is performed using a background thread and the main thread will try hard to perform writes when no fsync is in progress.) but you can only lose one second worth of writes.

The AOF log is an append only log, so there are no seeks, nor corruption problems if there is a power outage. Even if the log ends with an half-written command for some reason (disk full or other reasons) the redis-check-aof tool is able to fix it easily.

Redis is able to automatically rewrite the AOF in background when it gets too big. The rewrite is completely safe as while Redis continues appending to the old file, a completely new one is produced with the minimal set of operations needed to create the current data set, and once this second file is ready Redis switches the two and starts appending to the new one.

AOF contains a log of all the operations one after the other in an easy to understand and parse format. You can even easily export an AOF file. For instance even if you flushed everything for an error using a FLUSHALL command, if no rewrite of the log was performed in the meantime you can still save your data set just stopping the server, removing the latest command, and restarting Redis again.

AOF disadvantages

AOF files are usually bigger than the equivalent RDB files for the same dataset.

AOF can be slower than RDB depending on the exact fsync policy. In general with fsync set to every second performances are still very high, and with fsync disabled it should be exactly as fast as RDB even under high load. Still RDB is able to provide more guarantees about the maximum latency even in the case of an huge write load.

In the past we experienced rare bugs in specific commands (for instance there was one involving blocking commands like BRPOPLPUSH) causing the AOF produced to not reproduce exactly the same dataset on reloading. This bugs are rare and we have tests in the test suite creating random complex datasets automatically and reloading them to check everything is ok, but this kind of bugs are almost impossible with RDB persistence. To make this point more clear: the Redis AOF works incrementally updating an existing state, like MySQL or MongoDB does, while the RDB snapshotting creates everything from scratch again and again, that is conceptually more robust. However - 1) It should be noted that every time the AOF is rewritten by Redis it is recreated from scratch starting from the actual data contained in the data set, making resistance to bugs stronger compared to an always appending AOF file (or one rewritten reading the old AOF instead of reading the data in memory). 2) We never had a single report from users about an AOF corruption that was detected in the real world

10) Redis数据库RDBMS的不同点有哪些?

Redis是非关系型数据库的成员而RDBMS是关系型数据库.
Redis支持key-value的结构而RDBMS支持表的结构.
Redis速度极快RDBMS相对较慢.
Redis是内存型数据，将所有数据存放到内存而RDBMS存储数据到磁盘.
Redis用来存储小的频繁使用的的文件而RDBMS 用来存放较大的文件
Redis仅仅对Linux, BSD, Mac OS X, Solaris提供官方的支持. 当前对 Windows不提供官方支持，而RDBMS 支持所有的操作系统.

【1】 https://redis.io/topics/introduction

【2】https://redis.io/topics/whos-using-redis

【3】https://yq.aliyun.com/articles/531067

【4】https://redis.io/topics/persistence

【5】https://www.javatpoint.com/redis-interview-questions-and-answers

【6】https://redis.io/clients