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Technical Articles

Database

Redis

How to trim a list to a specific size using LTRIM?

The LTRIM command is used to preserve the list elements of the specified range and to delete the rest. 1. The usage is LTRIMkeystartstop. The index starts from 0 and supports negative numbers to represent the reciprocal; 2. The last N elements should be kept to use LTRIMkey-N-1; 3. Before use, confirm the importance of the data, combine LLEN to judge the length, and ensure atomicity in a concurrent environment; 4. The parameter order is consistent with the native command in redis-py. There will be no errors reported in the index that exceeds the boundary, and it will be automatically processed as a valid boundary when it exceeds the range.

Aug 19, 2025 am 09:45 AM

What is the minimum number of master nodes required for a stable Redis Cluster?

RedisCluster requires at least 3 master nodes, as this is the smallest number to form a majority consensus to prevent split brains and achieve secure failover. 1. When there is only 1 master, the cluster cannot be formed; 2.2 masters cannot reach a majority consensus after one of them goes down; 3.3 masters can still complete failover with 2/3 majority consensus when a node fails. In addition, it is recommended that each master configure at least one slave to improve availability, with a total of 6 nodes required; at the same time, all masters must uniformly cover 16384 hashslots to ensure balanced data distribution.

Aug 18, 2025 am 08:25 AM

主節(jié)點

What is an AOF rewrite and why is it necessary?

AOFrewrite compresses AOF files by removing redundant commands in Redis, and the necessity is to avoid slow recovery due to excessive files. 1.Redis performs rewrite through the fork child process, and the parent process continues to process the request and caches the newly written command. 2. The child process generates a sequence of commands for reconstructing data based on the parent process data set. 3. After the rewrite is completed, the parent process replaces the old file with the new file. The triggering conditions for AOF rewrite include: the AOF size increase exceeds the threshold and has passed a certain time since the last rewrite, or manually triggered by using the BGREWRITEAOF command. The importance of AOF rewrite is reflected in improving efficiency, saving disk space, speeding up recovery, and simplifying backup management to ensure Redis instances

Aug 18, 2025 am 04:18 AM

How Do You Install and Configure Redis on Ubuntu?

Installing and configuring Redis on Ubuntu requires the following steps: 1. Install using the commands sudoaptupdate and sudoaptinstallredis-server. 2. Verify that the installation is successful through redis-cliping. 3. Edit /etc/redis/redis.conf file for configuration, including binding address, setting password and persistence options. 4. Adjust parameters such as maxclients, maxmemory and maxmemory-policy according to application needs to optimize performance.

Aug 18, 2025 am 02:42 AM

What is the purpose of message acknowledgement (XACK) in Streams?

XACKinRedisStreamsensuresreliablemessageprocessingbyacknowledgingthataconsumerhassuccessfullyhandledamessage.1.Itremovesthemessagefromtheconsumergroup’spendingentrieslist(PEL).2.Itpreventsmessagelossandduplicationbytrackingwhichmessageshavebeenproces

Aug 18, 2025 am 01:16 AM

streams Message confirmation

How to perform set operations like union, intersection, and difference? (SUNION, SINTER, SDIFF)

Setoperationsincludeunion,intersection,anddifference.1.Union(SUNION)combinesalluniqueelementsfrommultiplesets.2.Intersection(SINTER)findselementscommontoallsets.3.Difference(SDIFF)retrieveselementsinonesetbutnotinothers.Theseoperationsareimplementedi

Aug 17, 2025 pm 12:24 PM

How to read messages from a Stream using XREAD and XREADGROUP?

XREAD is suitable for independent read stream messages, and XREADGROUP is used for message distribution of consumer groups. When using XREAD, the number of messages is specified by COUNT, and STREAMS is followed by the stream name and start ID (0 is the earliest, $ is the latest), and blocking and waiting can be combined with BLOCK; while XREADGROUP needs to create a consumer group first, specify the group name and consumer through GROUP, use > to read unassigned messages, and confirm that the processing is completed with XACK, XPENDING view the pending messages, and XCLAIM reassigns failed messages to ensure reliable consumption.

Aug 17, 2025 am 03:29 AM

XREAD

What are the common use cases for the String data type in Redis?

Redis strings are widely used and are suitable for a variety of scenarios. 1. Can be used to cache static or computed data, such as API response, HTML fragments and database query results, store and obtain through SET and GET commands, and set expiration time with EX parameters to improve application performance and reduce database load; 2. Support atomic operations, suitable for use as speed limit and temporary counters, such as tracking login attempts, API call frequency, etc., and use INCR, DECR and other commands to avoid concurrency problems; 3. Suitable as a session storage solution for web applications, save serialized session data in the form of a string, supports fast read and write and automatic expiration, and is suitable for distributed architectures; 4. It can be used as a functional switch or a simple configuration item to dynamically control application behavior, such as

Aug 16, 2025 am 08:59 AM

redis string

What is the impact of using many small keys versus fewer large keys in a hash?

Using fewer large keys usually has more advantages in memory efficiency, reducing hash conflicts, and improving cache behavior, but depending on the usage scenario. 1. Less large keys can reduce the metadata overhead of each key-value pair, thereby saving memory; 2. Large keys reduce the possibility of hash conflicts and improve search speed; 3. Large keys improve cache locality due to centralized data storage and optimize CPU access efficiency; 4. Selecting strategies requires weighing the needs of fast search, memory usage and fine-grained control, and different scenarios require different processing.

Aug 16, 2025 am 07:41 AM

Hash 鍵大小

How does Redis's single-threaded architecture work?

Redisisfastdespitebeingsingle-threadedbecauseitavoidsmulti-threadingoverheadandusesnon-blockingI/Owitheventloops.1.Itprocessescommandsinasinglethreadtoeliminatelockingandcontext-switchingcosts.2.IthandleshightrafficviaaneventloopthatusesI/Omultiplexi

Aug 16, 2025 am 07:10 AM

What is a 'fork' in the context of Redis persistence and replication?

Redis uses the fork mechanism to achieve persistence and replication to ensure that the main process is not blocked. 1. In RDB persistence, Redis creates a child process through fork to generate a data snapshot. During the child process writes an RDB file, the parent process continues to process the request; 2. In replication, the master node fork child process generates a snapshot and sends it to the slave node, and at the same time caches changes to ensure consistency; 3. Pay attention to memory peaks, disk I/O bottlenecks and delay issues, and reduces the impact through monitoring and reasonable scheduling.

Aug 16, 2025 am 01:44 AM

redis fork

What is the maximum number of elements in a Redis List?

Redislistscanholdupto2^32-1(4,294,967,295)elements.However,thereallimitisdeterminedbyavailablememory,aseachelementhasoverheadandquicklistsusecompressionsettingslikelist-max-ziplist-size.PerformanceconsiderationsincludeavoidinglargeLRANGEoperations,us

Aug 15, 2025 am 07:40 AM

最大元素數(shù)量

What is the HRANDFIELD command in Redis 6.2?

HRANDFIELDinRedis6.2allowsrandomselectionofhashfields,optionallywithvalues.1)Itefficientlyretrievesoneormorerandomfieldsfromahash.2)UsingWITHVALUESreturnsbothfieldandvalue,avoidingextracalls.3)Handlesedgecaseslikenon-existenthashesorcountexceedingfie

Aug 15, 2025 am 03:06 AM

redis

How to store an object-like structure in Redis using Hashes?

Using Hashes to store object structure is efficient and convenient in Redis, and is suitable for storing objects with fewer fields such as user information and product details. 1. Use the HSET command to store objects into Redis, for example, HSETuser:1name"Alice"email"alice@example.com"; 2. Use HGET to obtain the specified fields, and HGETALL can obtain all fields and values; 3. Use HSET to modify fields to overwrite them, delete fields to use HDEL, and check whether the fields exist with HEXISTS; 4. Applicable scenarios include frequently updating some properties of the object and saving memory, which is not suitable for nesting too deep or needing complexity.

Aug 15, 2025 am 02:22 AM

redis Hashes