Johnson Lin

The Redis PFMERGE command merges multiple HyperLogLogs into a single HyperLogLog. The cardinality estimate of the resulting HyperLogLog is calculated by taking the union of all the given HyperLogLogs. SyntaxThe basic syntax of the PFMERGE command is as follows: 1PFMERGE destkey [sourcekey [sourcekey ...]] The command merges multiple HyperLogLog values to approximate the cardinality of the union of the observed sets. The result is stored in a destination variable, which is created if it...

Redis is a popular, open-source, in-memory data structure store used as a database, cache, and message broker. Installing Redis on Ubuntu is straightforward, thanks to the Advanced Package Tool (APT), which simplifies installing software on Debian-based Linux systems like Ubuntu. Here’s a step-by-step guide on how to install Redis using APT. Step 1: Update Your SystemBefore installing any new software, it’s good practice to update your package lists to ensure you have the latest versions...

The Redis PFCOUNT command returns the cardinality estimate of the given HyperLogLog. SyntaxThe basic syntax of the PFCOUNT command is as follows: 1PFCOUNT key [key ...] Available SinceRedis version 2.8.9 and later. Time Complexity O(1) when called with a single key. O(N) when called with multiple keys, where N is the number of keys. ACL Categories@read, @hyperloglog, @slow Return ValueAn integer representing the cardinality of the given HyperLogLog. If multiple HyperLogLogs are provided,...

When running Apache Flink, you might encounter the following error message during startup or execution: 1java.lang.NoSuchMethodError: org.apache.commons.cli.CommandLine.hasOption For example, when I use Flink version 1.17.2 and start a job in Application Mode, the job is submitted successfully and runs as expected. The command and output information are as follows. 1234567891011121314151617$ /data/flink/flink-1.17.2/bin/flink run-application -t yarn-application -p 3...

The PFADD command in Redis adds one or more elements to a HyperLogLog data structure. SyntaxThe basic syntax for the PFADD command is as follows: 1PFADD key [element [element ...]] This command adds all the specified elements to the HyperLogLog data structure stored at the key provided as the first argument. Available SinceRedis version 2.8.9 and later. Time ComplexityO(1) to add every element. ACL Categories@write, @hyperloglog, @fast Return ValueReturns an integer: 1 if at least one...

HyperLogLog is a probabilistic data structure used to estimate the cardinality of a set. As a probabilistic data structure, HyperLogLog trades perfect accuracy for efficient space utilization. Redis added the HyperLogLog data structure in version 2.8.9. HyperLogLog in Redis is an algorithm used for cardinality estimation. Its main advantage is that, even when the number of input elements is extremely large, the memory required to calculate the cardinality remains fixed and small. In Redis,...

When using Ubuntu or any Debian-based system, you may come across the error message: “Could not get lock /var/lib/apt/lists/lock.” This is a common issue, particularly when managing software installations or updates via apt or similar commands. For example, I recently encountered this error while trying to install an application using the apt command on Ubuntu: 12345$ sudo apt updateReading package lists... DoneE: Could not get lock /var/lib/apt/lists/lock. It is held by process 1390...

Issue Description:After successfully installing Superset, running the command superset run -p 8088 -h 0.0.0.0 to start the application, and attempting to configure the Hive database connection through the interface, an error occurs. The server logs display the following: 123Unexpected error TTransportExceptionWARNING: superset.views.core: Unexpected error TTransportExceptionINFO: werkzeug:10.10.17.34 - - [11/Jan/2022 12:07:37] "POST /superset/testconn HTTP/1.1" 400 - Upon seeing...

When it comes to building data processing flows in Apache Flink, there are times you might want to run multiple independent pipelines, ideally sharing resources for efficiency. Here, we explore two common approaches to achieve this and provide a practical guide to help decide which one best suits your use case. Imagine you have two independent flows that look like this: Flow 1: Source1 -> operator1 -> Sink1 Flow 2: Source2 -> operator2 -> Sink2 The goal is to use a single...

The Redis BITCOUNT command count the number of set bits (population counting) in a string. SyntaxThe basic syntax of the BITCOUNT command is as follows: 1BITCOUNT key [start end [BYTE | BIT]] By default all the bytes contained in the string are examined. It is possible to specify the counting operation only in an interval passing the additional arguments start and end. Like for the GETRANGE command start and end can contain negative values in order to index bytes starting from the end of the...

In Redis, a bitmap is a data structure that represents a sequence of bits (0s and 1s). Each bit in the bitmap can be either on (1) or off (0), allowing it to represent binary data in a compact form. This is particularly useful when you need to handle large sets of boolean data, as bitmaps allow you to store information in a very space-efficient way. For example, if you have a system tracking whether users have completed a daily task, a bitmap can efficiently store whether each user has...

The Redis SUNIONSTORE command stores the union of the given sets in the specified destination set. If the destination set already exists, it will be overwritten. SyntaxThe basic syntax of the SUNIONSTORE command is as follows: 1SUNIONSTORE destination key [key ...] This command is equal to SUNION, but instead of returning the resulting set, it is stored in destination. If destination already exists, it is overwritten. Available SinceRedis version >= 1.0.0 Time ComplexityO(N) where N is...

The Redis SUNION command returns the union of the specified sets. Keys that do not exist are considered to be empty sets. SyntaxThe basic syntax of the SUNION command is as follows: 1SUNION key [key ...] For example: 1234key1 = {a,b,c,d}key2 = {c}key3 = {a,c,e}SUNION key1 key2 key3 = {a,b,c,d,e} Available SinceRedis version >= 1.0.0 Time ComplexityO(N) where N is the total number of elements in all given sets. ACL Categories@read, @set, @slow Return...

The Redis SSCAN command is used to iterate over the elements of a set. It is based on the SCAN command. SyntaxThe basic syntax of the SSCAN command is as follows: 1SSCAN key cursor [MATCH pattern] [COUNT count] Available SinceRedis version >= 2.8.0 Time ComplexityO(1) for every call. O(N) for a complete iteration, including enough command calls for the cursor to return back to 0. N is the number of elements inside the collection. ACL Categories@read, @set, @slow Return ValueReturns an...

The Redis SREM command removes one or more members from a set. Non-existent members are ignored. An error is returned if the key is not of set type. SyntaxThe basic syntax of the SREM command is as follows: 1SREM key member [member ...] In Redis version 2.4 and earlier, SREM only accepted a single member value. Available SinceRedis version >= 1.0.0 Time ComplexityO(N) where N is the number of members to be removed. ACL Categories@write, @set, @fast Return ValueReturns the number of...

Removes and returns one or more random members from the set value store at key. This operation is similar to SRANDMEMBER, that returns one or more random elements from a set but does not remove it. SyntaxThe basic syntax of the SPOP command is as follows: 1SPOP key [count] By default, the command pops a single member from the set. When provided with the optional count argument, the reply will consist of up to count members, depending on the set’s cardinality. Note: the count argument is...

The Redis SMOVE command moves the specified member element from the source set to the destination set. SMOVE operation is an atomic. If the source set does not exist or does not contain the specified member, the command does nothing and returns 0. Otherwise, the member is removed from the source set and added to the destination set. If the destination set already contains the member, the SMOVE command simply removes the member from the source set without adding it to the destination set...

The Redis SMISMEMBER command returns whether each member is a member of the set stored at key. For every member, 1 is returned if the value is a member of the set, or 0 if the element is not a member of the set or if key does not exist. SyntaxThe basic syntax of the SMISMEMBER command is as follows: 1SMISMEMBER key member [member ...] Available SinceRedis version >= 6.2.0 Time ComplexityO(N) where N is the number of elements being checked for membership ACL Categories@read, @set,...

The Redis SMEMBERS command returns all the members of a set. If the set does not exist, it is treated as an empty set. SyntaxThe basic syntax of the SMEMBERS command is as follows: 1SMEMBERS key This command returns all the members of the set value stored at key. It has the same effect as running SINTER with one argument key. Available SinceRedis version >= 1.0.0 Time ComplexityO(N) where N is the set cardinality. ACL Categories@read, @set, @slow Return ValueReturns a set with all the...

The Redis SISMEMBER command checks if a given element is a member of the set. SyntaxThe basic syntax of the SISMEMBER command is as follows: 1SISMEMBER key member It returns if member is a member of the set stored at key. Available SinceRedis version >= 1.0.0 Time ComplexityO(1) ACL Categories@read, @set, @fast Return Value 0 if the element is not a member of the set, or when the key does not exist. 1 if the element is a member of the set. Example123456127.0.0.1:6379> SADD key1 a b c...