Detailed explanation of priority queue implementation in Redis

Detailed explanation of Redis implementation of priority queue

Priority queue is a common data structure that can sort elements according to certain rules and maintain this order during queue operations, so that the elements taken out of the queue Elements are always processed according to a preset priority.

As an in-memory database, Redis also has advantages in implementing priority queues because of its fast and efficient data access capabilities. This article will introduce in detail the method and application of Redis to implement priority queue.

1. Basic principles of Redis implementation

The basic principle of Redis implementation of priority queue is to maintain an ordered list or ordered set. Each time an element is inserted, it is inserted in order according to the defined priority. ;Delete the first element directly every time an element pops up.

The following uses an ordered set as an example for demonstration. The same implementation method is also applicable to an ordered list. The following code and operations are performed in redis-cli.

1. Create an ordered collection
Use the ZADD command to create an ordered collection named priority_queue.

127.0.0.1:6379> ZADD priority_queue 5 "A"
(integer) 1
127.0.0.1:6379> ZADD priority_queue 3 "B"
(integer) 1
127.0.0.1:6379> ZADD priority_queue 4 "C"
(integer) 1
127.0.0.1:6379> ZADD priority_queue 2 "D"
(integer) 1
127.0.0.1:6379> ZADD priority_queue 1 "E"
(integer) 1

At this time, there are already five elements in priority_queue, and their values ​​​​and scores are: E (1), D (2), B (3), C (4), A (5) .

2. View the ordered set
Use the ZRANGE command to view the element list in priority_queue.

127.0.0.1:6379> ZRANGE priority_queue 0 -1 WITHSCORES
1) "E"
2) "1"
3) "D"
4) "2"
5) "B"
6) "3"
7) "C"
8) "4"
9) "A"
10) "5"

The result shows the list of elements of priority_queue, with the value and score of each element. where element E has a score of 1, D has a score of 2, and so on.

3. Compressed ordered set
Use the ZPOPMIN command to pop up the first element in priority_queue and delete it from the ordered set.

127.0.0.1:6379> ZPOPMIN priority_queue
1) "E"
2) "1"

The element E and its score 1 have been popped out. In the next step, E will no longer appear in the priority_queue.

The basic Redis principle of implementing priority queue is reflected in the above operations. Below are some further practical operations at the application level.

2. Application examples

1. Use priority queue to implement task scheduling
Task scheduling is an essential part of cluster computing. Considering that some tasks may require online interaction, We want to distribute the tasks on a node as evenly as possible to minimize task waiting time. At this time, priority queues can be used to implement task scheduling.

In the following example, we define two database instances, each instance handles different types of tasks. The priority queue is based on the list and uses the LPUSH and RPOP commands to implement a relatively simple task scheduling system.

127.0.0.1:6379> LPUSH db1 "task_1"
(integer) 1
127.0.0.1:6379> LPUSH db1 "task_2"
(integer) 2
127.0.0.1:6379> LPUSH db1 "task_3"
(integer) 3
127.0.0.1:6379> LPUSH db2 "task_4"
(integer) 1
127.0.0.1:6379> LPUSH db2 "task_5"
(integer) 2
127.0.0.1:6379> LPUSH db2 "task_6"
(integer) 3

In this example, db1 and db2 respectively represent two different database instances, each instance handles different types of tasks. Now we push the task into the corresponding queue.

127.0.0.1:6379> RPOP db1
"task_1"
127.0.0.1:6379> RPOP db1
"task_2"
127.0.0.1:6379> RPOP db2
"task_4"
127.0.0.1:6379> RPOP db1
"task_3"
127.0.0.1:6379> RPOP db2
"task_5"
127.0.0.1:6379> RPOP db2
"task_6"

Next, we use the RPOP command to remove tasks from the queue in sequence. Since the position of each task in the queue is uncertain, it does not have a clear priority. However, we can achieve priority control of different task types by using multiple queues.

2. Use priority queue to implement message filtering
Message filtering is a problem we often encounter in actual development. In a high-throughput system, messages need to be filtered and classified quickly, such as , group topics, mark important messages, etc. At this time, Redis' priority queue can be used to implement message filtering.

In the following example, we create two priority queues for filtering important and non-important messages respectively. The elements of each queue are message content and timestamp. Sorting by timestamp allows messages to be quickly sorted and filtered by time.

127.0.0.1:6379> ZADD important_messages 1628347641 "Important message 1"
(integer) 1
127.0.0.1:6379> ZADD important_messages 1628357641 "Important message 2"
(integer) 1
127.0.0.1:6379> ZADD important_messages 1628367641 "Important message 3"
(integer) 1
127.0.0.1:6379> ZADD important_messages 1628368641 "Important message 4"
(integer) 1
127.0.0.1:6379> ZADD important_messages 1628369641 "Important message 5"
(integer) 1
127.0.0.1:6379> ZADD normal_messages 1628367645 "Normal message 1"
(integer) 1
127.0.0.1:6379> ZADD normal_messages 1628368645 "Normal message 2"
(integer) 1
127.0.0.1:6379> ZADD normal_messages 1628369645 "Normal message 3"
(integer) 1
127.0.0.1:6379> ZADD normal_messages 1628370645 "Normal message 4"
(integer) 1

In this example, important_messages and normal_messages are the two priority queues we created, which are used for filtering important and non-important messages respectively. The elements of each queue are message content and timestamp.

127.0.0.1:6379> ZRANGE important_messages 0 -1
1) "Important message 1"
2) "Important message 2"
3) "Important message 3"
4) "Important message 4"
5) "Important message 5"
127.0.0.1:6379> ZRANGE normal_messages 0 -1
1) "Normal message 1"
2) "Normal message 2"
3) "Normal message 3"
4) "Normal message 4"

Next, we use the ZRANGE command to view the list of elements in the priority queue. The next step is to pop messages from the queue according to priority.

redis> ZPOPMIN important_messages
1) "Important message 1"
2) "1628347641"
redis> ZPOPMIN normal_messages
1) "Normal message 1"
2) "1628367645"

The above operations all use commonly used Redis commands to achieve fast and concise message filtering and sorting, which can meet relatively simple system requirements and can also be further expanded and optimized to complex scenarios.

3. Summary

Redis implementation of priority queue is a very useful technology. In actual development, we can use it to implement tasks scheduling, message filtering and other functions to improve system performance and reliability. Through the introduction of this article, we have learned about the basic implementation principles and application examples of Redis priority queue, and hope to help readers better master and apply this knowledge.

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