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This article mainly explains "what is the slow query and subscription mode in Redis". The content in the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "what is the slow query and subscription mode in Redis".

Slow query

The slow query log is the execution time of each command when the system executes the command, and when the threshold is exceeded, the command is recorded. [

Redis command execution flow

Send a command

Command queue

Command execution

Return the result

Among them, the execution of commands is the time of slow query statistics.

Slow query two configuration parameters

Slowlog-log-slower-than: default threshold (in milliseconds). If a "very slow" command is executed, the execution time exceeding the threshold will be recorded.

Slowlog-max-len: sets the maximum number of slow log entries to be stored

Publish and subscribe model

Redis provides publish and subscribe function, which can be used for message transmission. The publish and subscribe mechanism of Redis consists of three parts, publisher, subscriber and Channel.

Publish and subscribe function

Publish command is used to send messages.

Subscribe to a channel by using the subscribe command

Pattern matching: subscribe to multiple channels at the same time, and the command is PSUBSCRIBE

Redis expiration time processing

Active processing

Timed processing. Create a timer when setting the expiration time, and delete the operation immediately when the expiration time arrives. This operation is immediate. No matter how many expired Key or server operation is in this period, it will be deleted. It is not very friendly to CPU.

To delete periodically, to delete periodically is to set a time interval to detect whether there is an expired key in each period of time, and if so, to perform deletion.

Passive processing

When the expired key is accessed again, it will judge whether the key has expired or not. If it has expired, it will delete it and return NIL. This method is CPU-friendly and will not take up CPU on other expired key, but it is not memory-friendly. A key has expired, but it will not be deleted before it is operated, and it still takes up memory space. If a large number of expired Key is not operated again, a lot of memory space will be wasted.

The treatment of expired keys by 3.RDB and AOF

If you execute the save or bgsave command to create a RDB, the program will check the keys in the database, and expired keys will not be saved to the newly created RDB file.

When the expired key is lazy or periodically deleted, the program appends (append) a DEL command to the AOF file to explicitly record that the key has been deleted.

During AOF rewriting, the program checks the keys in the database, and expired keys are not saved to the rewritten AOF file.

Memory recovery

Noeviction: the default policy, which does not delete any data, rejects all write operations and returns a client-side error message, and Redis only responds to read operations.

Volatitle-rlu: delete the key with the timeout property set according to the LRU algorithm until enough space is freed. If there are no key objects to delete, fall back to the noeviction policy.

Allkeys-lru: delete the key according to the LRU algorithm, regardless of whether the data has set the timeout property or not, until enough space is freed.

Allkeys-random: randomly delete all keys until there is enough space available.

Volatitle-random: randomly delete expired keys until there is enough space available.

Volatitle-ttl: deletes the data that is about to expire recently based on the ttl property of the key object. If not, fall back to the noeviction policy

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