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This article is about how to improve the redis cache hit ratio. The editor thinks it is very practical, so share it with you as a reference and follow the editor to have a look.

Introduction of cache hit ratio

Hit: you can get the data you need directly through the cache.

Miss: you can't get the desired data directly through the cache. You need to query the database again or perform other operations. The reason may be that the cache does not exist at all or that the cache has expired.

Generally speaking, the higher the hit rate of the cache, the higher the benefit of using the cache, the better the performance of the application (the shorter the response time, the higher the throughput), and the stronger the ability to resist concurrency.

Thus it can be seen that in the Internet system with high concurrency, the hit rate of cache is an important indicator.

How to monitor the hit rate of the cache

In memcached, run the state command to view the status information of the memcached service, where cmd_get represents the total number of get and get_hits represents the total number of hits of the get, hit rate = get_hits/cmd_get.

Of course, we can also use some open source third-party tools to monitor the entire memcached cluster, the display will be more intuitive. Typical examples include: zabbix, MemAdmin and so on.

As shown in the figure: MemAdmin's monitoring statistics on the hit rate of memcached services

Similarly, in redis, you can run the info command to view the status information of the redis service, where keyspace_hits is the total number of hits, keyspace_misses is the total number of miss, hit rate = keyspace_hits/ (keyspace_hits+keyspace_misses).

The open source tool Redis-star can visualize the information related to redis services in a graphical way. At the same time, zabbix also provides related plug-ins to monitor redis services.

Several factors affecting cache hit rate

We mentioned the importance of cache hit ratio in the previous chapter. Here are several factors that affect cache hit ratio.

Business scenarios and business requirements

Caching is suitable for business scenarios where "read more and write less". On the contrary, the use of caching does not make much sense, and the hit rate will be very low.

Business requirements determine the timeliness requirements, which directly affect the cache expiration time and update strategy. The lower the timeliness requirements, the better for caching. In the case of the same key and the same number of requests, the longer the cache time, the higher the hit rate.

Caching is suitable for most business scenarios of Internet applications.

Cache design (granularity and strategy)

In general, the smaller the granularity of the cache, the higher the hit rate. Take a practical example to illustrate:

When caching a single object (for example, single user information), we need to update the cache or let the cache be removed only when the data corresponding to that object changes. When caching a collection (for example, all user data), when the data corresponding to any one of the objects changes, the cache needs to be updated or removed.

In another case, if you also need to get the data corresponding to the object elsewhere (for example, you also need to obtain a single user information elsewhere), if you are caching a single object, you can directly hit the cache, and vice versa, you can't hit it directly. This is more flexible and the cache hit rate is higher.

In addition, the cache update / expiration policy also directly affects the cache hit ratio. When the data changes, updating the cache value directly will have a higher hit rate than removing the cache (or letting the cache expire), and of course, the system will be more complex.

Cache capacity and infrastructure

If the cache capacity is limited, it is easy to cause cache invalidation and elimination (at present, most cache frameworks or middleware use LRU algorithm). At the same time, the technical selection of cache is also very important. for example, it is easy to have a stand-alone bottleneck by using the local cache built into the application, while using distributed cache is easy to expand after all. Therefore, it is necessary to make a good system capacity planning and consider whether it can be expanded or not. In addition, the efficiency and stability of different caching frameworks or middleware are also different.

Other factors

When a cache node fails, it is necessary to avoid cache invalidation and minimize the impact, which is also a special situation that architects need to consider. The typical practice in the industry is through consistent Hash algorithm, or through node redundancy.

Some friends may have this misunderstanding: since business requirements require high timeliness of data, and cache time will affect the cache hit rate, then the system should not use caching. In fact, this ignores an important factor-concurrency. Generally speaking, in the case of the same cache time and key, the higher the concurrency, the higher the cache benefit, even if the cache time is short.

The method of improving cache hit rate

From the architect's point of view, the application needs to get the data directly through the cache as much as possible and avoid cache invalidation. This is also a test of the architect's ability, which needs to be considered and weighed in terms of business requirements, cache granularity, cache strategy, technology selection, and so on. Focus on hot business with high frequency access and low timeliness requirements as much as possible, and improve the hit rate by means of cache preloading (preheating), increasing storage capacity, adjusting cache granularity, updating cache and so on.

For applications with high timeliness (or limited cache space), large content span (or random access), and low traffic, the cache hit rate may be very low for a long time, and the preheated cache may have expired before it is accessed.

Thank you for reading! This is the end of the article on "how to improve the hit rate of redis cache". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, you can share it for more people to see!

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