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Instructor: Our next technique is the caching.

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Caching is used to store

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frequently accessed data close to the API.

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So if we have a data that the API access frequently

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then it might be a good idea to store it somewhere close

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to the API code instead of a database

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that is usually stored on a different server

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or sometimes even in a different data center.

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Now, cache engines usually store the data in-memory

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so the retrieval of the data

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from the cache is extremely fast.

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There is no IO operation, there is no network.

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It simply goes to the memory and retrieve the data.

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Now, when working with caching,

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always make sure to set expiration and invalidation

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so that the data in the cache will automatically expire

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after a predefined interval.

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And also the data will be invalidated,

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which means deleted from the cache when something happened.

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For example, if you store in the cache

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the list of users of the system

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then when this list is modified

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and a new user is added,

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then the data in the cache should be invalidated

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because it's not relevant anymore.

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This data is called stale data,

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and you should always try your best

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to avoid having stale data in the cache.

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In this code example,

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you see an implementation of ResponseCache in ASP.NET core.

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And as you can see, this cache has a duration of 30 seconds.

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Now, what this means is that the response

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from this web API, the item web API,

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is cached on the web server for 30 seconds.

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So after the first request,

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any subsequent request for 30 seconds will get the response

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from the cache and not from the code.

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And this way, the time of executing the code

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and probably going to the database,

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is saved and the performance vastly improve.