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Back I am in the shell, still connected to my locally running mongod process.

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Now I want to introduce you to capped collections and explain what capped collections are. Capped collections

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are a special type of collection which you have to create explicitly where you limit the amount of data

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or documents that can be stored in there and old documents will simply be deleted

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when well this size is exceeded,

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so it's basically a store where oldest data is automatically deleted when new data comes in.

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This can be efficient for high throughput application logs where you only need the most recent logs

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or as a caching service where you cache some data and if the data then was deleted because it

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hasn't been used in a while, well then you're fine with that and you can just re-add it. To create such a

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collection,

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first of all you need to use a database of course,

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I'll use a new database, performance

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and then you create it with the create collection command, we saw that when we added validation earlier

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in the course.

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Now first of all, you define a name and I'll just name it capped but of course, the name is totally up to

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you,

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this collection can be anything, can also be users, whatever you like,

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so I'll name it capped. The important part

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here are now the options,

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so the document you pass as a second argument to create collection. Here, the option you want to set is

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capped to true

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and this will turn this into a capped collection. Now by default, it will have a size limit of 4 bytes but

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you can set size to any other value and it will then automatically be turned into a multiple of 256 bytes.

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So you could for example set this to 10000 bytes,

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it will be converted a bit and that will be your size and you can also add a Max key,

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the size is required, Max is optional

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and Max allows you to also define the amount of data that can be stored in there, measured in documents,

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so I could cap this to three documents at most.

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With that, I hit enter and I get this collection and now I can access capped as a collection

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and in that capped collection, we can now of course insert a document just as before. We can insert a document

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named Max and

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this worked.

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I can now also add Manu and this worked

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and I can also add Anna and this worked.

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Now I have three documents in there which is our max size.

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I can now of course also find my documents like this and I get back Max, Manu and

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Anna. The important thing is for a capped collection, the order in which we retrieve the documents is always

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the order in which they were inserted. For a normal collection,

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that may be the case but it's not guaranteed,

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so for a normal collection, you always need to sort if you want to sort for example by ID.

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You can do that and then you'll sort by that index but by default, in a capped collection, you'll always get

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the insertion order. If you want to change the order and sort and reverse,

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there is a special key you can use and that is $natural, the natural order by which it is

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sorted and then for example use -1 and then it's sorted the other way around,

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just as a little side note, this is important to understand or to keep in mind.

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You also can create indexes in there and you got an index on ID by default

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but of course you don't have to. Now with that, I found my documents,

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now let's insert another document,

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let's insert Maria and keep in mind that our size limit was three.

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Now if I insert Maria, I don't get an error because the idea of a capped collection is not that it gives

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me an error when I insert too many documents but that it clears the oldest document which in my case

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should be Max because that was the first document I added.

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So if I now output all entries again, indeed we see Max is gone, there's only Manu,

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Anna and Maria.

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So this is a capped collection and there are some use cases where this can be a good idea,

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why would you use a capped collection instead of a normal collection then?

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Well because of the automatic clear up. You can keep this collection fairly small and therefore, efficient

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to work with

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and you don't have to worry about manually deleting old data,

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so for use cases where you need to get rid of old data anyways with new data coming in or where you

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need high throughput and it's ok if you lose old data at some point, like for caching, then the capped

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collection is something you should keep in mind as a tool to improve performance.

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Obviously this is not a solution for storing anything like products or users or blog posts,

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that would be horrible because you would simply lose some users, some products or some blog posts.