Art of Codeartofcode
Lesson 1 of 16

11. Object / Blob Storage

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The picture to keep in your head
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11.1 What's the difference between block, file and object storage?
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11.2 Why not just store files in the database?
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11.3 What are buckets, keys and metadata?
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11.4 How do uploads work with presigned URLs?
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11.5 How do you upload very large files reliably?
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11.6 How does object storage avoid losing files?
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11.7 What are storage classes and lifecycle rules?
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11.8 How should files be served to users?
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11.9 What happens after a file is uploaded?
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Memory card and where the analogy breaks
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Practice: five levels
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Interview questions
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Solutions
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Sources
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Code lab
11. Object / Blob Storage

The picture to keep in your head

3 min read+10 XP

Every question below is answered twice: first at the cloakroom, so you can picture it, then in the system, so you know exactly how it works. Each answer ends with a memory code to lock it in.

The picture to keep in your head

At a wedding hall, you don't carry your coat around all evening. You hand it in at the cloakroom, and you get a small token: number 214. The attendant hangs the coat in a back room you never see, maybe with a tag ("wool, blue, belongs to Ravi"). Later you show the token and get your coat back, whole. You can't ask them to change one button on it while it's hanging there. And if you want your friend to collect it, you write a note: "please give coat 214 to the bearer, until 9 pm today".

That's object storage (also called blob storage): you hand over a whole file, you keep a small key to find it, and the service stores it safely in places you never see.

Our running example: the bookstore's files: book cover images, paid e-book PDFs, and photos customers attach to reviews. None of these belong in the orders database.

At the cloakroomIn the system
The coatThe object: an image, PDF, video, backup
Token number 214The object key, saved in your database
The tag on the coatMetadata: content type, size, your own labels
The back roomsThe storage service's machines and buildings
Getting the coat back wholeObjects are read and replaced whole, not edited in place
"Give coat 214 to the bearer until 9 pm"A presigned URL
Moving old coats to the basementStorage classes and lifecycle rules

the cloakroom mapped to object storage.

Figure: the cloakroom, mapped to the system. The database keeps tokens; object storage keeps coats.