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Microsoft Azure

Object Storage — Blob Storage

Store massive unstructured data in containers with hot/cool/archive tiers and fine-grained access.

Blob Storage holds objects in containers inside a storage account. It is an HTTP API, not a filesystem.

A warehouse chain with shelves at different price points: front-of-store shelves for fast-moving goods (Hot), a back room for slow items (Cool), and a distant low-cost depot for archives you rarely fetch (Archive) — with optional duplicate warehouses in another city (GRS) in case one burns down.

Key Concepts

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    https://myaccount.blob.core.windows.net/container/images/2026/photo.jpg
                                                      ^^^^^^^^^^^^^^^^^^^ one name
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There are no folders. images/2026/ is part of the blob name, which is why listing is a prefix scan and "renaming a folder" means copying every blob.
images/2026/
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Three blob types, and the choice is usually made for you.
    block   files, media, backups -- the normal case
    append  log files, where you only ever add to the end
    page    VHDs and random-access reads and writes
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Access tiers control cost.
    Hot      frequent access, highest storage price
    Cool     30-day minimum, cheaper storage, retrieval fee
    Cold     90-day minimum, cheaper still
    Archive  180-day minimum, lowest price, HOURS to rehydrate
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Archive is offline. You cannot read a blob there; you rehydrate it first, which takes up to 15 hours at standard priority. That surprises people who treat it as just another tier.
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Early deletion is charged. Delete a Cool blob after a week and you still pay the 30-day minimum.
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Lifecycle rules do the moving automatically, by age or last access — Hot to Cool at 30 days, Archive at 90, delete at a retention limit.
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Redundancy is chosen per account.
    LRS    three copies, one datacentre
    ZRS    across availability zones in one region
    GRS    LRS plus async replication to the paired region
    RA-GRS GRS where the secondary is readable
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Access, strongest first. Entra ID with RBAC; then user-delegation SAS; then service SAS; and account keys last, which are effectively root and should be disabled where possible.
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ADLS Gen2 is Blob with a hierarchical namespace, which makes directory rename and POSIX ACLs real operations — the foundation for a data lake.
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What the interviewer is probing.1. "You moved blobs to Archive and now a read fails. Why?" Probing: that Archive is offline. Stalls: "It should just be slower." Moves up: Archive is offline storage — you must rehydrate first, which takes hours at standard priority.
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2. "What does an early deletion charge mean?" Probing: the minimum durations. Stalls: "There is none." Moves up: Cool, Cold and Archive have 30, 90 and 180-day minimums, so deleting sooner still bills the remainder.
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3. "LRS, ZRS or GRS?" Probing: the redundancy options. Stalls: "Whichever is cheapest." Moves up: LRS keeps three copies in one datacentre, ZRS spreads across zones in the region, GRS replicates asynchronously to the paired region — availability against cost.
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4. "How should an application authenticate to Blob Storage?" Probing: the access hierarchy. Stalls: "The account key." Moves up: Entra ID with RBAC, or a user-delegation SAS; account keys are effectively root and should be disabled where possible.
Use cases:blob-storagetiersredundancysas