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The word blob is an acronym for binary large object. Blobs are basically files like those that you store on your computer (or tablet, mobile device, etc.). They can be pictures, Microsoft Excel files, HTML files, virtual hard disks (VHDs)—pretty much anything.

The Azure Blob service gives you the ability to store files and access them from anywhere in the world by using URLs, the REST interface, or one of the Azure SDK storage client libraries. Storage client libraries are available for multiple languages, including .NET, Node.js, Java, PHP, Ruby, and Python. To use the Blob service, you have to create a storage account. Once you have a storage account, you can create containers, which are similar to folders, and then put blobs in the containers. You can have an unlimited number of containers in a storage account and an unlimited number of blobs in each container, up to the maximum size of a storage account, which is 500 TB. The Blob service supports only a single-level hierarchy of containers; in other words, containers cannot contain other containers.

Azure Storage supports three kinds of blobs: block blobs, page blobs, and append blobs.

 Block blobs are used to hold ordinary files up to 195 GB in size (4 MB × 50,000 blocks). The primary use case for block blobs is the storage of files that are read from beginning to end, such as media files or image files for websites. They are named block blobs because files larger than 64 MB must be uploaded as small blocks, which are then consolidated (or committed) into the final blob.

 Page blobs are used to hold random-access files up to 1 TB in size. Page blobs are used primarily as the backing storage for the VHDs used to provide durable disks for Azure Virtual Machines (Azure VMs), the IaaS feature in Azure Compute. They are named page blobs because they provide random read/write access to 512-byte pages.

 Append blobs are made up of blocks like block blobs, but they are optimized for append operations. These are frequently used for logging information from one or more sources into the same blob. For example, you might write all of your trace logging to the same append blob for an application running on multiple VMs. A single append blob can be up to 195 GB.

Blobs are addressable through a URL, which has the following format:


The Blob service supports only a single physical level of containers. However, it supports the simulation of a file system with folders within the containers by allowing blob names to contain the '/' character. The client APIs provide support to traverse this simulated file system. For example, if you have a container called animals and you want to group the animals within the container, you could add blobs named cats/tuxedo.png, cats/marmalade.png, and so on. The URL would include the entire blob name including the “subfolder,” and it would end up looking like this:


When looking at the list of blobs using a storage explorer tool, you can see either a hierarchical directory tree or a flat listing. The directory tree would show cats as a subfolder under animals and would show the .png files in the subfolder. The flat listing would list the blobs with the original names, cats/tuxedo.png and cats/marmalade.png. You also can assign a custom domain to the storage account, which changes the root of the URL, so you could have something like this:


This eliminates cross-domain issues when accessing files in blob storage from a website because you could use the company domain for both. Blob storage also supports Cross-Origin Resource Sharing (CORS) to help with this type of cross-source usage.

Source of Information : Microsoft Azure Essentials Fundamentals of Azure Second Edition