Storage access keys and SAS tokens in Azure - Time & Space Complexity

Time Complexity: Storage access keys and SAS tokens

O(n)

Understanding Time Complexity

We want to understand how the time to access Azure Storage changes when using access keys versus SAS tokens.

Specifically, how the number of operations grows as we perform more storage requests.

Scenario Under Consideration

Analyze the time complexity of generating and using SAS tokens versus using storage access keys.

// Using storage access key directly
var client = new BlobServiceClient(storageAccountUrl, new StorageSharedKeyCredential(accountName, accountKey));
await client.GetBlobContainerClient(containerName).GetBlobClient(blobName).DownloadAsync();

// Using SAS token
var sasToken = GenerateSasToken(containerName, permissions, expiry);
var sasClient = new BlobServiceClient(storageAccountUrl + "?" + sasToken);
await sasClient.GetBlobContainerClient(containerName).GetBlobClient(blobName).DownloadAsync();
    

This code shows two ways to access blobs: one with a storage key, one with a SAS token.

Identify Repeating Operations

Look at what repeats when accessing storage multiple times.

• Primary operation: Blob download requests to Azure Storage service.
• How many times: Once per blob access, regardless of key or SAS token.
• Additional operation for SAS: SAS token generation, which happens once per token creation.

How Execution Grows With Input

Each blob download requires one request. SAS token generation is a one-time cost.

Input Size (n)	Approx. Api Calls/Operations
10	10 blob downloads + 1 SAS token generation (if using SAS)
100	100 blob downloads + 1 SAS token generation
1000	1000 blob downloads + 1 SAS token generation

Pattern observation: The number of blob download calls grows linearly with n; SAS token generation stays constant.

Final Time Complexity

Time Complexity: O(n)

This means the time grows directly in proportion to the number of storage accesses.

Common Mistake

[X] Wrong: "Generating SAS tokens for each request is necessary and scales well."

[OK] Correct: Generating a SAS token each time adds extra overhead and is inefficient; usually one token is generated and reused for many requests.

Interview Connect

Understanding how repeated storage access scales helps you design efficient cloud apps and shows you grasp practical cloud performance.

Self-Check

What if we generated a new SAS token for every blob access? How would the time complexity change?

Practice

1. What is the main difference between a storage access key and a SAS token in Azure Storage?

easy

A. SAS tokens provide full access; storage access keys provide limited access.

B. Both provide full access but SAS tokens expire faster.

C. Storage access keys provide full access; SAS tokens provide limited, temporary access.

D. Storage access keys are only for blobs; SAS tokens are for all storage types.

Solution

1. Step 1: Understand storage access keys

   Storage access keys give full control over the entire storage account without restrictions.

2. Step 2: Understand SAS tokens

   SAS tokens grant limited, time-bound access to specific resources or actions within the storage account.

3. Final Answer:

   Storage access keys provide full access; SAS tokens provide limited, temporary access. -> Option C

4. Quick Check:

   Access scope = Full vs Limited [OK]

Hint: Full control = access keys; limited control = SAS tokens [OK]

Common Mistakes:

• Confusing SAS tokens as full access keys
• Thinking SAS tokens never expire
• Assuming access keys are resource-specific

2. Which of the following is the correct way to generate a SAS token for a blob container using Azure CLI?

easy

A. az storage blob generate-sas --container-name mycontainer --permissions rwl --expiry 2024-12-31T23:59:00Z --account-name mystorage

B. az storage container generate-sas --name mycontainer --permissions rwl --expiry 2024-12-31T23:59:00Z --account-name mystorage

C. az storage account generate-sas --name mycontainer --permissions rwl --expiry 2024-12-31T23:59:00Z

D. az storage generate-sas --container mycontainer --permissions rwl --expiry 2024-12-31T23:59:00Z

Solution

1. Step 1: Identify the correct Azure CLI command for container SAS

   The command to generate a SAS token for a container is az storage container generate-sas.

2. Step 2: Check parameters

   az storage container generate-sas --name mycontainer --permissions rwl --expiry 2024-12-31T23:59:00Z --account-name mystorage correctly uses container name, permissions, expiry, and account name parameters.

3. Final Answer:

   az storage container generate-sas --name mycontainer --permissions rwl --expiry 2024-12-31T23:59:00Z --account-name mystorage -> Option B

4. Quick Check:

   Correct CLI command and parameters [OK]

Hint: Use 'container generate-sas' for containers, not 'blob generate-sas' [OK]

Common Mistakes:

• Using 'az storage blob generate-sas' for containers
• Omitting required parameters like --account-name
• Using incorrect command syntax

3. Given this SAS token snippet for a blob: ?sv=2023-01-01&ss=b&srt=sco&sp=rl&se=2024-07-01T00:00:00Z&st=2024-06-01T00:00:00Z&spr=https&sig=abc123, what permissions does it grant?

medium

A. Read and List permissions on blob service

B. Read and Write permissions on blob service

C. Full control on storage account

D. Only write permission on blob service

Solution

1. Step 1: Decode the SAS token parameters

   sp=rl means permissions are Read (r) and List (l).

2. Step 2: Understand service and resource types

   ss=b means the service is Blob storage.

3. Final Answer:

   Read and List permissions on blob service -> Option A

4. Quick Check:

   sp=rl means Read+List [OK]

Hint: Check 'sp' parameter for permissions letters [OK]

Common Mistakes:

• Confusing 'rl' with read-write
• Assuming full control from SAS token
• Ignoring service type 'ss' parameter

4. You created a SAS token with --permissions rwd but users report they cannot delete blobs. What is the likely cause?

medium

A. The SAS token lacks the 'Delete' permission 'x'.

B. The SAS token expired before use.

C. The storage account keys were regenerated.

D. The SAS token was generated for the wrong resource type.

Solution

1. Step 1: Understand permissions for delete

   Delete permission is represented by 'd', so 'rwd' includes delete.

2. Step 2: Check resource type relevance

   If the SAS token is generated for a container but used on blobs, or vice versa, permissions may not apply correctly.

3. Final Answer:

   The SAS token was generated for the wrong resource type. -> Option D

4. Quick Check:

   Permissions + resource type must match [OK]

Hint: Match SAS token resource type with intended use [OK]

Common Mistakes:

• Assuming 'x' is delete permission
• Ignoring resource type mismatch
• Not checking token expiry

5. You want to allow a third-party app to upload files to a specific blob container for 2 hours without exposing your storage account keys. Which approach is best?

hard

A. Generate a SAS token scoped to the container with write permission and 2-hour expiry.

B. Create a new storage account for the app.

C. Share the storage account access keys with the app.

D. Use a SAS token with full permissions and no expiry.

Solution

1. Step 1: Avoid sharing storage account keys

   Sharing keys gives full control and is insecure for third parties.

2. Step 2: Use SAS token with limited scope and expiry

   Generating a SAS token scoped to the container with write permission and a 2-hour expiry limits access and duration.

3. Step 3: Evaluate other options

   Creating a new storage account is unnecessary overhead; SAS token with full permissions and no expiry is insecure.

4. Final Answer:

   Generate a SAS token scoped to the container with write permission and 2-hour expiry. -> Option A

5. Quick Check:

   Limited scope + expiry = SAS token best practice [OK]

Hint: Use SAS tokens for limited, timed access, not keys [OK]

Common Mistakes:

• Sharing full access keys with third parties
• Using SAS tokens without expiry
• Creating unnecessary storage accounts