In high availability database setups, what is the key difference between clustering and mirroring in terms of how servers share data and handle failover?

Introduction / Context:
Database administrators often use techniques such as clustering and mirroring to improve availability and reliability. Although both approaches can reduce downtime, they work in different ways and protect different parts of the system. Interview questions about clustering versus mirroring test whether you understand these architectural differences rather than just the marketing names.

Given Data / Assumptions:

• We are dealing with relational database systems that support both clustering and mirroring features.
• Clustering usually involves multiple nodes that share common storage or appear as a single virtual instance.
• Mirroring involves maintaining one or more copies of a database on separate servers.

Concept / Approach:
In a typical database cluster, two or more server nodes share the same underlying storage, for example a storage area network. Only one node actively serves requests at a time. If that node fails, another node in the cluster can take over and attach the same database files, providing instance level failover. In mirroring, each server maintains its own set of database files that are kept in sync by continuously sending transaction logs. Failover occurs by switching clients from the principal database to the mirror database. The key difference is that clustering shares storage among nodes, while mirroring maintains separate copies on different storage systems.

Step-by-Step Solution:
Step 1: Identify that clustering groups servers around shared storage and presents a single database instance.Step 2: Recognise that mirroring replicates database content to a secondary server with its own storage.Step 3: Option A states that clustering uses shared storage and provides instance level failover, while mirroring keeps independent copies and fails over at database level.Step 4: Option B reverses details about how storage is shared and is not accurate.Step 5: Options C and D trivialise or equate the two methods, so option A is the correct answer.

Verification / Alternative check:
In platforms such as SQL Server, a failover cluster instance relies on Windows clustering and shared disks, while database mirroring maintains separate principal and mirror databases on different servers. Documentation explicitly states that clustering protects the entire instance and depends on shared storage, whereas mirroring focuses on individual databases and stores copies on separate disks. This confirms the explanation provided in option A.

Why Other Options Are Wrong:
Option B misrepresents clustering and mirroring by swapping their storage characteristics. Option C wrongly restricts them to unrelated environments such as printing services. Option D claims there is no difference, which is not accurate from either an implementation or a configuration perspective.

Common Pitfalls:
People sometimes assume that clustering automatically protects against storage failure because there are multiple servers. In reality, when nodes share the same storage, that storage can become a single point of failure and must be protected separately. Another pitfall is confusing replication or log shipping with mirroring. When answering exam questions, always emphasise that clustering involves shared storage and instance level failover, whereas mirroring keeps separate database copies and fails over at database level.

Final Answer:
In clustering, multiple servers share the same database storage and provide automatic failover at the instance level, while in mirroring each server maintains its own copy of the database and failover happens at the database level.