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🎯 Junior SRE Interview

Questions + Answers

🔹 Operating System & Linux

  • What does the top command show?

Answer: Running processes, CPU usage, memory usage, and load average.

  • What is the difference between a hard link and a soft link?

Answer:

  • A hard link points directly to the file’s inode;

  • A soft link (symlink) is a shortcut pointing to the file path.

  • What is the purpose of the /etc/hosts file?

Answer: Manual mapping of hostnames to IP addresses.

  • Difference between systemctl start and systemctl enable?

Answer:

  • start runs the service immediately;

  • enable makes it run automatically on boot.

  • Difference between process and thread?

Answer:

  • Process = independent program with its own memory space.

  • Thread = unit of execution inside a process sharing the same memory.

  • Difference between kill -9 and kill -15?

Answer:

  • -15 is a graceful termination signal (can be handled);

  • -9 forces immediate kill (cannot be handled).

  • What does df -h show?

Answer: Disk usage, total, used, and available space on partitions.

  • What is the difference between swap and RAM?

Answer:

  • RAM is fast volatile memory;

  • SWAP uses disk as backup memory, much slower.

  • Purpose of /etc/resolv.conf?

Answer: Configures DNS servers used by the system.

🔹 Networking

  • What does ping do?

Answer: Sends ICMP packets to check reachability and latency.

  • Difference between TCP and UDP?

Answer:

  • TCP is connection-oriented and reliable;

  • UDP is connectionless and faster but unreliable.

  • If DNS is not working but the network is fine, what do you check?

Answer: Verify /etc/resolv.conf, test with dig or nslookup, check the DNS server.

  • What is port 80 used for?

Answer: HTTP.

  • What is port 443 used for?

Answer: HTTPS (HTTP over TLS).

  • Difference between public IP and private IP?

Answer:

  • Public IP is routable on the internet;

  • private IP is only used in local networks.

  • What does NAT do?

Answer: Translates private IP addresses to public IPs for internet access.

  • If ping fails but traceroute shows a path, what’s the reason?

Answer: ICMP may be blocked by a firewall.

  • Difference between Layer 4 and Layer 7 load balancing?

Answer:

  • L4 → based on IP/port (TCP/UDP).
  • L7 → based on application data (HTTP headers, paths).

🔹 Databases

  • Difference between RDBMS and NoSQL?

Answer:

  • RDBMS (MySQL, PostgreSQL) stores structured data with ACID compliance;

  • NoSQL (MongoDB, Cassandra) handles unstructured data, scales horizontally.

  • What is database replication?

Answer: Keeping identical copies of data across multiple servers for availability and redundancy.

  • Difference between Backup and Snapshot?

Answer:

  • Backup = copy of data stored elsewhere;
  • Snapshot = point-in-time copy of a system/disk (usually in the same environment).

🔹 Security

  • Why is SSH key authentication better than passwords?

Answer: Stronger security, prevents brute-force, easier for automation.

  • Difference between symmetric and asymmetric encryption?

Answer:

  • Symmetric → same key for encryption/decryption (fast).

  • Asymmetric → public/private keys (more secure, slower).

  • Why is the Principle of Least Privilege important?

Answer: Reduces risk by limiting user and service permissions.

🔹 Monitoring & Observability

  • Difference between Metrics, Logs, and Traces?

Answer:

  • Metrics → numeric time-series data (CPU, latency)
  • Logs → textual events

  • Traces → request path across distributed systems

  • Difference between an Alert and a Notification?

Answer:

  • Alert is triggered when a condition is violated;

  • Notification is the delivery method to the team.

  • Difference between Histogram and Gauge?

Answer:

  • Histogram → distribution of values in buckets;

  • Gauge → current value of a metric.

  • What is a Canary Release?

Answer: Gradual rollout of a new version to a small subset of users to reduce risk.

🔹 CI/CD & DevOps

  • Difference between Continuous Integration and Continuous Deployment?

Answer:

  • CI = automatically testing and building code.

  • CD = automatically deploying code to production.

  • If a build fails in GitLab CI or Jenkins, what’s your first step?

Answer: Check logs, identify the failing stage, rollback or apply a quick fix.

  • What is Blue-Green Deployment?

Answer: Having two environments (blue & green) and switching traffic between them for zero-downtime deployment.

🔹 Cloud & Infrastructure

  • Difference between IaaS, PaaS, and SaaS?

Answer:

  • IaaS → Infrastructure as a Service (VMs, storage).
  • PaaS → Platform as a Service (Heroku, App Engine).

  • SaaS → Software as a Service (Gmail, Slack).

  • What is Auto Scaling?

Answer: Automatically adjusts the number of instances based on workload.

  • Difference between Region and Availability Zone?

Answer:

  • Region = geographic area (e.g., Europe West).
  • AZ = independent datacenters within a region.

🔹 SRE Culture

  • Difference between SLA, SLO, and SLI?

Answer:

  • SLA → agreement with customers
  • SLO → internal service quality target

  • SLI → numeric indicator (e.g., latency <200ms for 99% of requests)

  • What is a Postmortem and why is it important?

Answer: A report after an incident including timeline, root causes, and corrective actions; ensures learning and prevention.

  • What is an Error Budget?

Answer: Allowed amount of downtime/errors over a period, based on the SLO.

  • What is Blameless Culture?

Answer: Focusing on learning and fixing issues instead of blaming individuals.