Database troubleshooting challenges
In real failure scenarios such as a full disk, lock queues, replication lag and a broken backup, you find the cause, fix it and write a short incident report.
Level 301 · 15 lessons · 12 practice labs
Lessons
Connections and access
- The application can’t connect
- Too many connections
- Permission denied
- Authentication fails after an upgrade
- Short task: connection problems of three services
Performance
- A query that suddenly slowed down
- A lock queue
- The disk is full
- Redis keys are disappearing
- Short task: the lunchtime slowdown
Data and continuity
- The replica is falling behind
- The backup that won’t restore
- Garbled characters
- Duplicated records
- Capstone: a bad Monday morning
Practice labs
- Why can't the app connect to the database: Starting from the error message, check listen address, port and firewall to fix a connection problem.
- Relieve a server out of connections: Find which client causes "too many connections", clear idle ones and apply a lasting fix.
- Fix logins broken after an upgrade: Find that old clients cannot log in after an upgrade because of the auth method, and fix it securely.
- Find the query that suddenly slowed down: Compare statistics and plans to find why yesterday's fast report query changed its plan.
- Find the session at the head of a lock queue: Find the session holding an open transaction behind dozens of waiting queries and end it safely.
- Solve the lunchtime slowdown: Trace a slowdown that starts every day at noon through metrics, logs and scheduled jobs and remove the cause.
- Find what filled the disk: When the database can no longer write, find the large files on disk and free space safely.
- Solve the case of vanishing Redis keys: Link keys that should persist but disappear to the eviction policy and memory limit, then fix it.
- Stop log files that grow overnight: Find the source of transaction or log files that fill the disk every night and apply lasting limits and rotation.
- Repair mojibake in customer names: Find the encoding error behind customer names showing "ÅŸ" and convert the data to the right encoding.
- Find and prevent duplicate records: Find duplicate orders from double form submissions, merge them and stop repeats with a unique constraint.
- Sort out a bad Monday morning: The replica is behind, a backup is broken and connections are full; triage and fix all three and write an incident note.
About this course
In real failure scenarios such as a full disk, lock queues, replication lag and a broken backup, you find the cause, fix it and write a short incident report.
What you will learn
- Connections and access
- Performance
- Data and continuity
Prerequisites
You should be comfortable with the level 101 and 201 topics of the same path.
Lab environment
Lessons run in your own Kali Linux terminal in the browser, connected to a private lab network. Nothing to install.
Frequently asked questions
- Is this course right for my level?
- The course is at level 301; it assumes you know the topics of the lower-level courses.
- Is there anything to install?
- No. Every lesson runs in your own lab environment that opens in the browser.
- How do lessons work?
- Each lesson has 8 to 10 steps and a short quiz. Early steps show the command to type, later steps give less help; if two attempts fail, the solution opens.
- Can I try it for free?
- Yes. The first three lessons of every course are free.