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LEARN MONITORING SYSTEM IN C

Learn Systems Monitoring by Building a Zabbix Clone in C

Goal: To deeply understand the principles of modern IT infrastructure monitoring by building a simplified, Zabbix-like monitoring system from scratch in C. This project will teach you networking, concurrency, file I/O, and low-level systems programming on Linux.

Why Build a Monitoring System From Scratch?

Zabbix is a powerful, enterprise-grade monitoring solution. Using it is a valuable skill, but building it is a masterclass in systems programming. By creating your own simplified version, you will move beyond being a user of tools to being an architect of systems. You will learn how monitoring agents collect data, how servers process it, and how the entire client-server architecture functions at the lowest levels.

After completing these projects, you will:

  • Master socket programming in C for creating robust client-server applications.
  • Understand how to gather system metrics (CPU, memory, network) by reading from the /proc filesystem on Linux.
  • Implement both passive (server-polled) and active (agent-initiated) monitoring checks.
  • Design a simple time-series data storage system and a basic trigger/alerting engine.
  • Gain proficiency in multi-process programming using fork() and Inter-Process Communication (IPC).

Core Concept Analysis: The Architecture of a Monitoring System

A system like Zabbix has a clear client-server architecture. We will replicate this with our C program, “CZabb”.

  1. The Agent (czabb_agentd): A lightweight daemon running on the machine you want to monitor. Its only job is to collect and report data.
    • It listens on a TCP port for requests from the server (passive checks).
    • It can also proactively send data to the server (active checks).
    • It gets metrics by reading files like /proc/stat and /proc/meminfo.
  2. The Server (czabb_server): The central brain that collects, stores, and analyzes data from all agents.
    • Pollers: Processes that actively connect to agents to request data.
    • Trappers: Processes that listen for incoming data from active agents.
    • Database: A simple file-based system to store historical metric data.
    • Trigger Engine: Logic that analyzes incoming data and fires alerts.
                  ┌───────────────────────────┐
                  │       CZabb Server        │
                  │ ┌──────┐   ┌──────────┐ │
                  │ │Poller├─┐ │ Trigger  │ │
                  │ └──────┘ │ │  Engine  │ │
                  │ ┌──────┐ │ └──────────┘ │
┌──────────────┐  │ │Trapper|◀┘ ┌──────────┐ │  ┌──────────────┐
│ Agent (Host A) │◀─┼─└──────┘   │ Database │ │──▶│ Agent (Host B) │
└───────┬──────┘  │ (Passive)  │(Flat files)│ │  └───────▲──────┘
        │         └──────┬─────┴──────────┴─┘          │
        │ (Active Check) │ (Passive Check)              │
        └────────────────▶                              │

Environment Setup

  • OS: Linux is strongly recommended, as we will use the /proc filesystem.
  • Compiler & Tools: gcc (or clang) and make.
  • Lab: You need at least two machines. Vagrant with VirtualBox is a perfect way to create a simple lab with a “server” VM and an “agent” VM on your local machine.

Project List

Project 1: The Metric Collector

  • File: LEARN_MONITORING_SYSTEM_IN_C.md
  • Main Programming Language: C
  • Alternative Programming Languages: N/A
  • Coolness Level: Level 3: Genuinely Clever
  • Business Potential: 1. The “Resume Gold”
  • Difficulty: Level 2: Intermediate
  • Knowledge Area: Linux Systems Programming / File I/O
  • Software or Tool: C, /proc filesystem
  • Main Book: “Advanced Programming in the UNIX Environment” (APUE) by Stevens & Rago, Ch. 3-4 (File I/O, Files and Directories).

What you’ll build: A simple C program that, when run, reads specific files from /proc and prints formatted system metrics to the console. For example, it should be able to report the current memory usage and CPU load.

Why it teaches the core of data collection: This project isolates the first piece of the puzzle: getting the data. You’ll learn how Linux exposes kernel data through a virtual filesystem and how to parse simple text files in C to extract valuable information.

Core challenges you’ll face:

  • Reading and parsing /proc/meminfo → maps to opening a file, reading it line-by-line (getline or fgets), and using sscanf or strtok to extract values like MemTotal and MemFree.
  • Calculating CPU load → maps to reading the first line of /proc/stat, which contains aggregate CPU times. You must read it once, wait a second (sleep(1)), read it again, and calculate the percentage of time spent in different states (user, system, idle) during that interval.

Key Concepts:

  • The /proc Filesystem: man 5 proc
  • File I/O in C: fopen, fgets, fclose, sscanf.
  • CPU Statistics: A good blog post explaining how to interpret /proc/stat.

Difficulty: Intermediate Time estimate: Weekend Prerequisites: Solid C programming fundamentals.

Real world outcome: Running ./metric_collector will print:

Memory Free: 4123MB / 8096MB (50.9%)
CPU Load: 15.2%

Learning milestones:

  1. You can read and print the contents of a file from /proc → You understand basic file I/O.
  2. You can parse specific values from /proc/meminfo → You can process text and extract data.
  3. You can correctly calculate CPU load percentage over a time interval → You have implemented a slightly more complex, stateful check.

Project 2: The Agent Daemon (Passive Checks)

  • File: LEARN_MONITORING_SYSTEM_IN_C.md
  • Main Programming Language: C
  • Alternative Programming Languages: N/A
  • Coolness Level: Level 4: Hardcore Tech Flex
  • Business Potential: 1. The “Resume Gold”
  • Difficulty: Level 3: Advanced
  • Knowledge Area: Network Programming / Sockets
  • Software or Tool: C Sockets API
  • Main Book: “Beej’s Guide to Network Programming”.

What you’ll build: A TCP server that listens on a port (e.g., 10050). When a client connects, the server reads a string “key” (e.g., mem.free or cpu.load). Based on this key, it calls the appropriate logic from Project 1, and sends the resulting value back to the client as a string.

Why it teaches the agent model: This turns your simple collector into a network service. It’s the foundation of the passive check model, where the server dictates what information it wants. This project is a deep dive into fundamental TCP socket programming in C.

Core challenges you’ll face:

  • Opening a listening socket → maps to the socket, bind, and listen system calls.
  • Accepting connections → maps to the accept call, which blocks until a client connects and returns a new file descriptor for communication.
  • Reading and writing from a socket → maps to using read/recv and write/send on the new file descriptor.
  • Handling multiple requests → maps to putting the accept call in a loop to handle one client after another.

Key Concepts:

  • TCP Sockets: Beej’s Guide, Sections 5 & 6.
  • Client-Server Model: A fundamental networking concept.
  • Blocking System Calls: Understanding how accept and read can pause your program.

Difficulty: Advanced Time estimate: 1-2 weeks Prerequisites: Project 1, understanding of pointers and memory management in C.

Real world outcome: You will run ./czabb_agentd. It will sit and wait. From another terminal, you can use a simple tool like netcat or telnet to test it: echo "cpu.load" | nc localhost 10050. The agent will respond with the CPU load and close the connection.

Implementation Hints:

  • Follow Beej’s Guide closely for the server setup. The sequence of calls (socket, setsockopt, bind, listen, accept) is critical.
  • After accept returns a new socket descriptor, use that for read and write. Close it before looping back to accept again.
  • Create a simple if/else if structure or a function map to route the incoming key string to the correct metric-gathering function.

Learning milestones:

  1. Your program successfully listens on a TCP port → Your socket setup is correct.
  2. You can connect to it with netcat → The accept loop is working.
  3. Sending a valid key returns the correct metric value → You have successfully linked your network logic to your data collection logic.
  4. The server handles multiple connections sequentially without crashing → Your connection handling is robust.

Project 3: The Server Poller

  • File: LEARN_MONITORING_SYSTEM_IN_C.md
  • Main Programming Language: C
  • Alternative Programming Languages: N/A
  • Coolness Level: Level 4: Hardcore Tech Flex
  • Business Potential: 1. The “Resume Gold”
  • Difficulty: Level 3: Advanced
  • Knowledge Area: Network Programming / Sockets
  • Software or Tool: C Sockets API
  • Main Book: “Beej’s Guide to Network Programming”.

What you’ll build: The other half of the passive check system. This program will act as the Zabbix Server. It will read a simple configuration file that lists agent IPs and the metric keys to check. It will then loop through them, connect to the agent from Project 2, send the key, read the value, and print a formatted result.

Why it teaches the server model: This completes the full client-server loop. You’ll implement the client side of socket programming and build the logic for a “poller,” which is a core component of the Zabbix server.

Core challenges you’ll face:

  • Client-side networking → maps to the socket and connect system calls.
  • Parsing a configuration file → maps to reading a file and breaking down lines like 127.0.0.1:cpu.load into an IP address and a key.
  • Creating a main loop → maps to iterating through the list of checks, performing each one, and then sleeping for an interval before repeating.

Key Concepts:

  • TCP Client Sockets: Beej’s Guide, Section 5.
  • Address Resolution: Using getaddrinfo() to prepare the server address structure for connect.

Difficulty: Advanced Time estimate: 1 week Prerequisites: Project 2.

Real world outcome: With the agent from Project 2 running on one VM, you will run ./czabb_server on your other VM. The server will print output like this every 60 seconds:

[2023-10-27 10:00:00] Polling 192.168.56.11 for cpu.load... Value: 12.5
[2023-10-27 10:00:01] Polling 192.168.56.11 for mem.free... Value: 3876

Implementation Hints:

  • For each check, your program will socket(), connect(), write() the key, read() the value, close() the socket.
  • The main logic will be a while(1) loop that iterates through your list of configured checks and then calls sleep(60).
  • Start with a hardcoded check before you implement the config file parsing.

Learning milestones:

  1. The server can connect to the agent and get a single hardcoded metric → Your client socket logic is working.
  2. The server can parse a config file with multiple checks → Your config parsing logic is correct.
  3. The server runs in a continuous loop, polling all items every minute → You have a working poller.

Project 4: The Time-Series “Database” & Trigger Engine

  • File: LEARN_MONITORING_SYSTEM_IN_C.md
  • Main Programming Language: C
  • Alternative Programming Languages: N/A
  • Coolness Level: Level 4: Hardcore Tech Flex
  • Business Potential: 1. The “Resume Gold”
  • Difficulty: Level 4: Expert
  • Knowledge Area: Systems Programming / Data Management
  • Software or Tool: C
  • Main Book: “The C Programming Language” (K&R) by Kernighan & Ritchie.

What you’ll build: You will enhance the server from Project 3.

  1. Storage: Instead of just printing values, the server will append them to a specific file for that metric, creating a simple time-series database. For example, a check for cpu.load on host_a will append a line like 1698397200,12.5 to a file named data/host_a/cpu.load.csv.
  2. Triggers: The server will read a triggers.conf file with simple rules like host_a:cpu.load > 80.0. After writing a new value, it will check if it violates any trigger rule.
  3. Alerting: If a rule is violated, the server will perform a simple “action”: appending a message to an alerts.log file.

Why it teaches analysis and state: This project moves your server from a dumb data collector to an intelligent system. It’s the brain. You will have to manage files, handle timestamps, parse rule definitions, and maintain the state of alerts.

Core challenges you’ll face:

  • File and directory management → maps to programmatically creating directories (mkdir) and opening files for appending (fopen with “a” mode).
  • Working with time → maps to using time() to get a UNIX timestamp.
  • Parsing trigger rules → maps to reading a line and parsing it into a host, key, operator, and value.
  • Implementing the trigger logic → maps to writing a function that compares the latest received value against the configured threshold.

Key Concepts:

  • File I/O: APUE, Ch. 3.
  • Time and Date: time.h library functions.
  • String Manipulation: strtok, strcmp.

Difficulty: Expert Time estimate: 2 weeks Prerequisites: Project 3.

Real world outcome: Your server will be running. It will silently collect data and store it in CSV files. If you then create a heavy load on the agent machine (e.g., while :; do :; done &), the czabb_server will detect the high CPU usage, and you will see a new line appear in alerts.log: [PROBLEM] host_a:cpu.load is over threshold (95.4 > 80.0).

Learning milestones:

  1. The server correctly logs historical data to timestamped files → Your data storage system is working.
  2. The server can parse and load trigger configurations → The rule engine is configurable.
  3. A metric that violates a threshold correctly causes an alert to be written → Your trigger logic is functional.

Project 5: The Multi-Process Server

  • File: LEARN_MONITORING_SYSTEM_IN_C.md
  • Main Programming Language: C
  • Alternative Programming Languages: N/A
  • Coolness Level: Level 5: Pure Magic (Super Cool)
  • Business Potential: 1. The “Resume Gold”
  • Difficulty: Level 5: Master
  • Knowledge Area: Concurrency / Advanced Systems Programming
  • Software or Tool: C, fork()
  • Main Book: “Advanced Programming in the UNIX Environment” (APUE), Ch. 8 (Process Control) & Ch. 15 (Interprocess Communication).

What you’ll build: A refactored version of your czabb_server. On startup, the main process will fork() several “poller” child processes. The parent process will be responsible for reading the configuration and distributing checks to the pollers using an IPC mechanism (like a pipe). Each poller runs its own small polling loop. When a poller gets a result, it sends it back to the parent, which is responsible for writing to the database and checking triggers.

Why it’s the capstone project: This mimics the real architecture of Zabbix Server and teaches you how to build a scalable, concurrent C application. A single-threaded server cannot handle monitoring hundreds of hosts. This project forces you to confront the complexities of multi-process programming, including process creation, communication, and cleanup.

Core challenges you’ll face:

  • Process creation with fork() → maps to understanding how a new child process is created and the difference between parent and child execution paths.
  • Inter-Process Communication (IPC) → maps to using pipe() to create a channel where the parent can write work to the children, and another pipe for children to write results back.
  • Process management → maps to the parent process needing to handle child processes that might die (waitpid) and restart them.
  • Distributing work → maps to designing a simple protocol for the parent to tell a child which host and key to poll.

Key Concepts:

  • Process Model: APUE, Ch. 8.
  • Pipes for IPC: APUE, Ch. 15.
  • Concurrency vs. Parallelism: Understanding that you have multiple processes running in parallel.

Difficulty: Master Time estimate: 2-3 weeks

  • Prerequisites: All previous projects.

Real world outcome: Your ./czabb_server will now be much more powerful. Using ps -ef | grep czabb, you will see one parent server process and several child “poller” processes. The server will be able to poll multiple agents simultaneously, resulting in much faster data collection for a large number of configured checks.

Implementation Hints:

  • In the parent, create a number of pipes before you start forking.
  • After forking, the parent closes the ends of the pipes it doesn’t use, and the child does the same.
  • The parent can have a main select() loop to listen for results coming back from all its children’s pipes.
  • The children will have a simple loop: read a job from their incoming pipe, execute the poll, write the result to their outgoing pipe.

Learning milestones:

  1. The server successfully forks multiple child processes → You’ve mastered fork().
  2. The parent can send a “job” to a child via a pipe, and the child executes it → Your IPC work distribution is functional.
  3. The child can send a result back to the parent → Your result collection is functional.
  4. The overall system is more performant and can poll multiple hosts in parallel → You have successfully built a scalable, concurrent monitoring server in C.

Summary

Project Main Concept Core Skill Difficulty
1. The Metric Collector Reading /proc File I/O & Parsing Intermediate
2. The Agent Daemon Passive Check Server Socket Programming Advanced
3. The Server Poller Passive Check Client Socket Programming Advanced
4. Database & Triggers Data Storage & Analysis File I/O, Logic Expert
5. Multi-Process Server Concurrency & Scale fork(), IPC Master