Create a basic stats service API using C and FastCGI
- 中
- en
Preface
Was thinking about making my own stats page to monitor basic information for all my servers using the Primary back-end and Secondary Workers, where the Primary server will serve the front-end assets while other Worker back-ends provides API exposing the server stats.
This post is about the Secondary Worker written in C using FastCGI, other parts of the system will not be discussed here.
The FastCGI process will be expecting a maximum request rate of 1 request per 10 seconds - the downstream will be responsible to cache the information via Redis.
Deployed primary server can be viewed at stats.jixun.uk.
Environment Setup
I currently have multiple VPS running on different OS: Alpine Linux, Ubuntu and CentOS 7.
They all have their own package manager and package name conventions, I have wrote a simple script to automate it (which needs to be available):
setup_apt()
{
apt update
apt install -y gcc make libfcgi libfcgi-dev spawn-fcgi
}
setup_apk()
{
apk update
apk add gcc musl-dev make fcgi fcgi-dev spawn-fcgi
}
setup_yum()
{
yum -y install gcc make fcgi fcgi-devel spawn-fcgi
}
which apt && setup_apt
which apk && setup_apk
which yum && setup_yum
The Code
Talk is cheap, show me the code!
FastCGI related information is available on their archived home page, where you can also find a sample code to get started.
The C code emits contents via stdout and not much string variables to escape, the code did not use any third-party library to handle the JSON.
#include "fcgi_stdio.h"
#include <stdlib.h>
#include <string.h>
#include <inttypes.h>
#include <sys/statvfs.h>
#define JSON_KEY(str) "\"" str "\""
#define IS_DIGIT(c) (c >= '0' && c <= '9')
// Do not use shared_buff if multi-threaded.
char shared_buff[4096];
// CPU INFO
void print_cpu()
{
printf(JSON_KEY("cpu") ":");
FILE* f = fopen("/proc/loadavg", "r");
fread(shared_buff, 1024, 1, f);
fclose(f);
char* p = shared_buff;
while(*p != ' ') p++;
*p = 0;
printf("%s", shared_buff);
}
// RAM INFO
// Ram Keyword struct
struct kwd {
const char* word;
int len;
};
struct kwd ramKwds[] = {
{"MemTotal", 8},
{"MemFree", 7},
{"Cached", 6},
{"SwapTotal", 9},
{"SwapFree", 8},
{"SwapCached", 10},
};
#define RAM_KWD_N (sizeof(ramKwds)/sizeof(ramKwds[0]))
void print_ram()
{
printf(JSON_KEY("ram") ":{");
FILE* f = fopen("/proc/meminfo", "r");
fread(shared_buff, 4096, 1, f);
fclose(f);
int wrote = 0;
char* p = shared_buff;
while(*p)
{
char* line = p;
while(*p && *p != '\n') p++;
if (!*p) break;
*p++ = 0;
for (int i = 0; i < RAM_KWD_N; i++) {
if (memcmp(ramKwds[i].word, line, ramKwds[i].len) == 0) {
if (wrote) putchar(',');
printf("\"%s\":", ramKwds[i].word);
// skip past ':' char
line += ramKwds[i].len + 1;
// skip whitespace
while(*line == ' ') {
line++;
}
while(IS_DIGIT(*line)) {
putchar(*line++);
}
wrote = 1;
}
}
}
putchar('}');
}
char* storages[] = {
"/"
};
#define STORAGE_COUNT (sizeof(storages)/sizeof(storages[0]))
void print_storage()
{
printf(JSON_KEY("drives") ":[");
struct statvfs buf;
int wrote = 0;
for (int i = 0; i < STORAGE_COUNT; i++) {
// see: https://github.com/php/php-src/blob/001d43444944df0bef4e33a1876ba2818c188e11/ext/standard/filestat.c#L251
if (statvfs(storages[i], &buf)) continue;
uint64_t block_size = buf.f_frsize ? buf.f_frsize : buf.f_bsize;
uint64_t bytes_free = block_size * buf.f_bavail;
uint64_t bytes_total = block_size * buf.f_blocks;
if (wrote) putchar(',');
printf("{"
JSON_KEY("mnt") ":\"%s\","
JSON_KEY("free") ":%" PRIu64 ","
JSON_KEY("total") ":%" PRIu64 "}"
, storages[i], bytes_free, bytes_total
);
wrote = 1;
}
putchar(']');
}
int main(void) {
while (FCGI_Accept() >= 0) {
printf("Content-type: application/json\r\n");
printf("\r\n");
putchar('{');
print_cpu();
putchar(',');
print_ram();
putchar(',');
print_storage();
putchar('}');
}
return 0;
}
编译时带上 -lfcgi 即可。我使用的编译语句如下：
gcc main.c -Wall -Wpedantic -std=c11 -lfcgi -O2 -o stats-worker.cgi
To run it…
Since nginx does not spawn fastcgi process on-demand, spawn-fcgi is required to listen on socket.
Use -n flag to not fork the process, so it can be supervised with systemd or openrc easily.
# run as root and let spawn-fcgi drop privilege later.
spawn-fcgi -n -u www-data -g www-data -f /srv/slave.cgi -s /run/slave.sock -P /run/slave.pid
