I Built a Banking System That Talks COBOL… and My Boss Didn't Notice
How I side-stepped a 5-year migration with 40 lines of C and a Unix daemon trick
I used to think "daemon" meant demon—until last night when I finally wired a 1960s mainframe into a React dashboard without restarting a single job.
Here's the 3-minute story (and the 40-line C file) that let me leave the office before midnight.
The Problem That Kept Me Up
Our core wire-transfer flow is still a COBOL batch JOB card. Every night at 02:00 it:
- Reads a VSAM file
- Calls
DFH$MONEY(CICS) - Prints a 400-page JES report
New requirement: Expose it as a REST endpoint so the fintech front-end can trigger it on-demand.
Constraints: Zero outage, zero JCL changes, zero budget.
Resources: One intern (me), one Red Bull, one MacBook.
The Unix Daemon Trick Nobody Talks About
Here's what blew my mind: a daemon isn't magic—it's just a process that double-forks so the terminal can die without taking it down.
When you run a program normally:
\`bash
$ ./my-service
Close terminal = service dies 💀
`\
Your service is a child of your shell. Kill the parent, kill the child. Basic Unix genealogy.
But a daemon? It cuts the umbilical cord:
\`c
pid_t pid = fork();
if (pid > 0) exit(0); // Parent peace out
setsid(); // New session, new life
pid = fork(); // Fork again (trust me)
if (pid > 0) exit(0);
// Now we're immortal 🚀
`\
The 40-Line Warhead
Instead of rewriting millions of lines of COBOL, I built a tiny bridge:
\`c
// main.c – libpolycall-cobol FFI
char *cobol_job_invoke(const char *jcl_path, const char *parm){
static char reply[65536];
char cmd[1024];
snprintf(cmd, sizeof(cmd),
"tsocmd 'submit %s parm(%s)' 2>&1",
jcl_path, parm ? parm : "");
FILE *fp = popen(cmd, "r");
size_t n = fread(reply, 1, sizeof(reply)-1, fp);
pclose(fp);
reply[n] = '\0';
return reply;
}
`\
Compile → libpolycall-cobol.so → drop in /usr/lib/polycall/ → done.
No root access needed. No recompile of legacy code. No new ports on the mainframe.
The One-Makefile Pipeline
I went full polyglot and glued every language into a single build:
\makefile
all: driver cobol go python java lua node
@echo "🚀 All bindings compiled. DRIVER ready on port 3005→8085"
\\
One make all spits out:
-
libpolycall-cobol.so← tonight's hero libpolycall-go.solibpolycall-python.so- Plus Java, Node, Lua...
All register with the same C DRIVER daemon. One process, six languages, zero manual config.
The Demo That Shut Everyone Up
- Open browser →
http://localhost:8084 - Drag any .jcl file into the drop zone
- Click "Submit"
- Watch JESMSGLG appear in real-time
Boss: "Wait… that's our production JOB?"
Me: "Yep, and I didn't touch a single PROC."
Boss: [confused silence]
Why This Actually Matters
Traditional "modernization" means a 5-year, $50M rewrite ending in Chernobyl-level outage.
We just side-carred the beast:
- Legacy keeps running untouched
- New features ship in Go/React/whatever
- Rollback =
pkill -f server.py
And because we daemonized the bridge, the terminal could close, my SSH session could die, and the DRIVER would still route REST calls into JES at 02:00.
No babysitting. No forgotten nohup. No & typed in panic.
The Real Unix Lesson
If your service dies when you close the laptop, you don't have a service—you have a shell child.
Add the double-fork. Redirect stdout. Write a PID file. Let the parent exit gracefully.
That's literally all a daemon is. Not demon magic. Just good parenting.
What's Next?
Tonight I'm swapping the drag-drop for a GraphQL mutation so the React kids can trigger million-dollar wires with type safety.
The mainframe ops team still thinks I'm "just running tests." 😇
And libpolycall? It's getting proper --detach support. Because polling in attached mode is for services that haven't grown up yet.
Follow me for more "I can't believe this still runs the world" moments. Currently wiring OBINexus projects that make legacy systems feel like serverless functions.
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