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COBOL doesn't run alone. On real mainframes it lives inside an ecosystem.

CICS — Customer Information Control System

IBM's transaction monitor. You write CICS programs in COBOL with EXEC CICS ... END-EXEC blocks:

       EXEC CICS RECEIVE MAP('ORDER1') MAPSET('ORDERSET')
           INTO(WS-ORDER-IN)
       END-EXEC.

       PERFORM PROCESS-ORDER.

       EXEC CICS SEND MAP('ORDER1') MAPSET('ORDERSET')
           FROM(WS-ORDER-OUT)
       END-EXEC.

CICS handles:

• Multi-user concurrency (terminals)
• Database transactions (with COMMIT/ROLLBACK)
• Message queuing
• Recovery on failure

A "transaction" is one user interaction — submit a form, get a response. CICS shops do millions per minute.

DB2 / IMS

Mainframe databases. SQL embedded in COBOL:

       EXEC SQL
           SELECT NAME, BALANCE
           INTO :WS-NAME, :WS-BALANCE
           FROM CUSTOMERS
           WHERE ID = :WS-CUST-ID
       END-EXEC.

The :variable syntax is host variables — bridges between SQL and COBOL. The DB2 precompiler (DSNH) translates EXEC SQL into actual function calls.

IMS is hierarchical (pre-relational); DB2 is the relational standard. Both still in production at banks, insurance, and government.

JCL — Job Control Language

The shell that runs COBOL on z/OS. It's NOT COBOL — it's a separate language that defines batch jobs:

//MYJOB    JOB (ACCT),'AdaL',CLASS=A,MSGCLASS=H
//STEP1    EXEC PGM=MYPROG,PARM='REPORT'
//INFILE   DD DSN=PROD.CUST.DATA,DISP=SHR
//OUTFILE  DD DSN=PROD.RPT.OUT,DISP=(NEW,CATLG)
//SYSOUT   DD SYSOUT=*
//

JCL declares:

• A JOB (the batch job)
• One or more STEPs (programs to run)
• DD (Data Definition) — file allocations
• Conditional execution, parameters, return codes

Similar to a Bash script + Docker compose file rolled into one — but specific to z/OS.

VSAM

Virtual Storage Access Method — the file system. Three flavors:

• KSDS (Key-Sequenced Data Set) — indexed, sorted by key. Like a B-tree.
• ESDS (Entry-Sequenced) — append-only sequential.
• RRDS (Relative-Record) — fixed-position random access.

COBOL programs read/write VSAM files via FILE-CONTROL declarations.

RACF / ACF2

Mainframe access control. Every dataset, transaction, terminal user is permission-checked.

Migration: COBOL on modern systems

• GnuCOBOL — open source, runs on Linux/Mac/Windows. What Judge0 uses.
• Micro Focus Visual COBOL — commercial, integrates with .NET / Java.
• IBM COBOL for Linux on x86 — IBM's port to Linux, identical to z/OS

Most banks running COBOL are slowly migrating:

• Wrap COBOL with REST APIs (often via z/OS Connect)
• Move batch jobs to Linux (JCL → shell scripts, VSAM → DB)
• Rewrite gradually in Java/Go (NEVER "big bang")

Why COBOL persists

• 220+ billion lines of COBOL in production (Reuters estimate)
• Trillions of dollars of business logic encoded in it
• Migration risk: even a tiny bug in a payroll system affects millions
• Mainframe reliability: z/OS systems hit 5-nines uptime; rebuilding that on commodity hardware is non-trivial

Most COBOL maintenance is business knowledge transfer — the languages just isn't that hard. The hard part is understanding what 50 years of accreted business rules actually do.