Prologue — They said it was dead, but it's still there

"Mainframes are dead" has been a yearly headline since the 1990s. And yet in May 2026, the moment you pull money from an ATM, swipe a credit card, book a flight seat, pay your taxes, or register a birth at a government office, the last line of that transaction will, with very high probability, run through **a COBOL program on a mainframe**.

- **IBM Z** — z17 launched September 2025, Telum II processor, on-chip AI acceleration.

- **IBM i (formerly AS/400)** — 130,000+ companies still operate it. ERP, warehousing, insurance.

- **COBOL** — an estimated 200~220B (billion) lines run in production. Every day.

- **RPG / JCL / CICS / DB2 z/OS / IMS / PL/I** — all still in active duty.

- **Fujitsu BS2000 / NEC ACOS / Unisys ClearPath** — the world of non-IBM mainframes.

And since 2023 this enormous legacy has met a new variable: **AI**. IBM's **watsonx Code Assistant for Z** translates COBOL into Java. **GitHub Copilot** supports COBOL completion. **AWS Mainframe Modernization** lifts mainframe workloads to the cloud.

This article covers mainframes as of 2026: how they didn't die, what the new hardware looks like, how AI is chipping at this enormous legacy, and — if you're a developer — why some people decide right now to learn COBOL.

1. The Mainframe in 2026 — A Giant That Refused to Die

Let's start with numbers. Estimates of the mainframe ecosystem as of 2026:

| Item | Estimated scale |

| --- | --- |

| Production COBOL lines | 200~220B lines |

| IBM i installations | 130,000+ companies (Forrester/IBM estimate) |

| IBM Z systems deployed | ~10,000+ sites globally |

| Business transactions per day on IBM Z | ~30 billion |

| Fortune 500 using mainframes | ~70% |

| Top 50 global banks using mainframes | ~92% |

| Credit card transactions touching a mainframe | ~87% |

(Sources: IBM, Forrester, Gartner market estimates — exact numbers vary by source. All figures in this post are estimates based on 2024~2025 public reports.)

"Why didn't they switch" is the wrong question. The right question is **"why was not switching the rational decision?"** The answer compresses to three things:

1. **Proven reliability** — z/OS availability is often reported at 99.999% (5-nines) or higher. Around 5 minutes of downtime per year. Matching that on a new system takes years.

2. **Risk of untested change > value of change** — imagine rewriting a 20-million-line insurance core in Java. Two years. A hundred million dollars. And no new features in that window. CFOs don't sign.

3. **Data gravity** — DB2 z/OS, IMS, VSAM hold decades of accumulated data. Moving data is harder than moving code.

So the mainframe didn't die. Instead of dying, it **evolves slowly**. That's what this post is about.

2. IBM z17 (September 2025) — Telum II + Integrated AI

In September 2025, IBM announced **z17**. The previous generation (z16) launched in April 2022, so the cadence is roughly three years. The core of z17 is two things:

Telum II processor

- A roughly 5nm-class process (IBM does not generally disclose the exact node)

- 8 cores, clock in the 5 GHz range (per-core performance improved over Telum I in z16)

- **On-chip AI accelerator** — strengthened from Telum I's on-die AI inference. Allows ML inference (e.g., fraud detection) to be inlined into a transaction with sub-millisecond latency.

- Improved L2 cache structure, optimized NUMA topology.

In short: Telum II lets you **embed AI inference into the transaction itself**. The fraud-detection model runs on the same chip, at the moment the card payment happens. You don't ship data out to a GPU cluster.

Spyre accelerator (optional)

The **IBM Spyre AI accelerator** introduced with z16 (a separate PCIe card) is also available as an option for z17. Spyre handles larger models (generative AI inference) and can complement Telum II's on-chip acceleration.

What changed (vs z16)

| Item | z16 (April 2022) | z17 (September 2025) |

| --- | --- | --- |

| Processor | Telum I | Telum II |

| On-chip AI | Yes (1st gen) | Strengthened (2nd gen) |

| Spyre accelerator | Optional (later) | Optional (expanded) |

| Memory bandwidth | - | Improved (see IBM material for details) |

| Security | Crypto Express 8S, quantum-safe algorithms | Same plus enhanced |

| Availability | 99.999%+ | Same class |

z17 is deeper AI integration rather than a revolution. IBM's message — transactions plus AI inference plus security all on one chip — gets stronger.

Who buys z17

- **Global banks and card networks** — fraud detection inlined in transactions.

- **Large insurers** — claim processing plus risk scoring.

- **Governments and tax agencies** — massive batch plus security.

- **Airlines and logistics** — reservation and inventory backbones.

Prices aren't public. The usual line is "starts in the millions of dollars." And the companies writing those checks believe it's reasonable.

3. IBM z16 — Still in Active Service

The arrival of z17 doesn't retire z16. **Launched in April 2022, the z16 is still actively sold and installed in 2026.** Mainframe lifecycles are long — once a machine is in, it stays for a decade.

z16 highlights:

- **Telum I** processor — IBM's first generation with on-chip AI acceleration.

- **Quantum-safe cryptography** — Crypto Express 8S with NIST-standardized post-quantum algorithms (a subset).

- **Pentide (IBM Telum) cache structure** — a quirky design that uses L2 as virtual L3/L4 to expose roughly 256 MB of virtual L4 cache.

- **z/OS 2.5 / 3.1 support.**

If a bank just installed z16, the case to jump to z17 is weak. Many will run **z16 plus firmware updates** well into the early 2030s.

4. IBM i (formerly AS/400) — 130K+ Companies

Less famous than IBM Z but equally important: **IBM i**, the successor to the **AS/400** of 1988.

History of the name

- **System/38** (1978)

- **AS/400** (1988) — Application System/400. The icon of mid-market ERP in the 80s and 90s.

- **eServer iSeries** (2000)

- **System i** (2006)

- **IBM i** (2008~present) — both the OS name and the platform name.

The hardware runs on IBM **Power** servers. So IBM i in 2026 is **Power Systems hardware plus the IBM i OS**, with versions like IBM i 7.5 / 7.6 running on Power10.

Why 130,000 companies still use it

- **Single-object model** — files, programs, and DB objects live in the same system object model. Operations are simple.

- **TIMI (Technology Independent Machine Interface)** — hardware abstraction. New Power chips run old binaries unchanged.

- **Built-in Db2 for i** — no separate database to operate.

- **Lower running cost** — by mainframe standards. One box can run ERP, warehouse, and finance.

Who runs it

- US and European mid-market manufacturing

- Japanese food, retail, and logistics

- A number of Korean mid-market manufacturing and distribution (many specifics aren't public)

- European insurers

- Retail chains

ERP packages like **JD Edwards**, **Infor (formerly SSA Global)**, and **BPCS** run on IBM i.

RPG — the main language of IBM i

The dominant programming language on IBM i is **RPG (Report Program Generator)**. We'll dive in next.

5. COBOL — An Estimated 200~220B Lines in Production

COBOL was designed by a committee in 1959 that included Grace Hopper. **The name means COmmon Business Oriented Language.** A language older than 60 years still has 200B lines running.

Why so much

- **Banking core systems** — almost every large bank's core banking.

- **Insurance claims and policies** — auto, life, P&C.

- **Government tax and welfare** — the US IRS, parts of Korea's NTS, the UK's HMRC, and so on.

- **Airline reservations** — Sabre and Amadeus, the global distribution systems, still have COBOL in their backbone.

- **Retail and manufacturing ERP** — running on IBM i with RPG.

The 200B figure is **cumulative**. New COBOL written each year decreases, but old code rarely gets deleted. So the line count grows.

What COBOL code looks like

IDENTIFICATION DIVISION.

PROGRAM-ID. INTEREST-CALC.

AUTHOR. YJ.

*

ENVIRONMENT DIVISION.

CONFIGURATION SECTION.

SOURCE-COMPUTER. IBM-Z.

OBJECT-COMPUTER. IBM-Z.

*

DATA DIVISION.

WORKING-STORAGE SECTION.

01 WS-PRINCIPAL PIC 9(7)V99 VALUE 1000000.00.

01 WS-RATE PIC 9(3)V99 VALUE 3.50.

01 WS-YEARS PIC 9(2) VALUE 5.

01 WS-INTEREST PIC 9(9)V99.

01 WS-TOTAL PIC 9(9)V99.

*

PROCEDURE DIVISION.

CALC-INTEREST.

COMPUTE WS-INTEREST = WS-PRINCIPAL * (WS-RATE / 100) * WS-YEARS.

COMPUTE WS-TOTAL = WS-PRINCIPAL + WS-INTEREST.

DISPLAY "Principal : " WS-PRINCIPAL.

DISPLAY "Interest : " WS-INTEREST.

DISPLAY "Total : " WS-TOTAL.

STOP RUN.

COBOL hallmarks:

- **English-like syntax** — `COMPUTE`, `DISPLAY`, `STOP RUN`. Reads like sentences.

- **Fixed-column format** (traditional) — 1-6 sequence, 7 indicator, 8-11 area A, 12-72 area B. Modern COBOL also supports free-format.

- **PIC clauses** — define a data picture. `PIC 9(7)V99` means 7 integer digits and 2 decimals.

- **DIVISION structure** — four parts: IDENTIFICATION, ENVIRONMENT, DATA, PROCEDURE.

COBOL standards over time

- COBOL-60 → COBOL-68 → COBOL-74 → COBOL-85 (de facto standard for decades) → COBOL 2002 → COBOL 2014 → COBOL 2023.

- Modern standards add OOP, Unicode, and XML/JSON processing.

COBOL compilers

- **IBM Enterprise COBOL for z/OS** — the canonical compiler.

- **IBM COBOL for AIX / Linux on Z**

- **Micro Focus Visual COBOL** (now under OpenText) — Windows/Linux/mainframe-compatible.

- **GnuCOBOL** — open source. Transpiles to C. (More in a later chapter.)

6. RPG / JCL / CICS / DB2 z/OS / IMS / PL/I

COBOL is just the most famous mainframe language. The environment has other essentials.

RPG (Report Program Generator) — the main IBM i language

The name says "report generator" but modern RPG is a general-purpose business language. Timeline:

- **RPG II** (1960s+)

- **RPG III** (System/38)

- **RPG IV / RPG/ILE** (AS/400 era through today)

- **Free-format RPG** (modern) — not tied to column positions, similar to free-format COBOL.

Modern free-format RPG example:

**free

ctl-opt main(main);

dcl-proc main;

dcl-s principal packed(9:2) inz(1000000);

dcl-s rate packed(5:2) inz(3.50);

dcl-s years packed(2:0) inz(5);

dcl-s interest packed(11:2);

dcl-s total packed(11:2);

interest = principal * (rate / 100) * years;

total = principal + interest;

dsply ('Total: ' + %char(total));

end-proc;

RPG is **very tightly coupled to the database**. Embedded SQL inside RPG is the norm.

JCL (Job Control Language)

The language for defining batch jobs on z/OS. People call it "an unpleasant ancient shell script," but it is the lifeline of mainframe operations.

//PAYROLL JOB (ACCT1234),'PAYROLL JOB',

// CLASS=A,MSGCLASS=X,REGION=4M

//STEP1 EXEC PGM=PAYCALC

//STEPLIB DD DSN=PROD.LOADLIB,DISP=SHR

//INPUT DD DSN=PROD.PAYROLL.MASTER,DISP=SHR

//OUTPUT DD DSN=PROD.PAYROLL.NEWRUN,DISP=(NEW,CATLG,DELETE),

// SPACE=(CYL,(10,5)),

// DCB=(LRECL=200,BLKSIZE=27800,RECFM=FB)

//SYSPRINT DD SYSOUT=*

JCL features:

- **Column-position-sensitive syntax starting with `//`.**

- **JOB → EXEC → DD** card structure — job → execution step → dataset.

- **DSN** for dataset name, **DISP** for disposition (SHR/MOD/NEW), **SPACE** for allocation.

- **STEPLIB** for libraries, **SYSPRINT** for log output.

JCL is an operations asset. A good JCL line can save a company. A bad JCL line — like the eternal joke of using `DISP=NEW,DELETE` on a production master — can bankrupt one.

CICS (Customer Information Control System)

The **transaction monitor** on z/OS, in operation since 1969. CICS:

- Receives requests from terminals (traditionally 3270) or web/mobile clients.

- Executes business logic (COBOL/PL/I and others) as **transactions**.

- Returns the result.

- Handles distributed transactions (2PC), integrates with Db2 and IMS.

Modern CICS, called **CICS Transaction Server**, runs on z/OS and supports REST/JSON interfaces.

DB2 z/OS (IBM Db2 for z/OS)

The relational database of z/OS. It is a different codebase from Db2 in other environments (Db2 LUW, Db2 for i). As of 2026, **Db2 for z/OS V13** and later run in production.

Db2 for z/OS identity:

- **Transaction processing and data warehousing** on a single system.

- **Pentide cache** plus **Telum on-chip acceleration** plus AI Query Optimizer — performance fused to mainframe hardware.

- **Massive accumulated data** — petabyte-scale operation is common.

IMS (Information Management System)

Built in 1968 for the Apollo program, IMS is a **hierarchical database plus transaction monitor**. It predates relational databases (Db2) and a number of large banks, airlines, and manufacturers still run IMS DB and IMS TM. "Old" does not mean "unused" — old means **proven and fast**.

PL/I (Programming Language One)

In 1964 IBM tried to merge COBOL and Fortran into one language. It never replaced either, but parts of large systems (notably European banks and some operating-system code) still use it.

TEST: PROC OPTIONS(MAIN);

DCL PRINCIPAL FIXED DEC(9,2) INIT(1000000);

DCL RATE FIXED DEC(5,2) INIT(3.50);

DCL YEARS FIXED BIN(15) INIT(5);

DCL INTEREST FIXED DEC(11,2);

DCL TOTAL FIXED DEC(11,2);

INTEREST = PRINCIPAL * (RATE / 100) * YEARS;

TOTAL = PRINCIPAL + INTEREST;

PUT SKIP LIST('Total: ', TOTAL);

END TEST;

PL/I is more expressive than COBOL but has a steeper learning curve. "If it's there, it's there; nobody starts new in PL/I."

7. Fujitsu BS2000 / NEC ACOS / Unisys ClearPath — The Non-IBM Mainframes

People often equate "mainframe" with "IBM." That's inaccurate. Japan, parts of Europe, and parts of the US still run non-IBM mainframes.

Fujitsu BS2000 (Japan/Germany)

- Originally the **BS2000** operating system from Germany's **Siemens**. Fujitsu later absorbed Siemens' computer business.

- Today it is the Fujitsu **FUJITSU Server BS2000** line. The OS evolved into OSD/BC, BS2000/OSD, etc.

- Mostly operates at government agencies, banks, and insurers in Germany, Austria, and Switzerland.

- Supports COBOL, ASSEMBLER, C/C++. There are SQL DBMSes too (SESAM, UDS).

- Fujitsu also has Japan-domestic mainframe lines (such as the PRIMEFORCE series over the years — naming and lineup change over time, so refer to Fujitsu's official material).

NEC ACOS (Japan)

- NEC's **ACOS-4** OS runs on its mainframes. Parts of the Japanese government and some regional banks run NEC ACOS as their core.

- Japan Post and certain regional banks are example operators.

- It has its own COBOL plus 4GLs like ADL (ACOS Data Language).

Unisys ClearPath

- Unisys' mainframe line from the US, with two OS families:

- **MCP (Master Control Program)** — successor to Burroughs B5000 (1961). A fossil of OS history because it is ALGOL-based.

- **OS 2200** — successor to Sperry UNIVAC.

- ClearPath Dorado (the 2200 line) and ClearPath Libra (the MCP line) have been the brand names.

- Unisys has pivoted to **ClearPath Forward**, which emulates mainframe hardware on x86 servers.

- Still in operation at US airlines, government, and some banks.

Summary — what non-IBM mainframes share

| Item | Common pattern |

| --- | --- |

| Market share | Smaller than IBM, but non-zero |

| Where | Regional concentrations (Japan, Germany, US) |

| Migration pressure | Higher than IBM Z; next-gen support is less assured |

| Modernization path | Often to IBM Z, or to Linux and cloud |

8. AWS Mainframe Modernization

In 2022, AWS launched **AWS Mainframe Modernization (AWS MMA)** — a managed service to help move mainframe workloads onto AWS.

Two patterns

The two official migration strategies AWS pushes:

1. **Replatform** — leave the COBOL alone, but move the mainframe environment somewhere else (AWS containers or EC2). AWS exposes an option backed by the **Micro Focus** runtime (now OpenText).

2. **Refactor** — auto-convert COBOL into Java. AWS acquired and integrated **Blu Age**, a tool that translates COBOL to modern Java/Spring/Angular.

Replatform is the more common path

Most large migrations are replatform projects, because:

- **Business logic is untouched** — no risk of changing behavior.

- **Mainframe operational costs go down** — IBM license/MIPS cost replaced by AWS compute.

- **Modern tool integration** — CI/CD, containers, observability.

But there are traps:

- **Data migration** — Db2 z/OS to Aurora/RDS PostgreSQL is not easy. Triggers, stored procedures, and SQL dialect differences all surface.

- **Transaction monitor parity** — replicating exact CICS behavior is hard.

- **MIPS price comparison** — "savings" aren't always real. Matching equivalent availability and performance can land at similar cost.

Flow

[Mainframe] [AWS]

COBOL source ────────▶ COBOL source (or Java conversion)

JCL batch ────────▶ AWS Batch / EventBridge Scheduler

CICS ────────▶ Mainframe Modernization Runtime

Db2 z/OS ────────▶ Aurora PostgreSQL / Db2 LUW

VSAM ────────▶ DynamoDB / S3 / Aurora

IMS ────────▶ Custom mapping (hardest)

AWS provides assessment, analysis, and cutover support as a managed service. You still need mainframe veterans paired with AWS solution architects.

9. Micro Focus → OpenText (2022) → Visual COBOL

For a long time, the center of mainframe modernization was **Micro Focus** — a UK-headquartered company and one of the largest suppliers of COBOL compilers and runtimes.

2022 — The OpenText acquisition

In 2022, the Canadian information-management company **OpenText** acquired Micro Focus for roughly six billion dollars. The Micro Focus brand is being phased into OpenText as the deal completes.

Visual COBOL — the flagship product

Micro Focus's (now OpenText's) flagship is **Visual COBOL**. What it does:

- Lets mainframe COBOL run on **Windows/Linux/.NET/JVM**.

- **Visual Studio / IntelliJ / Eclipse** integration — edit, debug, and refactor COBOL inside modern IDEs.

- **CICS emulation** — provides an equivalent transactional environment so mainframe CICS apps can be lifted as-is.

- **JCL translation** — runs mainframe JCL outside the mainframe environment.

Enterprise Server / Enterprise Developer

OpenText's mainframe lineup usually includes:

- **Enterprise Developer** — the developer IDE plus compiler.

- **Enterprise Server** — production runtime (CICS-compatible, IMS-compatible environments).

- **AppMaster Builder, Enterprise Test Server, Modernization Workbench** — supporting tools.

Some replatform options inside AWS Mainframe Modernization run on this OpenText stack.

What it means

OpenText is **not in the business of killing the mainframe**. They are in the business of letting mainframe code keep running elsewhere. Their model assumes COBOL is alive into the 2030s.

10. IBM watsonx Code Assistant for Z (August 2023) — AI for COBOL Modernization

In August 2023, IBM announced **watsonx Code Assistant for Z**, an AI tool for helping convert mainframe COBOL into Java.

What it does

- **COBOL → Java conversion** — automatic or semi-automatic. Moves business logic into modern Java.

- **Code explanation** — uses an LLM to explain COBOL written in the 1980s. Natural-language queries like "what does this module do."

- **Test generation** — produces test cases for existing COBOL.

- **Incremental conversion** — module-by-module and service-by-service rather than big-bang.

How it works

watsonx Code Assistant for Z combines an LLM (built on IBM's `granite` code model family) with mainframe-specific domain knowledge. Not just generic ChatGPT, but a model **trained on COBOL/JCL/CICS patterns**.

Limits and reality

Don't take IBM marketing at face value. In practice:

- **Auto-converted Java needs review** — true of all LLM-generated code.

- **Subtle business logic** — COBOL's decimal arithmetic (PIC 9V99), EBCDIC, indicator semantics — these can be lossy.

- **Data migration is a separate problem** — moving code isn't the end of the project.

Still, as a **starting point**, it has value. Rather than starting from zero, humans refine an AI-generated first pass.

Trends in 2024~2025

- IBM generalized the watsonx Code Assistant family — Code Assistant for Java, for Ansible, and others.

- The IBM Z variant is being applied to larger cases — one insurer scoped 8 million lines of COBOL incrementally.

- Competitors: Amazon Q Developer (formerly CodeWhisperer) with mainframe support, GitHub Copilot's COBOL coverage.

11. GitHub Copilot COBOL Support

**GitHub Copilot** went GA in 2021, initially known for JavaScript, Python, and TypeScript. Language coverage has expanded over time, and **COBOL completion is supported**.

What it feels like

How Copilot helps in COBOL:

- **PIC clause completion** — type a variable name and Copilot suggests a PICTURE clause like `PIC 9(7)V99`.

- **Paragraph names and flow suggestions** — predicts the next PERFORM, MOVE, COMPUTE.

- **JCL generation** — a natural-language comment yields a JCL snippet.

- **Db2 embedded SQL** — autocompletes `EXEC SQL ... END-EXEC` blocks.

Limits

- **80-column format handling** — traditional column-based COBOL is harder for LLMs. Free-format works better.

- **Company coding standards** — Copilot knows generic patterns. It does not know your internal naming conventions or shop standards.

- **Test data semantics** — it doesn't know what your business data means.

Copilot vs watsonx Code Assistant for Z

- **Copilot** — generic coding assistant. Supports COBOL but is not specialized.

- **watsonx CA for Z** — mainframe-specialized. Tuned for code conversion and explanation.

These are less competitors than **different layers**. Use Copilot for everyday coding and watsonx for large conversion programs.

12. zCX (z/OS Containers) + Linux on Z + LinuxONE

Another stream pulling the mainframe into the cloud era is **containers and Linux**.

zCX (z/OS Container Extensions)

A z/OS feature introduced in 2019. It **runs Docker containers inside z/OS** by spinning up a Linux on Z environment as a built-in virtual machine and running containers inside that.

Why this matters:

- **Low-latency access** to mainframe data (Db2 z/OS, VSAM).

- Pull modern stacks — **Kafka, Elastic, Grafana** — next to the mainframe.

- Add modern interfaces **without touching the transaction backbone**.

Linux on Z

Linux running natively on z hardware. Coexists with z/OS via LPAR partitioning, so a single mainframe can host both environments.

- Major distributions: RHEL on Z, SUSE on Z, Ubuntu on Z.

- Mainframe reliability extended to Linux workloads.

- A single system can host hundreds or thousands of Linux VMs and containers.

IBM LinuxONE

A **Linux-only mainframe**. Same hardware family as IBM Z, but **no z/OS**. So: "mainframe reliability for Linux." Use cases:

- Large-scale payment processing (including blockchain nodes).

- Environments where data protection matters (combined with IBM Cloud Hyper Protect).

- Green data centers (less power for the same workload compared with x86).

IBM Cloud Hyper Protect

IBM Cloud's **confidential computing** offering. Brings mainframe security technology (Crypto Express, Secure Service Container) to the cloud as services — key management, trusted virtual servers, databases hosted at mainframe-grade security.

13. Hercules + GnuCOBOL — Open-Source Emulation

Can you try a mainframe at home? Yes.

Hercules — a System/370 / ESA/390 / z/Architecture emulator

An open-source **mainframe emulator** that mimics z/Architecture on regular PCs/Macs/Linux. Caveats:

- **Commercial z/OS requires a license** — so legally running z/OS at home is hard.

- **MVS 3.8j** and similar old IBM OSes are public domain — you can experience 1970s~80s mainframes.

- **Linux on Z** — free and legal. The s390x builds of RHEL/SUSE/Ubuntu run on Hercules.

GnuCOBOL

An open-source COBOL compiler. It **transpiles COBOL to C**, then builds with a normal C compiler.

Install (e.g., macOS Homebrew)

brew install gnu-cobol

Compile

cobc -x -o interest interest.cob

Run

./interest

- **Covers a large subset of COBOL 85 / 2002 / 2014.**

- Does not support IBM-OS-specific features (CICS, Db2 z/OS, etc.).

- Suitable for learning, education, and small-scale operation.

Learning path

- If you're starting COBOL: **GnuCOBOL plus VS Code (or Eclipse)**.

- For a mainframe experience: **Hercules plus MVS 3.8j**, plus IBM's **Z trial** or **IBM Z learning programs**.

- IBM has long operated free learning programs for students and beginners — **IBM Z Xplore**, **Master the Mainframe**, etc. (Names and content change over time; check IBM's official pages.)

14. Modernization Patterns — Strangler Fig and Anti-Corruption Layer

Replacing legacy systems is not unique to mainframes. The two most famous and practical patterns:

The Strangler Fig pattern (Martin Fowler)

Named after the Australian strangler fig — a vine that grows around another tree and eventually replaces it. Martin Fowler used the metaphor for system renewal in a 2004 article.

Strategy:

1. Put a **proxy/facade** in front of the legacy system.

2. Route some traffic to a **new system**.

3. Gradually route more traffic to the new system.

4. One day, legacy traffic is at 0%. Only then remove it.

[Client] ──▶ [Facade / Proxy] ──▶ [Legacy mainframe]

│

└──────────────▶ [New microservice]

(gradually more routes)

Pros:

- **No big-bang migration risk** — you don't switch 100% at once.

- **Rollbackable** — if the new system breaks, reverse the routing.

- **Incremental learning** — you grow operational know-how as traffic grows.

Cons:

- **Takes years** — not a fast answer.

- **Double operating cost** — you run both during the transition.

- **Data synchronization** — both systems share data, so you pay sync cost during transition.

Anti-Corruption Layer (Eric Evans, DDD)

A pattern from Domain-Driven Design. Place a **translation layer** between the legacy and the new system.

[New domain model] ──▶ [Anti-corruption layer] ──▶ [Legacy model]

(clean model) (translate/map/insulate) (old model)

Goals:

- Prevent legacy's odd concepts (60-year-old business assumptions) from **leaking into** the new system.

- The new system gets a clean model of its own.

- Translation cost is concentrated in one place.

Strangler Fig and Anti-Corruption Layer **go together**. Strangler is the routing pattern, Anti-Corruption is the domain-model insulation behind that routing.

In practice — where to start

1. **Draw boxes** — group modules running on the mainframe by domain.

2. **Prioritize by change frequency and risk** — split the most-changed and risky parts first.

3. **Start with reads** — moving query traffic to a new (read-only replica) database is the easiest.

4. **Slowly add writes** — dual-write (legacy plus new) for a period, then end on the new system only.

5. **Monitor and observe** — shadow traffic, compare responses across both systems.

6. **Rollback plan** — always.

15. Korean Financial Sector — KB / Shinhan / Woori / Hana / IBK / NongHyup Mainframes

Korean financial-sector mainframe operations are partly public, partly estimated. Based on 2024~2025 public information and industry estimates:

(The text below is a general summary based on publicly known patterns. Many bank-specific system details are non-public. Some content is estimated.)

The big picture

- **The five major commercial banks plus NongHyup and IBK** — most have operated IBM Z (or the prior generation of mainframe) alongside next-generation core systems.

- **Next-generation core projects** — large next-gen projects were run in the late 2000s and 2010s. The trend of reducing mainframe dependency coexists with the trend of preserving it.

- **2020s trend** — cloud transition under review, partial downsizing of some systems, integration with AI initiatives.

General per-bank flow (based on public reports and industry knowledge)

| Bank | General flow (industry knowledge) |

| --- | --- |

| KB Kookmin | Mainframe plus an in-house next-gen system. Actively reviewing cloud transition. |

| Shinhan | Mainframe operations. Active AI integration. |

| Woori | Next-gen system project underway. |

| Hana | In-house next-gen system plus some mainframe. |

| IBK Industrial | Mainframe operations. |

| NongHyup | IBM Z operations. Given its scale, one of the largest mainframe users. |

(Note: The table above is a general summary of publicly known patterns. Exact system configurations are largely non-public, and the contents include estimates.)

Korean mainframe talent market

- Many veterans in their 60s and 70s carry the core operations.

- New entrants are scarce — almost no university teaches mainframe.

- Hence salaries rise — anyone who knows COBOL/RPG/JCL and speaks Korean can essentially set the price.

- IBM Korea's **Master the Mainframe** / **Z Xplore** programs in Korea helped seed new entrants.

Korean non-financial

- **Public sector and tax** — some systems operate on mainframe.

- **Airlines** — parts of Korean Air, Asiana, and so on.

- **Telecom** — some backbones.

- **Manufacturing and distribution** — many IBM i sites reportedly operate.

16. Japan — MUFG / SMBC / Mizuho / NTT Data / Fujitsu BS2000

Japan is another stronghold of mainframes. Unlike Korea, **Japan has its own mainframe industry** (Fujitsu, NEC, Hitachi). That uniqueness shapes the story.

Three megabanks — general flow

(General summary based on public sources and industry knowledge. Exact system configurations are largely non-public.)

| Bank | General flow (industry knowledge) |

| --- | --- |

| MUFG (Mitsubishi UFJ) | Mainframe operations. Stable after a large integration in the 2010s. |

| SMBC (Sumitomo Mitsui) | Mainframe plus in-house next-gen systems. |

| Mizuho | Famous for system outages in 2002, 2011, and 2021. Has since focused on stability. |

NTT Data

The hub of Japan's SI market. **Partner for building and operating public-sector and financial core systems**. NTT Data executes mainframe migrations and outsourced operations at scale.

Fujitsu — its own mainframe

- **FUJITSU BS2000** — mainframe originating in Germany. Operates in parts of Europe and Japan.

- Japan-domestic mainframe lineups (names and lineups have shifted over the years; check Fujitsu's official material for the latest).

- In the 2020s Fujitsu announced partial wind-down of mainframe business — including reported decisions to stop new mainframe development and shift to x86-based GS / PRIMERGY lines (exact schedule and scope per Fujitsu's official announcements).

- This trend has notable impact on the Japanese mainframe market.

NEC ACOS

- Operated by parts of the Japanese government, certain prefectures, and some banks.

- NEC also pursues a gradual transition of its mainframe business.

Hitachi

- Once one of the pillars of Japan's mainframe market. Today the focus is more on **systems integration and operations services** than on the mainframe itself.

Japan's talent market

- Same pattern as Korea — veterans plus scarce new entrants.

- Japan's structure absorbs talent into large SIs (NTT Data, Fujitsu, NEC, Hitachi, IBM Japan), so the talent pool rotates at the company level.

17. Who Should Learn COBOL — Finance / Insurance / Government / Academia

If you're picking up a new language in 2026, COBOL probably isn't number one. But **for specific people, it can be number one**. Who and why.

Good candidates

- **People aiming to work in finance** — bank IT and insurance IT reward knowing mainframe operations. There aren't replacements.

- **Public-sector / tax / welfare IT** — Korea, Japan, and the US all have COBOL inside some government systems.

- **Information-systems consulting** — migration projects are a 10~20-year market.

- **Academic curiosity** — how a 60-year-old language survives is itself a CS case study.

- **People who optimize for compensation** — the per-hour rate can be very high (because the supply is tiny).

Less good candidates

- **People choosing their first programming language in 2026** — Python/JavaScript/Go are better.

- **Founders** — you don't build a new product in COBOL.

- **AI/ML engineers** — Python owns that space.

A practical learning path

1. **Install GnuCOBOL** — compile and run locally.

2. **COBOL fundamentals** — `IDENTIFICATION / ENVIRONMENT / DATA / PROCEDURE DIVISION`. PIC clauses. PERFORM.

3. **File handling** — sequential, indexed, relative files.

4. **DB integration** — embedded SQL.

5. **JCL** — how mainframes actually run work.

6. **CICS basics** — transaction processing.

7. **Mainframe environment experience** — IBM Z Xplore / Master the Mainframe (names and offerings change over time — check IBM's official pages).

8. **Modernization tools** — Visual COBOL, evaluation/demo of watsonx Code Assistant for Z.

Time investment

- Basic COBOL — 100~200 hours.

- Real mainframe operations (JCL/CICS/Db2 z/OS) — 1~2 years of on-the-job exposure.

The language itself isn't hard. **The environment is.** That's the barrier and, simultaneously, the opportunity.

Epilogue — The Giant Moves Slowly, Very Slowly

The mainframe isn't dying. Not until at least the mid-2030s.

| Time | Event |

| --- | --- |

| April 2022 | IBM z16 launched (Telum I, quantum-safe crypto) |

| August 2023 | IBM watsonx Code Assistant for Z announced |

| 2022 | OpenText acquired Micro Focus (new owner of Visual COBOL) |

| 2022 | AWS Mainframe Modernization GA |

| September 2025 | IBM z17 launched (Telum II) |

| 2026+ | AI-assisted modernization expands in earnest |

The big picture

- **Hardware evolves** — Telum II strengthens on-chip AI. z17 is a natural successor to z16.

- **Software is preserved** — 200B lines of COBOL don't disappear. They get moved.

- **AI is the accelerator** — watsonx, Copilot, and Amazon Q make modernization faster than human-only eras.

- **Cloud is optional** — AWS MMA, IBM Cloud Hyper Protect, Linux on Z, LinuxONE.

- **Operations are still people** — a great mainframe operator saves the company.

Next post ideas

Candidates:

- **JCL in practice — from the first line to SYSPRINT**

- **A COBOL → Java conversion walkthrough — watsonx Code Assistant for Z demo**

- **Db2 z/OS vs PostgreSQL — why migration is hard**

> "Old code is alive somewhere. We don't kill it — we plant a new tree beside it and let it slowly take over." — In the spirit of the metaphor that gave us the Strangler Fig.

— Mainframes 2026, fin.

References

- [IBM Z official](https://www.ibm.com/z)

- [IBM z17 announcement (IBM Newsroom)](https://newsroom.ibm.com/)

- [IBM z16 — Telum processor](https://www.ibm.com/products/z16)

- [IBM i official](https://www.ibm.com/products/ibm-i)

- [IBM Db2 for z/OS](https://www.ibm.com/products/db2/zos)

- [IBM CICS Transaction Server](https://www.ibm.com/products/cics-transaction-server)

- [IBM IMS](https://www.ibm.com/products/ims)

- [IBM Enterprise COBOL for z/OS](https://www.ibm.com/products/enterprise-cobol-zos)

- [IBM watsonx Code Assistant for Z](https://www.ibm.com/products/watsonx-code-assistant-z)

- [IBM LinuxONE](https://www.ibm.com/linuxone)

- [IBM Cloud Hyper Protect](https://www.ibm.com/cloud/hyper-protect-services)

- [Linux on Z (IBM)](https://www.ibm.com/it-infrastructure/z/os/linux)

- [AWS Mainframe Modernization](https://aws.amazon.com/mainframe-modernization/)

- [OpenText (acquired Micro Focus)](https://www.opentext.com/)

- [Visual COBOL — OpenText](https://www.opentext.com/products/visual-cobol)

- [Fujitsu BS2000](https://www.fujitsu.com/global/products/computing/servers/mainframe/bs2000/)

- [NEC ACOS](https://jpn.nec.com/acos/)

- [Unisys ClearPath](https://www.unisys.com/solutions/clearpath-forward/)

- [GitHub Copilot](https://github.com/features/copilot)

- [Hercules emulator](https://www.hercules-390.org/)

- [GnuCOBOL](https://gnucobol.sourceforge.io/)

- [Martin Fowler — StranglerFigApplication](https://martinfowler.com/bliki/StranglerFigApplication.html)

- [Eric Evans — Domain-Driven Design (Anti-corruption Layer)](https://www.domainlanguage.com/ddd/)

- [COBOL standard (ISO/IEC 1989)](https://www.iso.org/standard/74527.html)

- [IBM Z Xplore / Master the Mainframe (learning programs)](https://www.ibm.com/community/z/)