BBS SG Bank · modernisation walkthrough

From a nightly COBOL flat-file sweep to an always-on ledger

The same balances are processed two ways: the legacy batch that re-reads its account file for every single operation, and a modern engine that keeps the ledger indexed in memory. Everything below runs live against this machine.

Ledger total: —
Posted postings: —
Rejected postings: —
Invariant: —

Pre-modernisation

COBOL nightly batch

legacy/bank.cob

BBS SG BANK · CICS PROD · NBATCH 22:00

Open 1980s operator terminal ↗ Start legacy-ui/run.sh locally for live COBOL. This panel replays the output format.

Whole-file scan per operation. bank.cob reopens and reads every account row to validate, then again to rewrite it.
Batch window only. Balances move once a night, so the branch sees yesterday's position all day.
No audit trail. The program prints three counters and nothing an auditor can query later.
Fragile records. Fixed 8/12-character fields; a misaligned byte silently changes a balance.

Show the bottleneck in legacy/bank.cob

Loading source…

Post-modernisation

Modern ledger engine + site API

modern/bank.py

Engine latency — last batch, engine only

Postings/sec — rows ÷ engine time, last batch

Availability Always on postings API, no batch window

Largest balances

Waiting for the ledger…

Indexed ledger. Balances are applied in one pass; reads stay constant-time as rows grow.
Post any time. The UI and API post through the same engine the batch uses.
Auditable by default. Every accepted and rejected row is timestamped with a reason.
Typed boundaries. Amounts are integers of cents; ids are normalised before they reach the engine.

Side by side

What actually changes

Comparison of the legacy batch and the modern ledger
Concern	Legacy batch	Modern ledger
Posting window	Nightly batch only	Any time, per request
Per-operation file I/O	Whole-file read(s) and a rewrite	Single pass over an in-memory map
Interface	Green-screen job submission	Responsive web UI + JSON API
Audit	Three counters on a console	Per-row entries with reasons
Rejection handling	Counted, not explained	Counted and explained per row
Value conservation	Verified by eyeballing totals	Checked automatically every batch
Recovery	Rerun the night job and hope	Replay the audit trail

This table describes the two code paths in this repository. Performance claims live on the Benchmark tab and are only shown when they were measured on this machine.

Under the hood

How this demo is wired

1

Browser

Static HTML, CSS and vanilla JS. No build step, no CDN, no third-party scripts.
2

Local Python API

Standard-library HTTP server on localhost. Holds the demo ledger and an audit trail.
3

Shared engines

Postings are written to accounts.dat/operations.dat and applied by the unmodified modern/bank.py; benchmarks reuse scripts/benchmark.py fixtures.

Ledger

Accounts

Demo customer accounts. Balances reflect the seeded history replayed through the batch engine.

Demo accounts and balances
Account	Name	Product	Opening	Balance	Rows
Loading accounts…
Ledger total				—	—

Postings

Accounts & transfers

Post a deposit, withdrawal or transfer. Each posting is validated, applied by the engine and written to the audit trail.

Recent postings

No postings yet.

Interactive

Batch console

Generate a transfer-only batch across the live ledger and run it through the modern engine. Transfers move value between accounts, so the ledger total must come out unchanged.

Operations: 60 Random seed Include 3 known-bad rows (insufficient funds, unknown destination, self transfer)

Rows submitted—

Posted—

Rejected—

Engine time—

Rejections by reason

None

Rejected rows

Ref	From	To	Amount	Reason
—

Show accepted rows (0)

Ref	From	To	Amount	From balance after
—

Pick a size and run it. The console shows what the engine accepted, what it rejected and why.

Measured, not claimed

Benchmark

Both engines are timed as separate processes on identical fixtures generated by the shared harness. If GnuCOBOL is missing, the legacy side is reported as unavailable — never estimated.

Accounts Operations

Fixture size:

Legacy batch

legacy/bank.cob

Not measured yet.

Modern engine

modern/bank.py

Not measured yet.

Verdict

Run the measurement to see a comparison for this machine.

History

Audit trail

Every row the engine saw: posted and rejected, with the reason and the resulting balance.

Status Type

Audit trail of demo postings
Entry	Time (SGT)	Type	From	To	Amount	Status	Detail
Loading audit trail…