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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →A business-status API can tell you what it reports now. If you need to know what it reported yesterday—or when a company first appeared closed—you need to keep your own record. That is why Yoon built a scheduled job to check 211,243 companies each day, save a snapshot, and compare it with the previous one.
Why take a snapshot if the API already answers the question?
The Korean National Tax Service registration-status service returns current information such as operating status, tax type, and closure date. The Korean Public Data Portal describes the API as a free REST service, updated from NTS data every 30 minutes. It also cautions that newly opened businesses may take one to two days to appear. The portal listing, modified May 13, 2026, does not document an as_of parameter or historical endpoint; that does not prove no other historical facility exists. Yoon describes the upstream service as lacking a history endpoint, changelog, or way to request status as of a past date. Korean Public Data Portal API listing.
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The distinction is between a present answer and a record of what was observed over time. A one-time lookup is useful when you need a company’s current reported status. A recurring watchlist is useful when you need to detect changes and retain evidence of what the service returned on earlier dates. In this project, the valuable output is not just the latest status; it is the history created by keeping each observation.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11| Pattern | How it runs | What it returns or preserves | Best fit |
|---|---|---|---|
| One-time lookup | Started for an individual query | Current result for the requested business | Checking present status |
| Recurring watchlist | Runs on a schedule against a fixed list | A full daily snapshot and a separate comparison with the previous observation | Finding and retaining observed changes over time |
What the daily job records
Yoon’s workflow queries a fixed company list, stores the returned statuses as a daily snapshot, then compares that snapshot with the previous day’s. The full snapshot preserves the day’s observations; a separate changes file makes the differences easier to inspect. The Public Data Portal lists a maximum of 100 records per request, so the author estimates roughly 2,100 upstream calls per day for 211,243 records. That is an estimate for this workload, not the number of companies or results; the portal lists a daily limit of one million calls. Korean Public Data Portal API listing.
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In Yoon’s first three daily runs, each snapshot contained 211,243 observations. The second-day comparison found 70 status changes; the third found 94. These are results from the initial three-day sample, not a typical daily rate or an independently audited benchmark. Yoon reported files of approximately 42 MB for each daily snapshot and 1.7 MB for the changes file. Yoon’s October 1, 2026 article.
What a change record can—and cannot—prove
A change between two snapshots establishes that the service returned different states at two observation times. It does not establish the exact moment the underlying event occurred. If one observation says “active” and the next says “closed,” the closure could have happened at any point between those observations. Yoon’s record keeps the observation timestamps rather than claiming an exact closure time: “I can’t prove when it actually closed — only when I observed each state.” Yoon’s October 1, 2026 article.
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This distinction matters whenever the data is used to explain a timeline or support a later decision. A snapshot is evidence of what the API returned at that time, not necessarily a complete account of when a business event happened. The portal’s 30-minute update cycle and its warning about delays for new registrations are further reasons to avoid treating a returned value as a perfectly synchronized view of reality. Korean Public Data Portal API listing.
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Do not treat the first run as a diff test
On the first run there is no previous snapshot to compare against, so every row is new to the system. Yoon says the comparison logic could be meaningfully checked only on the second day, when the job had an earlier observation. Validation should account for that: a successful initial collection proves that data was fetched and stored, but not that the day-to-day change detection works.
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Check that scheduled execution did useful work
A scheduler’s green status is not enough. Yoon reports that a Linux entry-point guard caused a scheduled run to exit successfully without doing its work; manual execution exposed the problem. For a recurring data job, verify that expected output files or records were produced and that the comparison ran, rather than relying only on the process exit status. Yoon’s October 1, 2026 article.
Why the source list matters
The watchlist is not described as a collection of customer lookup queries. Yoon says it was assembled from public procurement and debarment registries and that runtime and test guards were used to keep the project within that scope. That provenance is part of the system’s design: a retained history is only as appropriate as the data collection purpose and source list behind it. Yoon’s October 1, 2026 article.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.The practical lesson for public-data projects
Before building an agent or service around a public API, ask what it cannot answer later. A current-state endpoint may be enough for a one-off decision. If a future question might be “what did this service report before the change?”, history has to be collected while the observations are available. Yoon’s point is not that every API needs a shadow database; it is that retention is a deliberate capability, with storage, validation, and provenance choices that should be made before the desired history is gone.
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