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One question
What remains usable when a service closes?
Separate exporting, verifying, continued access and running a replacement.
A photo service announces its closure. You can download the images, but album structure, notes and sharing relationships are missing. The files survive while part of their usefulness is lost. This hypothetical example makes “data portability” concrete.
Chapter 5 of the book puts user control and migration at the center of application design. Test that goal through four outcomes: obtaining the data, understanding its format, preserving necessary evidence and using it elsewhere. None requires every part of the solution to be blockchain-based.
| What you retain | What remains to be solved |
|---|---|
| Photo files | Reconstructing albums, notes and relationships |
| Hashes or on-chain records | Keeping the original files available |
| Encrypted backups | Retaining usable decryption keys and format information |
| Application source | Supplying dependencies, configuration, data and a working environment |
An IPFS content address helps check retrieved content. Continued retrieval still requires someone to retain and provide it. Pinning is a way to keep data from being garbage-collected by a node; an address is not a storage commitment.
Migration also involves other people. Taking your own copy does not transfer their accounts, consent or permissions to a new service. Old access grants need an exit process, and revoking future access cannot recall existing copies.
A useful exit test is to export a small sample while the service still operates, read it offline and restore it with another tool. Record the specific failures to identify remaining dependencies.
Book and sources
This lesson develops the questions in Chapter 5 of 《区块链实战》 with later standards. It is not an excerpt.
Check your understanding
Can file hashes and application source code restore a photo library?
No. You also need the photos, necessary keys and data structures, and a runnable environment with its dependencies. A hash cannot reconstruct the content.
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