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book/faqs: Talk about compression

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Zhaofeng Li 2023-01-01 20:59:02 -07:00
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book/src/faqs.md
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# FAQs
<!-- TODO: Write more about design decisions in a separate section -->
## Does it replace [Cachix](https://www.cachix.org)?
No, it does not.
@ -29,6 +31,37 @@ Authentication is done via signed JWTs containing the allowed permissions.
Each instance of `atticd --mode api-server` is stateless.
This design may be revisited later, with option for a more stateful method of authentication.
## How is compression handled?
Uploaded NARs are compressed on the server before being streamed to the storage backend.
We use the hash of the _uncompressed NAR_ to perform global deduplication.
```
┌───────────────────────────────────►NAR Hash
│
│
├───────────────────────────────────►NAR Size
│
┌─────┴────┐ ┌──────────┐ ┌───────────┐
NAR Stream──►│NAR Hasher├─►│Compressor├─►│File Hasher├─►File Stream─►S3
└──────────┘ └──────────┘ └─────┬─────┘
│
├───────►File Hash
│
│
└───────►File Size
```
At first glance, performing compression on the client and deduplicating the result may sound appealing, but has problems:
1. Different compression algorithms and levels naturally lead to different results which can't be deduplicated
2. Even with the same compression algorithm, the results are often non-deterministic (number of compression threads, library version, etc.)
When we do the compression on the server and use the hashes of uncompressed NARs for lookups, the problem of non-determinism is no longer a problem since we only compress once.
On the other hand, performing compression on the server leads to additional CPU usage, increasing compute costs and the need to scale.
Such design decisions are to be revisited later.
## On what granularity is deduplication done?
Currently, global deduplication is done on the level of NAR files.