53 lines
2.2 KiB
Markdown
53 lines
2.2 KiB
Markdown
---
|
|
published: false
|
|
---
|
|
|
|
# Architecture
|
|
|
|
## Design
|
|
**TODO(stevvooe):** Discuss the architecture of the registry, internally and externally, in a few different deployment scenarios.
|
|
|
|
### Eventual Consistency
|
|
|
|
> **NOTE:** This section belongs somewhere, perhaps in a design document. We
|
|
> are leaving this here so the information is not lost.
|
|
|
|
Running the registry on eventually consistent backends has been part of the
|
|
design from the beginning. This section covers some of the approaches to
|
|
dealing with this reality.
|
|
|
|
There are a few classes of issues that we need to worry about when
|
|
implementing something on top of the storage drivers:
|
|
|
|
1. Read-After-Write consistency (see this [article on
|
|
s3](http://shlomoswidler.com/2009/12/read-after-write-consistency-in-amazon.html)).
|
|
2. [Write-Write Conflicts](http://en.wikipedia.org/wiki/Write%E2%80%93write_conflict).
|
|
|
|
In reality, the registry must worry about these kinds of errors when doing the
|
|
following:
|
|
|
|
1. Accepting data into a temporary upload file may not have latest data block
|
|
yet (read-after-write).
|
|
2. Moving uploaded data into its blob location (write-write race).
|
|
3. Modifying the "current" manifest for given tag (write-write race).
|
|
4. A whole slew of operations around deletes (read-after-write, delete-write
|
|
races, garbage collection, etc.).
|
|
|
|
The backend path layout employs a few techniques to avoid these problems:
|
|
|
|
1. Large writes are done to private upload directories. This alleviates most
|
|
of the corruption potential under multiple writers by avoiding multiple
|
|
writers.
|
|
2. Constraints in storage driver implementations, such as support for writing
|
|
after the end of a file to extend it.
|
|
3. Digest verification to avoid data corruption.
|
|
4. Manifest files are stored by digest and cannot change.
|
|
5. All other non-content files (links, hashes, etc.) are written as an atomic
|
|
unit. Anything that requires additions and deletions is broken out into
|
|
separate "files". Last writer still wins.
|
|
|
|
Unfortunately, one must play this game when trying to build something like
|
|
this on top of eventually consistent storage systems. If we run into serious
|
|
problems, we can wrap the storagedrivers in a shared consistency layer but
|
|
that would increase complexity and hinder registry cluster performance.
|