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* Also add some new diagrams in the design doc * Fix some issues in ADR Signed-off-by: Jussi Kukkonen <[email protected]>
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| # Python-tuf repository API proposal: _minimal repository abstraction_ | ||
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| This is an attachment to ADR 10: _Repository library design built on top of | ||
| Metadata API_, and documents the design proposal in Dec 2020. | ||
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| ## Design principles | ||
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| Primary goals of this repository library design are | ||
| 1. Support full range of repository implementations: from command line | ||
| “repository editing” tools to production repositories like PyPI | ||
| 2. Provide canonical solutions for the difficult repository problems but avoid | ||
| making implementation decisions | ||
| 3. Keep python-tuf maintenance burden in mind: less is more | ||
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| Why does this design look so different from both legacy python-tuf code and | ||
| other implementations? | ||
| * Most existing implementations are focused on a specific use case (typically a | ||
| command line application): this is a valid design choice but severely limits | ||
| goal #1 | ||
| * The problem space contains many application decisions. Many implementations | ||
| solve this by creating functions with 15 arguments: this design tries to find | ||
| another way (#2) | ||
| * The Metadata API makes modifying individual pieces of metadata simpler. This, | ||
| combined with good repository API design, should enable more variance in | ||
| where things are implemented: The repository library does not have to | ||
| implement every little detail as we can safely let specific implementations | ||
| handle things, see goal #3 | ||
| * This variance means we can start by implementing a minimal design: as | ||
| experience from implementations is collected, we can then move implementation | ||
| details into the library (goals #2, #3) | ||
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| ## Design | ||
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|  | ||
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| The design expects a fully functional repository application to contain code at | ||
| three levels: | ||
| * Repository library (abstract classes that are part of python-tuf) | ||
| * The Repository abstract class provides an ergonomic metadata editing API | ||
| for all code levels to use. It also implements some core edit actions like | ||
| snapshot update | ||
| * A small amount of related functionality is also provided (private key | ||
| management API, maybe repository validation) | ||
| * is a very small library: possibly a few hundred lines of code | ||
| * Concrete Repository implementation (typically part of application code, | ||
| implements interfaces provided by the repository API in python-tuf) | ||
| * Contains the “application level” decisions that the Repository abstraction | ||
| requires to operate: examples of application decisions include | ||
| * _when should “targets” metadata next expire when it is edited?_ | ||
| * _What is the current “targets” metadata version? Where do we load it | ||
| from?_ | ||
| * _Where to store current “targets” after editing? Should the previous | ||
| version be deleted from storage?_ | ||
| * Actual application | ||
| * Uses the Repository API to do the repository actions it needs to do | ||
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| For context here’s a trivial example showing what “ergonomic editing” means -- | ||
| this key-adding code could be in the application or in the python-tuf library: | ||
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| ```python | ||
| with repository.edit(“targets”) as targets: | ||
| # adds a key for role1 (as an example, arbitrary edits are allowed) | ||
| targets.add_key(“role1”, key) | ||
| ``` | ||
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| This code loads current targets metadata for editing, adds the key to a role, | ||
| and handles version and expiry bumps before persisting the new targets version. | ||
| The reason for the context manager style is that it manages two things | ||
| simultaneously: | ||
| * Hides the complexity of loading and persisting metadata, and updating expiry | ||
| and versions from the editing code (by putting it in the repository | ||
| implementation – which may still be provided by the application) | ||
| * Still allows completely arbitrary edits on the metadata in question: now the | ||
| library does not need to anticipate what application wants to do and on the | ||
| other hand library can still provide e.g. snapshot functionality without | ||
| knowing about the application decisions mentioned in previous point. | ||
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| Other designs do not seem to manage both of these. | ||
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|  | ||
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| The core idea here is that because editing is ergonomic enough, when new | ||
| functionality (like “developer uploads new targets”) is added, _it can be added | ||
| at any level_: the application might add a `handle_new_target_files()` method | ||
| that adds a bunch of targets into the metadata, but one of the previous layers | ||
| could offer that as a helper function as well: code in both cases would look | ||
| similar as it would use the common editing interface. | ||
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| There are a few additional items worth mentioning: | ||
| * Private key management: the Repository API should come with a “keyring | ||
| abstraction” -- a way for the application to provide roles’ private keys for | ||
| the Repository to use. Some implementations could be provided as well. | ||
| * Validating repository state: the design is very much focused on enabling | ||
| efficient editing of individual metadata. Implementations are also likely to | ||
| be interested in validating (after some edits) that the repository is correct | ||
| according to client workflow and that it contains the expected changes. The | ||
| Repository API should provide some validation, but we should recognise that | ||
| validation may be implementation specific. | ||
| * Improved metadata editing: There are a small number of improvements that | ||
| could be made to metadata editing. These do not necessarily need to be part | ||
| of the repository API: they could be part of Metadata API as well | ||
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| It would make sense for python-tuf to ship with at least one concrete | ||
| Repository implementation: possibly a repo.py look alike. This implementation | ||
| should not be part of the library but an example. | ||
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| ## Details | ||
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| This section includes links to a Proof of Concept implementation in | ||
| [repository-editor-for-tuf](https://github.com/vmware-labs/repository-editor-for-tuf/): | ||
| it should not be seen as the exact proposed API but a prototype of the ideas. | ||
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| The ideas in this document map to POC components like this: | ||
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| | Concept | repository-editor-for-tuf implementation | | ||
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| | Repository API | [librepo/repo.py](https://github.com/vmware-labs/repository-editor-for-tuf/blob/main/tufrepo/librepo/repo.py), [librepo/keys.py](https://github.com/vmware-labs/repository-editor-for-tuf/blob/main/tufrepo/librepo/repo.py) | | ||
| | Example of repository implementation | [git_repo.py](https://github.com/vmware-labs/repository-editor-for-tuf/blob/main/tufrepo/git_repo.py) | | ||
| |Application code | [cli.py (command line app)](https://github.com/vmware-labs/repository-editor-for-tuf/blob/main/tufrepo/cli.py), [keys_impl.py (keyring implementation)](https://github.com/vmware-labs/repository-editor-for-tuf/blob/main/tufrepo/keys_impl.py) | | ||
| | Repository validation | [verifier.py (very rough, not intended for python-tuf)](https://github.com/vmware-labs/repository-editor-for-tuf/blob/main/tufrepo/verifier.py) | ||
| | Improved Metadata editing | [helpers.py](https://github.com/vmware-labs/repository-editor-for-tuf/blob/main/tufrepo/helpers.py) | ||
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| ### Repository API | ||
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| Repository itself is a minimal abstract class: The value of this class is in | ||
| defining the abstract method signatures (most importantly `_load`, `_save()`, | ||
| `edit()`) that enable ergonomic metadata editing. The Repository class in this | ||
| proposal includes concrete implementations only for the following: | ||
| * `sign()` -- signing without editing metadata payload | ||
| * `snapshot()` -- updates snapshot and timestamp metadata based on given input. | ||
| Note that a concrete Repository implementation could provide an easier to use | ||
| snapshot that does not require input (see example in git_repo.py) | ||
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| More concrete implementations (see cli.py for examples) could be added to | ||
| Repository itself but none seem essential at this point. | ||
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| The API requires a “Keyring” abstraction that the repository code can use to | ||
| lookup a set of signers for a specific role. Specific implementations of | ||
| Keyring could include a file-based keyring for testing, env-var keyring for CI | ||
| use, etc. Some implementations should be provided in the python-tuf code base | ||
| and more could be implemented in applications. | ||
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| _Prototype status: Prototype Repository and Keyring abstractions exist in | ||
| librepo/repo.py._ | ||
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| ### Example of Repository implementation | ||
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| The design decisions that the included example `GitRepository` makes are not | ||
| important but provide an example of what is possible: | ||
| * Metadata versions are stored in files in git, with filenames that allow | ||
| serving the metadata directory as is over HTTP | ||
| * Version bumps are made based on git status (so edits in staging area only | ||
| bump version once) | ||
| * “Current version” when loading metadata is decided based on filenames on disk | ||
| * Files are removed once they are no longer part of the snapshot (to keep | ||
| directory uncluttered) | ||
| * Expiry times are decided based on an application specific metadata field | ||
| * Private keys can be stored in a file or in environment variables (for CI use) | ||
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| Note that GitRepository implementation is significantly larger than the | ||
| Repository interface -- but all of the complexity in GitRepository is really | ||
| related to the design decisions made there. | ||
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| _Prototype status: The GitRepository example exists in git_repo.py._ | ||
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| ### Validating repository state | ||
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| This is mostly undesigned but something built on top of TrustedMetadataSet | ||
| (currently ngclient component) might work as a way to easily check specific | ||
| aspects like: | ||
| * Is top-level metadata valid according to client workflow | ||
| * Is a role included in the snapshot and the delegation tree | ||
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| It’s likely that different implementations will have different needs though: a | ||
| command line app for small repos might want to validate loading all metadata | ||
| into memory, but a server application hosting tens of thousands of pieces of | ||
| metadata is unlikely to do so. | ||
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| _Prototype status: A very rough implementation exists in verifier.py : this is | ||
| unlikely to be very useful_ | ||
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| ### Improved metadata editing | ||
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| Currently the identified improvement areas are: | ||
| * Metadata initialization: this could potentially be improved by adding | ||
| default argument values to Metadata API constructors | ||
| * Modifying and looking up data about roles in delegating metadata | ||
| (root/targets): they do similar things but root and targets do not have | ||
| identical API. This may be a very specific use case and not interesting | ||
| for some applications | ||
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| _Prototype status: Some potential improvements have been collected in | ||
| helpers.py_ |