initial commit for archtecture doc

Signed-off-by: Ramkumar Chinchani <[email protected]>

modules/kb/pages/arch.md

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+# Architecture
+
+`zot` is a OCI-native container image registry, and in this document we discuss
+the overall architecture with an emphasis on the various design choices that
+have been made.
+
+# Design Goals
+
+## OCI-first
+
+* **Strongly conforms** to the [OCI Distribution Specification](https://github.com/opencontainers/distribution-spec)
+
+`zot` intends to be **the** reference implementation for the OCI Distribution
+Specification. In fact, it does not support any other vendor protocol or spec.
+
+* **[OCI Image Specification](https://github.com/opencontainers/image-spec) storage layout**
+
+The default and only supported storage layout is the OCI Image Layout. The
+implications of this choice are that any OCI image layout can be served by
+`zot` and conversely, `zot` converts data on-the-wire into a OCI image
+layout.
+
+## Single binary model
+
+`zot` is a single binary with all features included so that deployment by
+itself is extremely simple in various environments (bare-metal, cloud and
+embedded devices) while behavior is controlled by configuration.
+
+## Enable Only What You Need
+
+There is a clear separation between the core OCI-compliant HTTP APIs and
+storage functionality and the other add-on features modeled as _extensions_.
+The additional features can be selectively enabled both at build-time and run-time.
+[NOTE: add cross ref to the right article about minimal/full builds]
+
+# Overall Architecture
+
+[NOTE: include the image here]
+
+## Interaction
+
+There are two types of external interaction with `zot` - 1) clients performing
+data or meta-data queries, and 2) configuration. All client-side interaction
+happens over HTTP APIs. The core data path queries are governed by the
+Distribution Specification. All additional meta-data queries by a graphQL
+interface if the `search` extension is enabled and if not falls back to the
+core APIs.
+
+## Configuration
+
+There is a single configuration file that governs `zot` instance behavior.
+Although note that some parts of the configuration which contain sensitive
+credentials are stored in their own files and these files are referenced in the
+main configuration file. This is done so that stricter permissions can be
+enforced on those files if stored locally, and also modeling as external files
+allows for Kubernetes
+[`Secrets`](https://kubernetes.io/docs/concepts/configuration/secret/).
+
+The configuration itself is separated into `http`, `storage`, `log` and
+`extension` governing the behavior of various components.
+
+## Authentication and Authorization
+
+Both authentication and authorization are supported natively in `zot`. These
+controls are enforced before access is allowed into the storage layer. More
+details are available here. [NOTE: add a cross-ref to authN doc]
+
+## Storage Driver Support
+
+`zot` supports any modern local filesystem, and if deduplication is desired
+then a filesystem with hard-link support is required.
+
+There is also support for remote filesystems such as AWS S3 or any AWS
+S3-compatible storage systems. Additional driver support is planned in the roadmap.
+Note that `zot` does support deduplication even for remote filesystems.
+
+## Security Scanning
+
+`zot` integrates with the [`trivy`](https://github.com/aquasecurity/trivy)
+security scanner to scan container images for any vulnerabilities. The database
+is kept up to date by periodically (also controlled by a configuration
+parameter) downloading any vulnerability database updates. The user remains
+agnostic of the actual scanner implementation which may change over time.
+
+## Extensions
+
+The following extensions are currently supported. Wherever applicable
+extensions can be dynamically discovered using OCI Distribution Specification
+[extensions support](https://github.com/opencontainers/distribution-spec/tree/main/extensions).
+
+* **Search**
+
+One of the key functions for a container image registry which is essentially a
+graph of blobs is to be able to perform interesting image and graph traversal
+queries such as "does an image exist?", "what is its size?", "does an image
+depend on this image via its layers?", "what vulnerabilities in an image? or
+its dependent images?" and so on.
+
+The user interacts with this extension via a graphQL endpoint and the schema is
+published with every release.
+
+* **Sync**
+
+You can set up a local mirror pointing to an upstream `zot` instance with
+various container image download policies, including on-demand and periodic
+downloads. This is useful when you do not want to overwhelm the upstream
+instance or the upstream instance has rate-limited access. 
+
+Note that `docker.io` as an upstream mirror is supported.
+
+* **Lint**
+
+The _lint_ extension can enforce certain policies about the image or the image
+metadata. Currently, it can enforce if the uploaded image has the right
+annotations (such as the author or the license). This helps with image
+compliance issues.
+
+* **Scrub**
+
+Although container images are content-addressable with their SHA256 checksums
+and validations are performed during storage and retrieval, it is possible that
+bit-rot sets in when not in use. The _scrub_ extension actively scans container
+images in background to proactively detect an errors.
+
+## Background Tasks
+
+There are several periodic tasks that occur such as garbage collection, sync
+mirroring and scrubbing, and a task scheduler handles these tasks in the
+background while being mindful to not degrade or interrupt foreground tasks
+running in the context of HTTP APIs.