design: add wasm extension supports OCI image code source

site/content/en/contributions/design/wasm-extension.md

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+---
+title: "Wasm OCI Image Support"
+---
+
+## Motivation
+Envoy Gateway (EG) should support Wasm OCI image as a remote wasm code source.
+This feature will allow users to deploy Wasm modules from OCI registries, such as Docker Hub, 
+Google Container Registry, and Amazon Elastic Container Registry, to Envoy proxies managed by EG.
+Deploying Wasm modules from OCI registries has several benefits:
+* **Versioning**: Users can use the tag feature of the OCI image to manage the version of the Wasm module.
+* **Security**: Users can use private registries to store the Wasm module.
+* **Distribution**: Users can use the existing distribution mechanism of the OCI registry to distribute the Wasm module.
+
+## Goals
+* Define the system components needed to support Wasm OCI images as remote Wasm code sources.
+
+## Architecture
+
+![](/img/wasm-extension.png)
+
+### Control Plane Wasm File Cache
+
+Envoy lacks native OCI image support, therefore, EG needs to download Wasm modules from their original OIC registries, 
+cache them locally in the file system, and serve them to Envoy over HTTP.
+
+#### HTTP Code Source
+
+For HTTP code source, we have two options: serve Wasm modules directly from their 
+original HTTP URLs, or cache them in EG (as with OCI images). Caching both the HTTP Wasm modules and OCI 
+images inside EG can make UI consistent, for example, sha256sum can be calculated on the EG side and made 
+optional in the API. This will also make the Envoy proxy side more efficient as it won’t have to download the
+Wasm module from the original URL every time, which can be slow.
+
+#### Resource Consumption
+
+Memory: Since we cache Wasm modules in the file system, we can optimize the memory usage 
+of the cache and the HTTP server to avoid introducing too much memory consumption by this feature. 
+For example, when receiving a Wasm pulling request form the Envoy, we can open the Wasm file and write 
+the file content directly to response, and then close the file. There won’t be significant memory consumption involved. 
+
+Disk: Though it's possible to mount a volume to the container for the Wasm file cache, the current implementation just
+stores the Wasm files in the EG container’s file system. The disk space consumed by the cache is limited to 1GB by default, 
+it can be made configurable in the future. 
+
+#### Caching Mechanism
+
+Cached files will be evicted based on LRU（Last recently used）algorithm. 
+If the image’s tag is latest, then they will be updated periodically. The cache clean and update periods 
+will be configurable.
+
+#### Restrict Access to Private Images
+
+* **Client Authn with mTLS:** To prevent unauthorized proxies from accessing the Wasm modules, the communication between 
+  the Envoy and EG will be secured using mTLS.
+* **User Authn with Registry Credentials:** To prevent unauthorized users from accessing the Wasm modules, the user who 
+  creates the EEP must have the appropriate permissions to access the OCI registry. For example, if two users create EEPs
+  in different namespaces (ns1, ns2) accessing the same OCI image, each must also create a unique secret with registry 
+  credentials (secret1 for user1 in ns1, secret2 for user2 in ns2) and provide it in the EEP configuration. EG will validate
+  the provided secret against the OCI registry before serving the Wasm module to the target HTTPRoute/Gateway of that EEP.
+* **Unguessable Download URLs:** It's possible that users who have no permission to access a private OCI image could create
+  an EnvoyPatchPolicy to bypass the EG permission check. For example, a user could create an EPP, inject a Wasm filter, 
+  and put the download URL of a private OCI image in the Wasm filter configuration. To prevent this, we need to make the 
+  download URL unguessable. The download URL will be generated by EG and will be a random string that is impossible to 
+  guess. If a user can get the config dump the Envoy Proxy, they can still get the download URL. However, they won’t be 
+  able to do so without the permission to access the config dump of Envoy Proxy, which is a more restricted permission (usually Admin role).
+
+## Alternative Considered
+
+### Inline Bytes
+
+EG downloads the Wasm modules, caches them in memory, and pushes the Wasm code through xDS as inline bytes. 
+This could inflate the xDS, potentially causing memory issues for EG and Envoy.
+
+### Data Plane Agent
+Mount Wasm modules in the local file system of the Envoy container. We’ll need an agent deployed in the 
+same pod as the Envoy for this. It would be too expensive to implement this as we’ll need to intercept 
+the xDS at the agent.
+
+### Standalone Wasm HTTP Server
+Deploying the Wasm HTTP server as a standalone service. While this has no obvious benefits, it increases 
+operational costs.
+
+### Wait for Envoy OCI image support
+We could wait indefinitely for Envoy to support OCI imag as a remote wasm code source.