Add Extension Store chapter for Extension loading RFC

docs/rfcs/024-extension-loading.md

@@ -207,3 +207,95 @@ Cons:
 The extension store does not have to be S3 directly, but could be a Node-local
 caching service on top of S3. This would reduce the load on the network for
 popular extensions.
+
+## Extension Store implementation
+
+Extension Store in our case is a private S3 bucket.
+Extensions are stored as tarballs in the bucket. The tarball contains the extension's control file and all the files that the extension needs to run.
+
+We may also store the control file separately from the tarball to speed up the extension loading.
+
+`s3://<the-bucket>/extensions/ext-name/sha-256+1234abcd1234abcd1234abcd1234abcd/bundle.tar`
+
+where `ext-name` is an extension name and `sha-256+1234abcd1234abcd1234abcd1234abcd` is a hash of a specific extension version tarball.
+
+To ensure security, there is no direct access to the S3 bucket from compute node.
+
+Control plane forms a list of extensions available to the compute node 
+and forms a short-lived [pre-signed URL](https://docs.aws.amazon.com/AmazonS3/latest/userguide/ShareObjectPreSignedURL.html) 
+for each extension that is available to the compute node.
+
+so, `compute_ctl` receives spec in the following format
+
+```
+"extensions": [{
+  "meta_format": 1,
+  "extension_name": "postgis",
+  "link": "https://<the-bucket>/extensions/sha-256+1234abcd1234abcd1234abcd1234abcd/bundle.tar?AWSAccessKeyId=1234abcd1234abcd1234abcd1234abcd&Expires=1234567890&Signature=1234abcd1234abcd1234abcd1234abcd",
+  ...
+}]
+```
+
+`compute_ctl` then downloads the extension from the link and unpacks it to the right place.
+
+### How do we handle private extensions?
+
+Private and public extensions are treated equally from the Extension Store perspective.
+The only difference is that the private extensions are not listed in the user UI (managed by control plane).
+
+### How to add new extension to the Extension Store?
+
+Since we need to verify that the extension is compatible with the compute node and doesn't contain any malicious code, 
+we need to review the extension before adding it to the Extension Store.
+
+I do not expect that we will have a lot of extensions to review, so we can do it manually for now.
+
+Some admin UI may be added later to automate this process.
+
+The list of extensions available to a compute node is stored in the console database.
+
+### How is the list of available extensions managed? 
+
+We need to add new tables to the console database to store the list of available extensions, their versions and access rights.
+
+something like this:
+
+```
+CREATE TABLE extensions (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(255) NOT NULL,
+    version VARCHAR(255) NOT NULL,
+    hash VARCHAR(255) NOT NULL, // this is the path to the extension in the Extension Store
+    supported_postgres_versions integer[] NOT NULL, 
+    is_public BOOLEAN NOT NULL, // public extensions are available to all users
+    is_shared_preload BOOLEAN NOT NULL, // these extensions require postgres restart
+    is_preload BOOLEAN NOT NULL,
+    license VARCHAR(255) NOT NULL,
+);
+
+CREATE TABLE user_extensions (
+    user_id INTEGER NOT NULL,
+    extension_id INTEGER NOT NULL,
+    FOREIGN KEY (user_id) REFERENCES users (id),
+    FOREIGN KEY (extension_id) REFERENCES extensions (id)
+);
+```
+
+When new extension is added to the Extension Store, we add a new record to the table and set permissions.
+ 
+In UI, user may select the extensions that they want to use with their compute node.
+
+NOTE: Extensions that require postgres restart will not be available until the next compute restart.
+Also, currently user cannot force postgres restart. We should add this feature later.
+
+For other extensions, we must communicate updates to `compute_ctl` and they will be downloaded in the background.
+
+### How can user update the extension?
+
+User can update the extension by selecting the new version of the extension in the UI.
+
+### Alternatives
+
+For extensions written on trusted languages we can also adopt
+`dbdev` PostgreSQL Package Manager based on `pg_tle` by Supabase.
+This will increase the amount supported extensions and decrease the amount of work required to support them.