blossom/buds/11.md

BUD-11

Local Blob Cache

draft optional

This document defines the specification for a local blob cache server that can be hosted on 127.0.0.1:24242 to provide fast, local access to cached blobs or proxy requests to other public Blossom servers.

Server Address

A local blob cache server MUST be accessible on http://127.0.0.1:24242. The port 24242 is chosen to align with the Nostr event kind used for Blossom authorization events.

Health Check Endpoint

The server MUST respond with a 2xx HTTP status code (typically 200 OK) on the HEAD / endpoint to allow local applications to easily detect if the cache server is available.

The HEAD / endpoint MUST return a 2xx HTTP status code. The response MUST NOT include a response body per RFC 7231.

This endpoint enables simple health checks:

curl -I http://127.0.0.1:24242/

Access Control

The server SHOULD NOT require any authentication for blob downloads or uploads. All requests MUST be served without requiring an Authorization header.

If an implementation needs to add access control, it SHOULD restrict access based on IP address to ensure requests only come from 127.0.0.1. This ensures that local applications can freely access cached blobs without the overhead of signing authorization events.

Blob Retrieval

The server MUST implement the GET /<sha256> and HEAD /<sha256> endpoints as defined in BUD-01:

1. The server MUST accept optional file extensions in the URL (e.g., /<sha256>.pdf)
2. The server MUST set appropriate Content-Type headers or default to application/octet-stream

Range Requests

To better support mobile devices, video files, or low bandwidth connections, implementations SHOULD support range requests (RFC 7233 section 3) on the GET /<sha256> endpoint and signal support using the accept-ranges: bytes and content-length headers on the HEAD /<sha256> endpoint.

See MDN docs for more details.

Proxy Hints

The server SHOULD accept BUD-10 query parameters xs (server) and as (author) hints on the GET /<sha256> endpoint to enable proxying blob requests to other Blossom servers when the blob is not available in the local cache.

Query Parameter Format

When a blob is not found in the local cache, the server MAY use the following query parameters to attempt retrieval from other Blossom servers:

• xs=<server> - One or more server hints where the blob may be available
• as=<pubkey> - One or more author pubkeys whose server lists may contain the blob

Example Request

GET /b1674191a88ec5cdd733e4240a81803105dc412d6c6708d53ab94fc248f4f553.pdf?xs=cdn.example.com&as=ec4425ff5e9446080d2f70440188e3ca5d6da8713db7bdeef73d0ed54d9093f0

Proxy Behavior

When the server receives a request with proxy hints and the blob is not in the local cache:

1. The server SHOULD attempt to retrieve the blob from the servers specified in the xs parameters
2. If xs hints fail, the server MAY attempt to retrieve the blob using the as parameters by:
   • Fetching the author's BUD-03 server list (kind:10063)
   • Attempting to retrieve the blob from servers in the author's list
3. If the blob is successfully retrieved, the server SHOULD:
   • Cache the blob locally for future requests
   • Return the blob to the client with appropriate headers
4. If the blob cannot be retrieved from any hint, the server MUST return a 404 Not Found response

This proxy functionality allows the local cache server to act as a transparent proxy, automatically fetching and caching blobs from remote Blossom servers when needed.

CORS Headers

The server MUST set the Access-Control-Allow-Origin: * header on all responses to ensure compatibility with web applications, as specified in BUD-01.

Use Cases

A local blob cache server enables several use cases:

• Fast Local Access: Applications can check the local cache first before making network requests to remote Blossom servers
• Offline Access: Previously cached blobs remain accessible even when network connectivity is unavailable
• Bandwidth Savings: Reduces redundant downloads of frequently accessed blobs
• Development: Provides a simple local server for testing Blossom applications without requiring remote server access

Implementation Notes

• The server SHOULD implement efficient blob storage, such as using the sha256 hash as the filename or storage key
• The server SHOULD implement cache eviction policies (e.g., LRU, size-based limits) to manage storage
• The server SHOULD validate that downloaded blobs match their sha256 hash before caching
• The server MAY implement the HEAD /<sha256> endpoint to allow clients to check blob availability without downloading

Example Implementation Flow

1. Client requests blob: GET http://127.0.0.1:24242/<sha256>
2. Server checks local cache
3. If found: Return blob immediately
4. If not found and proxy hints provided:
   • Attempt to fetch from xs server hints
   • If that fails, attempt to fetch using as author hints
   • Cache the blob if successfully retrieved
   • Return the blob to the client
5. If not found and no proxy hints: Return 404 Not Found