Shrex client server specification

specs/src/shrex/client-server.md

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+# SHREX Client/Server Specification
+
+## Abstract
+
+This specification defines a request-response protocol that enables nodes to retrieve data from peers. The protocol provides a simple interface for requesting specific data and receiving responses.
+
+## Overview
+
+It implements a client-server architecture for data exchange:
+
+- **SHREX Client**: Makes requests to retrieve data from a specific peer
+- **SHREX Server**: Responds to requests by serving data from local storage
+
+## Client
+
+The SHREX client is responsible for:
+
+- Connecting to a specified peer
+- Sending a data request
+- Receiving and validating the status response
+- Returning the result to the caller
+
+### Input and Output
+
+**Input**:
+
+- Target peer ID
+- Request parameters (height, type-specific data)
+
+**Output**:
+
+- Retrieved data (on success)
+- Error information (on failure)
+
+### Request Flow
+
+```text
+┌──────────┐         ┌────────────┐         ┌────────┐
+│  Getter  │         │   Client   │         │ Server │
+└────┬─────┘         └─────┬──────┘         └───┬────┘
+     │                     │                    │
+     │ Request(peer, data) │                    │
+     ├────────────────────>│                    │
+     │                     │                    │
+     │                     │ Connect            │
+     │                     ├───────────────────>│
+     │                     │                    │
+     │                     │ Send Request       │
+     │                     ├───────────────────>│
+     │                     │                    │
+     │                     │ Receive Status     │
+     │                     │<───────────────────┤
+     │                     │                    │
+     │                     │ Receive Data       │
+     │                     │<───────────────────┤
+     │                     │                    │
+     │ Return Data/Error   │                    │
+     │<────────────────────┤                    │
+     │                     │                    │
+```
+
+## Server
+
+- Accepting incoming connections
+- Reading and validating requests
+- Retrieving data from local storage
+- Sending responses back to clients
+
+### Request Handling
+
+```text
+                Incoming Request
+                       │
+                       ▼
+                 ┌──────────────┐
+                 │ Validate     │
+                 │ Request      │
+                 └──────┬───────┘
+                        │
+                ┌───────┴────────┐
+                │                │
+              Invalid          Valid
+                 │                │
+                 ▼                ▼
+            ┌──────────┐    ┌─────────────┐
+            │ Reject   │    │ Get Data    │
+            └──────────┘    │ from Store  │
+                            └──────┬──────┘
+                                   │
+                           ┌───────┴────────┐
+                           │                │
+                          Found          Not Found
+                            │                │
+                            ▼                ▼
+                       ┌──────────┐    ┌──────────┐
+                       │ Send OK  │    │ Send NOT │
+                       │ + Data   │    │ FOUND    │
+                       └──────────┘    └──────────┘
+```
+
+## Response Format
+
+All server responses follow a two-phase pattern:
+
+1. **Status Message**: Indicates whether the request can be fulfilled
+    - **OK**: Data is available and will follow
+    - **READ REQUEST ERROR** - error happened during reading a request from the stream
+    - **SEND STATUS ERROR** - errod during sending a status back the client
+    - **SEND RESPONSE ERROR** - errod during sending a status back the client
+    - **BAD REQUEST** - request did not pass validation
+    - **NOT FOUND**: Requested data is not available
+    - **INTERNAL ERROR**: Server encountered an error
+
+2. **Data (if OK)**: The actual requested data
+
+## Error Handling
+
+### Client Errors
+
+The client reports errors to its caller:
+
+- Connection failures
+- Timeouts
+- Invalid data
+
+### Server Errors
+
+The server handles errors by:
+
+- Sending appropriate status codes
+- Rejecting invalid requests
+
+## Resource Protection
+
+The protocol includes mechanisms to protect node resources:
+
+- **Timeouts**: Requests have time limits to prevent resource exhaustion
+- **Validation**: Invalid requests are rejected early to save resources
+
+## Requirements Language
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.