logos-liblogos/docs/spec.md

Logos Core Library

Overall Description

Logos Core is a modular runtime platform for hosting and orchestrating independently developed modules. It provides a C-API shared library (liblogos_core) and a pluggable runtime system that together enable a module-based architecture for decentralised applications. The default runtime (SubprocessManager) spawns a logos_host_qt process per module, but the architecture allows registering alternative runtimes for different module formats.

The platform is designed to:

• Load, start, stop, and introspect modules at runtime
• Isolate each module in its own process for robustness and security
• Provide transparent inter-module RPC via a remote object registry
• Support token-based authentication for secure module-to-module communication
• Expose a C API so that host applications in any language can drive the runtime

Definitions & Acronyms

Term	Definition
Module	An independently developed module that implements PluginInterface and is dynamically loaded by the core
Core Library	liblogos_core — the shared library that provides the C API for module management
Module Host	logos_host_qt — a lightweight executable that loads a single Qt module in its own process (a logos_host symlink exists for backward compatibility)
Module Runtime	An abstract interface (ModuleRuntime) that encapsulates a strategy for loading and managing modules (e.g. subprocess, in-process, WASM)
Runtime Registry	The central registry (RuntimeRegistry) that selects the appropriate ModuleRuntime for a given module based on its format descriptor
Core Manager	A built-in module that exposes core functionality via RPC, allowing modules to manage the core without linking against the C API
Capability Module	A built-in module that handles authorization tokens for inter-module communication
RPC	Remote Procedure Call — the mechanism by which modules invoke methods on each other
IPC	Inter-Process Communication — the underlying transport
Token	A UUID-based authentication credential issued by the core or capability module for securing RPC calls
SDK	The logos-cpp-sdk — client library that abstracts connection management, token handling, and asynchronous invocation

Domain Model

System Architecture

At a high level, the Logos Core consists of:

Core Library — The C/C++ shared library (liblogos_core) that provides the API functions for lifecycle management, module loading/unloading, and introspection.

Runtime Registry — A central registry of ModuleRuntime implementations. When a module is loaded, the registry selects the first runtime whose canHandle() returns true for the module's descriptor. This decouples the core from any specific loading mechanism.

Module Runtimes — Pluggable implementations of the ModuleRuntime interface. The default SubprocessManager spawns logos_host_qt processes, but alternative runtimes (in-process, WASM, etc.) can be registered.

Core Manager — A built-in module that runs in the core process and exposes core functionality as RPC methods, allowing remote modules to manage the core without linking against the C API directly.

Module Host — A lightweight executable (logos_host_qt) that loads a single Qt module in its own process. It communicates with the core over a local socket to receive an authentication token and registers the module's object with the remote registry.

Capability Module — A built-in module that handles authorization for inter-module communication by issuing tokens and notifying both communicating parties.

Remote Object Registry — A registry that maintains a mapping of module names to remote object replicas and forwards method calls/events.

Process Architecture

Each module runs in its own process for isolation:

┌──────────────────────────────────────────────────────┐
│  Host Application                                    │
│  ┌────────────────────────────────────────────────┐  │
│  │  liblogos_core                                 │  │
│  │  ├─ RuntimeRegistry                            │  │
│  │  │   └─ SubprocessManager (default runtime)     │  │
│  │  ├─ Core Manager (built-in module)             │  │
│  │  ├─ Capability Module (built-in module)        │  │
│  │  └─ Remote Object Registry                     │  │
│  └────────────────────────────────────────────────┘  │
│           │ IPC (local socket)                        │
│     ┌─────┼─────────┐                                │
│     ▼     ▼         ▼                                 │
│  ┌─────┐ ┌─────┐ ┌─────┐                             │
│  │host │ │host │ │host │  (logos_host_qt processes)   │
│  │mod A│ │mod B│ │mod C│                               │
│  └─────┘ └─────┘ └─────┘                             │
└──────────────────────────────────────────────────────┘

• The core uses a RuntimeRegistry to select the appropriate ModuleRuntime for each module
• The default SubprocessManager runtime spawns a logos_host_qt process per module
• Modules with format == "qt-plugin" or no explicit format are handled by SubprocessManager
• Communication happens via the Logos API. Each module's transport set is configured per-module by the host: by default modules listen on a LocalSocket only, but the host can register a LogosTransportSet (LocalSocket, TCP, TCP+TLS) per module via logos_core_set_module_transports and the runtime threads it through to the child
• Faulty or untrusted modules cannot crash the core or other modules
• Modules can be written in different languages as long as they implement the RPC protocol
• Alternative runtimes can be registered for different module formats (e.g. in-process, WASM)

Token-Based Authentication

Since the remote object registry has no built-in security mechanisms, all RPC calls require an authentication token. This is transparent to module developers when using the SDK:

1. Core → Module: When a module is loaded, the core generates a UUID token and sends it to the module process via local socket. The module uses this token to authenticate calls from the core.
2. Module → Module: When modules need to communicate, they request authorization from the Capability Module, which issues a token and notifies both parties. The modules then use this token for subsequent requests.
3. Token Storage: Each module stores tokens in a thread-safe TokenManager (part of the SDK). ModuleProxy validates tokens before dispatching method calls.

Module Metadata

Every module ships a metadata.json referenced by Qt's Q_PLUGIN_METADATA macro. Required fields:

Field	Purpose
name	Unique module identifier (must match name() return value)
version	Semantic version string
description	Human-readable description
author	Module author or organization
type	Module type (e.g., "core")
category	Module category for organization
main	Main module class name
dependencies	Array of required module names
capabilities	Array of capabilities this module provides
include	Optional array of extra files (shared libs, resources) to bundle

Features & Requirements

Module Lifecycle

Discovery

1. Core scans configured module directories for .so, .dylib, or .dll files
2. For each file, metadata is extracted via QPluginLoader
3. Modules are added to the "known" list without being loaded
4. Multiple module directories can be configured

Loading

1. Core locates the module file for the requested module name
2. Core resolves dependencies and loads them first (topological sort with circular dependency detection)
3. If a persistence base path is configured, core resolves an instance ID and persistence directory for the module (reusing an existing instance or creating a new one)
4. Core builds a ModuleDescriptor (name, path, format, module dirs, persistence path, transport-set JSON if registered via logos_core_set_module_transports)
5. Core asks the RuntimeRegistry to select() a runtime for the descriptor (the default SubprocessManager handles "qt-plugin" format and modules with no explicit format)
6. The selected runtime's load() spawns the appropriate process (e.g. logos_host_qt for the Qt subprocess runtime)
7. Core generates a UUID authentication token
8. Core sends the token to the module via the runtime's sendToken()
9. Host process loads the module and calls initLogos(LogosAPI*)
10. The LogosAPI instance exposes modulePath, instanceId, and instancePersistencePath as properties
11. Host process registers the module with the remote object registry
12. Core waits for registration and records the module as loaded (along with the runtime and handle)

Unloading

1. The module's host process is terminated
2. The module is removed from the loaded modules list
3. Associated tokens and state are cleaned up

Cascade Unloading

logos_core_unload_module_with_dependents() unloads the named module together with every currently loaded module that transitively depends on it. Teardown order is leaves-first (dependents before dependencies) so no process is left briefly pointing at a terminated parent. The call is serialised with ordinary load/unload operations under a single lock span — a late-arriving load cannot interleave between tearing down the dependents and the target.

Dependency Resolution

• Dependencies are declared in each module's metadata.json
• logos_core_load_module_with_dependencies() performs topological sort
• Circular dependencies are detected and reported as errors
• Missing dependencies produce warnings
• Dependencies are loaded in correct order before the requesting module
• The core maintains an in-process dependency graph with both forward and reverse edges. The reverse edges are re-derived from the forward edges at the tail of every discovery or metadata-processing pass, so cascade unload and dependent queries answer from memory without re-reading manifests from disk.

Process Monitoring

• CPU percentage, CPU time, and memory usage tracked per module process
• Statistics returned as JSON via logos_core_get_module_stats()
• Core Manager process is excluded from stats
• Not available on iOS

Thread Safety

The C API is designed to be safe for use from multi-threaded host applications:

• Load/unload operations (load_module, load_module_with_dependencies, unload_module, unload_module_with_dependents) are serialised — only one runs at a time, so rapid concurrent load/unload cycles on the same or different modules do not produce data races. The cascade variant holds the lock for its full leaves-first teardown.
• Read-only queries (get_known_modules, get_loaded_modules) use a shared reader-writer lock and may execute concurrently with each other and with load/unload operations.
• Module discovery (refresh_modules) is protected by the registry's own write lock.
• Lifecycle functions (init, start, cleanup) are not thread-safe and must be called from a single thread.

Dev vs Portable Builds

The platform supports two build variants:

• Dev build (default): Module loading looks for LGX variants with -dev suffix (e.g., linux-amd64-dev). Used in Nix/development environments.
• Portable build: Looks for portable variants without suffix (e.g., linux-amd64). Used in self-contained distributed applications.

API Description

Core Lifecycle

Function	Purpose
logos_core_init(argc, argv)	Initialize global state, optionally set module directory. Creates a QCoreApplication if one does not exist.
logos_core_set_modules_dir(path)	Set the module directory. Must be called before starting.
logos_core_add_modules_dir(path)	Add an additional module directory to scan.
logos_core_start()	Scan module directories, process metadata, create Core Manager, load built-in modules, start remote object registry.
logos_core_exec()	Run the Qt event loop. Returns when the application exits.
logos_core_cleanup()	Unload all modules, stop processes, clean up global state.
logos_core_process_events()	Process Qt events without blocking, for integration with external event loops.

Module Management

Function	Purpose
logos_core_get_loaded_modules() → char**	Return null-terminated array of loaded module names. Caller must free.
logos_core_get_known_modules() → char**	Return null-terminated array of all discovered modules. Caller must free.
logos_core_load_module(name) → int	Load a module by name. Returns 1 on success, 0 on failure.
logos_core_load_module_with_dependencies(name) → int	Load a module and all its dependencies in correct order. Returns 1 if all succeed.
logos_core_unload_module(name) → int	Terminate the module's process and remove it. Returns 1 on success.
logos_core_unload_module_with_dependents(name) → int	Cascade unload: terminate the module together with every currently loaded transitive dependent, leaves-first. Returns 1 only if every step succeeded.
logos_core_get_module_dependencies(name, recursive) → char**	Return null-terminated array of modules that name depends on (forward edges). With recursive=true, walks the forward dependency graph transitively via BFS. Unknown names yield an empty array. Caller must free.
logos_core_get_module_dependents(name, recursive) → char**	Return null-terminated array of modules that depend on name (reverse edges). With recursive=true, walks the reverse dependency graph transitively via BFS. Unknown names yield an empty array. Caller must free.
logos_core_process_module(path) → char*	Read a module file's metadata and register it as known without loading. Returns the module name or NULL. Caller must free.
logos_core_set_module_transports(name, json)	Register a per-module LogosTransportSet (JSON, see logos-cpp-sdk shape) for the named module. The runtime forwards it to the child via --transport-set so the child's LogosAPIProvider binds every transport instead of only the global default LocalSocket. Must be called before the module is loaded. NULL or empty clears any previously-registered entry.

Token and Monitoring

Function	Purpose
logos_core_get_token(key) → char*	Return the auth token for a key. Caller must free. NULL if not found.
logos_core_get_module_stats() → char*	Return JSON array of CPU/memory stats per loaded module. Caller must free. Not available on iOS.

Core Manager Module (RPC Surface)

The Core Manager is a built-in module exposing core functionality to remote modules:

Method	Purpose
setModulesDirectory(directory)	Set the module search directory.
start()	Start the core's registry and load built-in modules.
cleanup()	Unload all modules and shut down.
getLoadedModules() → std::vector<std::string>	Return names of loaded modules.
getKnownModules() → QJsonArray	Return all known modules with loaded flag.
loadModule(name) → bool	Load a module by name.
unloadModule(name) → bool	Unload a module by name.
processModule(filePath) → std::string	Read a module file's metadata and register it.
getModuleMethods(name) → QJsonArray	Introspect a module's methods via Qt meta-object system.

Module Implementation

A complete module must implement the Logos Module interface.

Inter-Module Communication

Modules communicate using the SDK's generated C++ wrappers:

logos-><module_name>.<method>(args...)       // call a remote method
logos-><module_name>.on("event", callback)   // subscribe to events
logos-><module_name>.trigger("event", data)  // emit an event

The SDK abstracts away registry lookup, token management, and async invocation.

Supported Platforms

• macOS (aarch64-darwin, x86_64-darwin)
• Linux (aarch64-linux, x86_64-linux)

Future Work

• Signature support — Signing and verifying module packages
• Additional runtimes — Register alternative ModuleRuntime implementations (e.g. in-process, WASM) alongside the default SubprocessManager
• Cross-language modules — Modules in languages other than C++
• Move away from Qt — Logos API will move away from Qt. Process management has been migrated from Qt (QProcess) to Boost.Process v2, and Qt container/utility types (QString, QStringList, QHash, QDir, QFile, QUuid) have been replaced with standard C++ and Boost equivalents (std::string, std::vector, std::unordered_map, std::filesystem, boost::uuids). The runtime abstraction (ModuleRuntime / RuntimeRegistry) decouples the core from any specific loading strategy. Remaining Qt dependencies (event loop, module loading, remote objects) are isolated in SubprocessManager and the host binary.