nim-namespaced-logging/README.md

# Namespaced Logging for Nim

`namespaced_logging` provides a high-performance, thread-safe logging framework
similar to [std/logging][std-logging] with support for namespace-scoped logging
similar to [log4j][] or [logback][] for Nim. It has four main motivating
features:
- Hierarchical, namespaced logging
- Safe and straightforward to use in multi-threaded applications.
- Native support for structured logging.
- Simple, autoconfigured usage pattern mirroring the [std/logging][std-logging]
  interface.

## Getting Started

Install the package from nimble:

```bash
nimble install namespaced_logging
```

## Usage Patterns

### Simple, Autoconfigured Setup
```nim
import namespaced_logging/autoconfigured

# Zero configuration of the LogService required, appender/logger configuration
# is immediately available
addLogAppender(initConsoleLogAppender())
info("Application started")

# Set global threshold
setRootLoggingThreshold(lvlWarn)

# Namespaced loggers, thresholds, and appenders supported
addLogAppender(initFileLogAppender(
  filePath = "/var/log/app_db.log",
  formatter = formatJsonStructuredLog, # provided in namespaced_logging
  namespace = "app/db",
  threshold = lvlInfo))

# in DB code
let dbLogger = getLogger("app/db/queryplanner")
dbLogger.debug("Beginning query plan...")

# native support for structured logs (import std/json)
dbLogger.debug(%{
  "method": "parseParams",
  "message": "unrecognized param type",
  "invalidType": params[idx].type
})
```

### Manual Configuration
```nim
import namespaced_logging

# Manually creating a LogService. This is an independent logging root fully
# isolated from subsequent LogServices initialized
var ls = initLogService()

# Configure logging
ls.addAppender(initConsoleLogAppender())
ls.addAppender(initFileLogAppender("app.log"))
ls.setThreshold("api", lvlWarn)

# Create loggers
let localLogSvc = threadLocalRef(ls)
let apiLogger = localLogSvc.getLogger("api")
let dbLogger = localLogSvc.getLogger("db")
```

### Manual Multithreaded Application
```nim
import namespaced_logging

# Main thread setup
var logService = initLogService()
logService.addAppender(initConsoleLogAppender())

var localLogSvc = threadLocalRef(logService) # for use on main thread

# Worker thread function
proc worker(ls: LogService) {.thread.} =
  let localLogSvc = threadLocalRef(ls)
  let logger = localLogSvc.getLogger("worker")

  # Runtime configuration changes
  localLogSvc.setThreshold("worker", lvlDebug)
  logger.debug("Worker configured")

# Safe thread creation
createThread(workerThread, worker, logService)
```

### Dynamic Configuration
```nim
# Configuration can change at runtime
proc configureLogging(localLogSvc: ThreadLocalLogService, verbose: bool) =
  if verbose:
    localLogSvc.setRootThreshold(lvlDebug)
    localLogSvc.addAppender(initFileLogAppender("debug.log"))
  else:
    localLogSvc.setRootThreshold(lvlInfo)

# Changes automatically propagate to all threads
```

## Loggers and Appenders

The logging system is composed of two main components: loggers and appenders.
Loggers are used to create log events, which are then passed to the appenders.
Appenders take log events and write them to some destination, such as the
console, a file, or a network socket. Appenders also have a logging level
threshold, which determines which log events are acted upon by the appender,
and, optionally, a namespace filter, which determines from which loggers the
appender accepts log events.

### Heirarchical Logging Namespaces

Loggers are organized hierarchically, with the hierarchy defined by the logger
scope. A logger with the scope `app/service/example` is conceptually a child of
the logger with the scope `app/service`. By default, appenders accept log
events from all loggers, but this can be restricted by setting a namespace
filter on the appender. An appender with a namespace set will accept log events
from all loggers with scopes that start with the namespace. For example, an
appender with the namespace `app` will accept log events from the loggers
`app`, `app/service`, and `app/service/example`, but not from `api/service`.

The other impact of the logger heirarchy is in the effective logging level of
the logger. An explicit logging level threshold can be set for any scope. Any
scope that does not have an explicit inherits its threshold from ancestor
loggers upwards in the scope naming heirarchy. This pattern is explained in
detail in the [logback documentation][effective logging level] and applies in
the same manner to loggers in this library.

## Notes on Use in Multi-Threaded Applications

The loggers and appenders in this library are thread-safe and are intended to
behave more intuitively in a multi-threaded environment than
[std/logging][std-logging] while presenting a similar API. This is particularly
true in environments where the logging setup code may be separated from the
thread-management code (in an HTTP server, for example).

The *LogService* object is the main entry point for the logging system and
should be initialized on the main thread. The *LogService* contains the "source
of truth" for logging configuration and is safe to be shared between all threads.
Internally all access to the shared *LogService* configuration is protected by
a mutex.

Individual threads should use the *threadLocalRef* proc to obtain a
*ThreadLocalLogService* reference that can be used to create *Logger* objects.
*ThreadLocalLogService* objects cache the global *LogService* state locally to
avoid expensive locks on the shared state. Instead an atomic configuration
version number is maintained to allow the thread-local state to detect global
configuration changes via an inexpensive [load][atomic-load] call and
automatically synchronize only when necessary.

This thread-local caching mechanism is the primary advantage of this logging
system over std/logging in a multi-threaded environment as it means that
the logging system itself is responsible for making sure appenders are
configured for every thread where loggers are used, even if the thread
initialization context is separated from the logging setup code.

As described in the [Getting Started](#getting-started) section, you can use
the `namespaced_logging/autoconfigured` import to use a simplified interface
that more closely matches the contract of [std/logging][std-logging]. In this
case all thread and state management is done for you. The only limitation is
that you cannot create multiple global *LogService* instances. In practice this
is an uncommon need.

If you do need or want the flexibility to manage the state yourself, import
`namespaced_logging` directly. In this case, the thread which initialized
*LogService* must also be the longest-living thread that uses that *LogService*
instance. If the initializing thread terminates or the *LogService* object in
that thread goes out of scope while other threads are still running and using
the *LogService*, the global state may be harvested by the garbage collector,
leading to use-after-free errors when other threads attempt to log (likely
causing segfaults).


## Architectural Design

### Overview

The namespaced logging library is built around a thread-safe architecture that
attempts to balance performance, safety, and usability in multithreaded
environments. The design centers on two key types (*LogService* and
*ThreadLocalLogService*) that work together to provide both thread-safe
configuration management and efficient logging operations.

### Core Architecture Components

#### GlobalLogService (Internal)

At the heart of the system is the `GlobalLogService`, a heap-allocated object
that serves as the single source of truth for logging configuration. This
internal type is not exposed to library users but manages:

- **Shared configuration state**: Appenders, thresholds, and root logging level
- **Synchronization primitives**: Locks and atomic variables for thread
  coordination
- **Background I/O threads**: Dedicated writer threads for console and file
  output
- **Configuration versioning**: Atomic version numbers for efficient change
  detection

The `GlobalLogService` ensures that configuration changes are safely propagated
across all threads while maintaining high performance for logging operations.

#### LogService vs ThreadLocalLogService

The library exposes two distinct types for different usage patterns:

##### LogService (Value Type)
```nim
type LogService* = object
  configVersion: int
  global: GlobalLogService
  appenders: seq[LogAppender]
  thresholds: TableRef[string, Level]
```

The *LogService* object is intended to support uses cases such as:
- **Main thread initialization**: a mutable *LogService* supports all of the
  configuration functions you would typically need when initializing logging
  for an application on the main thread.
- **Cross-thread communication**: Being an `object` type, *LogService* follows
  value semantics and can be safely copied between threads.
- **Service composition**: independently initialized *LogService* objects are
  truly independent and multiple can be created and embedded in larger
  application contexts.

> [!TIP]
> The *LogService* object is the object that is intended to be shared across
> threads.

##### ThreadLocalLogService (Reference Type)
```nim
type ThreadLocalLogService* = ref LogService
```

The *ThreadLocalLogService* is a reference to a thread-local copy of a
*LogService* and can be obtained via *threadLocalRef*. We purposefully use
reference semantics within the context of a thread so that *Logger* objects
created within the same thread context share the same *ThreadLocalLogService*
reference, avoiding the need to synchronize every *Logger* individually.

The *ThreadLocalLogService* is the object that users are expected to interact
with during regular operation and support both the configuration functions of
*LogService* and the creation of *Logger* objects.

> [!CAUTION]
> *ThreadLocalLogService* objects should **never** be shared outside the
> context of the thread in which they were initialized.

### Thread Safety Model

#### Safe Cross-Thread Pattern
```nim
# Main thread setup
let logService = initLogService()
logService.addAppender(initConsoleLogAppender())

# Safe: value semantics allow crossing thread boundaries
proc workerThread(ls: LogService) {.thread.} =
  # Convert to thread-local reference for efficient operations
  let tlls = threadLocalRef(ls)
  let logger = tlls.getLogger("worker")
  logger.info("Worker thread started")

createThread(worker, workerThread, logService)
```

#### Unsafe Pattern (Avoided by Design)
```nim
# DON'T DO THIS - unsafe reference sharing
# ThreadLocalLogService should not be shared across threads
let tlls = threadLocalRef(initLogService())
createThread(worker, someProc, tlls)  # ❌ Potential GC issues
```

### Configuration Synchronization

#### Atomic Version Checking

The library uses atomic version numbers to efficiently detect configuration
changes:

```nim
proc ensureFreshness*(ls: var LogService) =
  # Cheap atomic check first
  if ls.configVersion == ls.global.configVersion.load():
    return  # No changes, return immediately

  # Only acquire lock and copy if versions differ
  withLock ls.global.lock:
    ls.configVersion = ls.global.configVersion.load
    # Sync state...
```

This design ensures that:
- **Hot path is fast**: Most logging operations skip expensive synchronization
- **Changes propagate automatically**: All threads see configuration updates
- **Minimal lock contention**: Locks only acquired when configuration changes

#### Thread-Local Caching

Each thread maintains its own copy of the logging configuration:

- **Appenders**: Thread-local copies created via `clone()` method
- **Thresholds**: Complete copy of namespace-to-level mappings
- **Version tracking**: Local version number for change detection

This caching strategy provides:
- **High performance**: No locks needed for normal logging operations
- **Consistency**: All threads eventually see the same configuration
- **Isolation**: Thread-local state prevents cross-thread interference

## Error Handling

### Overview

The namespaced logging library implements a callback-based error handling system
designed to gracefully handle failures that may occur during logging
operations. Since logging is typically a non-critical operation, the library
prioritizes application stability over guaranteed log delivery, but provides
mechanisms for applications to monitor and respond to logging failures.

### Error Handler Pattern

The library uses a callback-based error handling pattern where applications can
register custom error handlers to be notified when logging operations fail. The
error handler receives:
- `error`: The exception that caused the failure
- `msg`: A descriptive message providing context about where the error occurred

```nim
type ErrorHandlerFunc* = proc(error: ref Exception, msg: string) {.gcsafe, nimcall.}
```

#### Thread-Safe Error Reporting
All error handling is thread-safe and uses a separate lock to prevent deadlocks:

```nim
proc reportLoggingError(gls: GlobalLogService, err: ref Exception, msg: string) =
  var handler: ErrorHandlerFunc

  # Quickly grab the handler under lock
  withLock gls.errorHandlerLock:
    handler = gls.errorHandler

  # Call handler outside the lock to avoid blocking other threads
  if not handler.isNil:
    try: handler(err, msg)
    except:
      # If custom handler fails, fall back to default
      try: defaultErrorHandlerFunc(err, msg)
      except Exception: discard
```

### Default Error Handling

#### Default Behavior
When no custom error handler is configured, the library uses `defaultErrorHandlerFunc`, which:

1. **Attempts to write to stderr**: Most likely to be available and monitored
2. **Includes full context**: Error message, stack trace, and context
3. **Fails silently**: If stderr is unavailable, gives up gracefully

```nim
proc defaultErrorHandlerFunc*(err: ref Exception, msg: string) {.gcsafe, nimcall.} =
  try:
    stderr.writeLine("LOGGING ERROR [" & msg & "]: " & err.msg)
    stderr.writeLine($err.getStackTrace())
    stderr.flushFile()
  except Exception:
    discard # If we can't write to stderr, there's nothing else we can do
```

### Configuration

#### Setting Custom Error Handlers
```nim
# During initialization
var logService = initLogService(errorHandler = myCustomErrorHandler)

# Or at runtime on either the LogService...
logService.setErrorHandler(myCustomErrorHandler)

# ... or on a ThreadLocalLogService
var localLogSvc = threadLocalRef(logService)
localLogSvc.setErrorHandler(myCustomErrorHandler)
```

#### Disabling Error Reporting
```nim
proc silentErrorHandler(err: ref Exception, msg: string) {.gcsafe, nimcall.} =
  discard # Do nothing

logService.setErrorHandler(silentErrorHandler)
```

### Error Handling Examples

#### Example 1: Monitoring and Metrics
```nim
import std/atomics

var errorCount: Atomic[int]
var lastError: string

proc monitoringErrorHandler(err: ref Exception, msg: string) {.gcsafe, nimcall.} =
  errorCount.atomicInc()
  lastError = msg & ": " & err.msg

  # Still log to stderr for immediate visibility
  try:
    stderr.writeLine("LOGGING ERROR [" & msg & "]: " & err.msg)
    stderr.flushFile()
  except: discard

# Usage
let logService = initLogService(errorHandler = monitoringErrorHandler)

# Later, check error status
if errorCount.load() > 0:
  echo "Warning: ", errorCount.load(), " logging errors occurred"
  echo "Last error: ", lastError
```

#### Example 2: Alternative Logging Destination
```nim
proc fileErrorHandler(err: ref Exception, msg: string) {.gcsafe, nimcall.} =
  try:
    var f: File
    if open(f, "logging_errors.log", fmAppend):
      f.writeLine($now() & " [" & msg & "]: " & err.msg)
      f.writeLine($err.getStackTrace())
      f.writeLine("---")
      f.close()
  except:
    # If file logging fails, fall back to stderr
    try:
      stderr.writeLine("LOGGING ERROR [" & msg & "]: " & err.msg)
      stderr.flushFile()
    except: discard

let logService = initLogService(errorHandler = fileErrorHandler)
```

#### Example 3: Development vs Production
```nim
proc createErrorHandler(isDevelopment: bool): ErrorHandlerFunc =
  if isDevelopment:
    # Verbose error reporting for development
    proc devErrorHandler(err: ref Exception, msg: string) {.gcsafe, nimcall.} =
      stderr.writeLine("=== LOGGING ERROR ===")
      stderr.writeLine("Context: " & msg)
      stderr.writeLine("Error: " & err.msg)
      stderr.writeLine("Stack Trace:")
      stderr.writeLine($err.getStackTrace())
      stderr.writeLine("====================")
      stderr.flushFile()

    return devErrorHandler
  else:
    # Minimal error reporting for production
    proc prodErrorHandler(err: ref Exception, msg: string) {.gcsafe, nimcall.} =
      try:
        var f: File
        if open(f, "/var/log/myapp/logging_errors.log", fmAppend):
          f.writeLine($now() & " " & msg & ": " & err.msg)
          f.close()
      except: discard

    return prodErrorHandler

# Usage
when defined(release):
  let logService = initLogService(errorHandler = createErrorHandler(false))
else:
  let logService = initLogService(errorHandler = createErrorHandler(true))
```

### Best Practices

#### Provide Fallbacks
```nim
proc robustErrorHandler(err: ref Exception, msg: string) {.gcsafe, nimcall.} =
  # Primary: Send to monitoring system
  if not sendToMonitoring(err, msg):
    # Secondary: Write to dedicated error log
    if not writeToErrorLog(err, msg):
      # Tertiary: Use stderr as last resort
      try:
        stderr.writeLine("LOGGING ERROR [" & msg & "]: " & err.msg)
        stderr.flushFile()
      except: discard
```

#### Keep Error Handlers Simple
```nim
# Good: Simple and focused
proc simpleErrorHandler(err: ref Exception, msg: string) {.gcsafe, nimcall.} =
  errorCounter.atomicInc()
  try:
    stderr.writeLine("LOG ERROR: " & msg)
    stderr.flushFile()
  except: discard

# Avoid: Complex operations that might themselves fail
proc complexErrorHandler(err: ref Exception, msg: string) {.gcsafe, nimcall.} =
  # Don't do heavy operations like database writes,
  # complex network operations, or file system operations
  # that might fail and cause cascading errors
```

[log4j]: https://logging.apache.org/log4j/2.x/
[logback]: https://logback.qos.ch/
[effective logging level]: https://logback.qos.ch/manual/architecture.html#effectiveLevel
[atomic-load]: https://nim-lang.org/docs/atomics.html#load%2CAtomic%5BT%5D%2CMemoryOrder
[std-logging]: https://nim-lang.org/docs/logging.html