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@ -1,11 +1,14 @@
# Namespaced Logging for Nim
`namespaced_logging` provides a logging framework similar to [log4j][] or
[logback][] for Nim. It has three main motivating features:
`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 (old-style string logging is also
supported).
- Native support for structured logging.
- Simple, autoconfigured usage pattern mirroring the [std/logging][std-logging]
interface.
## Getting Started
@ -15,22 +18,92 @@ Install the package from nimble:
nimble install namespaced_logging
```
Then, in your application, you can use the logging system like so:
## 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
# On the main thread
let logService = initLogService()
logService.addAppender(initConsoleAppender(LogLevel.INFO))
# Manually creating a LogService. This is an independent logging root fully
# isolated from subsequent LogServices initialized
var ls = initLogService()
# On any thread, including the main thread
let logger = logService.getLogger("app/service/example")
logger.info("Log from the example service")
# Configure logging
ls.addAppender(initConsoleLogAppender())
ls.addAppender(initFileLogAppender("app.log"))
ls.setThreshold("api", lvlWarn)
# Only get logs at the WARN or higher level from the database module
let logger = logService.getLogger("app/database", threshold = some(Level.lvlWarn))
logger.error("Database connection failed")
# 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
@ -43,140 +116,408 @@ 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 and Namespaces
### Heirarchical Logging Namespaces
Loggers are organized hierarchically, with the hierarchy defined by the logger
name. A logger with the name `app/service/example` is a child of the logger
with the name `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 names 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`.
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. Any logger can have an explicit logging level set, but if it does
not, the effective logging level is inherited from ancestor loggers upwards in
the logger 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.
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 behaves more
intuitively in a multi-threaded environment than `std/logging`, particularly in
environments where the logging setup code may be separated from the
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 shared between all threads.
Internally all access to the *LogService* is protected by a mutex.
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.
Logging can be very noisy and if the *LogService* needed to be consulted for
every log event, it could easily become a performance bottleneck. To avoid
this, the *getLogger* procedure makes a thread-local copy of the logging system
configuration (loggers defined and appenders attached).
**Note** that this means that the thread-local cache of the logging system
configuration can become stale if the logging system configuration is changed
after the thread-local copy is made (if another appender is added, for
example). This is a trade-off to avoid the performance penalty of consulting
the *LogService* for every log event.
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
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.
If you find yourself needing to change the logging configuration after the
logging system has been initialized, the *reloadThreadState* procedure can be
used to update the thread-local cache of the logging system configuration, but
it must be called on the thread you wish to update.
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.
As a final note, the advice to initialize the *LogService* on the main thread
is primarily to simplify the configuration of the logging service and avoid the
need to manually reload caches on individual threads. A *LogService* reference
is required to call *getLogger*, but it can be created on any thread.
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).
## Custom Appender Implementations
Due to the thread-safety of the logging system, there are a few additional
considerations when implementing custom appenders. The *LogAppender* abstract
class is the base class for all appenders. To implement a custom appender, two
methods must be implemented:
## Architectural Design
### `appendLogMessage`
### 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
method appendLogMessage*(appender: CustomLogAppender, msg: LogMessage): void {.base, gcsafe.}
type LogService* = object
configVersion: int
global: GlobalLogService
appenders: seq[LogAppender]
thresholds: TableRef[string, Level]
```
This is the primary appender implementation that takes a LogMessage and
writes it to the appender's destination. As the signature suggests, the
implementation must be GC-safe. As a multi-method, the *CustomLogAppender* type
should be replaced by the actual name of your custom appender.
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.
Because the *LogAppender* uses multi-methods for dynamic dispatch, the
custom appender class must also be a `ref` type.
### `initThreadCopy`
> [!TIP]
> The *LogService* object is the object that is intended to be shared across
> threads.
##### ThreadLocalLogService (Reference Type)
```nim
method initThreadCopy*(app: LogAppender): LogAppender {.base, gcsafe.}
type ThreadLocalLogService* = ref LogService
```
This method is used to create a thread-local copy of the appender. It is called
by the *reloadThreadState* procedure to update the thread-local cache of the
logging system configuration. The implementation will be passed the appender
instance that was provided to the *addAppender* procedure and must return a
thread-local copy of that appender.
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 `initThreadCopy` implementations for the built-in *ConsoleLogAppender* and
*FileLogAppender* provide simple examples of how to implement this method by
simply copying state into the local thread, but this method can also be used
to perform any other thread-specific initialization that may be required for
the appender implementation.
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.
### Example Custom Appender
> [!CAUTION]
> *ThreadLocalLogService* objects should **never** be shared outside the
> context of the thread in which they were initialized.
The following defines a simple custom appender that writes log messages to a
database table. It uses the [waterpark][] connection pooling library to manage
database connections as waterpark is also thread-safe and makes implementation
straight-forward.
### Thread Safety Model
#### Safe Cross-Thread Pattern
```nim
import db_connectors/db_postgres
import namespaced_logging, waterpark, waterpark/db_postgres
# Main thread setup
let logService = initLogService()
logService.addAppender(initConsoleLogAppender())
type DbLogAppender = ref object of LogAppender
dbPool: PostgresPool
# 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")
let dbPool: PostgresPool = newPostgresPool(10, "", "", "", connectionString)
method initThreadCopy*(app: LogAppender): LogAppender =
result = DbLogAppender(dbPool: dbPool) # copy semantics as PostgresPool is an object
method appendLogMessage*(appender: DbLogAppender, msg: LogMessage): void {gcsafe.} =
appender.withConnection conn:
conn.insert(
"INSERT INTO log_events " &
" (level, scope, message, error, timestamp, custom_fields) " &
"VALUES " &
" (?, ?, ?, ?, ?, ?)",
msg.level,
msg.scope,
msg.message,
if msg.error.isSome: msg.error.msg
else: "",
msg.timestamp,
msg.additionalData)
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