# Class LoggerNDC

The NDC class implements nested diagnostic contexts.

NDC was defined by Neil Harrison in the article "Patterns for Logging Diagnostic Messages" part of the book "Pattern Languages of Program Design 3" edited by Martin et al.

A Nested Diagnostic Context, or NDC in short, is an instrument to distinguish interleaved log output from different sources. Log output is typically interleaved when a server handles multiple clients near-simultaneously.

This class is similar to the `LoggerMDC` class except that it is based on a stack instead of a map.

Interleaved log output can still be meaningful if each log entry from different contexts had a distinctive stamp. This is where NDCs come into play.

Note that NDCs are managed on a per thread basis.

NDC operations such as `LoggerNDC::push()`, `LoggerNDC::pop()`, `LoggerNDC::clear()`, `LoggerNDC::getDepth()` and `LoggerNDC::setMaxDepth()` affect the NDC of the current thread only. NDCs of other threads remain unaffected.

For example, a servlet can build a per client request NDC consisting the clients host name and other information contained in the the request. Cookies are another source of distinctive information. To build an NDC one uses the `LoggerNDC::push()` operation.

Simply put,

- Contexts can be nested.
- When entering a context, call `LoggerNDC::push()` As a side effect, if there is no nested diagnostic context for the current thread, this method will create it.
- When leaving a context, call `LoggerNDC::pop()`
- When exiting a thread make sure to call `LoggerNDC::remove()`

There is no penalty for forgetting to match each `push` operation with a corresponding `pop`, except the obvious mismatch between the real application context and the context set in the NDC.

If configured to do so, LoggerPatternLayout and LoggerLayoutTTCC instances automatically retrieve the nested diagnostic context for the current thread without any user intervention. Hence, even if a servlet is serving multiple clients simultaneously, the logs emanating from the same code (belonging to the same category) can still be distinguished because each client request will have a different NDC tag.

Example:

{@example ../../examples/php/ndc.php 19}

With the properties file:

{@example ../../examples/resources/ndc.properties 18}

Will result in the following (notice the conn and client ids):

    2009-09-13 19:04:27 DEBUG root conn=1234: just received a new connection in src/examples/php/ndc.php at 23
    2009-09-13 19:04:27 DEBUG root conn=1234 client=ab23: some more messages that can in src/examples/php/ndc.php at 25
    2009-09-13 19:04:27 DEBUG root conn=1234 client=ab23: now related to a client in src/examples/php/ndc.php at 26
    2009-09-13 19:04:27 DEBUG root : back and waiting for new connections in src/examples/php/ndc.php at 29

Package: log4php
Version: $Revision: 1350602 $
Since: 0.3
Located at LoggerNDC.php

Methods summary
`public static` | # `clear( )` Clear any nested diagnostic information if any. This method is useful in cases where the same thread can be potentially used over and over in different unrelated contexts. Clear any nested diagnostic information if any. This method is useful in cases where the same thread can be potentially used over and over in different unrelated contexts.

This method is equivalent to calling the `LoggerNDC::setMaxDepth()` method with a zero maxDepth argument.

`public static array` | # `get( )` Never use this method directly, use the `LoggerLoggingEvent::getNDC()` method instead. Never use this method directly, use the `LoggerLoggingEvent::getNDC()` method instead.

#### Returns

`array`
`public static integer` | # `getDepth( )` Get the current nesting depth of this diagnostic context. Get the current nesting depth of this diagnostic context.

#### Returns

`integer`

#### See

``LoggerNDC::setMaxDepth()``
`public static string` | # `pop( )` Clients should call this method before leaving a diagnostic context. Clients should call this method before leaving a diagnostic context. The returned value is the value that was pushed last. If no context is available, then the empty string "" is returned.

#### Returns

`string`
The innermost diagnostic context.
`public static string` | # `peek( )` Looks at the last diagnostic context at the top of this NDC without removing it. Looks at the last diagnostic context at the top of this NDC without removing it. The returned value is the value that was pushed last. If no context is available, then the empty string "" is returned.

#### Returns

`string`
The innermost diagnostic context.
`public static` | # `push( string $message )` Push new diagnostic context information for the current thread. Push new diagnostic context information for the current thread. The contents of the message parameter is determined solely by the client.

#### Parameters

$message
  `string`
  $message The new diagnostic context information.

`public static` | # `remove( )` Remove the diagnostic context for this thread. Remove the diagnostic context for this thread.
`public static` | # `setMaxDepth( integer $maxDepth )` Set maximum depth of this diagnostic context. If the current depth is smaller or equal to maxDepth, then no action is taken. Set maximum depth of this diagnostic context. If the current depth is smaller or equal to maxDepth, then no action is taken.

This method is a convenient alternative to multiple `LoggerNDC::pop()` calls. Moreover, it is often the case that at the end of complex call sequences, the depth of the NDC is unpredictable. The `LoggerNDC::setMaxDepth()` method circumvents this problem.

#### Parameters

$maxDepth
  `integer`
  $maxDepth

#### See

``LoggerNDC::getDepth()``