## Class LoggerNDC

Description

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 push(), pop(), clear(), getDepth() and 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 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 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:

With the properties file:

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

- version: $Revision: 1350602 $
- since: 0.3

Located in /log4php/LoggerNDC.php (line 96)

Method Summary
static void clear ()
static array get ()
static integer getDepth ()
static string peek ()
static string pop ()
static void push (string $message)
static void remove ()
static void setMaxDepth (integer $maxDepth)
Methods
static method clear (line 109)

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 setMaxDepth() method with a zero maxDepth argument.

- access: public

static void clear ()
static method get (line 117)

Never use this method directly, use the LoggerLoggingEvent::getNDC() method instead.

- access: public

static array get ()
static method getDepth (line 127)

Get the current nesting depth of this diagnostic context.

- see: LoggerNDC::setMaxDepth()
- access: public

static integer getDepth ()
static method peek (line 156)

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.

- return: The innermost diagnostic context.
- access: public

static string peek ()
static method pop (line 140)

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.

- return: The innermost diagnostic context.
- access: public

static string pop ()
static method push (line 172)

Push new diagnostic context information for the current thread.

The contents of the message parameter is determined solely by the client.

- access: public

static void push (string $message)

- string $message: The new diagnostic context information.

static method remove (line 179)

Remove the diagnostic context for this thread.

- access: public

static void remove ()
static method setMaxDepth (line 197)

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 pop() calls. Moreover, it is often the case that at the end of complex call sequences, the depth of the NDC is unpredictable. The setMaxDepth() method circumvents this problem.

- see: LoggerNDC::getDepth()
- access: public

static void setMaxDepth (integer $maxDepth)

- integer $maxDepth

Documentation generated on Mon, 02 Feb 2026 13:59:51 +0900 by phpDocumentor 1.4.4