add support for peekByKey in MDC

[youjie23]

In some scenarios, after using MDC.pushByKey(key, value) to push a value into the deque (stack), we may need to retrieve the top value without popping the stack referenced by 'key', similar to a ​​peekByKey​​ operation. However, the current SLF4J implementation requires developers to invoke getCopyOfDequeByKey(key) and manually peek from the returned deque (stack), which involves creating a copy of the data structure and leads to unnecessary performance overhead. Would it be more efficient to add native support for peekByKey in MDC to avoid this redundancy?

Thank you for your attention and support.

[ppkarwasz]

Just my two cents — the MDC should not be treated as a general-purpose context value store. It is a utility class specifically designed for logging, and as such, it should not expose methods like MDC.get() or MDC.peekByKey() for general use. Application logic should not depend on values stored in the MDC.

That said, MDC.get() is unfortunately necessary in cases where you need to restore a previous value for a key:

String oldValue = MDC.get("foo");
try {
  MDC.put("foo", "bar");
  // business code
} finally {
  MDC.put("foo", oldValue);
}

On the other hand, MDC.peekByKey() is not essential, since the same goal can be achieved more cleanly with:

try {
  MDC.pushByKey("foo", "bar");
  // business code
} finally {
  MDC.popByKey("foo");
}

[youjie23]

Just my two cents — the MDC should not be treated as a general-purpose context value store. It is a utility class specifically designed for logging, and as such, it should not expose methods like MDC.get() or MDC.peekByKey() for general use. Application logic should not depend on values stored in the MDC.

That said, MDC.get() is unfortunately necessary in cases where you need to restore a previous value for a key:

String oldValue = MDC.get("foo");
try {
MDC.put("foo", "bar");
// business code
} finally {
MDC.put("foo", oldValue);
}
On the other hand, MDC.peekByKey() is not essential, since the same goal can be achieved more cleanly with:

try {
MDC.pushByKey("foo", "bar");
// business code
} finally {
MDC.popByKey("foo");
}

Thank you for your response.
In the scenario I mentioned earlier, the code requiring peekByKey is ​​independent of the business logic​​ (which you noted includes pushByKey), meaning it is ​​not within the same try-finally block​​. For example, this could involve extension code such as log collection utilities that need to output the latest value of a specific key ​​(user-customizable)​​

[ppkarwasz]

What about adding peekByKey to MDCAdapter? Since it is a SPI class, it gives users enough warning that they are entering the details of a logging implementation. With one notable exception (o.s.spi.LoggingEventBuilder) most users don't need access to o.s.spi. Those that do (logging implementations and utilities) should be aware that their code could require changes between minor version of SLF4J.

[youjie23]

What about adding peekByKey to MDCAdapter? Since it is a SPI class, it gives users enough warning that they are entering the details of a logging implementation. With one notable exception (o.s.spi.LoggingEventBuilder) most users don't need access to o.s.spi. Those that do (logging implementations and utilities) should be aware that their code could require changes between minor version of SLF4J.

ACK.Thanks for the feedback!
I'll proceed with:

• Remove peekByKey from MDC

• Retain it in:

  • MDCAdapter
  • BasicMDCAdapter
  • ThreadLocalMapOfStacks

  Will push changes shortly.