[Gluster-devel] GlusterFS and the logging framework

Wed Apr 30 07:06:26 UTC 2014

Hi,

I have attached some DHT files to demonstrate the 2 logging approaches. (*_1 is the original approach, *_2 is the proposed approach).I personally think the 2 approach leads to better code readability and propose that we follow approach 2. Please let me know of any concerns with this.

To consolidate all the points raised in the earlier discussions:

What are we trying to solve?
Improving gluster logs to make end user debugging easier by providing a sufficient information and a consistent logging mechanism and message format .

The new logging framework already logs the function name and line, msgid and strerror, which improves the log messages and debug-ability. However, there are some potential issues with the way it is getting used. Please note - there are no changes being proposed to the underlying logging framework.

Current approach (approach 1):

Define message_ids for each log message (except Trace and Debug) and associate both id and string with a msg_id macro
Replace all calls to gf_log with gf_msg passing in the message_id for the message. This message_id will be printed as part of the log message.
Document each log string with details of what caused it/how to fix it.

Issues:
1. Code readability - It becomes difficult to figure out what the following is actually printing and can cause issues with incorrect params being passed or params being passed in the wrong order:
gf_msg ("dht", GF_LOG_ERROR, 0, dht_msg_23, param1, param2, param3);

2.Code Redundancy -multiple messages for the same thing differing in small details can potentially use up a large chunk of allocated ids as well as making it difficult for end users - they will need to search for multiple string formats/msgids as they could all refer to more or less the same thing. For example:

dht_msg_1   123, "Failed to get cached subvol for %s"
dht_msg_2   124, "Failed to get cached subvol for %s on %s"

3. Documentation redundancy -

The proposed format for documenting these messages is as follows:

Msg ID
Message format string
Cause
Recommended action

This could potentially lead to documentation like:

Msg ID : 123
Message format string : Failed to get cached subvol for <path>
Cause : The subvolume might not be reachable etc etc
Recommended action : Check network connection  etc etc

Msg ID : 124
Message format string : Failed to get cached subvol for <path> on <path2>
Cause : The subvolume might not be reachable etc etc
Recommended action : Check network connection  etc etc

The end user now has to search for multiple msgids and string formats to find all instances of this error.

NOTE: It may be possible to consolidate all these strings into a single one, say, "Failed to get cached subvol for %s on %s" and mandate that it be used in all calls which are currently using variations of the string. However, this might not be possible in all scenarios - some params might not be available or might not be meaningful in a particular case or a developer might want to provide additional info in a particular scenario.

Proposed approach (approach 2):
Define meaningful macros for message_ids for a class of message (except Trace and Debug) without associating them to a message string. For example
#define DHT_CACHED_SUBVOL_GET_FAILED 123
#define DHT_MEM_ALLOC_FAILED         124

Replace all calls to gf_log with gf_msg but pass in the msg id and string separately. The string is defined by the developer based on an agreed upon format.

Define a log message format policy that all developers need to follow. This will need to be enforced by reviews. For example, we could mandate that all log messages must start with the name of the file on which the operation is performed and end with the strerror if it exists.This can also include rules as to sentence structure and wording - whether to use "failed", "unable to", "could not" etc.

Consolidate existing messages and reword them if necessary to make them more meaningful. If a single message will work in multiple instances, use that one everywhere.

Add your documentation writer as a reviewer for all patches. S/he will be responsible for ensuring that all newly introduced log messages are meaningful, consistent and follow the agreed upon format.

Devs will define new message classes ids as and when required.

Ideally, common message classes like dict-set-failed or memory-alloc-failed should be defined in a common file and included by others - no point having each component define a memory_alloc_failed id.

With the proposed approach:

#define DHT_CACHED_SUBVOL_GET_FAILED 123
#define DHT_HASHED_SUBVOL_GET_FAILED 124

Calls would then look like:

gf_msg ("dht", GF_LOG_ERROR, DHT_CACHED_SUBVOL_GET_FAILED, "Failed to get cached subvolume for path %s", param1);
...
gf_msg ("dht", GF_LOG_ERROR, DHT_CACHED_SUBVOL_GET_FAILED, "Failed to get cached subvolume for path %s on %s", param1, param2);

Documentation would be as follows:

Msg ID : 123
Description : Failed to get the cached subvolume for the specified path
Cause : The subvolume might not be reachable etc etc
Recommended action : Check network connection  etc etc

Admins could just search for [MSGID 123] and find all instances of where an operation failed to get a cached sub volume.

Issues raised with proposed approach:

1. Internationalization: Having the strings in a single file is required to make L10N easy.
While i18n is very important for user tools, utils etc, log messages are usually targeted at developers and sys admins who usually know English. Plus it seems unlikely that log messages will be localized in the near future. However, the document describing the msgid can be localized so the msg id mapping information can still be used.

2. Having the strings in a header file makes it easier to change the format later.
This is a valid point. However, IMHO, the code readability is more important especially in the case where we pass arguments to the format string.

3. Having a string defined in a single header file can make it easier for a dev to reuse it if necessary
I would suggest searching on the message id instead and copy the string from elsewhere if it suits his purpose as those will already have been reviewed by the doc writer.

Regards,
Nithya
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