Why?

In complex projects the logging infrastructure, uniform error handling mechanisms, automated tests, continuous integration,etc ar every important.
For best results, exceptions, logging and invariant checks like asserts should work together as smoothly as possible. This module is experimental and it is indented to be used inside projects derived from SwarmESB project (but does not have other dependencies). Given the distributed nature of SwarmESB projects we decided to build inside double-check module a base to put together logs, exceptions, asserts, checks and other type of semantic checks. This module is a foundation to grow and control yourself all these aspects. SwarmCore contains practical examples of how a real project should use this module. Also check our tests to get some usage examples.

What is double-check module?

DoubleCheck is a node.js module that can be extended to create your own "specific language/API" for

• extensible logging infrastructures

• extensible exception handling mechanisms connected with your logging and asserts infrastructure

• runtime validations called checks that can be added during developement and disabled in production

##Logging approach

We this module we try to make logging booth simple to use by programmer during development but also useful at runtime for operations. The API we offer our logger is extensible, you can add your own verbs (cases as we call them) To be fully useful, a logger will perform many roles:

• keeps a complete history with all relevant events that happened at runtime
• filters or aggregates semantically related log entries to be easily presented to system administrators or for other audit purposes
• can help monitoring tools to trigger other events and actions in the system

Therefore, all the extensions you can declare should also declare a semantic category for each parameter, declared as an array of objects in logger.addCase calls. We identified the following semantic categories having booleans as values:

• 'category ': the field is usable to create indexes for logs. The mandatory field "type" is such ca category but you can add other. One can see ca category

• 'level' : number friom 1-9, identify the level of attention that a log entry should get from operations point of view 0 system level critical error: hardError 1 potentially causing user's data loosing error: error 2 minor annoyance, recoverable error: logError
  3 user experience causing issues error: uxError 4 warning,possible isues but somehow unclear behaviour: warn
  5 store general info about the system working: info 6 system level debug: debug
  7 local node/service debug: ldebug 8 user level debug; udebug 9 development time debug: ddebug

• 'time' : a value that uniquely identifies in time the log. This parameter is mandatory and normally automatically instantiated by core logging functions

• 'description' : part of the description, environment values relevant for values. All parameters have this aspect so you don't declare

• 'stack' : describe an execution stack that caused the log entry

• 'filename' : path of a file causing or related to the log entry

• 'key part' : a set of fields that together create a key (category)

• 'swarm' : the current swarm name (SwarmESB specific)

• 'phase' : the current phase name (SwarmESB specific)

• 'mainGroup' : the type of the addapter (SwarmESB specific)

• 'adapter' : the uid of the current adapter instance ((SwarmESb specific))

• 'process' : the uid of a swarm process (or a process) that automatically stick multiple logs entries together

• 'var args' : potential variable number of args

Additionally,each parameter declaration should have a name, identified with field named 'name' in parameter descriptions

##APIs:

    var assert      = require("double-check").assert;      //get the assert singleton
    var check      = require("double-check").check;      //get the assert singleton
    var throwing    = require("double-check").exceptions;  //get the exceptions singleton
    var firewall    = require("double-check").firewall;    //get the firewall singleton
    var logger      = require("double-check").logger;      //get the logger singleton
    
    /* proposed, not implemented in the current version */
    

###Add new type of assert checks: addCheck

    assert.addCheck("notNull", function(item){
      if(item == null || item == ""){
        throw new Error("Null reference found");
    })

Check: assert.notNull("test");

Mechanism to control exception types, log important ones

###Add new type of exception: register throwing.register("randomFail", function(explanation){
throw new Error("explanation"); //it is mandatory to throw an expcetion, in order to preserve the semantic of throw keyword })

Usage: throwing.randomFail("Why not!?");

##Logger ###Provide an implementation for rawLogging logger.rawLogging = function(type, level, rawObject, timeStamp, stack){...}

Observations: 
- if stack is undefined it should be created from current stack. 
Give false or other value and the stack will not be saved.
- if timeStamp is undefined it should be taken from current time, unix time

###Add new type of logging function: addCase

logger.addCase("type", level,  loggingFunction, argument types, checkFunctions) 

Observations: additionaly to make the logging API mor apropiate for each case, the loggingFunction has a chance to add other contextual information before calling logger.rawLogging checkFunctions has a chance to trigger actions caused by current log entry or for thresholds violations from previous entries

Example:

    logger.addCase("warning", function(explanation){
                this.rawLogging(...)        
          }, [
                {
                    'name':'explanation'            
                },
                {
                    'name':'fileName',
                    'category':true                
                }
            ], 
            undefined      //we can let it undefined
          })          
    logger.warning("RandomFail happens in this file",__filename);

##Other

###alias assert.alias("isDocumentId", "notNull"); exceptions.alias("randomBreak", "randomFail"); exceptions.alias("warn", "warning");

      Usages:
      assert.isDocumentId("myDocumentId");
      exceptions.randomBreak();

##Conclusions

• Start with proper logging policies from begining: If you don't control how exceptions, asserts and logging code is writeln from the beginning, it can get ugly to modify your code in hundreds of places.

• Use checks in production code: We encourage use of asserts (called checks) even in production code (to check important invariants) but they should be properly integrated with logging and exceptions.

• Early crushes in a controled environment represent a better option than loosing money because security issues or other ugly bugs.