journaling-hash

journaling-hash

Immutable hash that can back itself up to a file and rebuild itself

"I get knocked down, but I get up again" - William Shakespeare

Quick use

npm install journaling-hash

Here is a simple example that will increment number_of_cookies every time you run it. If it crashes for some reason while incrementing the cookie count or writing to the file, the next time it is run it will recover to the last good state.

var journalingHash = require('journaling-hash');

journalingHash('./state.log').then(function(state){
  if (!state.has('number_of_cookies')) {
    state.update({
      number_of_cookies: 0
    });
  } else {
    state.update({
      number_of_cookies: state.get('number_of_cookies') + 1
    });
  }
});

Slow use

Here is a less contrived example. It aquires a new cookie every 5 seconds, but if you kill the process it won't lose any of the cookies it's kept.

var journalingHash = require('journaling-hash');

var cookieTypes = ['Chocolate Chip Cookie', 'Macadamia Nut Cookie', 'Double Chocolate Cookie', 'Pretend Cookie'];
var downloadCookie = function(callback){
  console.log('Starting cookie download...');
  setTimeout(function(){
    //give a random cookie
    callback(cookieTypes[Math.floor(Math.random()*cookieTypes.length)]);
  }, 5000);
};

journalingHash('./state.log').then(function(state){

  if (!state.has('number_of_cookies')) {
    console.log('First run. Prepare for cookies.');
  } else {
    console.log('Resuming a previous cookie aquisition operation.');
    console.log(state.get('number_of_cookies')+' cookies found in '+state.journalPath);
  }

  var getCookie = function(){
    downloadCookie(function(newCookie){
      var cookieCount = state.get('number_of_cookies') || 0;
      cookieCount++;

      console.log('got a '+newCookie, 'bringing the total to '+cookieCount);

      update = {
        number_of_cookies: cookieCount,
        cookies: {}
      };
      update['cookies'][cookieCount] = newCookie;

      state.update(update).then(getCookie);
    });
  };

  getCookie();
});

How it works

When you run journalingHash(path) it opens that path, reads it, and reconstructs an immutable hash from it.

The promise is resolved with an object which serves as your interface to the hash.

You can use any of the methods that immutable hash provides.

In order to make changes to the hash, you must use the update function which is provided. This will patch the hash, and persist the changes to the filesystem.

Note

hash.patch(...) will return a new immutable hash.

hash.update(...) will call hash.patch and use the result to replace the internally stored hash, returning a promise.