Optimistic and Pessimistic Creation

Say one needs to create objects in one namespace in a controlled fashion (i.e. avoid duplicates, avoid loss); what strategy can one employ?

The Problem

The basic issue one has to deal with comes from the fact that possibly multiple processes are actively accessing a given namespace thereby collisions can occur: in namespace N, process 1 creates object O1 whilst process 2 does the same (i.e. creates object O1).


The left-hand side diagram illustrates the problem: the competing processes race to win the creation event during which time a meta-stable state exists i.e. no one process can be certain (to a reasonable degree) which will win the race.




Two main strategies
There are (at least) two main strategies to address the problem: an optimistic and a pessimistic one.


The optimistic strategy can be used in patterns where a process is considered reasonably alone in accessing a given namespace. Example: a user accessing is private file folder.

The pessimistic strategy must be employed in situations where multiple processes potentially access a given namespace. Example: an application's user creation flow.

Optimistic Strategy
The optimistic strategy consists in a simple procedure. The system:

1. verifies if object[X] exists
2. creates object[X] if it does not exist

The notation object[X] just means: an object with for identifier X.

Pessimistic Strategy
The pessimistic strategy is outfitted with a probabilistic equivocation reduction process. The system:

1. verifies if object X exists in the namespace
   
2. generate a reasonable unique identifier UID
   
3. creates a new identifier [X;UID]
4. verifies if an object with for identifier [X;UID] exists in the namespace
5. following #4, if no object exists, wait for Y units of time
6. verifies if any object with identifier prefix X exists
7. if the process' object with [X:UID] comes as first created (assuming multiple hits), then the process wins the creation race

An Application: Amazon S3 WEB Service

Say an application must be crafted with a system for registering users. Obviously, this process must be controlled: the system must be designed to deal with conditions such as:
Use Case: user X tries to register to the application with user-name UN1 at the same time as user Y goes for UN1.

One way to deal with the creation of unique user records on Amazon S3 I propose is:

1. Generate a long unique identifier (e.g. 32bytes)
2. Prepare a string identifier I/UID where I is the wanted identifier
3. PUT-OBJECT with for URI /I/UID
4. Wait for Y seconds (e.g. 3seconds)
5. LIST-OBJECTS with for prefix /I
6. If the object with for prefix /I/UID comes out to be the first created, then the process wins

The advantage of this algorithm stems from the fact that only 2 HTTP requests are necessary for each creation instance.

Enabling APC for PHP CLI scripts

Include the following lines in the php.ini script corresponding to the one used by PHP for running CLI scripts:

apc.enabled = On

apc.enable_cli = 1

Don't forget that the initialization file for the CLI operation of PHP is most probably different than the one used with Apache.

Using Google Alerts as Forum Notification method

One irritant on the WEB I found a solution for: have you ever posted questions or comments to a site that didn't include automatic email notification when the question/comment got feedback? It happens to me all the time.

Google Alerts to the rescue!

I now use GoogleAlerts to track my whereabouts on the WEB. The usage pattern is simple: just form the habit of always signing your post with a distinctive mark and set an alert on this mark. With my name, this is easy as it is not so common.

Greasemonky Quick Search

I have to sift through countless RSS feeds all day seeking more information based on keywords I find. I wanted to automate the process just a bit so I devised a GreaseMonkey Firefox user script.

Usage is simple: just select the text to search and press " alt c " which will automatically bring-up the result page on Google Search.

One can find the script here on UserScripts.org.

Embedding FreeMind mindmaps

FreeMind is a very handy (and free ;-) tool. I recently started playing around Google Mashup Editor and wanted to document in my way the API; I thus used FreeMind yet again.

I have embedded the mindmap in a Google Page available here.

Embedding the mindmap is easy:
<div id="mm" src="http://jeanlou.dupont.googlepages.com/GME.mm" width="800" height="800"></div>

and place the Javascript code at the bottom of the page:

<script src="http://jeanlou.dupont.googlepages.com/freemind.js" type="text/javascript"></script>