Receiving asynchronous notifications of database changes

The following is directly pertaining to a PostgreSQL-based internet mail solution, but be not discouraged. Most, if not all, of the techniques can be applied elsewhere, and I intend to explain how.

For about six months in 2003 I worked for a company based out of Spearfish, SD, called Altaire Enterprises, Inc., a small floundering dialup ISP. This is where my first real experiences with PostgreSQL took place, up until this time I had been a die-hard user of MySQL for all of my database needs, whether it be as a backing store for a web application or otherwise. The largest project I took on while I was there was the implementation of a database backed mail system. That is to say, all mail accounts were entirely virtual, no system accounts, and all data associated with them was stored in PostgreSQL. It could just as easily have been LDAP, but it wasn’t, and that isn’t the point of this little ditty I’m writing now. Architecting the mail system was the easy part, as I found, plenty of mail applications are perfectly happy to talk to PostgreSQL. There are two hard parts. Performance and Management.

I will get to performance later. Management comprises more than one would think at first. Obviously, you need some sort of frontend or tools to add users, domains, etc. to your mail database. In this case it was a collection of C applications, rather than the typical web frontend, because they could be executed by the internal billing system (platypus). In consideration of the scope of this text, how the data is being entered is moot.

There is a flip-side to management, and specifically putting information into the database in this scenario. There are cases where you need to know when modifications are made to that data, so that you can perform operations on disk, for instance. Such operations may be setting quota’s, or creating a users Maildir if your MTA doesn’t handle that for you. The typical way to do this is to have your frontend perform that action as well, which is logical in a small installation, and is exactly the method used at Altaire. The C applications performed whatever on-disk operations were necessary.

What happens when your (web) fronted is hosted on a different server, though? I suppose you could export a set of web services, or similar, from the mailserver, allowing your frontend to connect to it and perform the necessary operations. What then if you have 10 mail servers? The frontend has to decide which one to connect to, and does its thing, fine, but what if one of those web services runners dies, what do you do? Write additional logic to record failures, and play them back later? Oh lords, there must be a better way. Yes, yes.. Of course there is. There is another method that has been used time and again, it is proven and reliable. My first major experience with it was during the time I helped architect ITMom.com, a web hosting provider, back in 99/00. You create an addition column in the tables that will require external operations, and when modifications happen, you toggle that field on with your frontend. You can then have a runner that polls those tables watching for modifications, and performing the correct operation when one is found. This works well, with a number of drawbacks. One, you are polling. Two, you are littering your carefully constructed, optimized and normalized schema with columns whose sole purpose is to notify external applications of changes. You could easily avoid this by logging changes to seperate tables, sure, but now we come to three. You are stilly relying on your frontend to dictate what actions the backend should take.

Would it not be better to just let the management application do what it should do best, and insulate it from the underlying technical details? This becomes even more important in an organization where the developer(s), database administrator(s) and system administrator(s) are all different people.

Enter Epidemic, a proof-of-concept framework to easily make such things possible.

To Be Continued…

PHP: Protecting your code (Zend Encoder/IonCube/SourceCop)

SourceCop Decoder

Personally, I have never found need to encode/encrypt/obfuscate any PHP. I do however know that there is a large audience of developers and/or organizations out there that do rely on such obfuscation to protect their works. Not being sure if it has hit the news or not, as I have been too busy of late to even open up my RSS aggregator to skim the headlines, know that there is at least one service in the wild that can successfully decode Zend Encoder and IonCube encoded files. It’s not perfect by any means, as it is reconstructing the code based on the opcodes, but it does return it in a format that is true to the original as far as execution and reasonably easy for a human to parse.

I wrote this little number the other day after running across a script I was wanting to use, in which one component was obviously dependant upon register_globals. My gosh, if I only had the code I could fix that! Fortunately it was obfuscated with an application called SourceCop, which provides very little in the way of protection. Come on guys, you could at least obfuscate the code itself first, munging whitespace, variable and function names. As it was, it took a mere 20 minutes to write a script that would replace an encoded file with a pristine copy of the original. At any rate, here is the script, do note that it was a quick hack and as such it may or may not work for you. It will also simply overwrite any SourceCop encoded files fed to it, so you will want to create a backup first, you have been warned.
Update: 2/23/2006, revised script

MySQL 5.0 standardized join syntax

I am sure the revised / SQL:2003 standardized join syntax in MySQL 5.0 is old news to many out there. My guess is they are in the minority, and most haven’t heard a thing about it. Some may have even upgraded only to be frustrated that their queries weren’t working as they should any longer. Here’s the skinny, taken directly from the MySQL manual.

Beginning with MySQL 5.0.12, natural joins and joins with USING, including outer join variants, are processed according to the SQL:2003 standard. These changes make MySQL more compliant with standard SQL. However, they can result in different output columns for some joins. Also, some queries that appeared to work correctly in older versions must be rewritten to comply with the standard.

I would like to applaud MySQL AB on their latest release. Throughout its history, MySQL has been vastly out-gunned in terms of useful features by many other commercial and free databases. It has also taken a great deal of heat on many occasions due to its poor standards conformance in comparison to the other options on the market. With this release, even if they have not completely closed that gap, they have narrowed it by an impressive margin.

I do have a major gripe, however. Whilst the previously mentioned changes improves standards conformance and portability, it breaks a large enough percentage of MySQL-bound applications to warrant serious scrutiny. Apparently MySQL AB has forgotten that not the entire world is open source, and that many of us must every day maintain databases accessed by scripts and applications controlled by a third party or to which the source code is simply not available. This makes it rather impossible for any of us in such a situation to move those databases to servers running 5.0.

In my particular case, I was looking forward to leveraging triggers, stored procedures and views to reduce my administrative burden and deprecate a number of external scripts (hacks) that we use to transform data for use by other applications. The primary application sitting on this database is commercial and Zend encoded, so “fixing” the broken queries is simply not an option. Yes, we have talked to the vendor. I find it hard to believe that I am the only one in this situation.

Seriously now, how hard would it have been to add an option to enable the legacy behavior?

Update: 2/19/2006, offending commit
http://mysql.bkbits.net:8080/mysql-5.0/patch%401.1886.80.1

PHP/AJAX file upload with progress bar

Over the past couple of days I have been pondering adding some file upload functionality to the form classes I have been using for a bit over a year now. History repeats itself, again, time spent pondering instead of just getting on with the nitty gritty means I start thinking about ideal functionality. So, as I pondered how to go about sanely handling file uploads features started coming to mind, and one of them just wouldn’t go away. A semi-realtime inline file upload progress indicator. Well, that doesn’t sound so hard.

I spent some time with Google doing the requisite research to find that there are a number of stumbling blocks. The first being client-side, when a browser window/frame is busy pushing a file or files up the pipe, it seems that it is just that, busy. Which makes it a bit difficult to talk it into displaying updates. This seems to be pretty easily solved by pushing the file upload through a hidden iframe referenced by the target attribute on the form.

That certainly isn’t where the problems end. As luck would have it, not only is the browser happy to be working against us, so is PHP, in more ways than one.

When the execution unit handling the upload gets hit with the POST, it would seem that it likes to make itself busy as well. Ok, so no way to get the status of the file upload from the thread/process actually handling the upload. Apparently there are some patches against PHP to rectify this situation, but until they get committed and see a release they are unusable for most people. I am all for gratuitously hacking my own PHP install, but it seemed like there must be a better way.

I then stumbled across another method. Scan the upload_tmp_dir (PHP INI variable) for files of a known naming scheme, looking for the one with the latest timestamp. The current size of this file could be pushed back to the browser so that it could calculate the upload progress. This method is also not without its glaring faults. The probability of a race condition is too high for any kind of production use. Oh wait, scratch that, I’m starting to sound like a PHP developer, let me rephrase… There is an unavoidable possibility of a race condition, so this method cannot be used. Well… Wait a minute, there is an upload_tmp_dir variable. Why don’t we just generate some kind of unique form id to be passed back to us when we get the POST, then it should be possible to create a directory to have PHP put the file(s) in of a known name, eliminating our race, no? I suppose upload_tmp_dir being read-only is a bit of a stumbling block with that idea, considering we already decided hacks to the PHP source were out. Not to mention PHP probably isn’t going to let us set the variable before it gets busy processing that form data anyway.

Google led me to a couple more resources for accomplishing this throughout the course of my research, but they all involved an external non-PHP script to handle the upload and drop status information somewhere accessible. Unacceptable I say! There must be a way to do it with PHP alone!

I have theorized a method, implementation forthcoming. Here is a brief summary. Have an onSubmit handler frob a PHP script and retrieve a URL to apply to the action property of the form, said PHP script will have just launched a PHP-based very simple webserver. This webserver’s sole purpose in life is to eat POST’s and parse multipart form data. This same PHP script will update an accessible location with the status of the upload. The hidden iframe trick gets used to free up the window with the form in it. This window can now pull upload status via XMLHttpRequest and update a progress bar accordingly. This method also has the benefit of being able to degrade gracefully in the event that JavaScript is unavailable on the client. The default action URL can be implemented as a standard file upload handler.