Category: PHP

When Amazon Web Services rolled out their version 4 signature we started seeing sporadic errors on a few projects when we created pre-authenticated link to S3 resources with a relative timestamp. Trying to track down the errors wasn’t easy. It seemed that it would occur rarely while executing the same exact code. Our code was simply to get a pre-authenticated URL that would expire in 7 days, the max duration V4 signatures are allowed to be valid. The error we’d get was “The expiration date of a signature version 4 presigned URL must be less than one week”. Weird, we kept passing in “7 days” as the expiration time. After the error occurred a couple of times over a few weeks I decided to look into it.

The code throwing the error was located right in the SignatureV4 class. The error is thrown when the end timestamp minus the start timestamp for the signature was greater than a week. Looking through the way the timestamps were generated it went something like this:

  1. Generate the start timestamp as current time for the signature assuming one is not passed.
  2. Do a few other quick things not related to this problem.
  3. Do a check to insure that the end minus start timestamp is less than a week in seconds.

So a rough example with straight PHP could of the above steps for a ‘7 days’ expiration would be as follows:

Straight forward enough, right? the problem lies when a second “rolls” between generating the `$start` and the end timestamp check. For example, if you generate the `$start` at `2017-08-20 12:01:01.999999`. Let’s say this gets assigned the timestamp of `2017-08-20 12:01:01`. Then the check for the 7 weeks occurs at `2017-08-27 12:01:02.0000` it’ll throw an exception as duration between the start and end it’s actually now for 86,401 seconds total. It turns outs triggering this error is easier than you’d think. Run this script locally:

That will throw an exception within a few seconds of running most likely.

After I figured out the error, the next step was to submit a issue to make sure I’m not misunderstanding how the library should be used. The simplest fix for me was to generate the end expiration timestamp before generating the start timestamp. After I made the PR, Kevin S. from AWS pointed out that while this fixed the problem, the duration still wasn’t guaranteed to always be the same for the same relative time period. For example, if you created 1000 presigned URLs all with ‘+7 days’ as the valid period, some may be 86400 in duration others may be 86399. This isn’t a huge problem, but Kevin made a great point that we could solve the problem by locking the relative timestamp for the end based on the start timestamp. After adding that to the PR it was accepted. As of release 3.32.4 the fix is now included in the SDK.

Posted In: Amazon AWS, PHP

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On one of our projects that I am working on I had the following problem: I needed to create an aggregate temporary table in the database from a few different queries while still using Doctrine2. I needed to aggregate the results in the database rather than memory as the result set could be very large causing the PHP process to run out of memory. The reason I wanted to still use Doctrine to get the base queries was the application passes around a QueryBuilder object to add restrictions to the query which may be defined outside of the current function, every query in the application goes through this process for security purposes.

After looking around a bit, it was clear that Doctrine did not support (and shouldn’t support) what I was trying to do. My next step was to figure out how to get an executable query from Doctrine2 without ever running it. Doctrine2 has a built in SQL logger interface which basically lets you to listen for executed queries and to see what the actual SQL and parameters were for the executed query.  The problem I had was I didn’t want to actually execute the query I had built in Doctrine, I just wanted the SQL that would be executed via PDO.  After digging through the code a bit further I found the routines that Doctrine used to actually build the query and parameters for PDO to execute, however, the methods were all private and internalized.  I came up with the following class to take a Doctrine Query and return a SQL statement, parameters, and parameter types that can be used to execute it via PDO.

In the ExampleUsage.php file above I take a query builder, get the runnable query, and then insert it into my temporary table. In my circumstance I had about 3-4 of these types of statements.

If you look at the QueryUtils::getRunnableQueryAndParametersForQuery function, it does a number of things.

  • First, it uses Reflection Classes to be able to access private member of the Query.  This breaks a lot of programming principles and Doctrine could change the interworkings of the Query class and break this class.  It’s not a good programming practice to be flipping private variables public, as generally they are private for a reason.
  • Second, Doctrine aliases any alias you give it in your select.  For example if you do “SELECT u.myField as my_field” Doctrine may realias that to “my_field_0”.  This make it difficult if you want to read out specific columns from the query without going back through Doctrine.  This class flips the aliases back to your original alias, so you can reference ‘my_field’ for example.
  • Third, it returns an array of parameters and their types.  The Doctrine Connection class uses these arrays to execute the query via PDO.  I did not want to reimplement some of the actual parameters and types to PDO, so I opted to pass it through the Doctrine Connection class.

Overall this was the best solution I could find at the time for what I was trying to do.  If I was ok with running the query first, capturing the actual SQL via an SQL Logger would have been the proper and best route to go, however I did not want to run the query.

Hope this helps if you find yourself in a similar situation!

Posted In: Doctrine, PHP, Symfony, Tips n' Tricks

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On many of our projects we use Gearman to do background processing.  One of problems with doing things in the background is that the web debug toolbar isn’t available to help with debugging problems, including queries.  Normally when you want to see your queries you can look at the debug toolbar and get a runnable version of the query quickly.  However, when its running in the background, you have to look at the application logs to see what the query is.  The logs don’t contain a runnable format of the query, for example they may look like this:

Problem is you can’t quickly take that to your database and run it to see the results. Plugging in the parameters is easy enough, but it takes time. I decided to quickly whip up a script that will take what is in the gist above and convert it to a runnable format. I’ve posted this over at http://code.setfive.com/doctrine-query-log-converter/ . This hopefully will save you some time when you are trying to debug your background processes.

It should work with both Doctrine 1.x/symfony 1.x and Doctrine2.x/Symfony2.x. If you find any issues with it let me know.

Good luck debugging!

Posted In: Doctrine, PHP, Symfony, Tips n' Tricks

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This simple tutorial will show you how to create a PhantomJS script that will scrape the state/population html table data from http://www.ipl.org/div/stateknow/popchart.html and output it in a PHP application.  For those of you who don’t know about PhantomJS, it’s basically a headless WebKit scriptable with a JavaScript API.

Prerequisites:

1.  Create the PhantomJS Script

The first step is to create a script that will be executed by PhantomJS. This script will do the following:

  • Take in a JSON “configuration” object with the site URL and a CSS selector of the HTML element that contains the target data
  • Load up the page based on the Site URL from the JSON configuration object
  • Include jQuery on the page (so we can use it even if the target site doesn’t have it!)
  • Use jQuery and CSS selector from configuration object to find and alert the html of the target element. You’ll notice on line 37 that we wrap the target element in a paragraph tag then traverse to it in order to pull the entire table html.
  • We can save this file as ‘phantomJsBlogExample.js’
  • One thing to note is that on line 24 below we set a timeout inside the evaluate function to allow for the page to fully load before we call the pullHtmlString function. To learn more about the ins and outs of PhantomJS functions read here http://phantomjs.org/documentation/

2.  Create PHP function to run PhantomJS script and convert output into a SimpleXmlElement Object

Next, we want to create a PHP function that actually executes the above script and converts the html to a SimpleXmlElement object.

  • On line 3 below you’ll construct a “configuration” object that we’ll pass into the PhantomJS script above that will contain the site url and CSS selector
  • Next on line 10 we’ll actually read in the base PhantomJs Script we created in step 1. Notice that we actually make a copy of the script so that we leave the base script intact. This becomes important if you are executing this multiple times in production using different site urls each time.
  • On line 20 we prepend the configuration object onto the copied version of the phantomJS script, make sure you json_encode this so it’s inserted as a proper json object.
  • Next on line 29 we execute the phantomJs script using the PHP exec function and save the output into an $output array.  Each time the PhantomJS script alerts a string, it’s added as an element in this array. Alerted html strings will split out as one line per element in the array. After we get the output from the script we can go ahead and delete the copied version of the script.
  • Starting on line 38, we clean up the $output array a bit, for example when we initially inject jQuery in PhantomJS a line is alerted into the output array which we do not want as it doesn’t represent the actual html data we are scraping. Similarly, want to remove the last element of the $output array where we alert (‘EXIT’) to end the script.
  • Now that it’s cleaned up, we have an array of individual html strings representing our target data. We’ll want to remove the whitespace and also join all the elements into one big html string to use for constructing a SimpleXmlElement on line 49.

3.  Call the function and iterate through the SimpleXmlElement Object to get to the table data

  • Call the function from step 2 making sure to pass in the target site url and CSS selector
  • Now that we have the SimpleXmlObject on line 7 we’ll want to iterate through the rows of the table body and pull out the state name and population table cells. It may help to var_dump the entire SimpleXmlObject to get a sense for what the structure looks like.
  • For purposes of this example we’ll just echo out the state name and population but you could really do anything you wanted with the data at this point (i.e., persist to database etc.)

4.  Final Output

Finally, running the function from step 3 should result in something like this.

Posted In: Javascript, jQuery, PhantomJS, PHP, Tips n' Tricks

Recently I was working on a project where part of it was doing data exports. Exports on the surface are quick and easy – query the database, put it into the export format, send it over to the user. However, as a data set grows, exports become more complicated. Now processing it in real time no longer works as it takes too long or too much memory to export. This is why I’ll almost always use a background process (notified via Gearman) to process the data and notify the user when the export is ready for download. On separate background threads you can have different memory limits and not worry about a request timeout. I suggest trying to not use Doctrine’s objects for the export, but get the query back in array format (via getArrayResult). Doctrine objects are great to work with, but expensive in terms of time to populate and memory usage; if you don’t need the object graph results in array format are much quicker and smaller memory wise.

On this specific export I was exporting an entity which had a foreign key to another table that needed to be in the export. I didn’t want to create a join over the entire data set as it was unnecessary. For example, a project which has a created by user as a relation. If I simply did the following:

I’d end up with an array which had all the project columns except any that are defined as a foreign key. This means in my export I couldn’t output the “Created by user id” as it wasn’t included in the array. It turns out that Doctrine already has this exact situation accounted for. To include the FK columns you need to set a hint on the query to include meta columns to true. The updated query code would look similar to:

Now you can include the foreign key columns without doing an joins on a query that returns an array result set.

Posted In: Doctrine, General, PHP, Symfony, Tips n' Tricks

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