I'm adding some testing to my current project which uses Azure blob storage to store telemetry data coming from a stream analytics job. I want to do testing of the routines that get the telemetry data, so I created a separate container for test data. I downloaded a sample set of data, modified the data to serve my needs and re-uploaded (using Azure storage explorer) everything back into the new container.
The tests were immediately failing and I quickly found out that this is because the LastModified date of the files changed into the date/time of upload. This is fine, but the sequence of the upload was also different. My code uses the modified date of the file to find out which one is the most recent, which would now return a different file based on the new dates.
I found that you cannot modify this property, although you can change another property to have it update. So I know the solution: I could write a quick script which gets the sequence of files from my production instance and then touches every file in the test instance in the same sequence.
But... I was wondering whether this is the best option. I also read it's 'best practice' to store a custom datetime in a separate property, but I don't think I can do that straight from Stream Analytics (which is writing the blobs). I also considered using an Azure Function to do this (new blob => update property), but I'm than adding complexity and something that might fail for whatever reason.
So I'm looking for the best way to solve this problem. Anyone?
Update: this one probably deserves a tiny bit more explanation. Apart from using the LastModified date to sort on, I also use it to filter blobs. The blobs themselves are CSV files containing ASA output data, so telemetry records. Each record has a timestamp, but that information is IN the file. When retrieving data, I don't want to have to dive into each file to find out what the timestamp is of those records. So I use a prefilter to filter out the blobs within a certain timespan, and then only download / open those file to the records inside.
This works perfectly as long as you do not touch any of the blob, but obviously it stops working as soon as any of the blobs gets modified for whatever reason. So I'm now convinced that I need a different / better way to solve this issue; but how?
The comment by @Vignesh deserves the credit here, but in order to get this one marked answer I'll provide it myself.
With ASA, you can set the output to be structured by date/time. That means in this case, data is written to the blob store with a directory structure such as:
2016 / 06 / 27 / 15 / 23 (= 27-06-2016 15:23)
2016 / 06 / 28 / 11 / 02 (= 28-06-2016 11:02)
The ASA output allow you to specify how granular you want the structure to be, in my case I chose to store it by day (so not including a time path). The ASA runtime will now ensure that data from a certain point in time is stored within a blob in that resides in the correct path.
Then I subsequently changed my logic to not use the datetime stamp of the individual blob files any more, but simply read just the files from the folders that are within the timerange I'm interested in. That assures we only get data that was produced within that timerange. And if there's more than one file in a folder, I need to load them both since both were in the same timerange anyway. As long as minutes are enough granularity for you, this works excellent even though it might feel a bit strange to use a folder structure for such a thing.
Having a seperate 'index' for blobs which tracks their datetime would work too of course, but adds complexity which in this case I don't really need.