In this chapter we will delve a bit more into the stack.
In this quite extensive post I will walk you through the process of creating from scratch a box in EC2 ready to use for deploying your Rails app using Ansible. In the process I will show how to write a simple module that, while not necessary, will illustrate some points as well.
If you’ve ever had to do a huge mysql import, you’ll probably understand the pain of not being able to know how long it will take to complete.
At work we use the backup gem to store daily snapshots of our databases, the main one being several gigabytes in size. This gem basically does a
mysqldumpwith configurable options and takes care of maintaining a number of old snapshots, compressing the data and sending notifications on completion and failure of backup jobs.
When the time comes to restore one of those backups, you are basically in the situation in which you simply have to run a
mysqlcommand with the exported
sqlfile as input, which can take ages to complete depending on the size of the file and the speed of the system.
The command used to import the database snapshot from the backup gem may look like this:
What this command does is
untarthe gzipped file and sending it as an input to a
mysqlcommand to the database you want to restore (passing it through
zcatbefore to gunzip it).
And then the waiting game begins.
There is a way, though, to get an estimate of the amount of work already done, which may be a big help for the impatiens like myself. You only need to make use of the good
procfilesystem on Linux.
The first thing you need to do is find out the
tarprocess that you just started:
This last command assumes that no other processes will have that string on their invocation command lines.
We are really interested in the
pidof the process, which we can get with some unix commands and pipes, appending them to the last command:
This will basically get the last line of the process list output (with
tail), separate it in fields using the space as a delimiter and getting the first one (
cutcommand). Note that depending on your OS and the
pscommand output you may have to tweak this.
After we have the
pidof the tar process, we can see what it is doing on the
procfilesystem. The information we are interested in is the file descriptors it has open, which will be in the folder
/proc/pid/fd. If we list the files in that folder, we will get an output similar to this one:
The important one for our purposes is the number
3in this case, which is the file descriptor for the file
We can get this number using a similar strategy:
With that number, we can now check the file
/proc/pid/fdinfo/fd_id, which will contain something like this:
The useful part of this list is the
posfield. This field is telling us in which position of the file the process is now on. Since
tarprocesses the files sequentially, having this position means we know how much percentage of the file
tarhas processed so far.
Now the only thing we need to do is check the original file size of the
tarfile and divide both numbers to get the percentage done.
To get the
posfield we can use some more unix commands:
To get the original file size, we can use the
Finally we can use
bcto get the percentage by just dividing both values:
To put it all together in a nice script, you can use this one as a template:
I developed this article and script following the tips in this stack overflow answer: http://stackoverflow.com/questions/5748565/how-to-see-progress-of-csv-upload-in-mysql/14851765#14851765
In chapter 10 we saw the basics to call a function. In this chapter we will cover more topics related to functions.
We have been doing a painful migration from Rails 2 to Rails 3 for several months at work, and refactoring some code the other day I had to do something in a non straightforward way, so I thought I’d share that.
Basically we had an action that would group several files into a zip file and return those zipped files to the user as a response. In the old code, a randomly named file was created on the
/tmpfolder of the hosting machine, being used as the zip file for the rubyzip gem, and then returned in the controller response as an attachment.
During the migration, we’ve replaced all those bespoken temp file generation for proper Tempfile objects. This was just another one of those replacements to do. But it turned out not to be that simple.
My initial thought was that something like this would do the trick:
But it did not. The reason for that is that the
openmethod, when used with the
Zip::File::CREATEflag, expects the file either not to exist or to be already a zip file (that is, have the correct zip structure data on it). None of those 2 cases is ours, so the method didn’t work.
So as a solution, you have to open the temporary file using the
Zip::OutputStreamclass and initialize it so it’s converted to an empty zip file, and after that you can open it the usual way. Here’s a full simple example on how to achieve this: