In a way, this is about an epic fail while attempting copy-and-paste coding. But really, this is about thinking outside the box. The issue -- to me -- comes from failing to see the box. Here's the body of the email, edited slightly.
"...how determine when a file has completed downloading. It would be helpful if code snippets in a unix shell and Python. I did Google but none seemed to address the fundamental race conditions. They all involve a variant of try, sleep, and try again. This is problematic for my particular case because the file sizes very significantly."
I'll ignore the grammar problems and focus on the intent of the "I did Google..." part. Based on some personal knowledge, I doubt there was more than a single search string tried. And I doubt that more than a single page of the response was looked at. Those are not important concerns.
The important concern is the shocking vagueness of the problem statement. These words are almost entirely meaningless:
" a file has completed downloading"
Imagine the variety of possible file transfer protocols that could be involved, and how many of them can be properly scripted. Take all the time you want. It can help to make a list of all the protocols that make this is a non-problem.
No protocol was named. Therefore, a protocol was assumed. And the presence of this kind of tacit assumption forms an implicit box restricting what they're doing. The restriction is so unyielding to them than they don't even need to mention it. It's as essential to them as air. They need it, but cannot see it, and refused to acknowledge it.
At this point, all we can do is make random guesses.
("Why didn't you ask them for clarification?" you ask. Good point. It's a personal failure in this case. The back-and-forth would take days. Eventually, they would send me useless explanations of deep ineptitude or a need to engage in corporate politics. Or both. I'll admit that I'm a jerk about requiring folks to take the first step and take a stab at coding. Without code, I find it largely impossible to determine what they're really talking about. The above question is a prime example of a disconnection from reality that's too exasperating to deal with except superficially.)
Identifying the Box
Guess #1. This may be about FTP (or SFTP) file transfers. Further, it may involve FTP file uploads to a server, where the client doesn't disclose a size. Yes, the word "downloading" seems to preclude this guess, but almost all other choices aren't even possible.
If it really was a client-side download, this is trivially automated using any of the available FTP client programs, including wget, curl, sftp, etc. The Python ftplib seems to be a fully automated client for FTP. The documentation is packed with examples. It seems unlikely that the question is actually client-side.
It's also possible that a single search failed to reveal all these automatable FTP clients.
Guess #2. "determine when"? Who actually cares when the upload finishes? An upload matters to the next client doing a download, or -- perhaps -- to a process that's supposed to consume the uploaded file. Is that what this is about?
Is the real question "how to trigger processing of an uploaded file when using FTP?"
In this case, we're left with stacks of follow-up questions. Primarily: "Why are you using FTP?"
If they replace their silly FTP (or SFTP) server with a RESTful API, they won't have these problems. It takes a few days to write a secure file-upload Flask container. With a swagger spec. And unit tests. And Gherkin feature definitions and a behave test suite to be sure it *really* works. It doesn't need very many routes. On completion of upload, it can fork off subprocesses to process the uploaded files. This is not hard. Really. Flask + Celery will do this.
Understanding the problem seems to require stepping outside of some box. It appears this is a struggle because of a poorly-defined box: a box assumed without being stated.
Working With the Box
At this point, we can only pretend the problem is about triggering processing after an upload. Let's further pretend the FTP is a legal requirement. Or we can pretend that SFTP is imposed by an inept IT department who also loves living inside some poorly-defined box. We're stuck with FTP for inexplicable reasons.
What can we do to game an FTP server to trigger processing of files of unknown sizes?
- Write our own FTP server. This isn't very hard. It is, however, far simpler to write a RESTful Flask service that handles the file upload as a POST request via curl or wget. Writing an FTP server's a pain in the ass because the FTP protocol is surprisingly complex. Even writing an FTP subset that serves very specific client needs can be painful.
- Poll the upload directory. This implies a race condition. Polling (and the race condition) have no practical consequences. If you want "real-time," write a RESTful API and don't use FTP. Since you're insisting on FTP, a delay is going to be part of the solution.
I'm more than a little shocked that search was considered as a viable design strategy to solving this problem. It doesn't seem like searching for solutions is required at all. I'm probably overstating this, but it seems sort of trivial and obvious that either a second file is required or a better file protocol is required. This seems to be simple "thinking" not "googling."
There are bunches of ways to approach this. Here are a few ways to use a second file and some kind of naming convention to show that two files are part of one transfer.
- Send a file with the size of the target file *before* the target file. When the target file matches the stated size, initiate processing.
- Send a file with the size and MD5 checksum of the target file. Etc.
- Send a file *after* the target file with the size and checksum. When this file shows up, simply confirm that the first file is all there.
Yes, polling is required. However, there's no race condition: there are two separate conditions which must both be met. The files are provided serially, the conditions are met serially.
Here are two approaches that use a file format that properly handles completeness.
- Gzip the file. The file receipt polling loop repeatedly tries to unzip it. If the unzip fails, the file is incomplete.
- Don't want to spend too much CPU time? Wait until the size has been stable for two polling intervals and then try to unzip them.
- Tar the file. Yes. A tar archive of a single file can be checked for integrity. When the archive can be checked and shown to be valid, the target element can be extracted and processed.
- Don't want to spend CPU time validating? Again. Wait for a stable size for a few polling intervals.
And, of course, it's possible to invent an entirely home-brewed file-wrapping protocol. Here's an approach.
- Wrap the content in MIME-style headers. These can provide a size or a terminator string to help identify the end of the transfer.
The point here is that googling for code isn't part of solving this problem. Indeed, it can't solve this problem. Merely thinking about the nature of the problem ("triggering processing"; "knowing the size") seemed necessary and sufficient to frame a solution.
- State the actual problem.
- Identify the boxes. Write them down. In words. There may be more than one.
- Locate code to work with the boxes. Find the libraries or packages. Install them. Write a hello world example to be sure that the code is understood.
Then -- and only then -- can we start to imagine solutions and ask questions about the boxes or the code that might manage the boxes.
It's impossible to state this strongly enough: We can't think outside the box if we refuse to acknowledge the box.