TweetFollow Us on Twitter

File-Based Dataflow

Volume Number: 19 (2003)
Issue Number: 3
Column Tag: Mac OS X

File-Based Dataflow

building robust systems without explicit file locking

by Rich Morin

Last month, I discussed "File Change Watcher", a Perl daemon that wakes up periodically, checks for file-system changes, copies files, and gathers metadata. To save space, I ignored a critical issue: how can another process know that a metadata file is "ready" to be read?

Race Conditions

This question comes up in the design of any system where one process is writing a file and another is accessing it. In this type of "race condition", the reading process can overtake the writing process, arriving prematurely at the end of the data (and ignoring any data the other process may write thereafter).

Alternatively, if two processes try to write the same file at the same time, assorted damage can ensue. Depending on the circumstances, changes might get lost, output could get intermingled, etc.

One way to deal with the problem is to use a "lock file". By common agreement, if the lock file is in place, the file it "locks" isn't available for access. An inquiring process can test for this by trying to create the lock file. If the process fails, it just waits for a while before trying again.

The reason that this test works is that the kernel won't let two processes open the same file with exclusive access (O_EXCL). So, the first attempt succeeds and all others fail (until the first process closes the file). The technique works quite well, as long as everyone plays nicely; the BSD side of OSX contains a number of lock files (e.g., /var/spool/lock).

In the general case, lock files (or some equivalent technique) are necessary. If, for instance, two users want to edit a file, you really don't want them doing it at the same time. So, some Unix editors (e.g., the BSD version of vi) implement file locking.

Unfortunately, lock files add complexity and room for error. All of the processes have to honor the lock files; what if an existing program doesn't want to play? Also, if the system crashes, the lock file has to be explicitly removed when things start up again. And, for all of this pain, they only solve one part (simultaneous access) of the file-based dataflow problem set.

Atomic Actions

Fortunately, there are a number of alternatives to lock files. Most of them are based on some kind of "atomic action"; that is, something that can only be done completely or not at all. BSD provides several atomic actions as system calls, including chmod(2), chown(2), link(2), open(2), and rename(2).

These same facilities are available from Perl, but its chmod and chown are only atomic for single file nodes. Corresponding shell commands are available, albeit with some caveats:

  • chgrp(1), chmod(1), chown(1) of a single file system node

  • link(1), but only for hard links

  • mv(1), to another name on the same file system

So, although you can't assume that a process will finish writing before some other process starts to read the output file, you can safely rename(2) an existing file (or mv(1) it to another name on the same file system) without worrying about race conditions.

Because any hard link to a file is simply a directory entry that points to a common inode(5), both the data and file system metadata are shared among a set of hard links. This lets a single atomic action (e.g., chmod) change the status of any number of links.

Finally, Cocoa's Application Kit Framework's NSFileWrapper class includes methods such as writeToFile:atomically:updateFilenames:. In short, a wealth of solutions is at hand.

Emitters and Consumers

The system I'm building is based on a data-flow model, similar to Unix pipelines, but it uses files instead of pipes. The method is far from new; mail transfer agents and print spoolers use variations of it in OSX. I am simply generalizing the idea into a framework for creating sets of file- and time-based tasks.

By specifying that only one program will ever write to a file, I can eliminate the issue of simultaneous writing. This means that I only need to prevent processes from opening or removing files prematurely and make sure that every file gets processed to completion.

The rules below allow the safe (i.e., no race conditions) use of files by any number of "emitters" and "consumers", without the need for lock files.

  • Files are created and written by emitters, read and deleted by consumers. No modification of files is allowed, including appending or read/write usage.

  • Files are created under temporary names (on the destination file system), then "published" (e.g., renamed) for use by consumers.

  • A file can only be used by one consumer, which removes it just before exiting.

    Note: This restriction applies only to consumers. Other programs (e.g., more) may read published files at any time. Also, the consumer is not required to read the file, just remove it.

  • If an emitter is creating files for multiple consumers, a separate link must be created for each consumer. All of the links must then be published in a single, atomic operation. One method uses a common directory:

    • Create a temporary directory.

    • For each file with N consumers, make N-1 links in the directory.

    • Rename the temporary file(s) into the directory, as the Nth file link(s).

    • Rename the temporary directory, publishing all of the links.

      Another method, which can only "protect" a single output file, has the advantage that the output links do not have to be in the same directory:

    • Turn off read access (using chmod) on the temporary file.

    • For a file with N consumers, make N-1 links.

    • Rename the temporary file as the Nth link.

    • Make any (and thereby, every) link readable.

Although these rules might complicate the life of programmers, the resulting programs aren't any more complicated. In fact, they tend to be quite simple: discover an input file, process it (writing any output to temporary files); when you're done, publish (e.g., rename) the output files and remove the input file.

Better yet, the file-discovery and -management details can be handed off to a scheduling daemon, allowing most of the "tasks" (working code) to be written as "filters": read from standard input; write to standard output.

A Data Collection System

Let's apply this to a data collection system and see how it plays out. A ps(1)-monitoring task is supposed to run once a minute, writing a report. Another task reads the report, writing a YAML (www.yaml.org) version. Other tasks produce hourly and daily summaries.

This is a fairly complex set of tasks, but it's only a small fraction of the workload for a full-scale operating system monitor. So, the amount of specification for each task should be as simple as possible. Here's a first cut at a configuration file:

# Every minute, collect raw ps(1) data.
{ ps_raw, type: cron, min: every,
  out: 'ps/raw/$time'
}
# Process the raw ps(1) data.
# Create two output links.
{ ps_rare, type: file, patt: 'ps/raw/*',
  out: ['ps/rare/1.$time',
        'ps/rare/2.$time' ]
}
# Every hour, create a summary.
{ ps_hour, type: cron, hour: every,
  out: 'ps/hour/$time'
}
# Every day, create a summary.
{ ps_day, type: cron, day: every,
  out: 'ps/day/$time'
}

This is fairly concise, but there's a lot of repetition. We could "boil it down" by taking advantage of the fact that it describes a tree of processes:

{ ps_raw, type: cron, min: every,
  out: 'ps/raw/$time',
  { ps_rare, type: file, patt: 'ps/raw/*',
    out: ['ps/rare/1.$time',
          'ps/rare/2.$time' ]
    { ps_hour, type: cron, hour: every,
      out: 'ps/hour/$time'
    },
    { ps_day, type: cron, day: every,
      out: 'ps/day/$time'
} } }

But that only kills off two lines (excluding comments), so it's not a huge win. Also, I'm not convinced that it's as easy to read, modify, etc. If we are willing to let the scheduling infrastructure generate file names for us, however, we can get away with "idioms" like:

{ ps_raw,      type: cron, min:  every,
  { ps_rare,   type: file,
    { ps_hour, type: cron, hour: every },
    { ps_day,  type: cron, day:  every }
} }

That brings the overhead back under control. And, if we need to access a file, we can follow standardized naming rules to find it. The output of ps_hour, for instance, would have a name of the form "ps_hour/1.<time>".

So much for theoretical hand-waving and speculative descriptions. Next month, I'll discuss some daemons that can actually make all of this work.


Rich Morin has been using computers since 1970, Unix since 1983, and Mac-based Unix since 1986 (when he helped Apple create A/UX 1.0). When he isn't writing this column, Rich runs Prime Time Freeware (www.ptf.com), a publisher of books and CD-ROMs for the Free and Open Source software community. Feel free to write to Rich at rdm@ptf.com.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Apple Pro Video Formats 2.0.2 - Updates...
Apple Pro Video Formats brings updates to Apple's professional-level codes for Final Cut Pro X, Motion 5, and Compressor 4. Version 2.0.2: Includes support for the following professional video codecs... Read more
Apple Final Cut Pro X 10.2.2 - Professio...
Apple Final Cut Pro X is a professional video editing solution.Completely redesigned from the ground up, Final Cut Pro adds extraordinary speed, quality, and flexibility to every part of the post-... Read more
Apple Compressor 4.2.1 - Adds power and...
Compressor adds power and flexibility to Final Cut Pro X export. Customize output settings, work faster with distributed encoding, and tap into a comprehensive set of delivery features. Powerful... Read more
Apple Motion 5.2.2 - Create and customiz...
Apple Motion is designed for video editors, Motion 5 lets you customize Final Cut Pro titles, transitions, and effects. Or create your own dazzling animations in 2D or 3D space, with real-time... Read more
A Better Finder Rename 10.00b1 - File, p...
A Better Finder Rename is the most complete renaming solution available on the market today. That's why, since 1996, tens of thousands of hobbyists, professionals and businesses depend on A Better... Read more
CrossOver 14.1.6 - Run Windows apps on y...
CrossOver can get your Windows productivity applications and PC games up and running on your Mac quickly and easily. CrossOver runs the Windows software that you need on Mac at home, in the office,... Read more
Printopia 2.1.14 - Share Mac printers wi...
Run Printopia on your Mac to share its printers to any capable iPhone, iPad or iPod Touch. Printopia will also add virtual printers, allowing you to save print-outs to your Mac and send to apps.... Read more
Google Drive 1.24 - File backup and shar...
Google Drive is a place where you can create, share, collaborate, and keep all of your stuff. Whether you're working with a friend on a joint research project, planning a wedding with your fiancé, or... Read more
Chromium 45.0.2454.85 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 45.0.2454.85: Note: Does not contain the "... Read more
OmniFocus 2.2.5 - GTD task manager with...
OmniFocus helps you manage your tasks the way that you want, freeing you to focus your attention on the things that matter to you most. Capturing tasks and ideas is always a keyboard shortcut away in... Read more

Zen Brush 2 (Entertainment)
Zen Brush 2 1.0 Device: iOS Universal Category: Entertainment Price: $2.99, Version: 1.0 (iTunes) Description: Zen Brush 2 is a drawing app focused on the strong yet beautiful feel of the East Asian ink brush. | Read more »
NetNewsWire (News)
NetNewsWire 4.0.0 Device: iOS iPhone Category: News Price: $3.99, Version: 4.0.0 (iTunes) Description: Follow the Web NetNewsWire 4, completely written from the ground up for iPhone. NetNewsWire is the best way to keep up with the... | Read more »
Huzzah! Farming Simulator 16 Supports Cl...
As though it weren't difficult enough to resist the siren call of Farming Simulator 16, now it's been updated with cloud saves - so you can jump between devices without missing any of that precious crop-harvesting time. [Read more] | Read more »
Don't Starve: Pocket Edition Finall...
I've made no effort to hide my enjoyment of Don't Starve: Pocket Edition, but I will admit I was a tiny bit disappointed that it was only available for the iPad. Well nuts to that, because Klei Entertainment has made it universal! [Read more] | Read more »
Goat Simulator Wasn't Enough? Then...
If you just didn't get enough goat in Goat Simulator - or if you've been wanting to play a simulated MMO as a microwave - then you're in luck! Goat Simulator MMO Simulator is out now, and it's a game about simulated goats in simulated MMOs. Plus... | Read more »
You Can Play Madfinger Games' Unkil...
Madfinger Games - probably best known for the Dead Trigger series - has officially launched their newest zombie shooter (that isn't called Dead Trigger), named Unkilled. [Read more] | Read more »
KORG iELECTRIBE for iPhone (Music)
KORG iELECTRIBE for iPhone 1.0.1 Device: iOS iPhone Category: Music Price: $9.99, Version: 1.0.1 (iTunes) Description: ** 50% OFF Special Launch Sale - For a Limited Time **The ELECTRIBE reborn in an even smaller form A full-fledged... | Read more »
Toca Life: City Just Got a Bunch of New...
Toca Life: City is Toca Boca's most popular app (number 1 in 47 different countries, apparently), and it's just had an update that adds a bunch of new content. [Read more] | Read more »
My Country 3D is More About Cities than...
My Country 3D is an upcoming city builder from Game Insight that looks pretty decent - although the name seems a tad out of whack for a city builder. Ah well, it is what it is. [Read more] | Read more »
I am Bread (Games)
I am Bread 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: ‘I am Bread’ is the latest quirky adventure from the creators of 'Surgeon Simulator', Bossa Studios. This isn't the best thing... | Read more »

Price Scanner via MacPrices.net

Near-Office Input Functionality Virtually Any...
Today Logitech introduced the Logitech K380 Multi-Device Bluetooth Keyboard and the Logitech M535 Bluetooth Mouse, giving users the freedom to work on any device, most anywhere. According to... Read more
College Student Deals: Additional $100 off Ma...
Take an additional $100 off all MacBooks and iMacs at Best Buy Online with their College Students Deals Savings, valid through September 4, 2015. Anyone with a valid .EDU email address can take... Read more
2.8GHz Mac mini available for $988, includes...
Adorama has the 2.8GHz Mac mini available for $988, $11 off MSRP, including a free copy of Apple’s 3-Year AppleCare Protection Plan. Shipping is free, and Adorama charges sales tax in NY & NJ... Read more
Will You Buy An iPad Pro? – The ‘Book Mystiqu...
It looks like we may not have to wait much longer to see what finally materializes as a new, larger-panel iPad (Pro/Plus?) Usually reliable Apple product prognosticator KGI Securities analyst Ming-... Read more
eFileCabinet Announces SMB Document Managemen...
Electronic document management (EDM) eFileCabinet, Inc., a hosted solutions provider for small to medium businesses, has announced that its SecureDrawer and eFileCabinet Online services will be... Read more
WaterField Designs Unveils American-Made, All...
San Francisco’s WaterField Designs today unveiled their all-leather Cozmo 2.0 — an elegant attach laptop bag with carefully-designed features to suit any business environment. The Cozmo 2.0 is... Read more
Apple’s 2015 Back to School promotion: Free B...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
128GB MacBook Airs on sale for $100 off MSRP,...
B&H Photo has 11″ & 13″ MacBook Airs with 128GB SSDs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 1.6GHz/128GB MacBook Air: $799.99, $100 off MSRP... Read more
13-inch 2.5GHz MacBook Pro (refurbished) avai...
The Apple Store has Apple Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.... Read more
27-inch 3.2GHz iMac on sale for $1679, save $...
B&H Photo has the 27″ 3.2GHz iMac on sale for $1679.99 including free shipping plus NY sales tax only. Their price is $120 off MSRP. Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Content Partner Engineer - *Apple* TV - App...
**Job Summary** The Apple TV team is looking for an experienced engineer with a passion for delivering first in class home entertainment solutions. The candidate will Read more
*Apple* Desktop Analyst - KDS Staffing (Unit...
…field and consistent professional recruiting achievement. Job Description: Title: Apple Desktop AnalystPosition Type: Full-time PermanentLocation: White Plains, NYHot Read more
*Apple* Retail - Multiple Customer Support P...
Job Description: Customer Support Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the Read more
*Apple* Desktop Analyst - KDS Staffing (Unit...
…field and consistent professional recruiting achievement. Job Description: Title: Apple Desktop AnalystPosition Type: Full-time PermanentLocation: White Plains, NYHot Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.