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

Logic Pro X 10.1.1 - Music creation and...
Apple Logic Pro X is the most advanced version of Logic ever. Sophisticated new tools for professional songwriting, editing, and mixing are built around a modern interface that's designed to get... Read more
VLC Media Player 2.2.0 - Popular multime...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more
Sound Studio 4.7.8 - Robust audio record...
Sound Studio lets you easily record and professionally edit audio on your Mac. Easily rip vinyls and digitize cassette tapes, or record lectures and voice memos. Prepare for live shows with live... Read more
LibreOffice 4.4.1.2 - Free, open-source...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
Freeway Pro 7.0.3 - Drag-and-drop Web de...
Freeway Pro lets you build websites with speed and precision... without writing a line of code! With its user-oriented drag-and-drop interface, Freeway Pro helps you piece together the website of... Read more
Cloud 3.3.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
Cyberduck 4.6.5 - FTP and SFTP browser....
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
Firefox 36.0 - Fast, safe Web browser. (...
Firefox for Mac offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals and... Read more
Thunderbird 31.5.0 - Email client from M...
As of July 2012, Thunderbird has transitioned to a new governance model, with new features being developed by the broader free software and open source community, and security fixes and improvements... Read more
VOX 2.4 - Music player that supports man...
VoxIt just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features & support for all audio formats you should ever need... Read more

Get The Whole Story – Lone Wolf Complete...
Get The Whole Story – Lone Wolf Complete is Now Available and On Sale Posted by Jessica Fisher on February 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Who Wore it Best? The Counting Dead vs....
Like it or not, the “clicker” genre, popularized by cute distractions like Candy Box and Cookie Clicker, seems like it’s here to stay. So Who Wore it Best? takes a look at two recent examples: The Counting Dead and AdVenture Capitalist. | Read more »
Card Crawl, the Mini Deck Building Game,...
Card Crawl, the Mini Deck Building Game, is Coming Soon Posted by Jessica Fisher on February 27th, 2015 [ permalink ] Tinytouchtales and Mexer have announced their new game, | Read more »
Witness an all new puzzle mechanic in Bl...
Well, BlastBall MAX is not one of those games and is bucking trends such as timers, elements of randomness, and tacked-on mechanics in favor of pure puzzle gameplay. When you first boot up the game you’ll see a grid made up of squares that are each... | Read more »
This Princess Has a Dragon and She isn’t...
This Princess Has a Dragon and She isn’t Afraid to Useit. | Read more »
Mecha Showdown Review
Mecha Showdown Review By Lee Hamlet on February 27th, 2015 Our Rating: :: IN A SPINUniversal App - Designed for iPhone and iPad Mecha Showdown replaces traditional buttons with a slot machine mechanic in this robot fighting game,... | Read more »
Reliance Games and Dreamworks Unveil Rea...
Reliance Games and Dreamworks Unveil Real Steel Champions Posted by Ellis Spice on February 27th, 2015 [ permalink ] Reliance Games and Dreamworks have announced that a third game in | Read more »
Sum Idea Review
Sum Idea Review By Jennifer Allen on February 27th, 2015 Our Rating: :: TAXING NUMBERSUniversal App - Designed for iPhone and iPad Sum Idea is a fairly charming but taxing puzzle game.   | Read more »
A New Badland Update Brings Daydream Lev...
A New Badland Update Brings Daydream Levels to Co-Op Posted by Ellis Spice on February 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Slashing Demons Review
Slashing Demons Review By Lee Hamlet on February 27th, 2015 Our Rating: :: IT'S A LONG WAY TO THE TOPUniversal App - Designed for iPhone and iPad Slashing Demons lacks the depth or scope to take it beyond the point of being just... | Read more »

Price Scanner via MacPrices.net

Save up to $600 with Apple refurbished Mac Pr...
The Apple Store is offering Apple Certified Refurbished Mac Pros for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The... Read more
Updated Mac Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
Apple CEO Tim Cook to Deliver 2015 George Was...
Apple CEO Tim Cook will deliver the George Washington University’s Commencement address to GWU grads on May 17, at which time he will also be awarded an honorary doctorate of public service from the... Read more
Apple restocks refurbished Mac minis for up t...
The Apple Store has restocked Apple Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz... Read more
Save up to $50 on iPad Air 2s, NY tax only, f...
 B&H Photo has iPad Air 2s on sale for $50 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $469.99 $30 off - 64GB iPad Air 2 WiFi: $549 $50 off - 128GB iPad Air 2... Read more
16GB iPad Air 2 on sale for $447, save $52
Walmart has the 16GB iPad Air 2 WiFi on sale for $446.99 on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale price for online orders only,... Read more
iMacs on sale for up to $205 off MSRP
B&H Photo has 21″ and 27″ iMacs on sale for up to $205 off MSRP including free shipping plus NY sales tax only: - 21″ 1.4GHz iMac: $1029 $70 off - 21″ 2.7GHz iMac: $1199 $100 off - 21″ 2.9GHz... Read more
Apple Takes 89 Percent Share of Global Smartp...
According to the latest research from Strategy Analytics, global smartphone operating profit reached US$21 billion in Q4 2014. The Android operating system captured a record-low 11 percent global... Read more
New Travel Health App “My Travel Health” iOS...
Rochester, Minnesota based Travel Health and Wellness LLC has announced that its new iOS app help safeguard the user’s health when traveling abroad — “My Travel Health” is now available on the Apple... Read more
Sale! MacBook Airs for up to $115 off MSRP
B&H Photo has MacBook Airs on sale for up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 128GB MacBook Air: $799 100 off MSRP - 11″ 256GB MacBook Air: $999 $100... Read more

Jobs Board

Sr. Technical Services Consultant, *Apple*...
**Job Summary** Apple Professional Services (APS) has an opening for a senior technical position that contributes to Apple 's efforts for strategic and transactional Read more
Event Director, *Apple* Retail Marketing -...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global engagement strategy and team. Delivering an overarching brand Read more
*Apple* Pay - Site Reliability Engineer - Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.