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

Things 3.1.3 - Elegant personal task man...
Things is a task management solution that helps to organize your tasks in an elegant and intuitive way. Things combines powerful features with simplicity through the use of tags and its intelligent... Read more
BetterTouchTool 2.292 - Customize Multi-...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
Bookends 12.8.3 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
Mellel 3.5.5 - The word processor for sc...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
Mellel 3.5.5 - The word processor for sc...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
Bookends 12.8.3 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
Carbon Copy Cloner 4.1.18 - Easy-to-use...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
Hopper Disassembler 4.2.14- - Binary dis...
Hopper Disassembler is a binary disassembler, decompiler, and debugger for 32- and 64-bit executables. It will let you disassemble any binary you want, and provide you all the information about its... Read more
BetterTouchTool 2.291 - Customize Multi-...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
Sound Studio 4.8.11 - Robust audio recor...
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

Meshi Quest beginner's guide - how...
Meshi Quest is Square Enix's newest free-to-play release, and it's a real charmer. You start off as the head of a sushi restaurant, upgrading your food and equipment as you serve visitors heaping helpings of your delicious meals. As you progress,... | Read more »
BUST-A-MOVE JOURNEY (Games)
BUST-A-MOVE JOURNEY 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: BUST-A-MOVE Features:- Shoot bubbles and match 3 or more bubbles of the same color to make them pop!- Complete your... | Read more »
The best card combos in Clash Royale
Clash Royale is all about building a deck of units that synergise well. To help you get off to a flying start, we've put together a list of unit combinations that are incredibly effective. Looking for some choice 2v2 combos? Check out our guide. [... | Read more »
The best 2v2 card combos in Clash Royale
2v2 is making it's grand return toClash Royalequite soon. 2v2 has quickly become one of the game's most popular gameplay modes, though they still have yet to make it a permanent fixture in the game. 2v2 is exciting and adds some new flavor to... | Read more »
The best games we played this week - Aug...
Another busy week has come to a close. We played a lot of excellent games this week and now it's time to look back and reflect on some our favorites. Here are our picks for the week of August 18. [Read more] | Read more »
War Wings beginner's guide - how to...
War Wings is the newest project from well-established game maker Miniclip. It's a World War II aerial dogfighting game with loads of different airplane models to unlock and battle. The game offers plenty of single player and multiplayer action. We... | Read more »
How to win every 2v2 battle in Clash Roy...
2v2 is coming back to Clash Royale in a big way. Although it's only been available for temporary periods of time, 2v2 has seen a hugely positive fan response, with players clamoring for more team-based gameplay. Soon we'll get yet another taste of... | Read more »
Roll to Win with Game of Dice’s new upda...
Joycity’s hit Game of Dice gets a big new update this week, introducing new maps, mechanics, and even costumes. The update sets players loose on an exciting new map, The Cursed Tower, that allows folks to use special Runes mid-match. If you feel... | Read more »
Bottom of the 9th (Games)
Bottom of the 9th 1.0.1 Device: iOS iPhone Category: Games Price: $4.99, Version: 1.0.1 (iTunes) Description: Play the most exciting moment of baseball in this fast-paced dice and card game! | Read more »
The best apps for viewing the solar ecli...
If you somehow missed the news, many parts of the United States will be witness to a total solar eclipse on August 21 for the first time in over 90 years. It'll be possible to see the eclipse in at least some capacity throughout the continental U... | Read more »

Price Scanner via MacPrices.net

Free Instant Translator 2.0 App For iOS Relea...
Mobile application development company, Neoappz has announced the release and immediate availability of Instant Translator 2.0 for iOS devices. Instant Translator is a user-friendly application which... Read more
2017 15-inch MacBook Pros on sale for $200 of...
Amazon has 2017 15″ MacBook Pros on sale for $200 off MSRP. Shipping is free: – 15″ 2.8GHz MacBook Pro Space Gray: $2199.99, $200 off MSRP – 15″ 2.8GHz MacBook Pro Silver: $2296, $103 off MSRP – 15″... Read more
Apple’s 2017 Back to School Promotion: Free B...
Purchase a new Mac using Apple’s Education discount, and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free. As part... Read more
Clearance 2016 12-inch Retina MacBooks, Apple...
Apple has Certified Refurbished 2016 12″ Retina MacBooks available starting at $1019. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The following... Read more
15-inch 2.2GHz Retina MacBook Pro, Apple refu...
Apple has Certified Refurbished 2015 15″ 2.2GHz Retina MacBook Pros available for $1699. That’s $300 off MSRP, and it’s the lowest price available for a 15″ MacBook Pro. An Apple one-year warranty is... Read more
Apple refurbished Mac minis available startin...
Apple has Certified Refurbished Mac minis available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: – 1.4GHz Mac mini: $419 $80 off MSRP – 2.6GHz Mac... Read more
Apple refurbished iPad Pros available startin...
Apple has Certified Refurbished 2016 12″ WiFi iPad Pros available starting at $589. An Apple one-year warranty is included with each model, and shipping is free: – 32GB 12″ iPad Pro WiFi: $589... Read more
Weekend sale: 13-inch MacBook Pros for up to...
Amazon has new 2017 13″ MacBook Pros on sale today for up to $200 off MSRP, each including free shipping: – 13″ 3.1GHz/256GB Space Gray MacBook Pro (MPXV2LL/A): $1599.99 $200 off MSRP – 13″ 3.1GHz/... Read more
Back To School With The Edge Desk All-in-one...
Back to school is just around the corner, and the ergonomically correct Edge Desk all-in-one portable kneeling desk is ideal for students living in dorms and small apartments, Edge Desk features:... Read more
Norton Core Secure Wi-Fi Router Now Available...
First introduced at the 2017 Consumer Electronics Show (CES), Norton Core, a secure, high-performance Wi-Fi router, fundamentally changed the concept of Wi-Fi routers by making security the primary... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Development Operations and Site Reliability E...
Development Operations and Site Reliability Engineer, Apple Payment Gateway Job Number: 57572631 Santa Clara Valley, California, United States Posted: Jul. 27, 2017 Read more
Frameworks Engineering Manager, *Apple* Wat...
Frameworks Engineering Manager, Apple Watch Job Number: 41632321 Santa Clara Valley, California, United States Posted: Jun. 15, 2017 Weekly Hours: 40.00 Job Summary Read more
*Apple* Solutions Consultant - Apple Inc. (U...
…about helping others on a team while also delighting customers? As an Apple Solutions Consultant (ASC), you will discover customers needs and help connect them Read more
*Apple* Solutions Consultant - Apple Inc. (U...
Job Summary As an Apple Solutions Consultant, you'll be the link between our future customers and our products. You'll showcase your entrepreneurial spirit as you Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.