On the usefulness of volatile memory

I have a strange hobby: Raising ants as pets. Not literally pets, because I don’t give them names and they probably don’t know I even exist, but as tended animals in a controlled and restricted environment, the formicarium. This doesn’t sound too strange if you know three additional things about me: I was fascinated by ants since kindergarten, I play dwarf fortress for fun and I’m interested in computers.

My main colony is a lasius niger nest with about 1500 individuals. Lasius niger or black garden ant is a very common and easy to raise ant that is small enough to not require too much space but big enough to be observed and tracked.

I could tell you hours and hours of ant facts, but let’s concentrate on the topic of this blog post: An ant colony can be perceived as one big organism. Each individual ant has a clear role in the hierarchy:

  • The queen ant is the sole egg layer. In an established colony, she won’t do anything else. She will be fed, cleaned and protected by her workers. She was born a queen and cannot be replaced. If she dies, the whole colony will slowly fade away because nobody produces new ants anymore. Worker ants don’t know if they still have a queen and don’t care anyway. If a queen dies, she will be fed to the remaining larvas as soon as her scent disappears.
  • The male ants are born, fed and kicked out of the nest, preferably when the young queens fly out to find a mate. They won’t do anything else and have a lifespan of days.
  • The worker ants are all sterile females and do all the work. Literally all the work. They are born workers and spend every moment of their lives contributing to the hive or just sitting around scrounging food. Other workers are too busy to judge them.

But keep in mind that ants only have short-range communication means (scent, antennae drumming and ground vibrations), so no single ant has complete overview about the situation. It wouldn’t have the brains to process that information anyway. Ants have limited capability to remember things but no long-time memory. It isn’t necessary for the single worker ant to store any information. The information is stored in the hive – literally.

If you abstract some details away, you can also perceive and describe an ant colony as a computer or at least as a massive parallel problem solver. The algorithms are ingrained in the worker ants and are executed ruthlessly. The problems are food, water, enemies, cleaning and nurture. If the environment (the problem space) is suitable for the algorithms, the colony will thrive. Else the colony will ultimately fail. An ant colony at the side of a busy road will lose many workers in sudden “enemy” attacks, while a colony in the middle of a meadow might find less insects killed by cars. Not a single ant in each colony is aware of these relations. But they have found a way to share information: They mark their position (and therefore their way) by special scent fluid. They use their scents to label the environment for other ants.

If you ever encountered an avid user of printed sticker labels, this is what ants do, too: They put a sticker label on everything they come in contact with. If you seem to be food, they label you as food and rush back to the hive, laying out a “food in this direction” lane. If you seem dead and inedible, they mark you as garbage and them or some other ant will pick you up and follow the garbage line (finding the garbage line often requires them to return near their hive so it seems they mistook the garbage for food). The garbage area is marked with its own scent, preferably at a cliff. My ants love to throw things down a cliff! If you didn’t get the relationship to dwarf fortress yet, it should be clear by now. There are countless more examples, but you get the idea. The environment is not only an area, it is the long-term memory of the colony. The colony’s “memory brain” is scent on dirt, stones and sticks. The ant colony has outsourced its complete knowledge about the surrounding into the surrounding itself.

The ants have invented something I would call an “inverse cloud”. The cloud in IT is a concept of data storage that exists mostly independent of physical location and provides access to that data from virtually anywhere. The ants’ inverse cloud is a concept of data storage that is tightly coupled to a physical location and provides access to that data only if you are in the immediate vincinity. If you remove a physical data storage part in the IT cloud, it gets replaced by other parts that contain mirrored copies of the data. The cloud never forgets. If you remove a physical data storage part in the ants’ inverse cloud, it is forgotten immediately. Ants accept their environment as it is right now and never look back.

Now think about what happens when it rains and all the scents are washed away.

After every rain, the ant colony enters a “new level”, a fresh environment to be discovered and labeled. They probably never grow old to rediscover the same cliff again and again. This is what happens to a computer when the power is lost. It loses its working memory. But keep in mind that the colony retains some memories: the hive is underground and often rain-proof by overlaying stones or plants. So the colony remembers that it is currently in the hive, because it always smells like hive. The colony remembers the queen chamber because it always smells like queen. The colony remembers the brood chambers because they always smell like teen spirit (SCNR).

In my formicarium, it never rains. The ants get enough water by drinking troughs, but the marked lanes are never erased. This leads to all sorts of silly situations that the ants don’t even recognize as such. For example, a strong “food here” scent lane exists long after the food is gone. So a lot of enthusiastic workers run around searching in the target area. And remember that the hive smell never fades? My ants have assimilated area after area as “hive” after enough ants have marked it. So now they react excessively to disturbances because they think they are defending the hive – and therefore the queen! – but are ant miles away from the colony. Even better, a lot of young ants that usually never leave the hive until they are older wander out into the open (“it’s still the hive, just less dark”) and panic as soon as they encounter something that shouldn’t be in a hive. The panic spreads by, you’ve guessed it already, alarm scent and soon hundreds of battle-ready ants are running around frantically without any one of them knowing why.

So, to speak in computer terms, the main memory never gets erased and the caches never flushed. A little error can spread like a wildfire (like the panic example above) and cause disadvantages like energy consumption without any real gain (no enemy to defend against) or a lot of delicate ants sitting around in plain sight of their predators. The whole system is fragile and erratic and probably wouldn’t survive in the wild. A good measure of rain would remove the odd memories and probably ease the ants because their hive, the area that needs to be defended at all costs, would shrink again.

I’ve raised neurotic ants in need of a cold shower.

How does this give us any insight into modern computing? Well, my train of thoughts is this: If modern systems are unable to forget, because memory is cheap and permanent, we might be prone to design software that acts neurotic and hyped up. The ability to forget, to really don’t remember at all, might be crucial in designing resilient parallel systems. There is the cost of losing valueable information, but the benefit of losing all results of errors seems to match it. So volatile memory might be a nuisance for us programmers, but it also provides a “blank slate” every time the system starts and is the reason for the most important question in IT: “Have you tried turning it off and on again?”

Systems that rely on “place oriented programming”  seem to have the need of regular reset phases where the working memory is cleared and the system goes into the next cycle fresh and rested. We might even call it sleep. And in case you wonder: The sleep of ants is an ongoing topic for research.

Disclaimer: I know that not all ants are as dumb as lasius niger. Some ants even teach each other facts about their environment. The wikipedia article mentions some wonderful examples. I had ant colonies with more complex ants and they were wonderful. But right now, as I’m typing this, there is a lasius niger worker that heaves a wasp husk part to the top corner of the formicarium, throws it down (did I mention they love throwing things?) and runs back down to heave it up again, probably because the corner is somehow marked as a garbage dump zone. It has repeated this process at least half a dozen times now. This is how a biological infinite loop looks like. Some ants even parallelize such a loop to exhaustion:

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Lightweight dependency management

Java’s classpath is a powerful concept – when used appropriate. As your project grows larger in terms of code and people, it gets harder to ensure that your classpath is correct. A great danger arises from JAR files containing different versions of the same resource. You might end up running different code than you think, leading to strange effects. If you build your classpath using wildcards, you can’t even control the order your JAR files are loaded.

Managing dependencies

To avoid the issues mentioned above, you need to manage your project dependencies. It’s a common practice to implement the build process of the project using maven or ant ivy. Both tools provide dependency mangement by declaration. But at a high cost. Especially maven has received some malice lately, criticizing its steep learning curve and complexity.

Scratching the biggest itch

We decided to try a different approach to dependency management, tackling only our biggest concern: The duplication of classpath resources. We take care of the scope of a third-party library, put required JARs in the repository (to us, third party binary artifacts are part of the project source) and update manually. The one thing we cannot assure manually is that every resource is unique. Sometimes, the same class is included in different JARs, as it seems to be common practice among java web frameworks.

Ant to the rescue

Thus, I wrote a custom ant task that, given the classpath, checks for duplicate entries. If it finds one, it lists the culprits and optionally aborts the build process. Included in our continuous integration system, it gets run every time somebody performs a change. You can’t forget to delete an old version of a library or check in the same library twice without breaking the build now.

Our ClasspathCollisionCheckTask

I provide this task here, without any warranty. The source code is included in the JAR alongside the classes, if you want to know what it does exactly.

Assuming you already know how to use custom tasks within an ant build script, here’s only a short usage description.

Import the custom task:

<taskdef
    name="check.collision"
    classname="com.schneide.internal.anttask.ClasspathCollisionCheckTask"
    classpath="${customtasks.library.directory}/schneidetasks.jar"
/>

Next, use it on your classpath:

<check.collision verbose="true" failOnCollision="true">
    <path>
        <fileset dir="${classpath.library.directory}">
            <include name="**/*.jar"/>
        </fileset>
        <fileset dir="${internal.library.directory}">
            <include name="**/*.jar"/>
        </fileset>
    </path>
</check.collision>

The task scans the whole path you give it and reports any collision it detects. You will see the warnings in your build log.

If the failOnCollision parameter is set to true (optional, defaults to false), the build will abort after a collision. If you want to have debug information, set the verbose parameter to true (optional, defaults to false).

Conclusion

If you manage your project dependencies manually, you might find our custom ant task useful. If you use maven or ant ivy, you already have this functionality in your build process.

Feedback

I’m very interested in hearing your opinion on the task or about your way of handling dependencies. Leave us a comment.