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What is a Distributed System?

Module by: Donal Fellows. E-mail the author

Summary: Discusses the differences between distributed systems and desktop computers, how the distributed nature has both benefits and some disadvantages, and points out the relevance of this for researchers. That is then followed by an example of how collections of distributed systems can be used to solve a substantive scientific challenge.

Key Concepts

  • distributed systems

Introduction

Over the past decades, as we have begun to explain, we have moved from processing the data that we can hold in a lab notebook to working with many thousands of terabytes of information. (For reference, a terabyte is a million megabytes, and a megabyte is a million letters. A plain textbook might be a few megabytes in size, as might a high-quality photograph — a terabyte is like a huge library.) And yet we keep striving to work with ever more: more genomic data; more high-energy physics data; ever more detailed astronomical photographs; ever richer seismographic measurements; ever more layers of interpretation of artistic details; ever greater volumes of financial data; ever more complex and realistic simulations. We drill down ever deeper into the details. How are we coping with this?

We are in the middle of a huge revolution in information processing, driven by the fact that our tool of choice for working with information — the computer — has been getting exponentially better ever since their invention during the Second World War. We live in the middle of an age of wonder. And yet, despite now being able to hold immense quantities of computation and storage in our hands, our desire to work with ever more has grown even faster.

Thankfully we have been living through another revolution at the same time; the telecommunications revolution. The telecommunications revolution started with the invention of the telegraph, but accelerated with the convergence of computers and telecoms to create the Internet. This not only allows people to share information, but also computers, and it has transformed the world. The first indication of just how amazing this would be came with the WorldWideWeb (WWW), the first internet system to really reflect everything that people do throughout society [link/reference here to history chapter]. But it will not be the last; the ripples from the second wave are now being felt, and it is the global research community that are in the lead. This second wave is Distributed Computing.

The Way Distributed Computing Works

Simply put, distributed computing is allowing computers to work together in groups to solve a single problem too large for any one of them to perform on its own. However, to claim that this is all there is to it massively misses the point.

Distributed computing is not a simple matter of just sticking the computers together, throwing the data at them and then saying “Get on with it”! For a distributed computation to work effectively, those systems must cooperate, and must do so without lots of manual intervention by people. This is usually done by splitting problems into smaller pieces, each of which can be tackled more simply than the whole problem. The results of doing each piece are then reassembled into the full solution.

The power of distributed computing can clearly be seen in some of the most ubiquitous of modern applications: the Internet search engines. These use massive amounts of distributed computing to discover and index as much of the Web as possible. Then when they receive your query, they split it up into fast searches for each of the words in the query. The results of the search are then combined in the twinkling of an eye into your results. What about locating computers on which to execute the web search? That is itself a distributed computing problem, both in the process of looking up computer addresses and also in finding an actual computer to respond to the message sent on that address.

Early distributed systems worked over short distances, perhaps only within a single room, and all they could really do was to share a very few values at set points of the computation. Since then, things have evolved: networks have got faster, numbers of computers have got larger and the distances between the systems have got larger too.

The speeding up of the networks (from the telecommunications revolution) has been extremely beneficial as it has allowed many more values to be shared effectively, and more often. The larger number of computers has only partially helped; while it has meant that it is possible to use more total computation and to split the problems into smaller pieces (allowing a larger overall problem), it has also increased the amount of time and effort that needs to be spent on communication between the computers, since the number of ways to communicate can increase (see Figure 1).

Figure 1: The growth in the number of links as the number of computers goes up.
Number of links in distributed systems

There are, of course, ways to improve communication efficiency, for instance by having a few computers specialize in handling the communications (like a post office) and letting all others focus on the work, but this does not always succeed when the overall task requires much communication.

The distance between computers has increased for different reasons. Computers consume power and produce heat. A single PC normally only consumes a small amount of power and produces a tiny amount of heat; it is typically doing nearly nothing, waiting for its users to tell it to take an action. With a computational task, it would be far busier and will be consuming electrical energy in the process; the busier it is, the more it consumes and produces heat. Ten busy PCs in a room can produce as much heat as a powerful domestic electric heater. With thousands in one place, very powerful cooling is required to prevent the systems from literally going up in smoke. Distributing the power consumption and heat production reduces that problem dramatically, but at a cost of more communications delay due to the greater distances that the data must travel.

There are many ways that a distributed system can be built. You can do it by federating traditional supercomputers (themselves the heirs to the original distributed computing experiments) to produce systems that are expensive but able to communicate within themselves very rapidly; this remains favoured for dealing with problems where the degree of internal communication is very high, such as weather modelling or fluid flow simulations. You can also make custom clusters of more traditional PCs that are still dedicated to being high-capability computers; these have slower internal communications but are cheaper, and are suited for many “somewhat-parallel” problems, such as statistical analysis or searching a database for matches (e.g., searching the web). And you can even build them by , in effect, scavenging spare computer cycles from across a whole organization through a special screen saver (e.g., Condor, BOINC); this is used by many scientific projects to analyse large amounts of data where each piece is fairly small and unrelated to the others (e.g., Folding@Home, SETI@Home, Malaria Control).

Special Challenges for Distributed Computing

We are now moving from having the number of computers working on a problem being small enough to fit in a building or two to having tens of thousands of computers working on single problems. This brings many special challenges that mark distributed computing as being a vastly more complex enterprise than what has gone before.

The first of these special problems is security. The first aspect of security is the security of the computers themselves, since few people feel like giving some wannabe digital mobster a free pass to misuse their computers. The second aspect is the security of the data being processed, much of which may be highly confidential or a trade secret (e.g., individual patients’ medical data, or the designs for products under development). The third aspect of security is the security of the messages used to control the other computers, which are often important in themselves and could be used to conduct a wide range of other mischief if intercepted.

The second special problem of distributed computing is due to the use of systems owned by others, either other people or other organizations. The issues here are to do with the fact that people ultimately retain control over their own systems; they do not like to cede it to others. This behaviour could be considered just a matter of human nature, but it does mean that it is extremely difficult to trust others in this space. The key worries relate to either the computer owner lying about what actions were taken on their systems (for pride, for financial gain, for spite, or just out of straight ignorance) or the distributed computer user using the system for purposes other than those that the computer owner wants to permit.

Interoperability presents the third challenge for distributed computing. Because the systems that people use to provide large-scale computing capabilities have grown over many years, the ways in which they are accessed are quite diverse. This does mean that a lot of effort has been put into finding out access methods that balance the need for efficiency with those of security and flexibility, but it also means that frequently it is extremely difficult to make these systems work together as one larger system. Past attempts by hardware and software vendors to lock people in to specific solutions have not been helpful here either; researchers and practitioners want to solve a far more diverse collection of challenges than the vendors have imagined there to be. After all, the number of things that people wish to do is limited only by the human imagination.

These challenges can be surmounted though, even if the final form of the solutions is not yet clear. We know that the demands of security can be met through a combination of encryption, digital signatures, firewalls, and placing careful constraints on what can be done by any program. The second challenge is being met through the use of techniques from digital commerce like formal contracts, service level agreements, and appropriate audit and provenance trails. The third, which will become ever more important as the size of problems people wish to tackle expands, is primarily dealt with through standardization of both the access mechanisms (whether for computation or for data) and the formal understanding of the systems being accessed by common models, lexicons and ontologies.

The final major challenge of distributed computing is managing the fact that neither the data nor the computations are open to relocation without bounds. Many datasets are highly restricted in where they can be placed, whether this is through legal constraints (such as on patient data) or because of the sheer size of the data; moving a terabyte of data across the world can take a long time, and in no time the most efficient technique becomes sending disks by courier, despite the large quantity of very high capacity networks that exist out there.

This would seem to indicate that it makes sense to move the computations to the location of the data, but that is not wholly practical either. Many applications are not easy to relocate: they require particular system environments (such as specialized hardware) or direct access to other data artefacts (specialized databases are a classic example of this) or are dependent on highly restricted software licenses (e.g., Matlab, Fluent, Mathematica, SAS, etc.; the list is enormous). This problem does not go away even when the users themselves develop the software — it is all too easy for them to include details of their development environment in the program so that it only works on their own computer or in their own institution — writing truly portable software is a special and rare talent.

Solving Problems using Distributed Computing

Because of these fundamental restrictions that will not go away any time soon, it is important for someone tackling a problem (often an otherwise intractable problem) with distributed computing to take them into account when working out what they wish to do. In particular, they need to bear in mind the restrictions on where their data can be, where their applications can be, and what sort of computational patterns they are using in their overall workflow.

As a case in point, when performing drug discovery in relation to a disease, the first stage is to discover a set of potential candidate receptors for the drug to bind to in or on the cell (typically through a massive database search of the public literature, plus potentially relevant patient data, which is much akin to searching the web). This is then followed by a search for candidate substances that might bind to the receptor in a useful way, first coarsely (using a massive cycle scavenging pool) and then in depth (by computing binding energies using detailed quantum chemistry simulations on supercomputers). Once these candidates have been identified, they then have to be screened to see if there are warning areas associated with them that might make their use inadvisable (another database search, though this time probably with more ontology support so that related substances such as metabolites are also checked; this step will probably involve real patient data). If we look at the data-flow between these steps, we see that the amount of data actually moved around is kept relatively small; the databases being searched are mostly not relocated, despite their massive size. However, once these steps are completed, the scientist can have a much higher level of confidence that their in silico experiments will mean that follow-up clinical trials of the winning candidate will succeed, and in many cases it may be possible to skip some parts of the trials (for example, it might be the case that the literature search uncovers the fact that a toxicity trial has already been performed).

This use-case has other interesting aspects in terms of distributed computing, in that it involves the blending of both public and private information to produce a result. The initial searches for binding receptors relating to a particular disease will often involve mainly public data — archives of scientific papers — and much of the coarse fit checking that identifies potential small molecules for analysis will benefit from being farmed out across such large numbers of computers that the use of public cycle scavengers makes sense; it would be difficult to backtrack from the pair of molecules being matched to exactly what was being searched for. On the other hand, there are strong reasons for being very careful with the later stages of the discovery process; scientists are looking at that stage in great depth at a small number of molecules, making it relatively easy for a competitor to act pre-emptively. Moreover, the use of detailed patient data means that care has to be taken to avoid breaches of privacy. In addition, the applications for the detailed analysis steps are often costly commercial products. This means that the overall workflow has both public and private parts, both in the data and computational domains, and so there are inherent complexities. On the other hand, this is also an application area that was impossible to tackle until very recently, and distributed computing has opened it up.

It should also be noted that distributed computing has many benefits at the smaller scale. For example, it is a key component of providing acceleration for many more commonplace problems, such as recalculating a complex spreadsheet or compiling a complex program. These tasks also use distributed computing, though only within the scope of a workgroup or enterprise. And yet there is truly a continuum between them and the very large research Grids and commercial Clouds, and that continuum is founded upon the fact that bringing more computational power together with larger amounts of data allows the discovery of finer levels of detail about the area being studied. The major differences involve how they respond to the problems of security, complex ownership of the parts, and interoperability. This is because those smaller scale solutions can avoid most of the security complexity, only needing at most SSL-encrypted communications, and they work within a single organization (which in turn allows imposition of a single-vendor solution, thus avoiding the interoperability problems). Of course, as time goes by this gap may be closed from both sides; from the lower end as the needs for more computation combine with the availability of virtual-machines-for-hire (through Cloud computing) and from the upper end as the benefits of simplified security and widespread standardized software stacks make adoption of scalable solutions easier.

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Definition of a lens

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A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

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What are tags? tag icon

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'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

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Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

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