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Margo Seltzer and Keith Bostic. Conway’s Law states that a design reflects the structure of the organization that produced it. Stretching d a bit, we might anticipate that a software artifact designed and initially produced download win2k x3daudio driver two people might страница reflect, not merely the structure of the organization, but the internal biases and philosophies each brings to the table.

One of us Seltzer has spent her career between the worlds of filesystems and database management berkeley db free download for windows. If questioned, she’ll argue the two bwrkeley fundamentally the same thing, and furthermore, operating systems and database management systems are essentially both resource managers and providers of convenient abstractions. The differences are “merely” implementation details. The other Bostic believes in the tool-based approach to software engineering and in the construction of components based on simpler building blocks, because such systems berkeley db free download for windows invariably superior to monolithic architectures in the important “-bilities”: understandability, extensibility, maintainability, testability, and flexibility.

When you combine those two perspectives, it’s not surprising to learn that together we spent much of the last two decades working on Berkeley DB—a software library that provides fast, flexible, reliable and scalable data management.

Berkeley Ffor provides much of the same functionality that people expect from more conventional systems, such as relational databases, but packages it berkeley db free download for windows. For example, Berkeley DB provides fast berkeley db free download for windows access, both keyed and sequential, as well as transaction support and recovery from failure.

However, it provides those features in a berkeley db free download for windows that links directly with the application wibdows needs those services, rather than being made available by a standalone server application. In this chapter, we’ll take a deeper look at Berkeley DB and see that it is berkeley db free download for windows of a collection of modules, each of which embodies the Unix “do one thing well” philosophy.

Applications that embed Berkeley DB can use those components directly or they can simply use them implicitly via the more familiar operations to get, put, and delete data items. We’ll focus on architecture—how we got started, what we were designing, and where we’ve ended up and why. Designs can взято отсюда certainly will!

We will also briefly consider the code evolution of long-term software projects. Berkeley DB berkrley over two decades of on-going development, and that inevitably takes its toll winfows good design. It boasted a clever scheme allowing a constant time mapping between hash values and page addresses, as well as the ability to handle large data—items larger than the underlying hash bucket or filesystem page size, typically four to eight kilobytes.

If hash tables were good, then Btrees and hash tables would be better. Mike Olson, also a graduate student at the University of California, Berkeley, had written a number of Btree implementations, and agreed to write one more. The three of us transformed Margo’s hash software and Mike’s Btree software into an access-method-agnostic API, where applications reference hash tables or Btrees via database handles that had handle methods to read and modify data.

Berkeley DB brekeley. The Berkeley DB 1. This effort produced the first transactional version of Berkeley DB, посетить страницу 2. Berkeley DB 4. At the time of writing, Berkeley DB is the most widely used database toolkit in the world, with hundreds of millions of deployed copies running in everything from routers and browsers to mailers and operating systems.

Although more than twenty years old, the Berkeley DB tool-based and object-oriented approach has allowed it to incrementally improve and re-invent itself to match the requirements of the software using it. It is vital for any complex software package’s testing and maintenance that the software be ответ wwe games for pc free download full version 2014 что and built as a cooperating set of modules with well-defined API boundaries.

The boundaries can and should! The existence of those boundaries prevents the software from becoming an unmaintainable pile of spaghetti. Butler Lampson once said that all problems in computer science can be solved by another level of indirection. More to the point, when asked what it meant for something to be berkeley db free download for windows, Lampson said it meant being читать to have multiple implementations behind an API. The Berleley DB design and implementation embody this approach of permitting multiple implementations behind a common interface, providing an object-oriented look and feel, even though the library is written in C.

Figure 4. As is discussed in Нажмите чтобы увидеть больше 4. The difference between the design and the actual gor db The recovery subsystem is shown in gray.

Recovery includes both the berkeley db free download for windows infrastructure, depicted in the recovery box, as well as a set of recovery redo and undo routines that recover the operations performed by the access methods. These berkeley db free download for windows represented by the circle labelled “access method recovery routines.

This general purpose design also produces a much richer interface between the various modules. The numbers in the diagram reference the APIs listed in the table in Table 4. Although the original architecture is still visible, the current architecture shows its age with the addition of new modules, the decomposition of old modules e.

Over a decade of evolution, dozens of commercial berkeley db free download for windows, and hundreds berkeley db free download for windows new features later, we see that the architecture is significantly more complex than its ancestors. The key things to note are: First, replication adds an entirely new layer to the system, but it does so cleanly, interacting with the rest of the system via the same APIs as does the historical code.

Second, the log module is split into log and dbreg database registration. This is discussed in more detail in Section 4. Third, we have placed all inter-module calls into a namespace identified with leading underscores, so that applications won’t collide with our function names.

We discuss this further in Design Lesson 6. Historically, Berkeley DB never berkkeley more than one thread of control reading or writing the log at berkeley db free download for windows instant in time, so the library had widows single notion of the current seek pointer in the log. This was never a good abstraction, but with replication it became unworkable. Just as the application API supports iteration using xb, the log now supports iteration using cursors.

Fifth, the fileop module inside of the access methods provides support for transactionally protected database create, delete, and rename operations. It took us multiple attempts to make the implementation palatable it is still not as clean as we would likeand after reworking it numerous time, we pulled berkeley db free download for windows out into its own module.

A software design is simply one of several ways to force yourself to think through the entire problem before attempting to solve it. Skilled programmers use different techniques to this end: some write a first version and throw it away, some write extensive manual pages or design documents, others fill out a code template where every requirement is identified and assigned to a specific function or comment.

For example, in Berkeley DB, we created a complete set of Unix-style manual pages for the access methods and underlying components before writing any code. Regardless of the technique used, it’s difficult to think clearly about program architecture after code debugging begins, not to mention that large architectural changes often waste previous debugging effort.

Software architecture requires a different mind set from debugging code, and the architecture you berkeley db free download for windows when you begin debugging is usually the architecture you’ll deliver in that release.

Why architect the transactional library out of components rather than tune it to a single anticipated use? There are three answers to this question. First, it forces a more disciplined design. Second, without strong boundaries in the code, complex software packages inevitably degenerate into unmaintainable piles of glop.

Third, you can never anticipate all the ways customers will use your software; if you empower users by giving them access to software components, they will use them in ways you never considered. In subsequent sections we’ll consider each component of Berkeley DB, understand what it does and how it fits into the larger picture.

The Downoad DB access methods provide both keyed lookup of, and iteration over, variable and fixed-length byte strings. The main difference between Btree and Hash access methods is that Btree offers locality of reference for keys, while Hash does not.

This implies that Btree is the right access method for almost all data sets; however, как сообщается здесь Hash access method is appropriate for data sets so large that not even the Btree indexing structures fit into memory.

At that point, it’s better to use the memory for data than for indexing structures. This trade-off made a lot more sense in when main memory was typically much smaller than today. The difference between Recno and Queue is that Queue supports record-level locking, at the cost of requiring fixed-length values. Recno supports variable-length objects, but like Btree and Hash, supports only page-level locking.

We originally designed Berkeley DB such that the CRUD хорошо. socom 4 pc download tpb очень create, read, update and delete was key-based and the primary interface for applications.

We subsequently added cursors to support iteration. That ordering led to the confusing and wasteful case of largely duplicated code paths inside the library. Over time, this became unmaintainable and we converted all keyed operations to cursor operations keyed operations now allocate a cached cursor, perform the operation, and return the cursor to the cursor pool. This is an application of one of the endlessly-repeated rules of software development: don’t optimize a code path in any жмите that detracts from clarity and simplicity until you know that it’s necessary to do so.

Software architecture does not age gracefully. Software architecture degrades in direct proportion to the number of changes made to the software: foor fixes corrode the layering and new features stress design. Deciding when the software architecture has degraded sufficiently that you should re-design or re-write a module is a hard decision. On one hand, as the architecture degrades, maintenance and development become more difficult and at the end of that path is a legacy piece of software maintainable only by having an army of brute-force testers for every release, because nobody understands how the software works inside.

On the other hand, users will bitterly complain winodws the instability and incompatibilities that result from fundamental changes. As a software architect, your only guarantee downloadd that someone will be angry with you no matter which path you choose. We omit detailed http://replace.me/1429.txt of the Berkeley Узнать больше здесь access method internals; they implement fairly well-known Btree and hashing algorithms Recno is a layer on top of the Btree code, and Queue is a file block lookup function, albeit complicated by the addition of record-level locking.

Over time, as we added additional functionality, we discovered that both applications and internal code needed the same top-level functionality for berkeley db free download for windows, a table join operation uses multiple cursors to iterate over the rows, just as an application might use a cursor to down,oad over those same rows.

It doesn’t matter how you name your variables, methods, functions, or what comments or code style you use; that is, there are a large number of formats and styles that are “good enough. Skilled programmers derive a tremendous amount of information from code format and object naming. You should view naming and style inconsistencies as some programmers investing time and effort to frwe to the other programmers, and vice versa.

Failing to follow house coding conventions is a firing offense. For this reason, we decomposed the access method APIs into precisely defined layers. These layers of interface routines perform all of the necessary generic error checking, function-specific error checking, interface tracking, and other tasks such as automatic transaction management.

When applications call into Berkeley DB, they call the first level of interface routines based on methods in the object handles. One of the Berkeley DB tasks performed in berkeley db free download for windows interface layer is tracking what threads are running inside the Berkeley DB library.

This is necessary because some frew Berkeley DB operations may be performed only when no threads are running inside the library. Berkeley DB tracks threads in the library by marking that a thread is executing inside the library at the beginning of every library API and clearing that flag when the API call returns. The obvious question is “why not pass a thread identifier into the library, wouldn’t that be easier? But, that change would have modified every single Berkeley DB application, most of every application’s calls into Berkeley DB, downloar in many cases would have required doownload re-structuring.

Software architects must choose their upgrade battles ссылка на страницу users will accept minor больше информации to upgrade to new releases if you guarantee compile-time errors, that berkeley db free download for windows, obvious failures until the upgrade is complete; upgrade changes should never fail in subtle ways. But to make truly fundamental changes, you must admit it’s a new code base and requires a port of your berkeley db free download for windows base.

Obviously, new code bases and application ports are not cheap in time or resources, but neither is angering your user base by telling them a huge overhaul is really a minor upgrade.