Varnish is a reverse caching proxy used as HTTP accelerator for content-heavy dynamic web sites as well as APIs . In contrast to other web accelerators , such as Squid , which began life as a client-side cache, or Apache and nginx , which are primarily origin servers , Varnish was designed as an HTTP accelerator. Varnish is focused exclusively on HTTP , unlike other proxy servers that often support FTP , SMTP , and other network protocols .
42-637: VCL may refer to: Computing [ edit ] Varnish Configuration Language, a domain-specific language used for configuring the Varnish Proxy / Server Video Coding Layer, a layer in H.264/AVC and HEVC Virus Creation Laboratory , an MS-DOS program designed to create computer viruses Visual Component Library , a programming library for Delphi and C++Builder Visual Class Library, an internal part of OpenOffice.org and LibreOffice Voluntary collective licensing , an alternative approach to solve
84-425: A metacompiler itself compiles a domain-specific metalanguage specifically designed for the domain of metaprogramming . Besides parsing domain-specific languages, metacompilers are useful for generating a wide range of software engineering and analysis tools. The meta-compiler methodology is often found in program transformation systems . Metacompilers that played a significant role in both computer science and
126-419: A round robin and a random director, both with a per-backend weighting. Basic health-checking of backends is also available. Varnish Cache also features: Domain-specific language A domain-specific language ( DSL ) is a computer language specialized to a particular application domain . This is in contrast to a general-purpose language (GPL), which is broadly applicable across domains. There are
168-406: A scripting language , and is often used in a way analogous to a programming library . The boundaries between these concepts are quite blurry, much like the boundary between scripting languages and general-purpose languages. Domain-specific languages are languages (or often, declared syntaxes or grammars) with very specific goals in design and implementation. A domain-specific language can be one of
210-568: A shell to perform more complex programming tasks. The line between domain-specific languages and scripting languages is somewhat blurred, but domain-specific languages often lack low-level functions for filesystem access, interprocess control, and other functions that characterize full-featured programming languages, scripting or otherwise. Many domain-specific languages do not compile to byte-code or executable code, but to various kinds of media objects: GraphViz exports to PostScript , GIF , JPEG , etc., where Csound compiles to audio files, and
252-1038: A domain-specific language approach to software engineering involves both risks and opportunities. The well-designed domain-specific language manages to find the proper balance between these. Domain-specific languages have important design goals that contrast with those of general-purpose languages: In programming, idioms are methods imposed by programmers to handle common development tasks, e.g.: General purpose programming languages rarely support such idioms, but domain-specific languages can describe them, e.g.: Examples of domain-specific programming languages include HTML , Logo for pencil-like drawing, Verilog and VHDL hardware description languages, MATLAB and GNU Octave for matrix programming, Mathematica , Maple and Maxima for symbolic mathematics , Specification and Description Language for reactive and distributed systems, spreadsheet formulas and macros, SQL for relational database queries, YACC grammars for creating parsers , regular expressions for specifying lexers ,
294-434: A host language as a library and tend to be limited to the syntax of the host language, though this depends on host language capabilities. There are several usage patterns for domain-specific languages: Many domain-specific languages can be used in more than one way. DSL code embedded in a host language may have special syntax support, such as regexes in sed, AWK, Perl or JavaScript, or may be passed as strings. Adopting
336-414: A language for decorating models with assertions or QVT , a domain-specific transformation language. However, languages like UML are typically general-purpose modeling languages. To summarize, an analogy might be useful: a Very Little Language is like a knife, which can be used in thousands of different ways, from cutting food to cutting down trees. A domain-specific language is like an electric drill: it
378-469: A ray-tracing domain-specific language like POV compiles to graphics files. A data definition language like SQL presents an interesting case: it can be deemed a domain-specific language because it is specific to a specific domain (in SQL's case, accessing and managing relational databases), and is often called from another application, but SQL has more keywords and functions than many scripting languages, and
420-561: A separate company, Varnish Software. Varnish is free and open-source software , available under a two-clause BSD license . Commercial support is available from Varnish Software, amongst others. Version 1.0 of Varnish was released in 2006, Varnish 2.0 in 2008, Varnish 3.0 in 2011, Varnish 4.0 in 2014, Varnish 5.0 in 2016, Varnish 6.0 in March 2018, and Varnish 7.0 in September 2021. Varnish stores data in virtual memory and leaves
462-562: A set of tags that can be used in ColdFusion pages to interact with data sources, manipulate data, and display output. CFML tag syntax is similar to HTML element syntax. FilterMeister is a programming environment, with a programming language that is based on C, for the specific purpose of creating Photoshop -compatible image processing filter plug-ins; FilterMeister runs as a Photoshop plug-in itself and it can load and execute scripts or compile and export them as independent plug-ins. Although
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#1732791026863504-435: A specific domain. For example, Perl was originally developed as a text-processing and glue language, for the same domain as AWK and shell scripts , but was mostly used as a general-purpose programming language later on. By contrast, PostScript is a Turing-complete language, and in principle can be used for any task, but in practice is narrowly used as a page description language . The design and use of appropriate DSLs
546-485: A standalone tool to spot-check the health of the cache. In order to graph metrics over time, correlate with other statistics from across an infrastructure, and set up alerts about any problems that may arise, monitoring services can integrate with Varnish and collect varnishstat metrics. Varnishlog is a tool that can be used to debug or tune Varnish's configuration, as it provides detailed information about each individual request. Varnish supports load balancing using both
588-611: A visual diagramming language, such as those created by the Generic Eclipse Modeling System , programmatic abstractions, such as the Eclipse Modeling Framework , or textual languages. For instance, the command line utility grep has a regular expression syntax which matches patterns in lines of text. The sed utility defines a syntax for matching and replacing regular expressions. Often, these tiny languages can be used together inside
630-493: A wide variety of DSLs, ranging from widely used languages for common domains, such as HTML for web pages, down to languages used by only one or a few pieces of software, such as MUSH soft code. DSLs can be further subdivided by the kind of language, and include domain-specific markup languages , domain-specific modeling languages (more generally, specification languages ), and domain-specific programming languages . Special-purpose computer languages have always existed in
672-424: Is Varnish Configuration Language (VCL), a domain-specific language (DSL) used to write hooks that are called at critical points in the handling of each request. Most policy decisions are left to VCL code, making Varnish more configurable and adaptable than most other HTTP accelerators. When a VCL script is loaded, it is translated to C , compiled to a shared object by the system compiler, and loaded directly into
714-456: Is a domain-specific language targeted at novice programmers to easily be able to learn programming. While the language serves as a blend of multiple languages including Delphi , C++ , and BASIC . Most of functions in that language after compiling in fact calls runtime functions written in language specific for targeted platform, so their final implementation is not visible to user. The language primarily serves to make it easy for anyone to pick up
756-453: Is a key part of domain engineering , by using a language suitable to the domain at hand – this may consist of using an existing DSL or GPL, or developing a new DSL. Language-oriented programming considers the creation of special-purpose languages for expressing problems as standard part of the problem-solving process. Creating a domain-specific language (with software to support it), rather than reusing an existing language, can be worthwhile if
798-520: Is a language designed to define test cases to check the behavior of software, without specifying how that behavior is implemented. It is meant to be read and used by non-technical users using a natural language syntax and a line-oriented design . The tests defined with Gherkin must then be implemented in a general programming language. Then, the steps in a Gherkin program acts as a syntax for method invocation accessible to non-developers. Other prominent examples of domain-specific languages include: Some of
840-765: Is a powerful tool with a wide variety of uses, but a specific context, namely, putting holes in things. A General Purpose Language is a complete workbench, with a variety of tools intended for performing a variety of tasks. Domain-specific languages should be used by programmers who, looking at their current workbench, realize they need a better drill and find that a particular domain-specific language provides exactly that. DSLs implemented via an independent interpreter or compiler are known as External Domain Specific Languages . Well known examples include TeX or AWK. A separate category known as Embedded (or Internal) Domain Specific Languages are typically implemented within
882-462: Is an example of this. The toolkit is a suite of utilities including a specification editor to create a requirements specification , a dependency graph browser to display variable dependencies, a consistency checker to catch missing cases in well-formed formulas in the specification, a model checker and a theorem prover to check program properties against the specification, and an invariant generator that automatically constructs invariants based on
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#1732791026863924-423: Is created specifically to solve problems in a particular domain and is not intended to be able to solve problems outside of it (although that may be technically possible). In contrast, general-purpose languages are created to solve problems in many domains. The domain can also be a business area. Some examples of business areas include: A domain-specific language is somewhere between a tiny programming language and
966-563: Is often thought of as a language in its own right, perhaps because of the prevalence of database manipulation in programming and the amount of mastery required to be an expert in the language. Further blurring this line, many domain-specific languages have exposed APIs, and can be accessed from other programming languages without breaking the flow of execution or calling a separate process, and can thus operate as programming libraries. Some domain-specific languages expand over time to include full-featured programming tools, which further complicates
1008-421: The Generic Eclipse Modeling System for creating diagramming languages, Csound for sound and music synthesis, and the input languages of GraphViz and GrGen , software packages used for graph layout and graph rewriting , Hashicorp Configuration Language used for Terraform and other Hashicorp tools, Puppet also has its own configuration language . The GML scripting language used by GameMaker Studio
1050-490: The S language), Bugs , Jags , and Stan . These languages provide a syntax for describing a Bayesian model and generate a method for solving it using simulation. Generate object handling and services based on an Interface Description Language for a domain-specific language such as JavaScript for web applications, HTML for documentation, C++ for high-performance code, etc. This is done by cross-language frameworks such as Apache Thrift or Google Protocol Buffers . Gherkin
1092-456: The transclusion (inclusion by reference) of MediaWiki pages into other MediaWiki pages. There has been much interest in domain-specific languages to improve the productivity and quality of software engineering . Domain-specific language could possibly provide a robust set of tools for efficient software engineering. Such tools are beginning to make their way into the development of critical software systems. The Software Cost Reduction Toolkit
1134-486: The FilterMeister language reproduces a significant portion of the C language and function library, it contains only those features which can be used within the context of Photoshop plug-ins and adds a number of specific features only useful in this specific domain. The Template feature of MediaWiki is an embedded domain-specific language whose fundamental purpose is to support the creation of page templates and
1176-421: The accelerator which can thus be reconfigured without a restart. A number of run-time parameters control things such as the maximum and the minimum number of worker threads, various timeouts, etc. A command-line management interface allows these parameters to be modified, and new VCL scripts to be compiled, loaded and activated, without restarting the accelerator. In order to reduce the number of system calls in
1218-534: The computer age, but the term "domain-specific language" has become more popular due to the rise of domain-specific modeling . Simpler DSLs, particularly ones used by a single application, are sometimes informally called mini-languages . The line between general-purpose languages and domain-specific languages is not always sharp, as a language may have specialized features for a particular domain but be applicable more broadly, or conversely may in principle be capable of broad application but in practice used primarily for
1260-492: The computer industry include Meta-II , and its descendant TreeMeta . Unreal and Unreal Tournament unveiled a language called UnrealScript . This allowed for rapid development of modifications compared to the competitor Quake (using the Id Tech 2 engine). The Id Tech engine used standard C code meaning C had to be learned and properly applied, while UnrealScript was optimized for ease of use and efficiency. Similarly,
1302-452: The development of more recent games introduced their own specific languages, one more common example is Lua for scripting. Various business rules engines have been developed for automating policy and business rules used in both government and private industry. ILOG , Oracle Policy Automation , DTRules, Drools and others provide support for DSLs aimed to support various problem domains. DTRules goes so far as to define an interface for
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1344-446: The fast path to a minimum, log data is stored in shared memory , and the task of monitoring, filtering, formatting and writing log data to disk is delegated to a separate application. Varnish Cache can speed up information delivery by a factor of several hundred. To ensure proper operation and performance, Varnish exposes metrics that can be monitored in the following areas: Varnish Cache ships with monitoring and logging tools. One of
1386-404: The language allows a particular type of problem or solution to be expressed more clearly than an existing language would allow and the type of problem in question reappears sufficiently often. Pragmatically, a DSL may be specialized to a particular problem domain, a particular problem representation technique, a particular solution technique, or other aspects of a domain. A domain-specific language
1428-580: The language and develop a game, and thanks to GM runtime which handles main game loop and keeps implementation of called functions, few lines of code is required for simplest game, instead of thousands. ColdFusion 's associated scripting language is another example of a domain-specific language for data-driven websites. This scripting language is used to weave together languages and services such as Java, .NET, C++, SMS, email, email servers, http, ftp, exchange, directory services, and file systems for use in websites. The ColdFusion Markup Language (CFML) includes
1470-442: The most used is varnishstat which gives a detailed snapshot of Varnish's current performance. It provides access to in-memory statistics such as cache hits and misses, resource consumption, threads created, and more. Running varnishstat from the command line returns a continuously updating list of all available Varnish metrics. If the -1 flag is added, varnishstat will exit after printing the list one time. Varnishstat can be used as
1512-592: The problem of software piracy Other uses [ edit ] Vinculin , a mammal protein Vickers-Carden-Loyd tankette , a British tankette Voluntary Committee of Lawyers , a former organization to repeal prohibition of alcohol in the US Vampire Cheerleaders , a manga series Chu Lai Airport (IATA code: VCL) Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with
1554-460: The question of whether a language is domain-specific or not. A good example is the functional language XSLT , specifically designed for transforming one XML graph into another, which has been extended since its inception to allow (particularly in its 2.0 version) for various forms of filesystem interaction, string and date manipulation, and data typing. In model-driven engineering , many examples of domain-specific languages may be found like OCL ,
1596-485: The requirements. A newer development is language-oriented programming , an integrated software engineering methodology based mainly on creating, optimizing, and using domain-specific languages. Complementing language-oriented programming , as well as all other forms of domain-specific languages, are the class of compiler writing tools called metacompilers . A metacompiler is not only useful for generating parsers and code generators for domain-specific languages, but
1638-588: The task of deciding what is stored in memory and what gets paged out to disk to the operating system . This helps avoid the situation where the operating system starts caching data while it is moved to disk by the application. Varnish is heavily threaded , with each client connection being handled by a separate worker thread. When the configured limit on the number of active worker threads is reached, incoming connections are placed in an overflow queue; when this queue reaches its configured limit incoming connections will be rejected. The principal configuration mechanism
1680-468: The title VCL . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=VCL&oldid=1063325619 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages Varnish (software)#Architecture The project
1722-632: The use of multiple DSLs within a rule set. The purpose of business rules engines is to define a representation of business logic in as human-readable fashion as possible. This allows both subject-matter experts and developers to work with and understand the same representation of the business logic. Most rules engines provide both an approach to simplifying the control structures for business logic (for example, using declarative rules or decision tables ) coupled with alternatives to programming syntax in favor of DSLs. Statistical modelers have developed domain-specific languages such as R (an implementation of
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1764-538: Was initiated by the online branch of the Norwegian tabloid newspaper Verdens Gang . The architect and lead developer is Danish independent consultant Poul-Henning Kamp (a well-known FreeBSD developer), with management, infrastructure and additional development originally provided by the Norwegian Linux consulting company Linpro . The support, management and development of Varnish was later spun off into
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