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nginx (pronounced 'engine x') is a free open source web serverwritten by Igor Sysoev, a Russian software engineer. Since its publiclaunch in 2004, nginx has focused on high performance, highconcurrency and low memory usage. Additional features on top of theweb server functionality, like load balancing, caching, access andbandwidth control, and the ability to integrate efficiently with avariety of applications, have helped to make nginx a good choice formodern website architectures. Currently nginx is the second mostpopular open source web server on the Internet.

14.1. Why Is High Concurrency Important?

These days the Internet is so widespread and ubiquitous it's hard toimagine it wasn't exactly there, as we know it, a decade ago. It has greatly evolved,from simple HTML producing clickable text, based on NCSA and then onApache web servers, to an always-on communication medium used by morethan 2 billion users worldwide. With the proliferation of permanentlyconnected PCs, mobile devices and recently tablets, the Internetlandscape is rapidly changing and entire economies have becomedigitally wired. Online services have become much more elaborate witha clear bias towards instantly available live information andentertainment. Security aspects of running online business have alsosignificantly changed. Accordingly, websites are now much more complexthan before, and generally require a lot more engineering efforts tobe robust and scalable.

One of the biggest challenges for a website architect has always beenconcurrency. Since the beginning of web services, the level ofconcurrency has been continuously growing. It's not uncommon for apopular website to serve hundreds of thousands and even millions ofsimultaneous users. A decade ago, the major cause of concurrency wasslow clients—users with ADSL or dial-up connections. Nowadays,concurrency is caused by a combination of mobile clients and newerapplication architectures which are typically based on maintaining apersistent connection that allows the client to be updated with news,tweets, friend feeds, and so on. Another important factor contributingto increased concurrency is the changed behavior of modern browsers,which open four to six simultaneous connections to a website toimprove page load speed.

To illustrate the problem with slow clients, imagine a simpleApache-based web server which produces a relatively short 100 KBresponse—a web page with text or an image. It can be merely afraction of a second to generate or retrieve this page, but it takes10 seconds to transmit it to a client with a bandwidth of 80 kbps (10KB/s). Essentially, the web server would relatively quickly pull 100KB of content, and then it would be busy for 10 seconds slowly sendingthis content to the client before freeing its connection. Now imaginethat you have 1,000 simultaneously connected clients who haverequested similar content. If only 1 MB of additional memory isallocated per client, it would result in 1000 MB (about 1 GB) ofextra memory devoted to serving just 1000 clients 100 KB ofcontent. In reality, a typical web server based on Apache commonlyallocates more than 1 MB of additional memory per connection, andregrettably tens of kbps is still often the effective speed of mobilecommunications. Although the situation with sending content to a slowclient might be, to some extent, improved by increasing the size ofoperating system kernel socket buffers, it's not a general solution tothe problem and can have undesirable side effects.

With persistent connections the problem of handling concurrency iseven more pronounced, because to avoid latency associated withestablishing new HTTP connections, clients would stay connected, andfor each connected client there's a certain amount of memory allocatedby the web server.

Consequently, to handle the increased workloads associated withgrowing audiences and hence higher levels of concurrency—and to beable to continuously do so—a website should be based on a number ofvery efficient building blocks. While the other parts of the equationsuch as hardware (CPU, memory, disks), network capacity, applicationand data storage architectures are obviously important, it is in theweb server software that client connections are accepted andprocessed. Thus, the web server should be able to scale nonlinearlywith the growing number of simultaneous connections and requests persecond.

Isn't Apache Suitable?

Apache, the web server software that still largely dominates theInternet today, has its roots in the beginning of the1990s. Originally, its architecture matched the then-existingoperating systems and hardware, but also the state of the Internet, where a website was typically a standalone physical serverrunning a single instance of Apache. By the beginning of the 2000s itwas obvious that the standalone web server model could not be easilyreplicated to satisfy the needs of growing web services. AlthoughApache provided a solid foundation for future development, it wasarchitected to spawn a copy of itself for each new connection, whichwas not suitable for nonlinear scalability of a website. EventuallyApache became a general purpose web server focusing on having manydifferent features, a variety of third-party extensions, and universalapplicability to practically any kind of web applicationdevelopment. However, nothing comes without a price and the downsideto having such a rich and universal combination of tools in a singlepiece of software is less scalability because of increased CPU andmemory usage per connection.

Thus, when server hardware, operating systems and network resourcesceased to be major constraints for website growth, web developersworldwide started to look around for a more efficient means of runningweb servers. Around ten years ago, Daniel Kegel, a prominent softwareengineer, proclaimed that 'it's time for web servers to handle tenthousand clients simultaneously' and predicted what wenow call Internet cloud services. Kegel's C10K manifest spurred anumber of attempts to solve the problem of web server optimization tohandle a large number of clients at the same time, and nginx turnedout to be one of the most successful ones.

Aimed at solving the C10K problem of 10,000 simultaneous connections,nginx was written with a different architecture in mind—one which ismuch more suitable for nonlinear scalability in both the number ofsimultaneous connections and requests per second. nginx isevent-based, so it does not follow Apache's style of spawning newprocesses or threads for each web page request. The end result is thateven as load increases, memory and CPU usage remain manageable. nginxcan now deliver tens of thousands of concurrent connections on aserver with typical hardware.

When the first version of nginx was released, it was meant to bedeployed alongside Apache such that static content like HTML, CSS,JavaScript and images were handled by nginx to offload concurrency andlatency processing from Apache-based application servers. Over thecourse of its development, nginx has added integration withapplications through the use of FastCGI, uswgi or SCGI protocols, andwith distributed memory object caching systems likememcached. Other useful functionality like reverse proxy withload balancing and caching was added as well. These additionalfeatures have shaped nginx into an efficient combination of toolsto build a scalable web infrastructure upon.

In February 2012, the Apache 2.4.x branch was released to the public. Althoughthis latest release of Apache has added new multi-processing core modules andnew proxy modules aimed at enhancing scalability and performance, it's too soonto tell if its performance, concurrency and resource utilization are now on parwith, or better than, pure event-driven web servers. It would be very nice tosee Apache application servers scale better with the new version, though, as itcould potentially alleviate bottlenecks on the backend side which still oftenremain unsolved in typical nginx-plus-Apache web configurations.

Are There More Advantages to Using nginx?

Handling high concurrency with high performance and efficiency hasalways been the key benefit of deploying nginx. However, thereare now even more interesting benefits.

In the last few years, web architects have embraced the idea ofdecoupling and separating their application infrastructure from theweb server. However, what would previously exist in the form of a LAMP(Linux, Apache, MySQL, PHP, Python or Perl)-based website, might nowbecome not merely a LEMP-based one (`E' standing for `Enginex'), but more and more often an exercise in pushing the web server tothe edge of the infrastructure and integrating the same or a revampedset of applications and database tools around it in a different way.

nginx is very well suited for this, as it provides the key featuresnecessary to conveniently offload concurrency, latency processing, SSL(secure sockets layer), static content, compression and caching,connections and requests throttling, and even HTTP media streamingfrom the application layer to a much more efficient edge web serverlayer. It also allows integrating directly with memcached/Redis orother 'NoSQL' solutions, to boost performance when serving a largenumber of concurrent users.

With recent flavors of development kits and programming languagesgaining wide use, more and more companies are changing theirapplication development and deployment habits. nginx has become one ofthe most important components of these changing paradigms, and ithas already helped many companies start and develop their web servicesquickly and within their budgets.

The first lines of nginx were written in 2002. In 2004 it was releasedto the public under the two-clause BSD license. The number of nginxusers has been growing ever since, contributing ideas, and submittingbug reports, suggestions and observations that have been immenselyhelpful and beneficial for the entire community.

The nginx codebase is original and was written entirely from scratchin the C programming language. nginx has been ported to manyarchitectures and operating systems, including Linux, FreeBSD,Solaris, Mac OS X, AIX and Microsoft Windows. nginx has its ownlibraries and with its standard modules does not use much beyond thesystem's C library, except for zlib, PCRE and OpenSSL which can beoptionally excluded from a build if not needed or because of potentiallicense conflicts.

A few words about the Windows version of nginx. While nginx works in aWindows environment, the Windows version of nginx is more like aproof-of-concept rather than a fully functional port. There arecertain limitations of the nginx and Windows kernel architectures thatdo not interact well at this time. The known issues of the nginxversion for Windows include a much lower number of concurrentconnections, decreased performance, no caching and no bandwidthpolicing. Future versions of nginx for Windows will match themainstream functionality more closely.

14.2. Overview of nginx Architecture

Traditional process- or thread-based models of handling concurrentconnections involve handling each connection with a separate processor thread, and blocking on network or input/outputoperations. Depending on the application, it can be very inefficientin terms of memory and CPU consumption. Spawning a separate process orthread requires preparation of a new runtime environment, includingallocation of heap and stack memory, and the creation of a newexecution context. Additional CPU time is also spent creating theseitems, which can eventually lead to poor performance due to threadthrashing on excessive context switching. All of these complicationsmanifest themselves in older web server architectures likeApache's. This is a tradeoff between offering a rich set of generallyapplicable features and optimized usage of server resources.

From the very beginning, nginx was meant to be a specialized tool toachieve more performance, density and economical use of serverresources while enabling dynamic growth of a website, so it hasfollowed a different model. It was actually inspired by the ongoingdevelopment of advanced event-based mechanisms in a variety ofoperating systems. What resulted is a modular, event-driven,asynchronous, single-threaded, non-blocking architecture which becamethe foundation of nginx code.

nginx uses multiplexing and event notifications heavily, and dedicatesspecific tasks to separate processes. Connections are processed in ahighly efficient run-loop in a limited number of single-threadedprocesses called workers. Within each worker nginx canhandle many thousands of concurrent connections and requests persecond.

Code Structure

The nginx worker code includes the core and the functionalmodules. The core of nginx is responsible for maintaining a tightrun-loop and executing appropriate sections of modules' code on eachstage of request processing. Modules constitute most of thepresentation and application layer functionality. Modules read fromand write to the network and storage, transform content, do outboundfiltering, apply server-side include actions and pass the requests tothe upstream servers when proxying is activated.

nginx's modular architecture generally allows developers to extend theset of web server features without modifying the nginx core. nginxmodules come in slightly different incarnations, namely core modules,event modules, phase handlers, protocols, variable handlers, filters,upstreams and load balancers. At this time, nginx doesn't supportdynamically loaded modules; i.e., modules are compiled along with thecore at build stage. However, support for loadable modules and ABI isplanned for the future major releases. More detailed information aboutthe roles of different modules can be found in Section 14.4.

While handling a variety of actions associated with accepting,processing and managing network connections and content retrieval,nginx uses event notification mechanisms and a number of disk I/Operformance enhancements in Linux, Solaris and BSD-based operatingsystems, like kqueue, epoll, and event ports. Thegoal is to provide as many hints to the operating system as possible,in regards to obtaining timely asynchronous feedback for inbound andoutbound traffic, disk operations, reading from or writing to sockets,timeouts and so on. The usage of different methods for multiplexingand advanced I/O operations is heavily optimized for every Unix-basedoperating system nginx runs on.

A high-level overview of nginx architecture is presented inFigure 14.1.

Workers Model

As previously mentioned, nginx doesn't spawn a process or thread forevery connection. Instead, worker processes accept new requestsfrom a shared 'listen' socket and execute a highly efficientrun-loop inside each worker to process thousands of connectionsper worker. There's no specialized arbitration or distribution ofconnections to the workers in nginx; this work is done by the OS kernel mechanisms. Upon startup, an initial set oflistening sockets is created. workers then continuously accept,read from and write to the sockets while processing HTTP requests andresponses.

The run-loop is the most complicated part of the nginx workercode. It includes comprehensive inner calls and relies heavily on theidea of asynchronous task handling. Asynchronous operations areimplemented through modularity, event notifications, extensive use ofcallback functions and fine-tuned timers. Overall, the key principleis to be as non-blocking as possible. The only situation where nginxcan still block is when there's not enough disk storage performancefor a worker process.

Because nginx does not fork a process or thread per connection, memoryusage is very conservative and extremely efficient in the vastmajority of cases. nginx conserves CPU cycles as well because there'sno ongoing create-destroy pattern for processes or threads. What nginxdoes is check the state of the network and storage, initialize newconnections, add them to the run-loop, and process asynchronouslyuntil completion, at which point the connection is deallocated andremoved from the run-loop. Combined with the careful use ofsyscalls and an accurate implementation of supportinginterfaces like pool and slab memory allocators, nginx typicallyachieves moderate-to-low CPU usage even under extreme workloads.

Because nginx spawns several workers to handle connections, itscales well across multiple cores. Generally, a separate workerper core allows full utilization of multicore architectures, andprevents thread thrashing and lock-ups. There's no resource starvationand the resource controlling mechanisms are isolated withinsingle-threaded worker processes. This model also allows morescalability across physical storage devices, facilitates more diskutilization and avoids blocking on disk I/O. As a result, serverresources are utilized more efficiently with the workload sharedacross several workers.

With some disk use and CPU load patterns, the number of nginxworkers should be adjusted. The rules are somewhat basic here,and system administrators should try a couple of configurations fortheir workloads. General recommendations might be the following: ifthe load pattern is CPU intensive—for instance, handling a lot ofTCP/IP, doing SSL, or compression—the number of nginx workersshould match the number of CPU cores; if the load is mostly disk I/Obound—for instance, serving different sets of content from storage,or heavy proxying—the number of workers might be one and a halfto two times the number of cores. Some engineers choose the number ofworkers based on the number of individual storage unitsinstead, though efficiency of this approach depends on the type andconfiguration of disk storage.

One major problem that the developers of nginx will be solving in upcomingversions is how to avoid most of the blocking on disk I/O. At the moment, ifthere's not enough storage performance to serve disk operations generated by aparticular worker, that worker may still block on reading/writingfrom disk. A number of mechanisms and configuration file directives exist tomitigate such disk I/O blocking scenarios. Most notably, combinations ofoptions like sendfile and AIO typically produce a lot of headroom for diskperformance. An nginx installation should be planned based on the data set,the amount of memory available for nginx, and the underlying storage architecture.

Another problem with the existing worker model is related tolimited support for embedded scripting. For one, with the standardnginx distribution, only embedding Perl scripts is supported. There isa simple explanation for that: the key problem is thepossibility of an embedded script to block on anyoperation or exit unexpectedly. Both types of behavior wouldimmediately lead to a situation where the worker is hung, affectingmany thousands of connections at once. More work is planned to makeembedded scripting with nginx simpler, more reliable and suitable fora broader range of applications.

nginx Process Roles

nginx runs several processes in memory; there is a single masterprocess and several worker processes. There are also a coupleof special purpose processes, specifically a cache loader and cachemanager. All processes are single-threaded in version 1.x ofnginx. All processes primarily use shared-memory mechanisms forinter-process communication. The master process is run as theroot user. The cache loader, cache manager and workersrun as an unprivileged user.

The master process is responsible for the following tasks:

• reading and validating configuration
• creating, binding and closing sockets
• starting, terminating and maintaining the configured number of worker processes
• reconfiguring without service interruption
• controlling non-stop binary upgrades (starting new binary androlling back if necessary)
• re-opening log files
• compiling embedded Perl scripts

The worker processes accept, handle and process connectionsfrom clients, provide reverse proxying and filtering functionality anddo almost everything else that nginx is capable of. In regards tomonitoring the behavior of an nginx instance, a system administratorshould keep an eye on workers as they are the processesreflecting the actual day-to-day operations of a web server.

The cache loader process is responsible for checking the on-disk cacheitems and populating nginx's in-memory database with cachemetadata. Essentially, the cache loader prepares nginx instances towork with files already stored on disk in a specially allocateddirectory structure. It traverses the directories, checks cachecontent metadata, updates the relevant entries in shared memory andthen exits when everything is clean and ready for use.

The cache manager is mostly responsible for cache expiration andinvalidation. It stays in memory during normal nginx operation and itis restarted by the master process in the case of failure.

Brief Overview of nginx Caching

Caching in nginx is implemented in the form of hierarchical datastorage on a filesystem. Cache keys are configurable, and differentrequest-specific parameters can be used to control what gets into thecache. Cache keys and cache metadata are stored in the shared memorysegments, which the cache loader, cache manager and workerscan access. Currently there is not any in-memory caching of files,other than optimizations implied by the operating system's virtualfilesystem mechanisms. Each cached response is placed in a differentfile on the filesystem. The hierarchy (levels and naming details) arecontrolled through nginx configuration directives. When a response iswritten to the cache directory structure, the path and the name of thefile are derived from an MD5 hash of the proxy URL.

The process for placing content in the cache is as follows:When nginx reads the response from an upstream server, thecontent is first written to a temporary file outside of the cachedirectory structure. When nginx finishes processing the request itrenames the temporary file and moves it to the cache directory. If thetemporary files directory for proxying is on another file system, thefile will be copied, thus it's recommended to keep both temporary andcache directories on the same file system. It is also quite safe todelete files from the cache directory structure when they need to beexplicitly purged. There are third-party extensions for nginxwhich make it possible to control cached content remotely,and more work is planned to integrate this functionality in the maindistribution.

14.3. nginx Configuration

nginx's configuration system was inspired by Igor Sysoev's experiences withApache. His main insight was that a scalable configuration systemis essential for a web server. The main scaling problem wasencountered when maintaining large complicated configurations withlots of virtual servers, directories, locations and datasets. In arelatively big web setup it can be a nightmare if not done properlyboth at the application level and by the system engineer himself.

As a result, nginx configuration was designed to simplify day-to-dayoperations and to provide an easy means for further expansion of webserver configuration.

nginx configuration is kept in a number of plain text files whichtypically reside in /usr/local/etc/nginx or/etc/nginx. The main configuration file is usually callednginx.conf. To keep it uncluttered, parts of the configurationcan be put in separate files which can be automatically included inthe main one. However, it should be noted here that nginx does notcurrently support Apache-style distributed configurations (i.e.,.htaccess files). All of the configuration relevant to nginxweb server behavior should reside in a centralized set ofconfiguration files.

The configuration files are initially read and verified by the masterprocess. A compiled read-only form of the nginx configuration isavailable to the worker processes as they are forked from themaster process. Configuration structures are automatically shared bythe usual virtual memory management mechanisms.

nginx configuration has several different contexts for main,http, server, upstream, location (and alsomail for mail proxy) blocks of directives. Contexts neveroverlap. For instance, there is no such thing as putting alocation block in the main block of directives. Also, toavoid unnecessary ambiguity there isn't anything like a 'global webserver' configuration. nginx configuration is meant to be clean andlogical, allowing users to maintain complicated configuration filesthat comprise thousands of directives. In a private conversation, Sysoev said, 'Locations, directories, and other blocks in the global serverconfiguration are the features I never liked in Apache, so this is thereason why they were never implemented in nginx.'

Configuration syntax, formatting and definitions follow a so-calledC-style convention. This particular approach to making configurationfiles is already being used by a variety of open source andcommercial software applications. By design, C-style configuration is well-suited for nested descriptions, being logical and easy to create, readand maintain, and liked by many engineers. C-style configuration ofnginx can also be easily automated.

While some of the nginx directives resemble certain parts of Apacheconfiguration, setting up an nginx instance is quite a differentexperience. For instance, rewrite rules are supported by nginx, thoughit would require an administrator to manually adapt a legacy Apacherewrite configuration to match nginx style. The implementation of therewrite engine differs too.

In general, nginx settings also provide support for several originalmechanisms that can be very useful as part of a lean web serverconfiguration. It makes sense to briefly mention variables and thetry_files directive, which are somewhat unique tonginx. Variables in nginx were developed to provide an additional even-more-powerful mechanism to control run-time configuration of a webserver. Variables are optimized for quick evaluation and areinternally pre-compiled to indices. Evaluation is done on demand;i.e., the value of a variable is typically calculated only once andcached for the lifetime of a particular request. Variables can be usedwith different configuration directives, providing additionalflexibility for describing conditional request processingbehavior.

The try_files directive was initially meant togradually replace conditional if configuration statements in amore proper way, and it was designed to quickly and efficientlytry/match against different URI-to-content mappings. Overall, thetry_files directive works well and can be extremely efficientand useful. It is recommended that the reader thoroughly check thetry_files directive and adopt its use whenever applicable.

14.4. nginx Internals

As was mentioned before, the nginx codebase consists of a core and anumber of modules. The core of nginx is responsible for providing thefoundation of the web server, web and mail reverse proxyfunctionalities; it enables the use of underlying network protocols,builds the necessary run-time environment, and ensures seamlessinteraction between different modules. However, most of the protocol-and application-specific features are done by nginx modules, not thecore.

Internally, nginx processes connections through a pipeline, or chain,of modules. In other words, for every operation there's a module whichis doing the relevant work; e.g., compression, modifying content,executing server-side includes, communicating to the upstreamapplication servers through FastCGI or uwsgi protocols, or talking tomemcached.

There are a couple of nginx modules that sit somewhere between thecore and the real 'functional' modules. These modules arehttp and mail. These two modules provide anadditional level of abstraction between the core and lower-levelcomponents. In these modules, the handling of the sequence of eventsassociated with a respective application layer protocol like HTTP,SMTP or IMAP is implemented. In combination with the nginx core, theseupper-level modules are responsible for maintaining the right order ofcalls to the respective functional modules. While the HTTP protocol iscurrently implemented as part of the http module, thereare plans to separate it into a functional module in the future, dueto the need to support other protocols like SPDY (see 'SPDY: Anexperimental protocol for a faster web').

The functional modules can be divided into event modules, phasehandlers, output filters, variable handlers, protocols, upstreams andload balancers. Most of these modules complement the HTTPfunctionality of nginx, though event modules and protocols are alsoused for mail. Event modules provide a particular OS-dependentevent notification mechanism like kqueue or epoll. Theevent module that nginx uses depends on the operating systemcapabilities and build configuration. Protocol modules allow nginx tocommunicate through HTTPS, TLS/SSL, SMTP, POP3 and IMAP.

A typical HTTP request processing cycle looks like the following.

1. Client sends HTTP request.
2. nginx core chooses the appropriate phase handler based on the configured location matching the request.
3. If configured to do so, a load balancer picks an upstream server for proxying.
4. Phase handler does its job and passes each output buffer to the first filter.
5. First filter passes the output to the second filter.
6. Second filter passes the output to third (and so on).
7. Final response is sent to the client.

nginx module invocation is extremely customizable. It is performedthrough a series of callbacks using pointers to the executablefunctions. However, the downside of this is that it may place a bigburden on programmers who would like to write their own modules,because they must define exactly how and when the module shouldrun. Both the nginx API and developers' documentation are beingimproved and made more available to alleviate this.

Some examples of where a module can attach are:

• Before the configuration file is read and processed
• For each configuration directive for the location and the server where it appears
• When the main configuration is initialized
• When the server (i.e., host/port) is initialized
• When the server configuration is merged with the main configuration
• When the location configuration is initialized or merged with its parent server configuration
• When the master process starts or exits
• When a new worker process starts or exits
• When handling a request
• When filtering the response header and the body
• When picking, initiating and re-initiating a request to an upstream server
• When processing the response from an upstream server
• When finishing an interaction with an upstream server

Inside a worker, the sequence of actions leading to therun-loop where the response is generated looks like the following:

1. Begin ngx_worker_process_cycle().
2. Process events with OS specific mechanisms (such as epoll or kqueue).
3. Accept events and dispatch the relevant actions.
4. Process/proxy request header and body.
5. Generate response content (header, body) and stream it to the client.
6. Finalize request.
7. Re-initialize timers and events.

The run-loop itself (steps 5 and 6) ensures incremental generation ofa response and streaming it to the client.

A more detailed view of processing an HTTP request might look likethis:

1. Initialize request processing.
2. Process header.
3. Process body.
4. Call the associated handler.
5. Run through the processing phases.

Which brings us to the phases. When nginx handles an HTTP request, itpasses it through a number of processing phases. At each phase thereare handlers to call. In general, phase handlers process a request andproduce the relevant output. Phase handlers are attached to thelocations defined in the configuration file.

Phase handlers typically do four things: get the locationconfiguration, generate an appropriate response, send the header, andsend the body. A handler has one argument: a specific structuredescribing the request. A request structure has a lot of usefulinformation about the client request, such as the request method, URI,and header.

When the HTTP request header is read, nginx does a lookup of theassociated virtual server configuration. If the virtual server isfound, the request goes through six phases:

1. server rewrite phase
2. location phase
3. location rewrite phase (which can bring the request back to theprevious phase)
4. access control phase
5. try_files phase
6. log phase

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In an attempt to generate the necessary content in response to therequest, nginx passes the request to a suitable contenthandler. Depending on the exact location configuration, nginx may tryso-called unconditional handlers first, like perl,proxy_pass, flv, mp4, etc. If the request doesnot match any of the above content handlers, it is picked by one ofthe following handlers, in this exact order: random index,index, autoindex, gzip_static, static.

Indexing module details can be found in the nginx documentation, butthese are the modules which handle requests with a trailing slash. Ifa specialized module like mp4 or autoindex isn'tappropriate, the content is considered to be just a file or directoryon disk (that is, static) and is served by the static contenthandler. For a directory it would automatically rewrite the URI sothat the trailing slash is always there (and then issue an HTTPredirect).

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The content handlers' content is then passed to the filters. Filtersare also attached to locations, and there can be several filtersconfigured for a location. Filters do the task of manipulating theoutput produced by a handler. The order of filter execution isdetermined at compile time. For the out-of-the-box filters it'spredefined, and for a third-party filter it can be configured at thebuild stage. In the existing nginx implementation, filters can only dooutbound changes and there is currently no mechanism to write andattach filters to do input content transformation. Input filteringwill appear in future versions of nginx.

Filters follow a particular design pattern. A filter gets called,starts working, and calls the next filter until the final filter inthe chain is called. After that, nginx finalizes the response. Filtersdon't have to wait for the previous filter to finish. The next filterin a chain can start its own work as soon as the input from theprevious one is available (functionally much like the Unixpipeline). In turn, the output response being generated can be passedto the client before the entire response from the upstream server isreceived.

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There are header filters and body filters; nginx feeds the header andthe body of the response to the associated filters separately.

A header filter consists of three basic steps:

1. Decide whether to operate on this response.
2. Operate on the response.
3. Call the next filter.

Body filters transform the generated content. Examples of body filtersinclude:

• server-side includes
• XSLT filtering
• image filtering (for instance, resizing images on the fly)
• charset modification
• gzip compression
• chunked encoding

After the filter chain, the response is passed to the writer. Alongwith the writer there are a couple of additional special purposefilters, namely the copy filter, and the postponefilter. The copy filter is responsible for filling memorybuffers with the relevant response content which might be stored in aproxy temporary directory. The postpone filter is used forsubrequests.

Subrequests are a very important mechanism for request/responseprocessing. Subrequests are also one of the most powerful aspects ofnginx. With subrequests nginx can return the results from a differentURL than the one the client originally requested. Some web frameworkscall this an internal redirect. However, nginx goes further—not onlycan filters perform multiple subrequests and combine the outputs intoa single response, but subrequests can also be nested andhierarchical. A subrequest can perform its own sub-subrequest, and asub-subrequest can initiate sub-sub-subrequests. Subrequests can mapto files on the hard disk, other handlers, or upstreamservers. Subrequests are most useful for inserting additional contentbased on data from the original response. For example, the SSI(server-side include) module uses a filter to parse the contents ofthe returned document, and then replaces include directiveswith the contents of specified URLs. Or, it can be an example of makinga filter that treats the entire contents of a document as a URL to beretrieved, and then appends the new document to the URL itself.

Upstream and load balancers are also worth describingbriefly. Upstreams are used to implement what can be identified as acontent handler which is a reverse proxy (proxy_passhandler).Upstream modules mostly prepare the request to be sent to an upstreamserver (or 'backend') and receive the response from the upstreamserver. There are no calls to output filters here. What an upstreammodule does exactly is set callbacks to be invoked when the upstreamserver is ready to be written to and read from. Callbacks implementingthe following functionality exist:

• Crafting a request buffer (or a chain of them) to be sent to the upstream server
• Re-initializing/resetting the connection to the upstream server (which happens right before creating the request again)
• Processing the first bits of an upstream response and saving pointers to the payload received from the upstream server
• Aborting requests (which happens when the client terminates prematurely)
• Finalizing the request when nginx finishes reading from the upstream server
• Trimming the response body (e.g. removing a trailer)

Load balancer modules attach to the proxy_pass handler toprovide the ability to choose an upstream server when more than oneupstream server is eligible. A load balancer registers an enablingconfiguration file directive, provides additional upstreaminitialization functions (to resolve upstream names in DNS, etc.),initializes the connection structures, decides where to route therequests, and updates statsinformation. Currently nginx supports two standard disciplines forload balancing to upstream servers: round-robin and ip-hash.

Upstream and load balancing handling mechanisms include algorithms todetect failed upstream servers and to re-route new requests to theremaining ones—though a lot of additional work is planned to enhancethis functionality. In general, more work on load balancers isplanned, and in the next versions of nginx the mechanisms fordistributing the load across different upstream servers as well ashealth checks will be greatly improved.

There are also a couple of other interesting modules which provide anadditional set of variables for use in the configuration file. Whilethe variables in nginx are created and updated across differentmodules, there are two modules that are entirely dedicated tovariables: geo and map. The geo module is used tofacilitate tracking of clients based on their IP addresses. Thismodule can create arbitrary variables that depend on the client's IPaddress. The other module, map, allows for the creation ofvariables from other variables, essentially providing the ability todo flexible mappings of hostnames and other run-time variables. Thiskind of module may be called the variable handler.

Memory allocation mechanisms implemented inside a single nginxworker were, to some extent, inspired by Apache. A high-leveldescription of nginx memory management would be the following: Foreach connection, the necessary memory buffers are dynamicallyallocated, linked, used for storing and manipulating the header andbody of the request and the response, and then freed upon connectionrelease. It is very important to note that nginx tries to avoidcopying data in memory as much as possible and most of the data ispassed along by pointer values, not by calling memcpy.

Going a bit deeper, when the response is generated by a module, theretrieved content is put in a memory buffer which is then added to abuffer chain link. Subsequent processing works with this buffer chainlink as well. Buffer chains are quite complicated in nginx becausethere are several processing scenarios which differ depending on the moduletype. For instance, it can be quite tricky to manage the buffersprecisely while implementing a body filter module. Such a module canonly operate on one buffer (chain link) at a time and it must decidewhether to overwrite the input buffer, replace the buffer with a newlyallocated buffer, or insert a new buffer before or after the buffer inquestion. To complicate things, sometimes a module will receiveseveral buffers so that it has an incomplete buffer chain that it mustoperate on. However, at this time nginx provides only a low-level APIfor manipulating buffer chains, so before doing any actualimplementation a third-party module developer should become reallyfluent with this arcane part of nginx.

A note on the above approach is that there are memory buffers allocated for theentire life of a connection, thus for long-lived connections some extra memoryis kept. At the same time, on an idle keepalive connection, nginx spendsjust 550 bytes of memory. A possible optimization for future releases of nginxwould be to reuse and share memory buffers for long-lived connections.

The task of managing memory allocation is done by the nginx poolallocator. Shared memory areas are used to accept mutex, cachemetadata, the SSL session cache and the information associated withbandwidth policing and management (limits). There is a slab allocatorimplemented in nginx to manage shared memory allocation. To allowsimultaneous safe use of shared memory, a number of locking mechanismsare available (mutexes and semaphores). In order to organize complexdata structures, nginx also provides a red-black treeimplementation. Red-black trees are used to keep cache metadata inshared memory, track non-regex location definitions and for a couple ofother tasks.

Unfortunately, all of the above was never described in a consistent andsimple manner, making the job of developing third-party extensions fornginx quite complicated. Although some good documents on nginxinternals exist—for instance, those produced by Evan Miller—suchdocuments required a huge reverse engineering effort, and theimplementation of nginx modules is still a black art for many.

Despite certain difficulties associated with third-party moduledevelopment, the nginx user community recently saw a lot of usefulthird-party modules. There is, for instance, an embedded Luainterpreter module for nginx, additional modules for load balancing,full WebDAV support, advanced cache control and other interestingthird-party work that the authors of this chapter encourage and willsupport in the future.

14.5. Lessons Learned

When Igor Sysoev started to write nginx, most of the software enablingthe Internet already existed, and the architecture of such softwaretypically followed definitions of legacy server and network hardware,operating systems, and old Internet architecture in general. However,this didn't prevent Igor from thinking he might be able to improvethings in the web servers area. So, while the first lesson might seemobvious, it is this: there is always room for improvement.

With the idea of better web software in mind, Igor spenta lot of time developing the initial code structure and studyingdifferent ways of optimizing the code for a variety of operatingsystems. Ten years later he is developing a prototype of nginx version 2.0,taking into account the years of active development on version 1. It is clear that the initialprototype of a new architecture, and the initial code structure, are vitallyimportant for the future of a software product.

Another point worth mentioning is that development should befocused. The Windows version of nginx is probably a good example ofhow it is worth avoiding the dilution of development efforts onsomething that is neither the developer's core competence or thetarget application. It is equally applicable to the rewrite enginethat appeared during several attempts to enhance nginx with morefeatures for backward compatibility with the existing legacy setups.

Last but not least, it is worth mentioning that despite the fact thatthe nginx developer community is not very large, third-party modulesand extensions for nginx have always been a very important part of itspopularity. The work done by Evan Miller, Piotr Sikora, ValeryKholodkov, Zhang Yichun (agentzh) and other talented softwareengineers has been much appreciated by the nginx user community andits original developers.

LAMP the ubiquitous acronym for Linux, Apache, MySQL, and PHP, Python, or Perl has a couple cousins. They are OPAL (Oracle’s stack on Linux, and MAMP (Mac OS X, Apache, MySQL, and PHP et cetera). Perhaps another acronym on the horizon is: OPAM (Oracle, PHP, Apache, and Mac OS X). OPAM is a guess on my part. Nobody knows what Oracle’s marketing department may choose. Regardless of the acronym for it, Oracle has published instructions for configuring an Oracle/PHP stack on Mac OS X.

Master Of The Lamps Mac Os Catalina

I generally configure the OPAL stack with Zend Core for Oracle and the Oracle database on a virtual machine running Windows XP, Windows Vista, Ubuntu, or Red Hat Linux. If you follow my posts I prefer VMWare Fusion over Parallels. The MAMP stack I use is open source and provided by Living E. It follows the pattern of Mac OS X installations, which differs from the recent posting from Oracle. It’s easy to install, as you tell from the documentation. MAMP installs PHP 5.2.6 as the current release.

It’s a great choice when you incorporate the open source Sequel Pro tool. Isn’t it ashame that Sequel Pro doesn’t work natively with Oracle. If I find some time this summer, it might make a great project to extend it to Oracle. The interface to Sequel Pro looks like this:

When you create a connection, you should know the typical values. The database value can be left blank when connecting as the superuser root:

Here’s the connection dialog where you’ll enter the values:

Have fun playing with the MAMP stack.