Customer-Server Model

Internet, Overview

Raymond Greenlaw , Ellen M. Hepp , in Encyclopedia of Information Systems, 2003

III.E. Customer/Server Model

The client/server model provides many of the network benefits described in this department. A client makes a request to the server and the server responds by satisfying the client'southward asking. In the client/server model new clients and servers can be added incrementally as more users come up on-line and the need for services increases. That is to say, the client/server model is easily extensible and therefore scales well. Many clients tin share the resource provided past a single server. This eliminates the need for each client to have their own "copy" of the resource. Each Net service has its ain associated set of clients and servers. For example, in the Web domain browsers are clients and Spider web servers are the servers.

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Principles of Parallel and Distributed Calculating

Rajkumar Buyya , ... S. Thamarai Selvi , in Mastering Cloud Computing, 2013

2.4.iii.iii System architectural styles

System architectural styles comprehend the concrete organization of components and processes over a distributed infrastructure. They provide a set of reference models for the deployment of such systems and help engineers non simply have a common vocabulary in describing the physical layout of systems merely too quickly place the major advantages and drawbacks of a given deployment and whether it is applicable for a specific grade of applications. In this department, we introduce two fundamental reference styles: client/server and peer-to-peer.

Customer/server

This architecture is very popular in distributed computing and is suitable for a wide diverseness of applications. As depicted in Effigy ii.12, the client/server model features two major components: a server and a client. These two components interact with each other through a network connection using a given protocol. The communication is unidirectional: The client issues a request to the server, and afterwards processing the request the server returns a response. There could be multiple client components issuing requests to a server that is passively waiting for them. Hence, the important operations in the client-server paradigm are request, have (client side), and heed and response (server side).

Figure 2.12. Client/server architectural styles.

The client/server model is suitable in many-to-ane scenarios, where the data and the services of interest can be centralized and accessed through a single access signal: the server. In general, multiple clients are interested in such services and the server must be accordingly designed to efficiently serve requests coming from different clients. This consideration has implications on both client blueprint and server blueprint. For the client design, we identify two major models:

•

Sparse-client model. In this model, the load of information processing and transformation is put on the server side, and the client has a light implementation that is mostly concerned with retrieving and returning the data it is beingness asked for, with no considerable further processing.

•

Fat-client model. In this model, the client component is also responsible for processing and transforming the information before returning information technology to the user, whereas the server features a relatively light implementation that is generally concerned with the management of access to the data.

The 3 major components in the client-server model: presentation, application logic, and data storage. In the thin-customer model, the customer embodies only the presentation component, while the server absorbs the other ii. In the fatty-customer model, the client encapsulates presentation and near of the application logic, and the server is principally responsible for the data storage and maintenance.

Presentation, awarding logic, and data maintenance tin can be seen as conceptual layers, which are more appropriately chosen tiers. The mapping betwixt the conceptual layers and their physical implementation in modules and components allows differentiating among several types of architectures, which go under the proper name of multitiered architectures. 2 major classes exist:

•

Two-tier architecture. This architecture partitions the systems into two tiers, which are located 1 in the client component and the other on the server. The client is responsible for the presentation tier past providing a user interface; the server concentrates the application logic and the information store into a unmarried tier. The server component is generally deployed on a powerful machine that is capable of processing user requests, accessing data, and executing the application logic to provide a client with a response. This architecture is suitable for systems of limited size and suffers from scalability issues. In particular, equally the number of users increases the operation of the server might dramatically decrease. Another limitation is caused by the dimension of the data to maintain, manage, and access, which might be prohibitive for a single computation node or as well large for serving the clients with satisfactory performance.

•

3-tier architecture/N-tier architecture. The three-tier architecture separates the presentation of data, the awarding logic, and the data storage into 3 tiers. This architecture is generalized into an North-tier model in instance it is necessary to further divide the stages composing the awarding logic and storage tiers. This model is generally more scalable than the two-tier one because it is possible to distribute the tiers into several computing nodes, thus isolating the performance bottlenecks. At the same fourth dimension, these systems are besides more than circuitous to understand and manage. A classic instance of three-tier architecture is constituted by a medium-size Spider web awarding that relies on a relational database management arrangement for storing its data. In this scenario, the client component is represented by a Spider web browser that embodies the presentation tier, whereas the application server encapsulates the business logic tier, and a database server automobile (possibly replicated for high availability) maintains the data storage. Application servers that rely on third-party (or external) services to satisfy client requests are examples of N-tiered architectures.

The client/server architecture has been the dominant reference model for designing and deploying distributed systems, and several applications to this model can exist found. The most relevant is perhaps the Web in its original formulation. Nowadays, the client/server model is an important building block of more complex systems, which implement some of their features by identifying a server and a client process interacting through the network. This model is generally suitable in the case of a many-to-one scenario, where the interaction is unidirectional and started by the clients and suffers from scalability issues, and therefore it is not appropriate in very large systems.

Peer-to-peer

The peer-to-peer model, depicted in Figure 2.13, introduces a symmetric architecture in which all the components, called peers, play the same office and comprise both client and server capabilities of the client/server model. More precisely, each peer acts as a server when it processes requests from other peers and as a client when it issues requests to other peers. With respect to the client/server model that partitions the responsibilities of the IPC between server and clients, the peer-to-peer model attributes the aforementioned responsibilities to each component. Therefore, this model is quite suitable for highly decentralized architecture, which can scale better along the dimension of the number of peers. The disadvantage of this approach is that the direction of the implementation of algorithms is more complex than in the client/server model.

Effigy ii.13. Peer-to-peer architectural manner.

The nigh relevant instance of peer-to-peer systems [87] is constituted by file-sharing applications such every bit Gnutella, BitTorrent, and Kazaa. Despite the differences among these networks in analogous nodes and sharing data on the files and their locations, all of them provide a user client that is at the same time a server providing files to other peers and a client downloading files from other peers. To address an incredibly large number of peers, dissimilar architectures take been designed that divert slightly from the peer-to-peer model. For case, in Kazaa not all the peers have the same role, and some of them are used to group the accessibility information of a group of peers. Another interesting example of peer-to-peer architecture is represented by the Skype network.

The system architectural styles presented in this section establish a reference model that is further enhanced or diversified co-ordinate to the specific needs of the application to exist designed and implemented. For example, the customer/server architecture, which originally included only two types of components, has been further extended and enriched past developing multitier architectures as the complexity of systems increased. Currently, this model is still the predominant reference architecture for distributed systems and applications. The server and client brainchild can be used in some cases to model the macro calibration or the micro scale of the systems. For peer-to-peer systems, pure implementations are very hard to find and, every bit discussed for the case of Kazaa, evolutions of the model, which introduced some kind of hierarchy among the nodes, are common.

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The Internet and the Spider web

Stuart Ferguson , Rodney Hebels , in Computers for Librarians (3rd Edition), 2003

Client/server model

The term client/server model may sound a bit daunting for those unfamiliar with calculator concepts, but the concept itself is straightforward. For a user to access the Internet resource outlined earlier, in that location generally needs to exist cooperation betwixt two estimator systems: the client and the server. The client is the user's local organization, which will typically comprise a personal computer, calculator peripherals (for case, a computer screen on which to display the data exchanged) and software. These components allow the user to communicate with the customer and the client to communicate with the server. Note that if this book mentions 'clients' it is referring to such systems, and not to users (for which 'clients' is sometimes synonymous in librarianship literature). The server, which is a 'remote' organisation (information technology may be on another continent or in the side by side building), stores the information that the user wishes to access, retrieves data requested past the user and transmits them to the client. (The server is sometimes referred to every bit a host, especially in the example of the commercial servers discussed in the side by side affiliate.)

Figure i.3. The client/server model

The functioning of the client/server model, therefore, involves the utilise of two sets of computer software: client software, such every bit the software that helps the user communicate with the computer system (see, for example, the web browser discussed later in this chapter), and server software, such as the retrieval software (consider, for example, search engines, likewise discussed subsequently). The server is generally a more powerful computer system than the customer and would be expected to deal with more than one client simultaneously. One of the chief advantages of the customer/server model is the fact that, provided the clients and servers tin understand the information passing between them, it doesn't affair what kind of figurer system is used for the client. An IBM-compatible server (a computer designed co-ordinate to the same standards as those of the IBM visitor) may be accessed, for instance, by clients that are non IBM-compatible, such as Apple Macintosh computers. This depends on networking standards, one of the main topics of Chapter vii.

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VoIP Security

Xinyuan Wang , Ruishan Zhang , in Advances in Computers, 2011

2.2.1 SIP Messages

Based on client-server model, SIP uses request and response messages to constitute sessions between ii or more endpoints. To establish, manage or tear downwardly a VoIP session, UAC volition send to SIP server or UAS a SIP request message identified by one of the 13 SIP method names. The most commonly used SIP methods are the post-obit:

•

REGISTER is used by the UAC to inform its electric current location to the SIP server.

•

INVITE is used by the UAC to initiate a call session.

•

ACK acknowledges the successful receipt of some SIP messages.

•

BYE terminates an established session.

•

CANCEL quits from the ongoing setup of a session.

•

OPTIONS queries the capability of a server.

Upon receiving SIP request message, the SIP server or UAS will, when appropriate, respond with one or more SIP response letters identified by the following status codes:

•

1xx Provisional indicates that the asking has been successfully received, and it is in the process of processing the asking. For example, 100 Trying, 180 Ringing.

•

2xx Success indicates the successful completion of the activity requested. For example, 200 Ok.

•

3xx Redirection indicates that the request needs to be redirected to someone else. For example, 301 Moved Permanently, 302 Moved Temporarily.

•

4xx Client Fault indicates that the asking is invalid. For example, 404 Not Constitute, 410 Gone, 403 Forbidden.

•

5xx Server Fault indicates that the server cannot fulfill certain valid request. For instance, 503 Service Unavailable.

•

6xx Global Failure indicates that the request cannot be fulfilled at any server. For example, 600 Busy Everywhere.

Figures 2 and 3 show the INVITE and 200 OK messages of a call from a PSTN phone (703-30-9398) to our AT&T VoIP phone (703-xxx-0461). The sample SIP messages include important fields such as To, From, CSeq, Call-ID, Max-Forward, Via, Request-URI, and Contact as well as office of the message torso. Specifically, the message trunk part of the INVITE and 200 OK messages includes the IP addresses and UDP port numbers that the caller and callee choose to use for the upcoming existent-time transport protocol (RTP) voice stream. Effigy 2 shows that the caller is expecting to receive RTP voice stream from the callee at IP address 12.194.224.134 on port 22148. Figure three shows that the callee is expecting to receive RTP voice stream from the caller at IP accost 192.168.one.188 on port 10000.

Fig. two. An example of SIP INVITE message.

Fig. iii. An example of SIP 200 OK message.

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Asynchronous Transfer Mode: An Emerging Network Standard for High-Speed Communications

Ronald J. Vetter , in Advances in Computers, 1997

3.ane.3 LANE Components

LANE follows a customer/server model, in which multiple LANE clients (LECs) connect to LANE servers. LANE defines three dissimilar types of servers: LANE Server (LES), Broadcast and Unknown Server (Omnibus), and the LANE Configuration Server (LECS) [2,6] (see Fig. 15). The LES is responsible for resolving MAC addresses to ATM addresses, the BUS is responsible for flooding multicast and unicast packets with unknown destination ATM addresses among LECs on an emulated LAN, and the LECS provides functions necessary for configuring LECs with the addresses of the LES used by the fastened emulated LAN.

Fig. xv. LANE components and protocol interfaces.

Every Ethernet or token ring LAN station has an associated LEC that is used to handle ATM transfers. When a station on a traditional LAN segment wants to send data to another station on some other segment, over an ATM backbone network, it uses its LEC to transfer/receive data over the ATM part of the path. That is, suppose a sending station wants to send a LAN frame to another station located on the other side of an ATM network. The LEC sends the LES a MAC ATM address resolution query containing the target station's MAC address. The LES responds with the ATM address of the LEC that is associated with the target station. The originating LEC so sets up an ATM switched virtual excursion, converts the MAC frames to ATM cells, and transmits the cells over the network. At the receiving LEC, the ATM cells are converted to MAC frames and forwarded to the receiving station.

Although Fig. fifteen depicts the various LANE components as separate entities from the ATM network, in do the LES, Coach, and LECS are typically software components located in the ATM switch, rather than in stand up-alone host computers. The LEC component is required to be in every ATM host and device attached to the ATM network.

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Application Layer Protocols

Edward Insam PhD, BSc , in TCP/IP Embedded Internet Applications, 2003

SMTP

The simple mail transfer protocol (SMTP) is divers in RFC 821, and is used for uncomplicated electronic mail transmission; SMTP can simply be used to transmit mail. A different protocol, postal service role protocol iii (POP3) is required for more than advanced operations such every bit receiving, deleting, enumerating and managing mail items. Most ISPs provide both SMTP and POP3 hosting services on their dial-up lines.

In order to use e-mail with an ISP, a customer must take an email business relationship with the Internet access provider arranged. During a dial-upward session, the PPP protocol is used to authenticate the client (by sending its password and user number), and for the server to allocate an IP number to the customer, which is valid for the elapsing of the session. Once the IP connection to the Internet access provider is established, the customer opens a TCP connection on port 25, the nominal port allocated to SMTP. In one case the TCP link on this port is established, all subsequent ii-way communication on the line is via standard NVT-ASCII characters.

Functioning follows the standard client–server model, where the customer originates a response from the server. Requests are fabricated in the course of SMTP Commands. All commands follow a uncomplicated character format structure known every bit the 822-header format (afterward the RFC standard that defines information technology, RFC 822). This format uses standard NVT-ASCII characters delimited by normal carriage return – line feed character pair sequences <CR-LF>. This 'standard' text header format is also used in various other Internet application layer protocols. An SMTP transaction is straightforward, and hands implemented on a pocket-sized micro. The full general sequence of operations is as follows:

i.

The caller establishes a TCP connection on port 25 with the host (the caller itself may utilise for response whatsoever port information technology wishes). The host (Internet service provider) may reply with a welcome text message every bit follows (note the first three digit number or response code):

220 Sat, 28 Jan 2002 MYISP.COM Postal service server. Welcome

two.

The client logs onto the SMTP server by sending the following text string:

HELO myname <CR><LF>

Notation the unmarried '50' in 'HELO' This is followed by your registered e-mail 'id', followed by a single carriage return – line feed pair. Some hosts will require simply the id, some others will want your full east-mail address, and some others may but take the HELO string without a senders address at all, that is:

HELO <CR><LF>

iii. The client now needs to await for a answer. The reply from the server will also exist in ASCII course and typically will comprise the following message:

250 mail service – welcome [electronic mail protected]

The three-digit number is a unique reply code. The text post-obit it contains the corresponding plain text explanation for human consumption. Client software but needs to bank check the iii digits confronting a list (total details in the RFC). Code 250 for example is a general OK response. This indicates that the server has accepted the command and that the client can proceed to the adjacent phase. Uncomplicated client cease software could but answer to 250 codes and ignore the residuum, aborting the session otherwise. In any case, the client must wait for a reply earlier proceeding to the side by side phase. Not waiting for a response from the server may cause it to ignore the balance of the commands.

4.

On receipt of the correct fault lawmaking, the client can now send the email message envelope (which contains the sender and recipient addresses). The information in this envelope is required by the server in social club to route the bulletin to the final recipient. A typical envelope transaction is as follows:

Annotation that the server acknowledges every line sent individually (returned lines are shown here indented for clarity). The client must look for every reply before proceeding to the adjacent command. Multiple RCPT TO headers take been used here to show how the same e-mail can be sent to more than one recipient in 1 session. In a existent message, at to the lowest degree one recipient address is required. Annotation how the server checks every address entry as it arrives, and notifies the sender whether it can or cannot recognise the destination. Many other envelope commands are available, delight refer to the RFC for more information. The last line of the envelope contains the command Information on its own. This tells the server that whatsoever text post-obit this command volition exist the trunk of the text message.

5.

The client tin can now ship the text of the message. Information technology is common exercise to add a message header at the commencement of the message body. Message headers are used by receivers and browsers to classify message received and for displaying to remote users. They are non strictly necessary, and they exercise not need to exist nowadays for a message to achieve its destination.

The FROM command is followed by the common proper noun and the e-mail address of the sender. They could be annihilation you like; they are simply used for display purposes as the host will simply pass them on without processing. The second header command TO, takes the mutual name and the electronic mail accost of the recipient. Over again, this information is used by the recipient's browser or eastward-post director, and not processed by the mail arrangement. The SUBJECT line can be used to carry optional header information, which may be of use to a recipient browsing through a long list of incoming mail letters. Information on headers can exist very confusing and misleading as every computer in the chain may append its own piece of text to the header. This is washed then that a recipient can track the sequence of events back to the sender. Unfortunately, misleading information can be added to headers by spammers to misfile the recipient as to who the original sender was. The torso text follows the headers next. Information technology is common practice to send text in manifestly 7-bit ASCII. It is also possible to encode sections of text in dissimilar character formats by using MIME type encoding. This allows the inclusion of characters in non-standard character sets, special punctuation marks and special symbols. Computer or graphics files can also be sent as attachments, where text or binary files are converted using pinch and other encoding algorithms into character sets uniform with NVI-ASCII, without including characters that may upset transmission. RFC 1522 specifies 1 way of sending non-ASCII characters in RFC 822 message headers. The main use of this is to let boosted characters in the sender and receiver names, and in the subject lines. With this method, encoded characters are sent as multiple-graphic symbol encoded sequences, for example, the sequence =?ISO-8859-1?Q?Andr=E9? = results in the text André (annotation the addition of an accent in the terminal letter of the alphabet). The text subsequently the first question mark ISO-8859-1 specifies 1 of many grapheme sets to exist used (valid values are 'us-ascii' and 'iso-8859-Ten' where X is a unmarried digit, as in ISO-8859-1), Q denotes 'quoted printable' that is, characters are sent as the combination of the grapheme '=' followed by two hexadecimal digits corresponding to the wanted character position in the relevant table. For example, the character é (accented e), corresponding to the 8-bit grapheme 0xE9, is sent as the sequence '=E9' Spaces can be sent every bit an underscore or the three character sequence '=20'. The British pound currency sign is sent as '=A3' An culling to Q is B, which ways base of operations-64 encoding. In base-64 encoding, three consecutive bytes of original text (24 bits) are encoded every bit four 6-scrap values. The 6-flake values are mapped to a standard ASCII table of numbers and upper/lower case letters. In this fashion, text and data composed of eight-bit characters tin can be sent as a limited character set.

Multipurpose Net mail extensions (MIME) encoding is covered in RFC 1521. This applies mainly to the trunk of the text (as opposed to the headers). The purpose of MIME is to add extensions, in the form of control lines, to the body of a mail message to add some form of structure to it. In practice, MIME only adds some new RFC-822 fashion headers to the text, telling the recipient the structure of the body to follow. RFC-1522 encoding is normally applied to ensure the torso is transmitted using plain ASCII (NVT ASCII) characters in instance some of the original cloth contains 8-fleck data (i.east. binary files or images). A typical set of MIME headers are:

More information on character and graphic encoding can be found in the respective RFCs. The last item of body text must have a full stop (menstruum) character in a line of its ain. That is, the termination sequence <CR><LF><.><CR><LF> notifies the server that the message text is completed and message transmission is over.

6.

The client now waits for the host to send an acknowledge message. The reply will usually contain a unique message number

250 submitted and queued (msg.12345678)

seven.

The client can now shut the email session, close the TCP connectedness, or attempt to ship another e-mail message if needed. The client does this by sending the QUIT command on a line of its own:

Tables eight-3 and 8-4 give a list of SMTP commands and answer codes, respectively.

Tabular array 8-3. List of SMTP commands

Command Argument Description
HELO Sender'due south host proper noun Used for logging on
Postal service Sender of message Used to place the sender and therefore define the revere path back to the sender (also Post FROM)
RCPT Intended recipient The forward path to one or more final recipients
DATA Body of message Message is terminated with a full terminate on a single line of text
SEND Intended recipient Used to initiate a mail message that should be delivered direct to a user if they are currently logged on to the system
RSET The current mail transaction should be aborted, and that any stored messages should exist deleted
VRFY Recipient to be verified Used to confirm the identity of the user passed as an argument, this can be used to bank check if the user actually exists in the hosts listing.
NOOP Used to solicit an OK response from the remote
QUIT Specifies that the host should send an OK response, and so close the communications channel.
Turn Command the host to return an OK reply then contrary the roles of the machines in social club to ship mail the other mode

Table 8-4. SMTP reply codes (please refer to RFC 821 for consummate definitions)

Code Clarification
211 Organization status or system help respond
214 Help message
220 Service ready
221 Service closing manual aqueduct
250 OK, service activeness accepted
251 User is non local, will forward to given path
354 Starting time mail text input
421 Service not available – closing communications channel
450 Requested activity not taken, mailbox unavailable
451 Requested activeness not taken, local error in processing
452 Requested action non taken, insufficient storage
500 Syntax error – command unrecognized
501 Syntax fault in parameters or arguments
502 Command non implemented
503 Bad sequence of commands
504 Command parameter non implemented

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Interpreting Results

Russ Rogers , in Nessus Network Auditing (2nd Edition), 2008

The Nessus UI Basics

☑

Nessus works on a client–server model.

☑

There are currently ii open-source GUI clients available—Nessus Client for Windows and the Nessus Client for X. They are nearly exact.

☑

At that place is also a UNIX command-line client that is not discussed in this chapter.

☑

Both GUI clients discussed allow one to view scanner output, relieve and export reports in various formats, and import results from previously saved reports.

☑

Regardless of the client used, the Nessus scanner reports are detailed and complex and need to be carefully analyzed to exist fully understood.

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Introducing Nessus

Russ Rogers , in Nessus Network Auditing (Second Edition), 2008

Bones Components

☑

The Nessus Project adopted a customer/server model for its foundation. This client/ server model allows the security analyst to "detach" from the vulnerability scan and utilise his resources for other items while Nessus continues to do what it does best.

☑

The Nessus client can connect to the nessusd server in many means that employ both encryption and authentication.

☑

The Nessus Assail Scripting Language (NASL) allows security analysts to quickly create their own plugins for vulnerability checks.

☑

The Cognition Base allows Nessus to share data gleaned from one plugin with the processes of another plugin. In this manner, each plugin builds upon previously executed plugins.

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Introduction

John F. Buford , ... Eng Keong Lua , in P2P Networking and Applications, 2009

Bulwark to Entry

Compare the costs of the customer/server model with the P2P model for launching a new network application. For a number of meaning applications, the relative software complexity is comparable. For large user populations, client/server requires a server subcontract, that is, a set of server-course hosts maintained in a data center with sufficient network chapters to behave the aggregated traffic from all the clients to the servers. For error tolerance, the server farm is typically replicated at multiple locations. The servers are managed to provide 24 × 7 service. The servers are besides monitored for faults, security, and utilization versus capacity.

In the P2P model there are no server farms. The client machines or end systems contribute resources. Systems are not managed. Service quality may be depression. In real P2P deployments, servers can be used to download the initial client software, back up the bootstrapping of the P2P overlay, and provide user business relationship registration. These servers are typically minor in terms of costs compared to the server farms.

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The Information Integration System K2

Val Tannen , ... Scott Harker , in Bioinformatics, 2003

eight.7 USER INTERFACES

K2 has been developed using a client–server model. The K2 server listens for connections either through a socket or through Java RMI. Information technology is easy to develop a client that tin can connect to K2 through one of these paths, effect queries, and receive results. Three basic clients come with K2: a text-based client, an RMI client, and one that runs as a servlet.

The interactive, text-based customer connects to a K2 server through a socket connexion. It accepts a query in OQL through a command-line-fashion interface, sends it to the server, gets the result dorsum as formatted text, and displays it; and then it waits for the next query to be entered. This simple client generally is used to test the socket connection to K2 and to issue unproblematic queries during the development process. It is not intended to be an interface for end users.

The other blazon of user connection is through RMI. These connections are capable of executing multiple queries at in one case and can halt execution of queries in progress. This is the connexion method that K2 servers use to connect to other K2 servers to distribute the execution of a query.

There is a client that makes an RMI connectedness to a K2 server with administrator privileges. The server restricts these connections to certain usernames connecting from certain IP addresses and requires a countersign. Currently, an administrator can examine the country of the server, add and remove connections to individual drivers, end currently running queries, disconnect clients, and bring the server to a country where it tin be stopped safely. More functionality is planned for administrators in the futurity.

A client that runs as a servlet is likewise included with K2. Using code developed at the Computational Biology and Informatics Laboratory (at the Academy of Pennsylvania), this servlet allows entry of advertisement hoc K2 queries and maintains the results for each username individually.

A major component of any user interface is the representation of the data to the user. As exemplified previously, a user (perhaps one serving a larger grouping) tin define in K2MDL a transformed/integrated schema for a form of users and applications and can specify how the objects of this schema map to the underlying data sources. Users of this schema (chosen an ontology past some) can issue vastly simplified queries against it, without knowledge of the data sources themselves.

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