in the following , numerous specific details are set forth to provide a thorough description of various embodiments of the invention . certain embodiments of the invention may be practiced without these specific details or with some variations in detail . in some instances , certain features are described in less detail so as not to obscure other aspects of the invention . the level of detail associated with each of the elements or features should not be construed to qualify the novelty or importance of one feature over the others . referring to fig1 , in accordance with one embodiment , an exemplary operating environment may comprise crawler 110 , aggregator 120 , and one or more content providers 130 . crawler 110 and aggregator 120 may be implemented as a software applications running on one or more computing systems that are connected to each other and to content providers 130 in a communication network ( e . g ., the internet ). content providers 130 may include web servers or other computing systems configured for delivery or storage of content . in one implementation , crawler 110 may be configured to classify ( e . g ., index ) content stored on content providers 130 . aggregator 120 may be configured to act as a conduit between crawler 110 and content providers 130 to service requests submitted by crawler 110 to help retrieve one or more references ( e . g ., uniform resource locators ( urls )) that identify content available on one or more content providers 130 . the aggregator 120 may be implemented to run on one or more of content providers 130 or on other computing systems acting as proxies for servicing requests submitted by crawler 110 . as provided in further detail below , in some embodiments , more than one aggregator 120 ( not shown ) may be employed , for example , where several content providers 130 are designated as associated with one another related content providers ). in accordance with one embodiment , crawler 110 may start a crawling session by sending a request to aggregator 120 to access content on one or more content providers 130 . the request may comprise page information including , for example , a starting index and a range . as such , crawler 110 may request access to content ( e . g ., documents 51 to 60 ) of a content provider 130 by setting the starting index to a first value ( e . g ., 51 ) and the range to a second value ( e . g ., 10 ). the request may also comprise state or timing information . in one embodiment , state information ( e . g ., state ′, state ″, state ′″, state ″″) may comprise an opaque object generated by content providers 130 for passing internal information between content providers 130 . the object may be passed to aggregator 120 , which may insert additional information . crawler 110 may use state information to access content from more than one content provider 130 during a single crawling request . table 1 below provides an exemplary implementation of state information . in one embodiment , timing information ( e . g ., timestamp ′, timestamp ″, timestamp ′″, timestamp ″″) may comprise an opaque timestamp object that is created by an application and may be internal to the application . in another embodiment , timing information may comprise a timestamp that is internal to a particular content provider 130 . in yet another embodiment , timing information may comprise a document identification number for the last retrieved document , wherein the each update is associated with a new document identification number . crawler 110 may use timing information during an incremental crawling session . for example , crawler 110 may submit one or more crawling requests to access content on content providers 130 . however , if crawler 110 has already crawled the content on content providers 130 during a previous crawling session , crawler 110 may limit crawling to updated content on content providers 130 by using timing information . fig2 , in accordance with one embodiment , provides a state machine that describes state and timing information during full and incremental crawling sessions . at point a ( 0 , 0 ), crawler 110 starts a full crawling session with a first request for content . the first coordinate “ 0 ” indicates that there is no state information , and the second coordinate “ 0 ” indicates that there is no timing information . at point b ( 1 , 0 ), crawler 110 submits one or more subsequent requests for content . there is state information , but no timing information because it is the first crawling session . crawler 110 then completes the full crawling session and updates content on content providers 130 . at point c ( 0 , 1 ), crawler 110 starts an incremental crawling session with a first request for content . there is no state information because it is the first request , but there is timing information from the previous full crawling session . at point d ( 1 , 1 ), crawler 110 submits one or more subsequent requests for content . there is both state and timing information . timing information , however , does not change between requests . crawler 110 returns to point a and disregards timing information if crawler 110 starts another full crawling session . referring to fig1 and 3a , in accordance with one embodiment , upon receiving a request to crawl content on a plurality of associated content providers ( p 300 ), aggregator 120 may retrieve one or more first references to content on a first content provider 130 ( p 310 ) and one or more second references to content on one or more second content providers 130 during the same request ( p 320 ). once the first and second references are retrieved , aggregator 120 may aggregate the first and second references ( p 330 ). accordingly , crawler 110 may access content on the first content provider 130 to index content available on the first content provider 130 . as noted earlier , process p 320 may be repeated for subsequent content providers 130 , depending on implementation , if the crawling request is not satisfied by the first content provider 130 . for example , if 50 documents are requested and the first content provider 130 has 20 documents , aggregator 120 may provide the crawler 110 with 30 additional documents that may be available on other content providers 130 . if the other content providers 130 cannot fully satisfy the request , the request is satisfied to the maximum extent possible . in response to a crawling request , aggregator 120 may return to crawler 110 one or more references to content available on the first content provider 130 and subsequent content providers 130 , in addition to state information and timing information ( p 340 ). aggregator 120 ( or crawler 110 ) may be configured to associate the state information with the content available on content providers 130 . in one embodiment , the state information is not directly associated with the content by the content providers 130 . therefore , the content remains stateless and thus several crawlers 110 may access the content concurrently without risking corruption or deadlock . in other words , in one embodiment , the content is stored on content providers 130 , desirably , independent of the state information that is tracked by aggregator 120 ( or crawler 110 ). since the content is read - only and stateless , from the point of view of a content provider 130 , no locking mechanism may need to be implemented to prohibit a second crawler from concurrently accessing content being accessed by a first crawler . depending on implementation , the state information may be determined based on timing information associated with a crawler &# 39 ; s 110 access to content available on each content provider 130 . in one embodiment , for example , timing information may be associated with content accessed by crawler 110 to identify a certain point in time when a request for accessing the content was submitted by crawler 110 to aggregator 120 . for example , timing information associated with content during a first crawling session may be used during a subsequent crawling session to determine if the content has been updated since the previous crawling session . if so , the updated content is crawled , and the old content is not crawled . that is , if no content has been updated , then crawling the old content will have the same result as the first crawling session . in other words , crawling the old content will effectively be a waste of time . thus , advantageously , the timing information may be used to avoid unnecessary crawling . more detailed examples are provided below to further illustrate how the state information and timing information data may be utilized to allow for an incremental crawling process . referring to fig1 and 3b , in accordance with one embodiment , aggregator 120 may receive a request to crawl content on a plurality of associated content providers 130 , wherein the request comprises at least a starting index value and a range value ( p 350 ). the request may also comprise state information . the state information is valid if the starting index value is the same as the last requested index ( e . g ., lastindex ), which is stored in the state information . if there is no valid state information ( p 355 ), aggregator 120 forwards the request to a first content provider 130 on a list of associated content providers 130 , increments the range value by the starting index value , and passes timing information corresponding to the first content provider 130 , if any , with the request ( p 360 ). if there is valid state information ( p 355 ), aggregator 120 forwards the request to a first content provider 130 identified by the state information as the next content provider ( e . g ., nextsource ), sets the starting index value to a value indicated by the first state information as the next starting index ( e . g ., nextstartindex ), and passes the state information and timing information corresponding to the first content provider 130 , if any , with the request ( p 365 ). upon receiving references , state information , or timing information from the first content provider , aggregator 120 aggregates the received references , state information , and timing information with other references , state information ( e . g ., aggregated - state ), and timing information ( e . g ., aggregated - timestamp ), respectively ( p 370 ). if the request is not satisfied to the maximum extent possible by the first content provider 130 ( p 375 ), aggregator 120 forwards the request to a second content provider 130 on the list , decrements the range value with the number of references received from the first content provider 130 , and passes timing information corresponding to the second content provider , if any , with the request ( p 380 ). processes p 370 , p 375 , and p 380 are repeated until the request is satisfied to the maximum extent possible . if the request is satisfied to the maximum extent possible by the first content provider ( p 375 ), aggregator 120 updates the state information by setting the next content provider 130 ( e . g ., nextsource ) and corresponding next starting index ( e . g ., nextstartindex ) ( p 385 ). if there are no more references left to receive from the first content provider 130 , aggregator 120 sets the next content provider 130 to be the next content provider 130 on the list ( p 385 ). if there are more references left to receive from the first content provider 130 , aggregator 120 sets the next content provider to be the first content provider 130 ( p 385 ). upon determining that the request has been satisfied to the maximum extent possible , aggregator 120 returns the aggregated references , the updated state information , and the aggregated timestamp , if available ( p 390 ). as provided earlier , a request is satisfied to the maximum extent possible if the requested number of references has been returned or if there are no more associated content providers 130 . on the other end , upon receiving timing information , a content provider 130 reads the information and returns references that have been updated since the last crawling session . upon receiving state information , a content provider 130 uses the information for optimization purposes . upon receiving a request with a starting index value of zero , a content provider 130 , returns timing information . upon receiving a request with a starting index value of zero , a content provider 130 , does not return timing information because the request is not the first request in the current crawling session . fig4 a - 4f illustrate two exemplary crawling sessions , in accordance with one or more embodiments . the first crawling session is a crawling session that , for example , started at 01 / 01 / 2008 8 : 00 . as shown in fig4 a , in accordance with one embodiment , aggregator 1 is directly associated with content provider ( cp ) 1 , aggregator 2 , and cp 2 . through aggregator 2 , aggregator 1 is also associated with content providers cp 3 and cp 4 . in this example , each content provider has 10 documents , thus the first exemplary crawling session involves 3 sub - requests . referring to fig4 b , in accordance with one embodiment , crawler sends a request to access 15 content pages ( e . g ., documents ) to aggregator 1 . the range value is incremented by a starting index of 0 , so that the range value is 15 . there is no state information because the crawling session has just begun , and there is no timestamp because there have been no previous crawling sessions . aggregator 1 forwards the request to cp 1 , and cp 1 returns a list of references to 10 documents on cp 1 , and a timestamp ( e . g ., timestamp_a ). once aggregator 1 receives the list and the timestamp , aggregator 1 updates its state information as provided in the example pseudo - code ( i ). upon updating its state information , aggregator 1 decrements the range value by 10 and forwards the request to aggregator 2 . aggregator 2 , in turn , forwards the request to cp 3 . cp 3 returns a list of 5 documents and a timestamp ( e . g ., timestamp_b ), and aggregator 2 updates its state information as provided in the example pseudo - code ( ii ). upon updating its state information , aggregator 2 returns the list of 5 documents , along with state information ( e . g ., state_c ) and a timestamp ( e . g ., timestamp_c ). once aggregator 1 receives the list , state information , and timestamp , aggregator 1 returns a list of 15 documents ( i . e ., 10 from cp 1 and 5 from aggregator 2 ), along with state information ( e . g ., state_d ) and a timestamp ( e . g ., timestamp_d ), to the crawler . aggregator 1 also updates its state information as in the example pseudo - code ( iii ): referring to fig4 c , in accordance with one embodiment , crawler sends a request to aggregator 1 to access the next 15 documents , along with state information ( e . g ., state_d ) from the previous request . there is no timestamp because the request belongs to the same crawling session . upon receiving the request and state information , aggregator 1 reads the received state information , sets the request &# 39 ; s starting index to 5 and range value to 15 , and forwards the request to aggregator 2 . aggregator 2 , in turn , reads its state information and forwards the request to cp 3 . upon receiving the request and state information , cp 3 returns a list of the remaining 5 documents . cp 3 does not return a timestamp because the starting index is not 0 ( i . e ., the request is not the first request for this crawling session ). upon receiving the list of 5 documents , aggregator 2 forwards the request to cp 4 , since the request has not been satisfied . cp 4 returns a list of 10 documents and a timestamp ( e . g ., timestamp_f ) and indicates that there are no more documents to be accessed on cp 4 . once aggregator 2 receives the list of 10 documents , aggregator 2 returns a list of 15 documents ( i . e ., 5 from cp 3 and 10 from cp 4 ) and a timestamp ( timestamp_g ) and indicates that there are no more documents to be accessed on aggregator 2 . aggregator 2 also updates its state information as provided in the example pseudo - code ( iv ). upon receiving the list of 15 documents from aggregator 2 , aggregator 1 returns the list of 15 documents , along with state information ( e . g ., state_h ) and a timestamp ( e . g ., timestamp_h ) to crawler . aggregator 1 also updates its state information as provided in the example pseudo - code ( v ). referring to fig4 d , in accordance with one embodiment , crawler sends a request to aggregator 1 to access the next 15 documents , along with state information ( e . g ., state_h ) from the previous request . there is no timestamp because the request belongs to the same crawling session in this scenario . upon receiving the request and state information , aggregator 1 reads the received state information , sets the request &# 39 ; s starting index to 0 and range value to 15 , and forwards the request to cp 2 . cp 2 , in turn , returns a list of 10 documents and a timestamp ( e . g ., timestamp_i ) and indicates that there are no more documents to be accessed on cp 2 . once aggregator 1 receives the list of 10 documents , aggregator 1 returns the list of 10 documents and a timestamp ( timestamp_j ). aggregator 1 also updates its state information , as provided in the example pseudo - code ( vi ). the second crawling session is an incremental crawling session that starts at 01 / 01 / 2008 9 : 00 , for example . in the second crawling session , the crawler accesses the updates that occurred between the first crawling session and the start of the second crawling session ( e . g ., current time ). the second crawling session is built from 2 sub - requests , this scenario . as shown , in fig4 e , in accordance with one embodiment , crawler sends a request to aggregator 1 to access 15 updated documents , along with the latest timestamp from the first crawling session ( e . g ., timestamp_j ). the range value is incremented by a starting index of 0 , so that the range value is 15 . there is no state information because the crawling session has just begun . aggregator 1 forwards the request to cp 1 , along with the relevant timestamp for cp 1 ( e . g ., timestamp_a ). upon receiving the request and timestamp , cp 1 reads the timestamps and returns a list of 5 documents that were updated after 01 / 01 / 2008 8 : 05 and a timestamp ( e . g ., timestamp_k ). once aggregator 1 receives the list and timestamp , aggregator 1 updates its state information as provided in the exemplary pseudo - code ( vii ). upon updating its state information , aggregator 1 decrements the range value by 5 and forwards the request to aggregator 2 , along with the relevant timestamp for aggregator 2 ( e . g ., timestamp_g ). aggregator 2 , in turn , forwards the request to cp 3 , along with the relevant timestamp for cp 3 ( e . g ., timestamp_b ). cp 3 returns a list of 5 documents that were updated after 01 / 012008 8 : 10 and a timestamp ( e . g ., timestamp_l ). since the request is not satisfied , aggregator 2 then forwards the request to cp 4 , along with the relevant timestamp for cp 4 ( e . g ., timestamp_f ). cp 4 returns a list of 5 documents that were updated after 01 / 01 / 2008 8 : 25 and a timestamp ( e . g ., timestamp_m ) and indicates that there are no more documents to be accessed on cp 4 . once aggregator 2 receives the list and timestamp , aggregator 2 updates its state information , as provided in the example pseudo - code ( iix ). upon updating its state information , aggregator 2 returns a list of 10 documents ( i . e ., 5 from cp 3 and 5 from cp 4 ), along with state information ( e . g ., state_n ) and a timestamp ( e . g ., timestamp_n ). once aggregator 1 receives the list , state information , and timestamp , aggregator 1 returns a list of 15 documents ( i . e ., 5 from cp 1 and 10 from aggregator 2 ), along with state information ( e . g ., state_o ) and a timestamp ( e . g ., timestamp_o ), to crawler . aggregator 1 also updates its state information , as provided in the example pseudo - code ( ix ): referring to fig4 f , in accordance with one embodiment , crawler sends a request to aggregator 1 to access the next 15 updated documents , along with state information ( e . g ., state_o ) from the previous request and the latest timestamp from the first crawling session ( e . g ., timestamp_j ). upon receiving the request and state information , aggregator 1 reads the received state information , sets the request &# 39 ; s starting index to 0 and range value to 15 , and forwards the request to cp 2 , along with the relevant timestamp for cp 2 ( e . g ., timestamp_i ). cp 2 , in turn , returns a list of 5 documents that were updated after 01 / 01 / 2008 8 : 25 and a timestamp ( e . g ., timestamp_p ) and indicates that there are no more documents to be accessed on cp 2 . once aggregator 1 receives the list and timestamp , aggregator 1 returns the list of 5 documents and a timestamp ( e . g ., timestamp_q ) to crawler . aggregator 1 also updates its state information , as provided in example pseudo - code ( x ). in different embodiments , the invention may be implemented either entirely in the form of hardware or entirely in the form of software , or a combination of both hardware and software elements . for example , crawler 110 , aggregator 120 , and content providers 130 may comprise a controlled computing system environment that may be presented largely in terms of hardware components and software code executed to perform processes that achieve the results contemplated by the system of the present invention . advantageously , the present invention facilitates efficient incremental crawling . the systems and methods provided above do not require time zone synchronization and allow aggregators to save state information to optimize indexing or searching of content on multiple content providers . also , the transparent management of content providers allows for efficient handling of long , failed , or intentionally interrupted crawling sessions . referring to fig5 and 6 , a computing system environment in accordance with an exemplary embodiment is composed of a hardware environment 500 and a software environment 600 . the hardware environment 500 comprises the machinery and equipment that provide an execution environment for the software ; and the software provides the execution instructions for the hardware as provided below . as provided here , the software elements that are executed on the illustrated hardware elements are described in terms of specific logical / functional relationships . it should be noted , however , that the respective methods implemented in software may be also implemented in hardware by way of configured and programmed processors , asics ( application specific integrated circuits ), fpgas ( field programmable gate arrays ) and dsps ( digital signal processors ), for example . software environment 600 is divided into two major classes comprising system software 602 and application software 604 . system software 602 comprises control programs , such as the operating system ( os ) and information management systems that instruct the hardware how to function and process information . in one embodiment , crawler 110 and aggregator 120 may be implemented as system software 602 or application software 604 executed on one or more hardware environments to facilitate incremental crawling of multiple content providers using aggregation . application software 604 may comprise but is not limited to program code , data structures , firmware , resident software , microcode or any other form of information or routine that may be read , analyzed or executed by a microcontroller . in an alternative embodiment , the invention may be implemented as computer program product accessible from a computer - usable or computer - readable medium providing program code for use by or in connection with a computer or any instruction execution system . for the purposes of this description , a computer - usable or computer - readable medium can be any apparatus that can contain , store , communicate , propagate or transport the program for use by or in connection with the instruction execution system , apparatus or device . the computer - readable medium can be an electronic , magnetic , optical , electromagnetic , infrared , or semiconductor system ( or apparatus or device ) or a propagation medium . examples of a computer - readable medium include a semiconductor or solid - state memory , magnetic tape , a removable computer diskette , a random access memory ( ram ), a read - only memory ( rom ), a rigid magnetic disk and an optical disk . current examples of optical disks include compact disk read only memory ( cd - rom ), compact disk read / write ( cd - r / w ) and digital videodisk ( dvd ). referring to fig5 , an embodiment of the system software 602 and application software 604 may be implemented as computer software in the form of computer readable code executed on a data processing system such as hardware environment 500 that comprises a processor 502 coupled to one or more computer readable media or memory elements by way of a system bus 504 . the computer readable media or the memory elements , for example , may comprise local memory 506 , storage media 508 , and cache memory 510 . processor 502 loads executable code from storage media 508 to local memory 506 . cache memory 510 provides temporary storage to reduce the number of times code is loaded from storage media 508 for execution . a user interface device 512 ( e . g ., keyboard , pointing device , etc .) and a display screen 514 can be coupled to the computing system either directly or through an intervening i / o controller 516 , for example . a communication interface unit 518 , such as a network adapter , may be also coupled to the computing system to enable the data processing system to communicate with other data processing systems or remote printers or storage devices through intervening private or public networks . wired or wireless modems and ethernet cards are a few of the exemplary types of network adapters . in one or more embodiments , hardware environment 500 may not include all the above components , or may comprise other components for additional functionality or utility . for example , hardware environment 500 may be a laptop computer or other portable computing device embodied in an embedded system such as a set - top box , a personal data assistant ( pda ), a mobile communication unit ( e . g ., a wireless phone ), or other similar hardware platforms that have information processing and / or data storage and communication capabilities . in certain embodiments of the system , communication interface 518 communicates with other systems by sending and receiving electrical , electromagnetic or optical signals that carry digital data streams representing various types of information including program code . the communication may be established by way of a remote network ( e . g ., the internet ), or alternatively by way of transmission over a carrier wave . referring to fig6 , system software 602 and application software 604 may comprise one or more computer programs that are executed on top of an operating system after being loaded from storage media 508 into local memory 506 . in a client - server architecture , application software 604 may comprise client software and server software . for example , in one embodiment of the invention , client software is executed on one or more computing systems ( not shown ) and server software is executed on a server system ( not shown ). software environment 600 may also comprise browser software 608 for accessing data available over local or remote computing networks . further , software environment 600 may comprise a user interface 606 ( e . g ., a graphical user interface ( gui )) for receiving user commands and data . please note that the hardware and software architectures and environments described above are for purposes of example , and one or more embodiments of the invention may be implemented over any type of system architecture or processing environment . it should also be understood that the logic code , programs , modules , processes , methods and the order in which the respective steps of each method are performed are purely exemplary . depending on implementation , the steps may be performed in any order or in parallel , unless indicated otherwise in the present disclosure . further , the logic code is not related , or limited to any particular programming language , and may comprise of one or more modules that execute on one or more processors in a distributed , non - distributed or multiprocessing environment . therefore , it should be understood that the invention may be practiced with modification and alteration within the spirit and scope of the appended claims . the description is not intended to be exhaustive or to limit the invention to the precise form disclosed . these and various other adaptations and combinations of the embodiments disclosed are within the scope of the invention and are further defined by the claims and their full scope of equivalents .