Distributed Cooperative Web Servers
Scott M. Baker and Bongki Moon
Department of Computer Science, University of Arizona, Tucson, AZ 85721, U.S.A.
Traditional techniques for a distributed web server design rely on manipulation of central resources, such as routers or DNS services, to distribute requests designated for a single IP address to multiple web servers. The goal of the Distributed Cooperative Web Server (DCWS) system development is to explore application-level techniques for distributing web content. We achieve this by dynamically manipulating the hyperlinks stored within the web documents themselves. The DCWS system effectively eliminates the bottleneck of centralized resources, while balancing the load among distributed web servers. DCWS servers may be located in different networks, or even different continents and still balance load effectively. DCWS system design is fully compatible with existing HTTP protocol semantics and existing web client software products.
keywords: scalable web servers, document migration, load balancing.
With the explosive popularity of the internet and the world wide web (WWW), there is a rapidly growing need to provide unprecedented access to globally distributed data sources through the internet. Web accessibility will be an essential component of the services that future digital libraries should provide for clients. This need has created a strong demand for database access capability through the internet , and high performance scalable web servers [13,20]. As most popular web sites are experiencing overload from an increasing number of users accessing the sites at the same time, it is desired that scalable web servers should adapt to the changing access characteristics and should be capable of handling a large number of concurrent requests simultaneously, with reasonable response times and minimal request drop rates.
A collection of web documents may be viewed as a directed graph, where each document is a node and each hyperlink (or image reference) is a directed link from one node to another. If there is a way to distribute this graph amongst many server computers in such a way that the load is evenly distributed despite the dynamically changing web access patterns, then the problem of load balancing, one of the most important issues of creating a distributed web server, has been solved. Our solution will take this graph-based approach and will be based on the hypothesis that most web sites only have a few well-known entry points (, www.washingtonpost.com) from which users start navigating through the site's documents.
The proposed solution is to dynamically modify the web documents to change their hyperlink connectivity, and thereby distributing the document graph adaptively amongst several servers. The dynamic modifications will be performed automatically by the web servers and will require no user intervention. All the well-known entry points will be maintained at the home servers where the web documents originate, while less known internal documents may be migrated to alternate server computers which we call co-op servers for load balancing purposes. The home servers and co-op servers can serve collectively as a distributed cooperative web server (DCWS) for the need of web request processing with great flexibility and scalability.
There may be many possible situations where the distributed cooperative web server can be deployed to handle highly fluctuating web requests. Any stand-alone web server can be supported by several computers connected together by a local area network in the same organization. When the stand-alone web (home) server is overloaded, some of the computers can act as co-op servers by off-loading documents from the home server and delivering them on behalf of the home server. For another example, two or more departmental web server machines which work independently in the usual operational mode, can become a distributed cooperative web server; since the relative load may be different on each departmental web server depending on the time of year, project deadlines and so on, any of the lightly loaded servers can be a co-op server for any of the heavily loaded servers. The server machines can be geographically distributed. If an organization runs a number of independent web servers for branches in the east and west coasts of the United States and Asian and European countries, then the DCWS approach enables the web servers to adapt to the changes in geographic distribution of document requests and the changes due to different time zones. It also enables the web servers to take advantage of geographic caching of documents .
The distributed cooperative web server solution poses the following benefits over traditional systems based on packet-level manipulation, or domain name services (DNS) and distributed file systems:
- Network or packet level manipulation is not necessary. There is no entity (such as a router) that needs to touch every packet that is transferred between client and server. This eliminates a significant bottleneck present in traditional systems.
- Instead of implicit load balancing by using custom DNS servers, the cooperating servers make use of the connectivity of hyperlinks to directly control load balancing at the fine-grained level of documents.
- The cooperating servers do not need to be located within the same administrative domain or local area network. They may be geographically distributed and can distribute network traffic over multiple networks.
- Adding a new server is easy, flexible, and cost effective. Any available machine may be added as a cooperating server, without consideration as to the location of the machine relative to other existing servers.
In this paper, we present the design principles of the DCWS system and the detailed issues of its prototype implementation. We also demonstrate the scalable performance of the DCWS system by extensive experiments with real-life data sets. The rest of the paper is organized as follows. In Section 2, we briefly review related work for building scalable web servers. Section 3 presents the motivations behind the development of DCWS system, and describes the primary design issues. In Section 4, we describe the metrics for selecting documents to migrate, and present the detailed process of document migration and consistency considerations. Section 5 presents experimental results to demonstrate how the DCWS system works well with real-life data sets. Finally, in Section 6, we discuss the contributions of this paper and suggest future work.
2 Background and Related Work
Various load balancing techniques based on domain name service (DNS) have been proposed in the literature. The NCSA scalable web server is built on a cluster of identically configured servers, and uses round-robin DNS scheduling and Andrew file system (AFS) for load sharing among the servers [13.15]. The IBM scalable web server is built on an SP-2 parallel system, which is essentially a cluster of identical RS6000 workstations. The IBM web server uses a TCP router instead of DNS scheduling for improved load balancing , but its use is limited to tightly coupled systems such as SP-2.
The presence of heterogeneous web servers not only increase the complexity of the DNS scheduling, but also makes a simple round-robin scheduling not directly applicable. Numerous variations of the round-robin DNS scheduling have been proposed for heterogenous web servers and non-uniform client distribution. Two-tier round-robin DNS scheduling divides clients into two classes normal and hot to handle non-uniform distribution of client requests . Probabilistic and deterministic algorithms based on adaptive TTL (time-to-live) approach have been proposed . Lower TTL values are assigned when the DNS chooses a less capable server or an address mapping request comes from a hot client.
Another solution proposed in  attempts to develop a distributed scheduling heuristic based on a multi-variate cost function (CPU, disk and network utilization), which helps make the decision on task migration. Two techniques are used for load balancing: DNS rotation and HTTP URL redirection. DNS rotation is used for initial load distribution, and HTTP URL redirection is used to dynamically adjust network load based on server utilization. A potential problem with DNS rotation is the development of ``hot spots'', which lead to serious load imbalances. A detailed study of the techniques for an online digital library was reported in .
Dynamic server selection  is proposed as a client-based solution, in which clients automatically determine the best server for a given file without a priori knowledge of server performance. The technique relies on replication of web documents by proxy servers. It is assumed that a list of proxy servers exists which contain a given document. A hybrid buffer management algorithm  has been proposed to balance intra-cluster network traffic and disk access by dynamically controlling the amount of data replication. A centralized round-robin router is used to route requests amongst multiple servers.
The Cisco LocalDirector Cisco systems  uses a virtual server to handle incoming requests at a virtual IP address. The LocalDirector functions as an intelligent router, routing requests from the virtual IP address to physical servers at real IP addresses. It is intended to be a general purpose solution, capable of handling other services in addition to the web. No discussion is given in the white paper as to what extent the LocalDirector is a bottleneck of the system.
Fast Packet Interposing is a user-level technique developed in  and is used by the MagicRouter to distribute load. The MagicRouter is used to make a cluster of servers appear to have a single IP address without modifications to any servers. Fast Packet Interposing is used to modify network addresses within the data packets that pass through the MagicRouter. Fault tolerance and Load Balancing are stressed by the paper. The MagicRouter is expected to be a bottleneck as all packets must arrive through it as a central resource.
A technique called dynamic packet rewriting (DPR)  is used to distribute load. DPR attempts to route requests at the IP level, by manipulating the methods in which an IP address is mapped to a host. It is a distributed algorithm, and attempts to eliminate the bottleneck of a centralized solution, such as a centralized round-robin router. Each host acts as both a web server and a packet-level router for incoming requests. DNS rotation is used as an initial partitioning method to achieve a rough distribution.
3 Design Principles
In this section, we present the motivations behind the development of distributed cooperative web server (DCWS), and describe the primary design issues to construct a flexible, fully symmetric and scalable web server that achieves dynamic load balancing.
3.1 Entry-points Hypotheses
Most web sites only have a few well-known entry points for the site. A well-known entry point is a URL which is published to the rest of the world. For example, typical digital library applications such as newspaper sites, article archives and customer information services would publish the URL of their index pages to the world rather than publishing the URL of every single page in the sites. Specifically, the DCWS solution proposed in this paper is based on the following observations and generalizations about the organizations of and access patterns to the web documents:
- Web documents are either listed (well-known entry points) or not. Web pages that are not well-known are usually only accessed by a particular client by first accessing some well-known page and then following a path of hyperlinks from the well-known page to the destination page.
- The URL's of images embedded within a document are seldom published. Users need not know the URL's of the embedded images because the images are fetched automatically along with the corresponding web document without user intervention. Furthermore, images constitute a large portion of web bandwidth due to their comparatively large size.
- The use of frames in most modern web sites promotes a well-known frame template that is published along with several seldom-published internal frame pages. The frame template is usually small and easily hosted by the home server, while the internal frame pages may be large and can be migrated to other servers for load balancing purposes.
- In this setting, users do not care what the text of the URL is for internal pages within the site, as they primarily access the site through the well-known entry point.
The proposed solution will assume that most requests follow the common pattern of arriving at a well-known entry point, while a minority of requests may be bookmarks or search site retrievals, and thus the solution will attempt to optimize the common case while providing only a minimal penalty in the less-common case.
3.2 Document Migration
All documents originally reside on a home server. The home server is where the administrator or authors have placed the documents during the process of creating them. A permanent copy of the original document will always be maintained on the home server for consistency and robustness purposes. All well-known entry points will be maintained on the home server so that the users may see a consistent view of the web site. Documents or images other than the well-known entry points may be migrated to co-op servers. A co-op server is another server computer which has been designated as a server which will share the load of the home server. In principle, a document can be replicated to more than one co-op servers, but the current prototype implementation allows each document to be migrated to only one co-op server.
Pertaining to the document migration, it is important to note that the document hyperlinks are modified in such a way that the load is balanced among the home and co-op servers by redirecting user requests from one to another. In Figure 1, for example, documents A - E and F - K were initially hosted by two different servers. The numbers in parentheses presents the load (i.e., hits) associated with individual documents. Since the first server was overloaded, the document D was chosen to be migrated to the second server. In this example, the second server became the co-op server for the document D.
We adopt lazy migration policy in an attempt to minimize overhead incurred by physical data migration. Further details about the process of document migration will be discussed in Section 4.
Figure 1a: Initial Distribution of Documents
Figure 1b: After a document D is migrated from Server #1 to SErver #2
3.3 Document Graphs for Load Balancing
The distributed cooperative web server (DCWS) is designed to be fully symmetric, in the respect that each server may be both a home server for its own documents as well as a potential co-op server for sharing the load of some other home server. There are two key data structures that must be managed by the DCWS servers in order that they may share and balance the load amongst participating servers. First, each server must have the information of the local documents and the link structure among the documents. All the information is stored in a local document graph, and each server is responsible for maintaining the graph for any documents stored within it. Second, the servers must globally communicate load information amongst one another so that intelligent load-balancing decisions may be made. This information is global in nature, but each node maintains its own local view of the global state. The best-effort global load information is stored in a global load table on each server machine.
Local Document Graph.The local document graph (LDG) consists of a set of tuples
(Name, Location, Size, Hits, LinkTo, LinkFrom, Dirty).
The Name field is simply the name of the document, as it is requested by the user. It is also directly related to the name of the file on the server's local disk, so that the server knows where the contents of the document are located. The Location indicates which server presently is hosting the document, whether it be the home server where the document originated, or a co-op server that the document has been migrated to. The Size and Hits are used for load balancing computations, in order to determine which documents should be migrated. The LinkTo is a list of documents that the current document has hyperlinks pointing to; the LinkFrom is a lists of documents that have hyperlinks pointing to the current document. The Dirty bit is used to indicate whether some of the tuple's LinkTo documents have been migrated, thus requiring the system to regenerate the document with some altered hyperlinks. Figure 2 shows the local document graph entries stored in the first server after the document D is migrated to the second server. Note that some information such as document sizes is left out for visual clarity.
The local document graph is computed upon initialization of the web server by scanning its disk and parsing the documents. It is intended to be a dynamic structure and can be modified over time if the content of pages is changed by an administrator. Section 4 describes how the local document graph is updated when a document is migrated. A hash table is utilized to quickly find a tuple given the document name. It is important to optimize with a hash table because retrieving the tuple is necessary for each request that the server processes. The local document graph is assumed to be small enough that it can be stored entirely in memory. If this is not the case, then it should be a straightforward process to store the structure on disk and use memory as a cache for frequently accessed tuples.
Global Load Table The global load table (GLT) stores the overall state of the server group. Although the load information is global in nature, each server maintains a local copy of the load information and attempts to maintain data using a best-effort consistency mechanism. The global load table consists of a set of tuples
The Server is the name (or IP address) of the server computer. The LoadMetric is some measurement of the load that the server is experiencing and is used for load balancing purposes. For example, the total number of requests per minute could be used as a satisfactory load metric. Each server may trivially compute its own load information tuple by recording its LoadMetric as user requests bombard the server. The interesting point is how load information is communicated from one server to another.
Since the network is already presumably filled with many user requests and responses, it is desired not to initiate any additional data transfers simply for the purpose of communicating load information. Such a solution would be wasteful in a system where network bandwidth is an important resource. Instead, the solution that was chosen was to piggyback the load information onto existing HTTP transfers. The idea of piggybacking information has been used for cache coherency by server invalidation .
The HTTP protocol allows for inserting extension headers into the existing protocol semantics. These extension headers are ignored by any server which does not understand them, but may be interpreted by a server that does understand the extension header. Extension headers may be included in both the HTTP request (client to server) and the response (server to client). Thus, it is possible to insert an arbitrary amount of bi-directional information into an existing HTTP transaction. Amongst the DCWS servers, transfers are already occurring frequently to migrate documents between the servers, and these transfers provide an excellent opportunity to also communicate the load information by piggybacking. Thus, no additional communication channels are required for the means of communicating load information.
In the unlikely case that load information is not being communicated frequently enough, then it is possible to insert an artificial transfer to communicate load information. This would incur additional overhead since the transfer would not have occurred normally. A special pinger thread is present to watch for out-of-date information and automatically generate artificial transfers to bring the information up to date.
Figure 3 depicts the functional modules and data structures of the DCWS system, and illustrates access requests from web clients and interaction between home and co-op servers to process the requests. Further details of the multithreaded implementation of the DCWS prototype will be described in Section 5.
4 Process of Document Migration
In this section, we describe the metrics for selecting documents to migrate, and the detailed process of migrating documents including the lazy migration policy, hyperlink modification, load information update, and consistency considerations.
4.1 Metrics for Selecting Documents to Migrate
There may be several conditions and factors that should be considered in determining which documents be migrated. Among others, we have chosen a few criteria for selecting documents to migrate such that load balancing can be achieved with a small number of document migrations and low overhead. The algorithmic procedure for the document selection is presented in Algorithm 1 in Figure 4.
Figure 3: DCWS Architecture
Algorithm 1 Document Selection for Migration
Input: Given a local
document graph of a home server, and a threshold T of
Figure 4: Algorithm for Document Selection for Migration
It is typical that well-known entry points will provide users with an external view of the server. Users will use the well-known entry points to gain access to the server. If any of the well-known entry points is migrated to a co-op server, the home server should redirect every user request to the co-op server. Thus, step XXXstep2 is necessary to maintain a consistent view of the home server and to avoid burdensome request redirections.
Step3 is important because we want to balance the work load by migrating as few documents as we can. In other words, the access frequency of the document should be high enough to justify the migration process because migrating a document that receives only a few hits does not do much good for load balancing.
If a document is migrated from its home server to a co-op server, all the documents pointing to the migrated document (, the documents in the LinkFrom list) must be modified to update the hyperlinks and connectivity information. Step 4 seeks to minimize additional network traffic that would be required to update the hyperlinks in the LinkFrom documents on remote servers. Step 5 allows a document that points to a minimal number of LinkTo documents. This promotes future compliance with the goal of step 4.
4.2 Lazy Migration
If it is determined that one or more document migrations should occur, then the following process is used:
- The server with the lowest LoadMetric value is selected from the global load table. This can be done with a simple scan of the table. This server will become a co-op server for the home server and will host the migrated documents.
- The local document graph is updated accordingly. Specifically, the Location field of the tuple for the document is modified to reflect the new location. For each document referenced by the LinkFrom field of the tuple, the Dirty bit is set for that tuple. This will cause the documents referenced by the LinkFrom field to be regenerated next time a request arrives for one of them.
Through the above steps, the selected documents are migrated only logically. The physical document migration is deferred until it is actually required. We call this a lazy migration of documents.
To understand the physical migration process, it is necessary to discuss the behavior of the DCWS servers in detail. When a request arrives at a server, it may be one of two cases. Either it is a request for a document that is local to the server (, the server is a home server for the document), or the request is for a document that has been migrated to the server (, the server is a co-op server for the document).
If the request is for a migrated document, then there are two sub-conditions that exist:
- The co-op server does not have a copy of the document on it's local disk. In this case, the co-op server must initiate a HTTP session with the document's home server to retrieve the document. Once the document is retrieved from the home server, a copy is stored on the co-op server's local disk for future purposes, and the contents are also forwarded back to the user that initiated the original request.
- If the co-op server does have a copy of the document on it's local disk, then the document must have already been physically migrated, and the copy on the co-op server's local disk can be sent to the end user. Document Parsing and Reconstruction
To modify the hyperlinks embedded in a document, a HTML parser builds a simple parse tree from an HTML source file of the document. Any modified links are then replaced in the parse tree, the parse tree is turned back into a stream of HTML tokens, and then written back to its HTML source file. Since parsing and regeneration of documents is expected to be a time intensive process, it is desired to postpone the process until the latest time possible in order to eliminate any redundant or unnecessary work. The Dirty bit of a document's tuple in the local document graph is used as an indication as to whether the document needs to be parsed and regenerated with some modified links.
When a request arrives to retrieve a document, if the Dirty bit is not set, then the document is assumed to be up-to-date and a copy from disk is retrieved. If the Dirty bit is set, then the document is considered to be outdated and will be parsed, regenerated, a new copy will be written to disk, and the Dirty bit will be reset.
5.1 Prototype Development
The DCWS server is constructed in a modular fashion using multithreaded paradigm. Components include a multithreaded HTTP front-end, which is responsible for accepting and parsing requests, a worker module, which utilizes multiple threads to process and respond to requests from the front-end, and a statistics module, which is responsible for maintaining Global Load Table. The multithread support was implemented with portability as a chief concern. The server can run on Linux using the Posix pthreads library as well as Microsoft Windows products using the Win32 Thread API. The current Linux implemenetation has been tested using Linux kernel version 2.0.30 and pthreads version 0.5.
A general purpose HTML parser has been used to build simple parse trees for HTML source files. Although the parser is not optimized for the DCWS prototype, it is expected that an optimized parser would only improve performance by a constant amount and would not affect the speed up or scale up performance of the server.
5.2 Experimental Settings
Testing and benchmarks were performed on a cluster of 64 Intel Pentium workstations with 200 MHz clock rate. Each workstation has 128 MB of memory and 2 or 4 GB of disk storage. The workstations are connected by a 100 Mbps switched Ethernet network. The switch can handle an aggregate bandwidth of 2.4 Gbps in an all-to-all type communication. A server process (either home or co-op) ran on each of the Pentium workstations. Each server process contained 12 worker threads as well as a front-end thread and a pinger thread. Each home server was configured to migrate files at a maximum of one file per 10 seconds. No single co-op server was allowed to accept more than one migrated file every 60 seconds. This time interval is necessary to avoid overloading a co-op server by migrating documents too quickly, before it has a chance to adjust and recalculate its load statistics. The co-op servers were configured to validate migrated documents for consistency every 120 seconds. The pinger thread was assigned a sleep value of 20 seconds, in order to guarantee that all statistical data was accurate within 20 seconds. The server configuration parameters are summarized in Table 1.
|Number of front-end threads||1|
|Number of pinger threads||1|
|Number of worker threads||12|
|Socket queue length for backlogged requests||100|
|Statistcs re-calculation interval||10 seconds|
|Pinger thread activation interval||20 seconds|
|Co-op server document validation interval||120 seconds|
|Home server document re-migration interval||300 seconds|
|Minimum time for migrations to the same co-op server||60 seconds|
Client benchmark configuration. A custom benchmark was constructed due to the unique property of the distributed cooperative web servers that the hyperlinks of documents may be modified dynamically. Conventional benchmarks such as SPECweb96  are not suitable as they are designed to request documents without regard to the dynamically changeable hyperlinks contained within the delivered documents. For the same reason, we were not able to use server traces.
The benchmark is intended to correspond to the real-world behavior that most clients exhibit while they are accessing the web. Web clients running browsers typically maintain a client-side cache. This client side cache effects access patterns significantly and reduces temporal locality . With the custom client benchmark, we sought to simulate this caching behavior by building a client-side cache into the benchmark program. The cache is maintained for the duration of each simulated access sequence (1-25 document requests) of the benchmark and reset after each sequence.
The effects of the client cache on the DCWS system performance are expected to be two-fold: (1) Hot-spot behavior of images linked to multiple pages is reduced, and (2) redirections are increased due to increased stale link data being stored client-side. Both of these effects are real-world phenomena which increase the realism of the DCWS custom client benchmark.
The client benchmark program is multithreaded and includes one main thread to load a document and four additional threads to load images in parallel. The parallelism is intended to simulate the actions of existing web browsers which make use of multiple threads as well. Approximately eight instances of the client benchmark were configured to run on each client benchmark workstation. The number of client benchmark processes was selected to consume all available CPU and network resources of the client machine. The throughput produced per client workstation on the median dataset, LOD, was approximately 700 connections per second and 1.7 Mega bytes per second. The LOD data set will be described later in this section. Twenty-five workstations were configured as benchmark machines. The detailed procedures that performed by the custom benchmark is outline in Algorithm 2 in Figure 5.
|Algorithm 2 Custom
do forever begin
Figure 5: Custom Benchmark Algorithm
Request drop behavior It is likely that the request arrival rate might be often higher than the request service rate. The backlogged requests are queued in the socket queue at the server, and the socket queue may grow beyond the preset maximum queue length (see Table 1). Then, the connection is dropped gracefully with a 503 error response. This is the most graceful way of dropping requests, but also the most load intensive method for servers. When a 503 is received, a client has an exponential back-off and retry to minimize server load. That is, a client thread sleeps for a second at the first drop, sleeps for two seconds at the second drop, sleeps for four seconds at the third drop, and so forth.
Data sets We have chosen four real-world data sets with different link and hot spot characteristics in order to exercise the DCWS system under a variety of situations. Specific information and content of these data sets are available at http://www.cs.arizona.edu/dcws. The fourth data set is the Sequoia 2000 storage benchmark data  publicly available in http://epoch.cs.berkeley.edu:8000/sequoia/benchmark/.
- MAPUG Mailing List Archive: This data set is comprised of 1,534 formatted email messages containing a total of 28,998 links and an aggregate size of 5,918 Kbytes. The email messages are threaded and indexed by date, subject, and author. Six bitmapped images are utilized in the documents to represent navigational buttons. Since these six bitmapped images are linked to nearly all of the documents, they have a high request rate and form significant hot spots.
- SBLog Web Statistics: Generated by an automated statistics program, this data set includes 402 documents, 57,531 links, and has an 8,468 Kbyte aggregate size. The link graph of this data set is tree-like in nature with a few dozen summary and index pages as internal tree nodes and the bulk of the pages occupying leaf nodes. A single JPEG image is utilized in constructing bar-graphs in nearly all of the pages and forms a significant hot spot due to its high request rate.
- LOD Role-Playing Adventure Guide: This dataset is comparatively smaller than the others, with 349 documents, 1433 links, and 750 kbytes of data. It is highly graphical in nature, with 200 of the 349 documents being JPEG images. This dataset was chosen to represent the real-world shift of web site design to increasing graphical complexity. Although the large number of images do not present as severe hot spots as the other data sets, several of the index pages are significantly hotter than the remaining pages and images.
- Sequoia benchmark data: The raster data for Sequoia 2000 storage benchmark contains 130 AVHRR (Advanced Very High Resolution Radiometer) image files from NOAA satellite. The images are compressed and in the 1-2.8 Mbytes range. We created an HTML front-end page to the Sequoia raster data set that includes a hyperlink to each image file.
5.3 Experimental Results
Among many ways to evaluate the performance of web servers, there seems to have emerged a consensus that three most important measures are connections per second (CPS), bytes transferred per second (BPS), and round-trip time (RTT) [15,17]. However, the third measure, round-trip time is difficult to measure for an operational web server and depends on various performance factors such as network overhead and bottleneck, which are not directly related to the web server itself. Thus, in our experiments, we have decided to use the first two measures to evaluate the performance and scalability of the DCWS prototype system.
Peak load One of the most critical challenges for web servers is to deal with the peak load. To accurately measure the peak performance of the DCWS, we averaged the bytes per second (, throughput) and connections per second measures for a fixed number of concurrent simulated clients (, threads running on clients workstations). Increasing the number of concurrent clients from 16 up to 400, we repeated the same experiment and obtained averages of the measures. The third data set (LOD) was utilized for this set of experiments due to the fact that its graphical nature is most representative of real-world web sites. The large number of images provide an evenly distributed access pattern and help demonstrate the near-linear scalability of the DCWS system.
Figure 6(a) and Figure 6(b) show BPS and CPS measures, respectively, as compared to number of concurrent clients, with different numbers of servers being used. In both figures, the performance measures increased almost linearly until the peak was reached. After the peak was reached, the measures remained stable, presumably due to dropping excessive requests beyond the server capability. On the other hand, it is quite obvious that the DCWS prototype performs in a scalable way. Whenever the number of servers was doubled up, the peak performance was improved proportionally. For example, with 8 servers, the peak performance of about 18.6 Mega BPS and 7150 CPS was reached at 176 clients. With 16 servers, the peak performance of about 39.4 Mega BPS and 15150 CPS was reached at 368 clients.
Scalability and hot spots Hot spots (, extremely popular documents or images) limit the parallelism of most scalable web server systems. Since DCWS is able to control which documents are served by which co-op servers, extremely hot documents are automatically partitioned to disjoint co-op servers, effectively sharing the set of hot documents amongst many co-op servers. Figure 7(a) and Figure 7(b) show the peak performance of the DCWS prototype measured in BPS and CPS respectively, as the number of servers is increased, with four different data sets being used. With LOD and Sequoia data sets, both measures were found to be very close to linear up to 16 servers, which was the maximum number of available servers for the experiment.
However, we have observed less scalability performance from the other two data sets SBLog and MAPUG. For both of these datasets, the DCWS system scaled well up to eight server configuration, but suffered a significant degredation in scalability when the number of servers was increased from 8 to 16. We performed a detailed manual analysis of the MAPUG dataset and were able to determine that the cause of this was the extreme hot spot behavior of the six JPEG images present in the data set. Although the images were successfully migrated to disjoint co-op servers by DCWS, each image had a high access rate which caused several of the co-op servers to peak when operated in the 16 server test. We are presently working on a solution to extend scalability by replicating extremely hot documents on multiple co-op servers, which we expect will allow continued scalability in the presence of hot spots.
From this set of experiments, we acknowledge that data distribution and data access characteristics have significant impact on the performance, and hot spots can limit the potential parallelism of the DCWS system. We conjecture that the only way to get around this problem is to adopt replication of hot spots, which is not currently implemented in the DCWS prototype.
CPS vs. BPS The measurements of CPS and BPS are related by the size of the documents involved and connection overhead. Executing a web transaction via TCP connections requires exchange of several connection setup and tear-down packets in addition to the packets used to transfer the actual data. Thus, while a small file size increases CPS by reducing the number of bytes per transfer, it also decreases the overall BPS by wasting more bandwidth on additional connection overhead packets. This is corroborated by the results presented in Figure 7(a) and Figure 7(b). The highest BPS was observed from the Sequoia data set and followed by SBLog, MAPUG and LOD data sets, which is the decreasing order of the data sets in terms of average size of documents in the data sets. As we expected, in contrast, the CPS measures were observed in the reverse order.
Since real-world web transactions are fairly small . we chose to use CPS as a balancing metric rather than BPS in Section 4. On the other hand, it is possible that in a system which uses significantly larger file sizes (such as the Sequoia storage benchmark data set), BPS may be a better load balancing metric. Since the connection setup and tear-down overhead is amortized by the large file size, BPS can represent load more accurately in a more fine-grained measurement than CPS.
6 Conclusion and Future Work
We have designed and implemented a prototype system of the Distributed Cooperative Web Server (DCWS). The DCWS system is based on dynamic manipulation of the hyperlinks embedded in web documents in order to distribute access requests among multiple cooperating web servers. We have analyzed the performance of the DCWS prototype system with real-life data sets including the Sequoia storage benchmark data.
The experiments show that the DCWS system has a high potential to achieve linear scalability by effectively avoiding the bottleneck of centralized resources such as disk and network bandwidths. The overhead involved in hyperlink modifications was negligible compared with the improved performance gained by utilizing distributed servers. It has been shown that without the presence of the hot spots the peak performance was scaled almost linearly up to 16 cooperating servers. The load was evenly distributed among the servers and load balancing was achieved faster than linearly from a cold start. We conclude that the DCWS system proposed in this paper is a viable solution to building a scalable web server in both locally and geographically distributed environments. As an example, the DCWS system can be used to integrate a group of independent servers to build a federated web server in order to archive large-scale images and scientific data being produced and stored in geographically dispersed locations.
We recognize that there still remain several issues for further study of the DCWS system development. We have not taken into account the effects of user think time in the custom client benchmark and we have not used actual access logs for the experiments. These considerations may enhance the understanding of the DCWS system performance in more realistic situations. We plan to carry out further experiments in order to evaluate policies for document migration and consistency, and tune the performance parameters in geographically distributed and heterogenous environments. We also plan to extend the current implementation of the DCWS system so that it can handle hot spots by replicating popular documents in a controlled manner, and investigate the effects of initial data distribution on the potential parallelism and scalability.
Scott M. Baker is an independent software consultant and received his Bachelor and Master of Science degrees from the University of Arizona. His interests include networking, graphics, and distributed information systems.
Bongki Moon is an assistant professor in the Department of Computer Science at the University of Arizona. His current research interests include high performance spatial databases, scalable web servers, data mining and warehousing, and parallel and distributed processing. He received his PhD degree in Computer Science from University of Maryland, College Park, in 1996, and his MS and BS degrees in Computer Engineering from Seoul National University, Korea, in 1985 and 1983, respectively. He worked for Samsung Electronics Corp., Korea, as a member of the research staff at the Communication Systems Division from 1985 to 1990.
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