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DECENTRALIZED SYSTEMS PROJECT. MAY 2012                                                                                   ...
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High Availability of Services in Wide-Area Shared Computing Networks


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Highly available distributed systems have been widely used and have proven to be resistant to a wide range of faults. Although these kind of services are easy to access, they require an investment that developers might not always be willing to make. We present an overview of Wide-Area shared computing networks as well as methods to provide high availability of services in such networks. We make some references to highly available systems that are being used and studied at the moment this paper was written (2012).

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High Availability of Services in Wide-Area Shared Computing Networks

  1. 1. DECENTRALIZED SYSTEMS PROJECT. MAY 2012 1 High Availability of Services in Wide-Area Shared Computing Networks M´rio Almeida (, EMDC Student a Ozgur Seyhanli (, CANS Student Sergio Mendoza (, CANS Student Zafar Gilani (, EMDC Student Abstract—Highly available distributed systems have been level of operational performance will be met during a con-widely used and have proven to be resistant to a wide range tractual measurement period.of faults. Although these kind of services are easy to access,they require an investment that developers might not always Availability is related to the ability of the user to accessbe willing to make. We present an overview of Wide-Area the system. If a user is unable to access it then it is saidshared computing networks as well as methods to provide to be unavailable. The period in which the system is un-high availability of services in such networks. We make somereferences to highly available systems that are being used available is called downtime. Unscheduled downtime canand studied at the moment this paper was written. be due to multiple causes such as power outages, hardware Index Terms—High Availability, Wide-Area Networks, failures, security breaches or application/OS failures.Replication, Quorum Consistency, Descentralized Systems, As stated in the CAP theorem [1], a distributed com-File virtualization, Load balancing, Migration of Services puter system has to decide which two of these three prop- erties will be provided : Consistency, Availability and Par- I. Introduction tition tolerance. This formulation tends to oversimplify the tensions between properties since there is the need toH IGHLY available distributed systems have been widely used and have proven to be resistant to a widerange of faults, like power outages, hardware failures, secu- choose between consistency and availability when there are partitions. Recent distributed systems [22] show that thererity breaches, application failures, OS failures and even to is a lot of flexibility for handling partitions and recovering,byzantine faults. For example, services like Amazon Elas- even for highly available and somewhat consistent systems.tic Compute Cloud provide resizable computation capacity As an example of different distributed systems, one canin the cloud with an annual uptime percentage of 99.95%. think of the NoSQL movement that focus on availability Although these kind of services are easy to access, they first and consistency second, while the databases that pro-require an investment that developers might not always vide ACID properties focus more on willing to make. Also some distributed systems have Availability is usually expressed as a percentage of up-specific properties that make more sense when applied to time in a year. Services generally provide service levelshared non-dedicated computing networks. An example agreements (SLA) that refer to a contract on the mini-can consist of a file sharing peer-to-peer network, in which mum monthly downtime or availability. For example, ser-the developers might not want to be held responsible for vices like Amazon Elastic Compute Cloud provide resizablethe contents being shared. computation capacity in the cloud with an annual uptime In this report we present an overview of Wide-Area percentage of 99.95% [25]. This SLA agreements gener-shared computing networks as well as methods to provide ally have a backdrop since they generally cover the corehigh availability of services in such networks. We make instances and not the services on which the instances de-some references to highly available systems that are being pend. This was a big issue during EC2s Easter outage inused and studied at the moment this paper was written. 2011 [26]. As downtimes in distributed systems generally occur due II. Wide-Area Shared Computing Networks to faults, we will focus on a specific type of fault that is A Wide-Area shared computing network is an hetero- depicted by the Byzantine Generals Problem [8].geneous non-dedicated computer network. In these typesof networks, machines have varying and limited resources A. Byzantine faultsand can fail at anytime. Also, theyre often not properly A Byzantine fault is an arbitrary fault that occurs dur-designed to deal with machine failures and so it makes the ing the execution of an algorithm by a distributed system.challenge of having no planned downtimes or maintenance It can describe omission failures (crashes, lost communi-intervals even harder. These types of networks can be sim- cation...) and commission failures (incorrect processing,ulated using PlanetLab testbed. corrupt states or incorrect responses...). If a system is not tolerant to byzantine faults it might respond in unpre- III. High Availability of Services dictable ways. High availability is a system design approach and asso- Some techniques have been widely used since the paperciated service implementation that ensures a prearranged [8] that was published in 1999. Some open-source solu-
  2. 2. 2 DECENTRALIZED SYSTEMS PROJECT. MAY 2012tions like UpRight provide byzantine fault tolerance usinga Paxos-like consensus algorithm.B. High Availability in Wide-Area Networks Usually high availability clusters provide a set of tech-niques in order to make the infrastructure as reliable aspossible. Some techniques include disk mirroring, redun-dant network connections, redundant storage area networkand redundant power inputs on different circuits. In thecase of Wide-Area networks, only a few of these techniquescan be used since it relies on heterogeneous machines thatarent designed specifically for providing high availability. As the main property of this type of networks is the het-erogeneity of nodes and its varying resources, it is crucialto scale its capacities depending on the incoming requestsor actual resources available to the node. Due to the lim-itations of resources, it is important to be able to scale Fig. 1. Diagram of active replication architecture.the service to more nodes. This is one of the key pointsof availability of services, since if a node receives more re-quests than it can handle, it will stop being able to providethe service and therefore wont have high availability. Thismeans that a service needs to do load balancing and some-times partition data or state in several machines in orderto scale. Scaling the number of machines also increasesthe probability of some machines to fail. This can be ad-dressed by creating redundancy by means of replication totolerate failures.C. Load balancing Load balancing consists of a methodology to distributeworkload across multiple machines in order to achieve op-timal resource utilization, maximize throughput, minimizeresponse time and avoid overloading. A simple solutioncan be achieved through a domain name system, by asso-ciating multiple IP addresses with a single domain name. In order to determine how to balance the workload, the Fig. 2. Diagram of passive replication architecture.load balancer can also have other characteristics in accountsuch as reported server load, recent response times, keepingtrack of alive nodes, number of connections, traffic and of replicas also has an impact on the performance andgeographic location. complexity of the system. For example, higher number of The load balancing can be done at two levels: at the replicas imply more messages to keep consistency betweentracking of services at system level and at the node level. them.At the node level this load balancing can be achieved by Replication is important not only to create the neededeither redirecting requests or redirecting clients. Also, the redundancy to handle failures but also to balance the work-nodes could send tokens to each other in order to estimate load by distributing the client requests to the nodes de-how much requests they can redirect to each other. pending on their capacities. When we talk about replication two simple schemesD. Replication of Services come to our mind, active and passive replication [11]. The To explain how replicating a service can help it tolerate architectures of active and passive replication models arefailures, let’s consider the probability of failure of a sin- represented, respectively, in Figure 1 and Figure machine to be P and that machines fail independently. In active replication each request is processed by all theThen if we replicate data N times to survive N-1 failures, nodes. This requires that the process hosted by the nodesof replicas the probability of losing a specific data must be is deterministic, meaning that having the same initial stateP N . A desired reliability R can be picked by changing the and the same request sequence, all processes should pro-number so that P N < R. duce the same response and achieve the same final state. So we can provide smaller probabilities of having down- This also introduces the need of atomic broadcast proto-times of services by increasing the number of replicas. But cols that guarantee that either all the replicas receive thethis is not as easy as it seems as the increasing number message in the same order or none receives it.
  3. 3. ALMEIDA, SEYHANLI, MENDOZA AND GILANI: H. AVAILABILITY OF SERVICES IN W-A SHARED CNS 3 In passive replication there is a primary node that pro- This solution highly depends on the amount of data thatcesses client requests. After processing the request the the service manages.node replicates its state to the other backup nodes and A common way to simplify the access to remote files in asends back the response to the client. If the primary node transparent way is to perform file virtualization. File vir-fails, there is a leader election and one of the backups takes tualization eliminates the dependencies between the dataits place as primary. accessed at the file level and the location where the files In regular passive replication, secondary replicas should are physically stored. It allows the optimization of storageonly perform reads, while the writes are performed by the use and server consolidation and to perform non-disruptiveprimary replica and then replicated to the other replicas. file migrations.There could be better workload balancing if every node Caching of data can be done in order to improve its per-could receive requests but this also implies using another formance. Also there can be a single management interfacesystem to keep consistency between nodes. Also caching of for all the distributed virtualized storage systems. It allowsreads can greatly improve the overall performance of the replication services across multiple heterogeneous devices.system but one may have to relax consistency properties The data replication can also be done in an hybrid way,to achieve this. storing less important content in the heterogeneous nodes For passive replication, papers like A Robust and and more important content in a more reliable distributedLightweight Stable Leader Election Service for Dynamic file system. An example of a somewhat hybrid system canSystems [3] describe system implementations of fault- be Spotify, it makes use of client replicated files in ordertolerant leader election services that use stochastic failure to offload some work from its servers but when the clientsdetectors [10] and link quality estimators to provide some have low throughput or the files arent available, the Spotifydegree of QoS control. These systems adapt to changing servers can provide the files in a more reliable conditions and has proven to be robust and not Amazon S3 also provides storage options such as thetoo expensive. reduced redundancy storage system. This system reduces Active replication deals better with real time systems the costs by storing non-critical, reproducible data at lowerthat require faster responses, even when there are faults. levels of redundancy. It provides a cost-effective, highlyThe main disadvantage of active replication is that most available solution for distributing or sharing content thatservices are non-deterministic and the disadvantage of pas- is durably stored elsewhere, or for storing thumbnails,sive replication is that in case of failure the response is transcoded media, or other processed data that can be eas-delayed. ily reproduced. Passive replication can be efficient enough if we consider G. Migration of Servicesthat the type of services we want to provide perform sig-nificantly more reads than writes. Serializing all updates Another problem in Wide-Area networks is that nodethrough a single leader can be a performance bottleneck. resources can vary a lot. This means that although a node As replication also introduces costs in communication may have proven worth during a period of time, its avail-and resources, some techniques are generally used to reduce able resources such as CPU, bandwidth or memory canit. An example is the use of read and write quorum sets vary and affect the service sustainability. Also, if the levelas we will explain in sections ahead. of replication is not aware of the variation of resources of the nodes, we might see the number of needed replicas toE. Service Recovery provide a service growing to a point that it affects the per- formance of the service. Due to this aspect, a new concept Another important characteristic of this kind of net- has been researched lately that consists of resource-awareworks is that a node can be shutdown at any moment. migration of services [5] between nodes.Actually some studies show that most failures in Planet- It might seem that migration is a similar concept to repli-Lab are due to rebooting the machines, this means that cation as it consists of replicating the data from one nodenode regeneration capabilities would be crucial in such en- to another. However it is different since it also aims tovironment. It is noticeable that in this case, re-accessing transfer the current state of execution in the volatile stor-the data on the secondary storage instead of creating a age as well as its archival state in the secondary node and performing replication could definitively im- Moreover it also provides mechanisms for handling any on-prove the overall performance of the system (for systems going client-sessions.that keep states). This can also depend on the average Migration of services uses migration policies to decideobserved downtime of the machines that will be revisited when a service should migrate. These policies can bein the evaluation. locality-aware and resource-aware For example; available resources can be CPU, bandwidth, memory and more.F. Storage replication Migration of services also introduces some issues such Wide Area Shared Computing networks arent the most as the need of a tracking system to allow clients to accesspropitious type of networks for persistently storing files a location changing service. Also, during this migrationin a highly available way. Since nodes often reboot, the there is a period in which the service might be unable toeasiest way would be to replicate data like the services. attend requests and so it needs to delegate responsibilities
  4. 4. 4 DECENTRALIZED SYSTEMS PROJECT. MAY 2012to another replica. This period is called blackout period is therefore important to be able to verify and merge theand the aim of replication is to make this period negligible. existing versions. This can be achieved by requesting all Latest research papers such as Building Autonomically existing versions of data from the read quorum, and thenScalable Services on Wide-Area Shared Computing [4] aim waiting for the responses from all the replicas. If thereto provide models for estimating service capacity that is are multiple versions, it returns all the versions that arelikely to be provided by a replica in the near future . It also causally unrelated. Divergent versions are reconciled andprovides models for dynamic control of the degree of service written to the write quorum.replication. This is done in order to provision the required Quorums consistency is actually used in a variety of dis-aggregate service capacity based on the estimated service tributed systems and seem to perform well. An example iscapacities of the replicas. They also describe techniques the quorum consistency of replicas used by Amazons Dy-to provide reliable registry services for clients to locate namo [12]. Dynamo also manages group membership usingservice replicas. Their experimental evaluation shows the a gossip-based protocol to propagate membership changesimportance of this estimations and they claim to be able and maintain an eventually consistent view of predict correctness of 95%. One of the other methods of achieving consistency includes In conclusion, the system performance of this kind of techniques like fuzzy snapshots to perceive the global statesystems is highly dependent on the type of service pro- of the system composed by the replicas.vided. For services that make intensive use of the sec-ondary storage, migration is a very costly solution. One IV. Related workapproach could consist of pro-actively select and transfer Commercial approaches for replication have been evolv-secondary storage to a potential target node for any future ing towards increasing tolerance to fail-stop faults. Thisre-locations. is mainly because of falling hardware costs, the fact that replication techniques become better understood and eas-H. Quorum Consistency ier to adopt, and systems become larger, more complex, If we consider services that make large use of secondary and more and the properties of Wide-Area shared computing There appears to be increasingly routine use of doubly-networks such as the frequent shutdown of nodes, then we redundant storage. Similarly, although two-phase commitmust be able to recover these nodes so that we dont have is often good enough, it can be always safe and rarely un-to replicate the whole data again. If, on other hand, we live, increasing numbers of deployments pay the extra costassume that this data is small, then we can simply replicate to use Paxos three-phase commit to simplify their designthe server to a new node. or avoid corner cases requiring operator intervention. If we consider recovering services, we must have a way Distributed systems increasingly include limited Byzan-to keep track of the nodes alive and have an efficient way tine fault tolerance aimed at high-risk subsystems. For ex-to update them instead of copying the whole set of data. ample the ZFS [17], GFS [18], and HDFS [19] file systemsIf we consider a small and fix number of nodes, we can provide checksums for on-disk data. As another example,always do a simple heartbeat/versioning system, but for after Amazon S3 was afected for several hours by a flippedmore dynamic number of replicas, a group membership bit, additional checksums on system state messages wereprotocol would probably be more suitable for keeping track added.of the nodes. Some other systems that we have studied and will in- In order to perform efficient updates in a dynamic set of clude here are UpRight fault tolerance infrastructure andreplicas, a quorum system can be used to provide consis- Zookeeper coordination service. We have studied manytency. The main advantage of quorums is that it uses its other systems that we did not list here. Special mention toquorum sets properties to propagate changes and reduce amazons Dynamo storage system that provides advancedthe needed number of messages. It can reduce the needed techniques like the ones we have mentioned in previousnumber of messages to perform a critical action from three chapters.times the total number of nodes to three times the numberof nodes in its quorum (in the best case). A. UpRight For example, in the case of passive replication, if the Upright is an open-source infrastructure and library forprimary node needs to perform a write operation, it gen- building fault tolerant distributed systems [20]. It pro-erates the vector clock for the new data version and per- vides a simple library to ensure high availability and faultforms the write locally. Then it sends the new version to tolerance through replication. It claims to provide highthe nodes in its quorum, if all the nodes respond then the availability, high reliability (system remains correct even ifwrite is considered successful. Thanks to quorum proper- byzantine failures are present) and high performance. Inties, it doesnt need to contact all the backup nodes, only Figure 3 we show the architecture of UpRight.the nodes present at its quorum set. The latency is deter- As depicted in the previous architecture diagram, themined by the slowest node of this write quorum. application client sends its requests through the client li- As the primary node can also fail and it could hypothet- brary and these requests are ordered by the UpRight Core.ically (and depending on the design) have the most recent The application servers handle these ordered requests andversion of the data that it didn’t have time to replicate, it send replies back to the clients. The redundancy provided
  5. 5. ALMEIDA, SEYHANLI, MENDOZA AND GILANI: H. AVAILABILITY OF SERVICES IN W-A SHARED CNS 5 V. Practical work A. PlanetLab Overview PlanetLab [21] is a heterogeneous infrastructure of com- puting resources shared across the Internet. Established in 2002, it is a global network of computers available as a testbed for computer networking and distributed systems research. In December 2011, PlanetLab was composed of 1024 nodes at 530 sites worldwide. Accounts are available to people associated with com- panies and universities that host PlanetLab nodes. EachFig. 3. Diagram of Upright architecture. investigation project runs a ”slice”, which gives experi- menters access to a virtual machine on each node attached to that the UpRight replication engine guarantees that even if a Several efforts to improve the heterogeneity of PlanetLabgiven number of nodes are down, faulty, or even malicious, have been made. OneLab, an European Project funded bythe whole system can still work correctly. the European Commission, started in September 2006 with UpRight also uses some of the properties that we de- two overarching objectives: Extend the current PlanetLabscribed in previous chapters, such as the use of quorums. infrastructure and Create an autonomous PlanetLab Eu-Its purpose is to optimistically send messages to the min- ropeimum number of nodes and resend to more nodes only if PlanetLab Europe is a European-wide research testbedthe observed progress is slow. It also provides byzantine that is linked to the global PlanetLab through a peer-to-fault tolerance using a Paxos-like consensus algorithm. peer federation. During this Project different kinds of ac- cess technologies (such as UMTS, WiMax and WiFi) wereB. Zookeeper integrated, allowing the installation of new kinds of multi- Zookeeper [16] is an open-source coordination service homed PlanetLab nodes (e.g. nodes with an Ethernet plusthat has some similarities to Chubby [15]. It provides ser- a interface) [23].vices like consensus, group management, leader election, Since 2008, hundreds researchers at top academic insti-presence protocols, and consistent storage for small files. tutions and industrial research labs have tested their exper- Zookeeper guards against omission failures. However, imental technologies on PlanetLab Europe, including: dis-because a data center typically runs a single instance of a tributed storage, network mapping, peer-to-peer systems,coordination service on which many cluster services de- distributed hash tables, and query processing. As of Jan-pend, and because even a small control error can have uary 2012, PlanetLab Europe has 306 nodes at 152 sites.dramatic effects, it seems reasonable to invest additional B. PlanetLab Setupresources to protect against a wider range of faults. Considering u as the total number of failures it can tol- For using PlanetLab infrastructure an account is re-erate and remain live and r the number of those failures quired. To use the resources offered by various nodes, athat can be commission failures while maintaining safety, slice has to be created. A slice is a collection of resourcesZookeeper deployment comprises 2u + 1 servers. A com- distributed across multiple PlanetLab nodes. When a nodemon configuration is 5 servers for u = 2 r = 0. Servers is added to a slice, a virtual server for that slice is createdmaintain a set of hierarchically named objects in memory. on that node. When a node is removed from a slice, thatWrites are serialized via a Paxos-like protocol, and reads virtual server is destroyed. Each sites PI is in charge ofare optimized to avoid consensus where possible. A client creating and managing slices at that site [24].can set a watch on an object so that it is notified if the In order to measure a few metrics related to availabil-object changes unless the connection from the client to a ity, we deployed a sample application on PlanetLabs UPCserver breaks, in which case the client is notified that the slice (upcple sd). To run experiments, we added a total ofconnection broke. 8 nodes to our slice to create a virtual network over Plan- For crash tolerance, each server synchronously logs up- etLab. The following table shows hostnames of nodes anddates to stable storage. Servers periodically produce fuzzy their locations.snapshots to checkpoint their state: a thread walks the The map represented in Figure 4 shows the locations ofserver’s data structures and writes them to disk, but re- the nodes in Europe.quests concurrent with snapshot production may alter We deployed a simple application to these nodes to eval-these data structures as the snapshot is produced. If a uate number of requests generated against time. More im-Zookeeper server starts producing a snapshot after request portantly we evaluate the availability of nodes against timeSstart and finishes producing it after request send, the based on number of successful requests.fuzzy snapshot representing the system’s state after re- Apart from this we also had to set up a web server atquest send comprises the data structures written to disk IST in Lisbon, Portugal. A web server is necessary for stor-plus the log of updates from Sstart to send. ing messages from PlanetLab nodes. When a node sends
  6. 6. 6 DECENTRALIZED SYSTEMS PROJECT. MAY 2012 Fig. 5. Successfull requests between 2200 and 2259 hours (CEST).Fig. 4. Location of the nodes in Europe.a message to the web server, it is termed as a heartbeatmessage. In our experiments, we setted up each node tosend a heartbeat message once every 10 minutes. We tookmeasurements on data obtained for two periods of 6 hourseach. These two periods correspond to day and night timeusage in order to observe any difference in availability dur-ing day and night time hours. Percentage usage of a nodecan possibly affect availability of a node. Fig. 6. Successfull requests between 0900 and 0959 hours (CEST).C. Monitoring In order to automate the process of generating requests It can be observed from the bar chart that all the nodesand having nodes to send a heartbeat message, we used responded successfully to requests apart from the nodes incronjobs. Each node was instructed to execute a python Portugal and Greece, which failed for a request betweenscript once every 10 minutes. The python script performed 2200 and 2259 hours (CEST).three easy tasks: Figure 6 is similar to Fig 5 but shows for day time pe- Get Node Credentials: Get PlanetLab nodes credentials riod from 0600 to 1200 hours (CEST). It can be observed such as name and url. that the Norwegian node in our slice could not successfully Get Site Credentials: Get PlanetLab sites credentials reply to a request between 1000 and 1059 hours (CEST). such as site id, latitude, longitude and login base Similarly, node in Sweden failed to reply between 0900 and Post Data: Encode information this into a URL en- 0959 hours (CEST). coded string and send it to the web server as POST From these two bar charts we can conclude that most messages by calling a php script on the web server. of the requests in a given time period were handled suc- When called, the php script on a web server was used to cessfully and that failure of one or more nodes doesnt af-append node and site information sent by the PlanetLab fect the overall operation since the application had replicasnode to a text file. elsewhere. Figure 7 shows a bar chart for availability of nodes inD. Results our PlanetLab slice. The vertical axis represents availabil- Figure 5 below shows number of successful requests for ity in percentage as a function of successful requests foreach node for night time period. We represent each node each node. Each node shows two bars, dark for night andby its location (i.e. country). The vertical axis shows total light for day. As can be seen most of the nodes show morerequests. Each bar represents a different node and shows than 97 percent of availability. Some nodes such as thenumber of successful requests for night time period from ones in Portugal and Greece were unavailable for short pe-2100 to 0300 hours (CEST). This time period is divided riod of time during night hours. Others such as the ones inin 6 one-hour columns, as represented by the color-coded Norway and Sweden were shortly unavailable during day-legend. time.
  7. 7. ALMEIDA, SEYHANLI, MENDOZA AND GILANI: H. AVAILABILITY OF SERVICES IN W-A SHARED CNS 7 on PlanetLab. Moreover as mentioned in section E, we consistently experienced failure of tools such as CoDeploy and MultiQuery. Ultimately we had to accomplish things manually. Also we realized that some of the tools havent been up- dated for about ten years and some of their dependencies are already deprecated. We had to find a host in order to launch our server and group the results from the PlanetLab nodes. As this host did not have a fixed IP we had to constantly update our private/public keys to communicate with the nodes. If we opted for using PlanetLab tools it would have taken even longer to evaluate our project since it can take fromFig. 7. Availability of nodes in our PlanetLab slice. a few minutes to a few hours to commit changes to virtual machine configurations. For speeding up the process of development of a highlyE. Issues available distributed system, one can use either Amazons Standard approach to deploy software and applications EC2 for deploying highly available and resource elastic ser-on PlanetLab nodes is to use an application called CoDe- vices. As this is not always the most appropriate solu-ploy. However using CoDeploy [27] was neither convenient tion, one can set up its own network and use, for example,nor consistent. We observed that for most of the nodes, the the multiple open-source Hadoop technologies for reliabledeployment failed altogether. As a workaround we manu- and scalable distributed systems. But in the case of Wideally deployed scripts on PlanetLab nodes. Area Shared Computing networks, maybe solutions like the Similarly, standard method of registering cronjobs on open-source UpRight are more suitable since it can be inte-PlanetLab nodes is to use an application called MultiQuery grated either with Zookeeper or with Hadoops distributed[27] which is a part of CoDeploy application. We found file system.that even though MultiQuery registers cronjobs, it how- We have concluded that it is possible to provide highlyever fails to start the crond daemon. As a workaround we available distributed systems in wide area shared comput-manually registered our cronjobs on PlanetLab nodes. ing through the use of resource-aware replication [5] with reasonable results. Quorum sets help reducing the costs VI. Evaluation of Highly Available Systems of replication and the paxos algorithm can help tolerate The problem with theoretical reliability through replica- byzantine faults.tion is that it assumes that these failures are indeed inde- Finally, as an experiment we replicated a simple appli-pendent. If nodes share the same software and there can be cation over a small network of PlanetLab PLE nodes us-corrupt requests, there is always some correlation between ing active replication methodology. We found that evenfailures of nodes (At least WAN networks will less proba- though a few nodes might fail at any given time, the ap-bly share the same configuration of machines). P N is an plication can still work without major issues.upper bound of reliability that can never be approached inpractice. This is discussed in papers such as the one from ReferencesGoogle [4] that shows empirical numbers on group fail- [1] Nancy Lynch and Seth Gilbert, Brewer’s conjecture and theures that demonstrates rates of several orders of magnitude feasibility of consistent, available, partition-tolerant web services ACM SIGACT News, Volume 33 Issue 2, 2002.higher than the independence assumption would predict. [2] Mamoru Maekawa, Arthur E. Oldehoeft, Rodney R. Oldehoeft Reliability and high availability should not only be Operating Systems: Advanced Concept. Benjamin/Cummingsproved through theoretical methodologies, it has to be Publishing Company, Inc, 1987. [3] Nicolas Schiper, Sam Toueg, A Robust and Lightweight Sta-tested through empirical methods such as continuous hours ble Leader Election Service for Dynamic Systems University ofof successful operations. There are two metrics that are of- Lugano, 2008.ten difficult to measure on academic research projects but [4] V. Padhye, A. Tripathi, Building Autonomically Scalable Services on Wide-Area Shared Computing Platforms Network Computinggive a very good measurement of the availability and relia- and Applications (NCA), 10th IEEE International Symposium,bility of the system, mean time between failures and mean 2011.time between recoveries. [5] V. Padhye, A. Tripathi, D. Kulkarni, Resource-Aware Migratory Services in Wide-Area Shared Computing Environments Reliable Distributed Systems. SRDS 28th IEEE International Symposium, VII. Conclusion 2009. We expected to perform further evaluation over Planet- [6] A. Tripathi, V. Padhye, Distributed Systems Research with Ajanta Mobile Agent Framework 2002.Lab. However it took more to get an account and to get [7] Benjamin Reed, Flavio P. Junqueira, A simple totally orderedaccess to a slice and respective nodes. This was mainly due broadcast protocol LADIS ’08 Proceedings of the 2nd Workshopto the fact that each of this resources is managed by differ- on Large-Scale Distributed Systems and Middleware, 2008. [8] Miguel Castro, Barbara Liskov, Practical Byzantine Fault Toler-ent entities. Once we had an account, we were surprised ance Laboratory for Computer Science, Massachusetts Instituteby the time it takes for virtual machines to get configured of Technology, 1999.
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