Cloud computing Module 2 First Part
- 1. Module 2(1st Part) Cloud Computing Presented By- Soumee Maschatak MTECH(SCS) 1EW18SCS07
- 2. Contents 1. Challenges for cloud computing. 2. Architectural styles for cloud applications. 3. Workflows - coordination of multiple activities. 4. Coordination based on a state machine model.
- 3. Cloud Applications • Cloud computing is very attractive to the users: • Economic reasons. • low infrastructure investment. • low cost - customers are only billed for resources used. • Convenience and performance. • application developers enjoy the advantages of a just-in-time infrastructure; they are free to design an application without being concerned with the system where the application will run. • the execution time of compute-intensive and data-intensive applications can, potentially, be reduced through parallelization. If an application can partition the workload in n segments and spawn n instances of itself, then the execution time could be reduced by a factor close to n. • Cloud computing is also beneficial for the providers of computing cycles - it typically leads to a higher level of resource utilization.
- 4. • Ideal applications for cloud computing: • Web services. • Database services. • Transaction-based service. The resource requirements of transaction-oriented services benefit from an elastic environment where resources are available when needed and where one pays only for the resources it consumes. • Applications unlikely to perform well on a cloud: • Applications with a complex workflow and multiple dependencies, as is often the case in high-performance computing. • Applications which require intensive communication among concurrent instances. • When the workload cannot be arbitrarily partitioned.
- 5. Challenges Of Cloud Computing • Performance isolation - nearly impossible to reach in a real system, especially when the system is heavily loaded. • Reliability - major concern; server failures expected when a large number of servers cooperate for the computations. • Cloud infrastructure exhibits latency and bandwidth fluctuations which affect the application performance. • Performance considerations limit the amount of data logging; the ability to identify the source of unexpected results and errors is helped by frequent logging.
- 6. Architectural styles for cloud applications • Based on the client-server paradigm. • Stateless servers - view a client request as an independent transaction and respond to it; the client is not required to first establish a connection to the server. • Often clients and servers communicate using Remote Procedure Calls (RPCs). • SimpleObject Access Protocol (SOAP) - application protocol for web applications; message format based on the XML. UsesTCP or UDP transport protocols. • Representational StateTransfer (REST) - software architecture for distributed hypermedia systems. Supports client communication with stateless servers, it is platform independent, language independent, supports data caching, and can be used in the presence of firewalls.
- 7. Workflow • Process description - structure describing the tasks to be executed and the order of their execution. Resembles a flowchart. • Case - an instance of a process description. • State of a case at time t - defined in terms of tasks already completed at that time. • Events - cause transitions between states. • The life cycle of a workflow - creation, definition, verification, and enactment; similar to the life cycle of a traditional program (creation, compilation, and execution).
- 8. Workflow Description User Planning Engine Verification Engine Enactment Engine Component Database Workflow Description Case Activation Record Unanticipated Exception Handling User Programming Language Computer Program Workflow Description Language Automatic Programming Component Libraries Compiler Object Code Data Processor Running the Process Workflow Database (a) Workflow (b) Program Dynamic Workflows Static Workflows Static Programs Dynamic Programs Program Libraries Run-Time Program Modification Requests
- 9. Basic workflow patterns • Sequence - several tasks have to be scheduled one after the completion of the other. • AND split - both tasks B and C are activated when task A terminates. • Synchronization - task C can only start after tasks A and B terminate. • XOR split - after completion of task A, either B or C can be activated. • XOR merge - task C is enabled when either A or B terminate. • OR split - after completion of task A one could activate either B, C, or both.
- 10. • Multiple Merge - once task A terminates, B and C execute concurrently; when the first of them, say B, terminates, then D is activated; then, when C terminates, D is activated again. • Discriminator – wait for a number of incoming branches to complete before activating the subsequent activity; then wait for the remaining branches to finish without taking any action until all of them have terminated. Next, resets itself. • N out of M join - barrier synchronization. Assuming that M tasks run concurrently, N (N<M) of them have to reach the barrier before the next task is enabled. In our example, any two out of the three tasks A, B, and C have to finish before E is enabled. • Deferred Choice - similar to the XOR split but the choice is not made explicitly; the run-time environment decides what branch to take.
- 11. A B C a A B C AND b A B c AND C A B e XOR CA B C XOR d A B C OR f A B C AND g DXOR A B C AND h DDIS A B CAND i D 2/3 E A B C XOR j X Basic workflow patterns. (a) Sequence. (b) AND split. (c) Synchronization. (d) XOR split. (e) XOR merge. (f) OR split. (g) Multiple merge. (h) Discriminator. (i) N out of M join. (j) Deferred choice.
- 12. Coordination - ZooKeeper • Cloud elasticity distribute computations and data across multiple systems; coordination among these systems is a critical function in a distributed environment. • ZooKeeper • Distributed coordination service for large-scale distributed systems. • High throughput and low latency service. • Implements a version of the Paxos consensus algorithm. • Open-source software written in Java with bindings for Java and C. • The servers in the pack communicate and elect a leader. • A database is replicated on each server; consistency of the replicas is maintained. • A client connect to a single server, synchronizes its clock with the server, and sends requests, receives responses and watch events through a TCP connection.
- 13. Server Server Server Server Server Client ClientClientClientClientClientClient Client (a) Write processor Replicated database Atomic broadcast WRITE READ (b) Leader Follower Follower Follower Follower Follower (c) WRITE The ZooKeeper coordination service. (a) The service provides a single system image. Clients can connect to any server in the pack. (b) Functional model of the ZooKeeper service.The replicated database is accessed directly by read commands; write commands involve more intricate processing based on atomic broadcast. (c) Processing a write command
- 14. Zookeeper communication • Messaging layer responsible for the election of a new leader when the current leader fails. • Messaging protocols use: • Packets - sequence of bytes sent through a FIFO channel. • Proposals - units of agreement. • Messages - sequence of bytes atomically broadcast to all servers. • A message is included into a proposal and it is agreed upon before it is delivered. • • Proposals are agreed upon by exchanging packets with a quorum of servers, as required by the Paxos algorithm.
- 15. • Messaging layer guarantees: • Reliable delivery: if a message m is delivered to one server, it will be eventually delivered to all servers. • Total order: if message m is delivered before message n to one server, it will be delivered before n to all servers. • Causal order: if message n is sent after m has been delivered by the sender of n, then m must be ordered before n.
- 16. ZooKeeper service guarantees • Atomicity - a transaction either completes or fails. • Sequential consistency of updates - updates are applied strictly in the order they are received. • Single system image for the clients - a client receives the same response regardless of the server it connects to. • Persistence of updates - once applied, an update persists until it is overwritten by a client. • Reliability - the system is guaranteed to function correctly as long as the majority of servers function correctly.
- 17. Zookeeper API • The API is simple - consists of seven operations: • Create - add a node at a given location on the tree. • Delete - delete a node. • Get data - read data from a node. • Set data - write data to a node. • Get children - retrieve a list of the children of the node. • Synch - wait for the data to propagate.