One of the possible directions for my thesis work is going to be a stream data processing. That’s why it is worth reviewing some possible architectures, which at least can be used afterwords to compare my future system with existing ones.
Data stream processing system become quite popular due to the enormous growth (in particular in Volume, Variety, Velocity) of big data analytics on the fly. Starting from Machine Learning systems, such as Mahout, WEKA etc., continuing with a financial trading system, finally ending with realtime world process analysis. Among distributed systems for data stream processing, I would like to highlight two: S4(Simple Scalable Streaming System) and Storm.
Storm is a real time computation system for reliable processing of unbounded stream of data. Storm topology takes streams of data and process them in a complex way. A good thing about this system is that it is can be integrated with any queueing system and any database system. On the one hand, Storm is needed simply to connect to the database and in response it will handle the parallelization, partitioning and failures when necessary. Within storm tuples can be manipulated. Tuple is a names list of values.
Storm supports three abstractions: spouts, bolts and topologies. Spout – source of streams in a computation. Bolt – process input streams and produce another output streams. Topology – network of spouts and bolts, where the bolt is an edge subscribed to the other spout or bolt. Topologies are submitted using Apache Thrift. On another hand, Storm is scalable and can be run across a cluster of machines. At the same time, fault-tolerance is taken into account. Storm daemons are developed to be stateless and fail-fast. Nimbus and Supervisors are used to maintain fault-tolerance properties. What if:
- Worker dies? Supervisor will restart it. If it is still not responding with pings to Nimbus, Nimbus will reassign the worker to another machine.
- Node dies? Tasks, assigned to that machine, will time-out and Nimbus will reassign them to another machine.
- Nimbus (N) or Supervisor (S) dies? All states stored in Zookeeper or on disk. Usually N and S should be run over supervision of daemontools or monit. That’s why on restart they will start function as nothing had happened. Additionally, no worker processes are effected by the fail of N or S.
- Nimbus dies? Nothing critical will happen if N fails. However the ability to reassign processes to other machines won’t be able (which is equivalent to losing an opportunity to reassign tasks to other machines).
Each message in the system, coming out of the spout, is guaranteed to be fully processed. Storm assumes a tuple that is fully processed when the tuple tree processed all the messages.
The system has a master and workers. The master holds Nimbus, which perform code distribution, task assignment and failure monitoring. Each worker has a Supervisor, which listen to the incoming tasks, start and ends the processes. Each worker execute part of a topology, assigned to it. Finally, the coordination between the master and workers is handled by ZooKeeper. All Nimbus and Supervisors states are stored either in ZooKeeper or Local Disks.
As it was said before, the whole computation is based on a built computational tree – topologie. Over topologie a stream of data is circling. Each node in Storm topology executes in parallel.
Stream grouping is one of the distinctive feature of a Storm system. It manages topology to send tuples between two components in a required fashion (send it to a specific task in a component).
S4 is a simple, scalable and flexible processing platform for data streams. Processing elements in the system can be specified and form directed graphs when a data stream is supposed to flow. Nodes in the graph is called Processing Elements(PE) through which data is sent in the form of events. ZooKeeper is used for managing the configuration and tasks assignment. This way the system can be dynamically reconfigured and resolve failures by reassigning tasks to idle nodes.
PE is the basic computation units in the system and uniquely identified by functionality, types of events, keyed attribute and a value. However a keyless PEs are also possible and can accept all events to which they are associated, while keyed PE accept only events with corresponding key. Usually keyless are the first elements in a processing path. Each node usually have one instance of any keyless PE. Each PE has a state which can be changes as a reaction on received event.
Processing Node (PN) is hosting a running PE. Through PN different PEs can be reached, interruction with a communicatino leyer is estabblished and emmiting output messages are supported. Listener component in PN is used to receive events. Events are routed to PN based on a hash function for keyed attribute .
Cluster management and automatic failover provided by the communication layer.
The biggest difference between Storm and S4 is that Storm guarantees messages will be processed even in the face of failures whereas S4 will sometimes lose messages. However the state recovery is not supported by Storm.