Although demand for large scale distributed computing solutions has existed for decades, the term Big Data did not get a lot of public attention till Google published its data processing programming model, MapReduce, back in 2004. The Java-based Hadoop framework further popularized the term a couple of years later, partly due to the ubiquitous popularity of Java.
From Batch to Real-time
Hadoop has proven itself a great platform for running MapReduce applications on fault-tolerant HDFS clusters, typically composed of inexpensive servers. It does very well in the large-scale batch data processing problem domain. Adding a distributed database system such as HBase or Cassandra helps extend the platform to address the needs for real-time (or near-real-time) access to structured data. But in order to be able to use feature-rich messaging or streaming functionality, one will need some suitable systems that operate well on a distributed platform.
I remember feeling the need for such a real-time Big Data system when I was with a cleantech startup, EcoFactor, a couple of years ago seeking solutions to handle the increasingly demanding time-series data processing in a near-real-time fashion. It would have saved me and my team a lot of internal development work had such a system been available. One of my recent R&D projects after I left the company was to adopt suitable software components to address such real-time distributed data processing needs. The following highlights the key pieces I picked and what prompted me to pick them for the task.
Over the past couple of years, Kafka and Storm have emerged as two maturing distributed data processing systems. Kafka was developed by LinkedIn for high performance messaging, whereas Storm, developed by Twitter (through the acquisition of BackType), addresses the real-time streaming problem space. Although the two technologies were independently built, some real-time Big Data solution seekers see advantages bringing the two together by integrating Kafka as the source queue with a Storm streaming system. According to a published blog earlier this year, Twitter also uses the Kafka-Storm combo to handle its demanding real-time search.
High-performance Distributed Messaging
Kafka is a distributed publish-subscribe messaging system equipped with robust semantic partitioning and high messaging throughput by leveraging kernel-managed disk cache. High performance is evidently a key initiative in Kafka’s underlying architecture. It adopts the design principle that leverages kernel page caching to minimize data copying and context switching for higher messaging throughput. It also uses message grouping (MessageSet) to reduce network calls. At-least-once message processing is guaranteed. If exactly-once is a business requirement, one approach is to programmatically keep track of the messaging state by coupling the data with the message offset to eliminate duplication.
Kafka flows data by having the publishers push data to the brokers (Kafka servers) and subscribers pull from the brokers, giving the flexibility of a more diverse set of message consumers in the messaging system. It uses ZooKeeper for auto message broker discovery for non-static broker configurations. It also uses ZooKeeper to maintain message topics and partitions. Messages can be programmatically partitioned over a server cluster and consumed with ordering preserved within individual partitions. There are two APIs for the consumer – a high-level consumer (ConsumerConnector) that heavily leverages ZooKeeper to handle broker discovery, consumer rebalancing and message offset tracking; and a low-level consumer (SimpleConsumer) that allows users to manually customize all the key messaging features.
Setting up Kafka on a cluster is pretty straight forward. The version used in my project is 0.7.2. Each Kafka server is identified by a unique broker id. Each broker comes with tunable configurations on the socket server, logs and connections to a ZooKeeper ensemble (if enabled). There are also a handful of configurable properties that are producer-specific (e.g. producer.type, serializer.class, partitioner.class) and consumer-specific (e.g. autocommit.interval.ms).
The following links detail Kafka’s core design principles:
Real-time Distributed Streaming
Storm is a distributed streaming system that streams data through a customizable topology of data sources (spouts) and processors (bolts). It uses ZooKeeper, as well, to coordinate among the master and worker nodes in a cluster and manage transactional state as needed. Streams (composed of tuples), spouts and bolts constitute the core components of a Storm topology. A set of stream grouping methods are provided for partitioning a stream among consuming bolts in accordance with various use cases. The Storm topology executes across multiple configurable worker processes, each of which is a JVM. A Thrift-based topology builder is used to define and submit a topology.
On reliability, Storm guarantees that every tuple from a spout gets fully processed. It manages the complete lifecycle of each input tuple by tracking its id (msgId) and using methods ack(Object msgId) and fail(Object msgId) to report processing result. By anchoring the input tuple from the spout to every associated tuple being emitted in the consuming bolts, the spout can replay the tuple in the event of failure. This ensures at-least-once message processing.
Transactional Stream Processing
Storm’s transactional topology goes another step further to ensure exactly-once message processing. It processes tuples by batches, each identified by a transaction id which is maintained in a persistent storage. A transaction is composed of two phases – processing phase and commit phase. The processing phase allows batches to be proceeded in parallel (if the specific business logic warrants it), whereas the commit phase requires batches to be strongly ordered. For a given batch, any failure during the processing or commit phases will result in a replay of the entire transaction. To avoid over-update to a replayed batch, the current transaction id is examined against the stored transaction id within the strong-ordered commit phase and persisted update takes place only when the the id’s differ.
Then, there is this high-level abstraction layer, Trident API, on top of Storm that helps internalize some state management effort. It also introduces opaque transaction spouts to address failure cases in which loss of partial data source forbids replaying of the batch. It achieves such fault tolerance by maintaining in persistent storage a previous-state computed value (e.g. word count) in additional to the current computed value and transaction id. The idea is to reliably carry over partial value changes across strong-ordered batches, allowing the failed tuples in a partially failed batch to be processed in a subsequent batch.
Deploying Storm on a production cluster requires a little extra effort. The version, 0.8.1, used in my project requires a dated version of ZeroMQ – a native socket/messaging library in C++, which in turn needs JZMQ for Java binding. To build ZeroMQ, UUID library (libuuid-devel) is needed as well. Within the cluster, the master node runs the “Nimbus” daemon and each of the worker nodes runs a “Supervisor” daemon. It also comes with a administrative web UI that is handy for status monitoring.
The following links provide details on the topics of Storm’s message reliability, transactional topology and Trident’s essentials:
And, 1 + 1 = 1 …
Both Kafka and Storm are promising technologies in the real-time Big Data arena. While they work great individually, their functionalities also complement each other well. A commonly seen use case is to have Storm being the center piece of a streaming system with Kafka spouts providing queueing mechanism and data processing bolts carrying out specific business logic. If persistent storage is needed which is often the case, one can develop a bolt to persist data into a NoSQL database such as HBase or Cassandra.
These are all exciting technologies and are part of what makes contemporary open-source distributed computing software prosperous. Even though they’re promising, that doesn’t mean they’re suitable for every company that wants to run some real-time Big Data systems. At their current level of maturity, adopting them still requires some hands-on software technologists to objectively assess needs, design, implement and come up with infrastructure support plan.