Programming model for Apache Beam

Apache Beam is an open source, unified model for defining both batch- and streaming-data parallel-processing pipelines. The Apache Beam programming model simplifies the mechanics of large-scale data processing. Using one of the Apache Beam SDKs, you build a program that defines the pipeline. Then, one of Apache Beam's supported distributed processing backends, such as Dataflow, executes the pipeline. This model lets you concentrate on the logical composition of your data processing job, rather than the physical orchestration of parallel processing. You can focus on what you need your job to do instead of exactly how that job gets executed.

The Apache Beam model provides useful abstractions that insulate you from low-level details of distributed processing, such as coordinating individual workers, sharding datasets, and other such tasks. Dataflow fully manages these low-level details.

Concepts

This section contains summaries of fundamental concepts. On the Apache Beam website, the Apache Beam Programming Guide walks you through the basic concepts of building pipelines using the Apache Beam SDKs.

Basic concepts

Pipelines
A pipeline encapsulates the entire series of computations involved in reading input data, transforming that data, and writing output data. The input source and output sink can be the same or of different types, allowing you to convert data from one format to another. Apache Beam programs start by constructing a Pipeline object, and then using that object as the basis for creating the pipeline's datasets. Each pipeline represents a single, repeatable job.
PCollection
A PCollection represents a potentially distributed, multi-element dataset that acts as the pipeline's data. Apache Beam transforms use PCollection objects as inputs and outputs for each step in your pipeline. A PCollection can hold a dataset of a fixed size or an unbounded dataset from a continuously updating data source.
Transforms
A transform represents a processing operation that transforms data. A transform takes one or more PCollections as input, performs an operation that you specify on each element in that collection, and produces one or more PCollections as output. A transform can perform nearly any kind of processing operation, including performing mathematical computations on data, converting data from one format to another, grouping data together, reading and writing data, filtering data to output only the elements you want, or combining data elements into single values.
ParDo
ParDo is the core parallel processing operation in the Apache Beam SDKs, invoking a user-specified function on each of the elements of the input PCollection. ParDo collects the zero or more output elements into an output PCollection. The ParDo transform processes elements independently and possibly in parallel.
Pipeline I/O
Apache Beam I/O connectors let you read data into your pipeline and write output data from your pipeline. An I/O connector consists of a source and a sink. All Apache Beam sources and sinks are transforms that let your pipeline work with data from a number of different data storage formats. You can also write a custom I/O connector.
Aggregation
Aggregation is the process of computing some value from multiple input elements. The primary computational pattern for aggregation in Apache Beam is to group all elements with a common key and window. Then, it combines each group of elements using an associative and commutative operation.
User Defined Functions (UDFs)
Some operations within Apache Beam allow executing user-defined code as a way of configuring the transform. For ParDo, user-defined code specifies the operation to apply to every element, and for Combine, it specifies how values should be combined. A pipeline might contain UDFs written in a different language than the language of your runner. A pipeline might also contain UDFs written in multiple languages.
Runner
Runners are the software that accepts a pipeline and executes it. Most runners are translators or adapters to massively parallel big-data processing systems. Other runners exist for local testing and debugging.

Advanced concepts

Event time
The time a data event occurs, determined by the timestamp on the data element itself. This is contrasted with the time the actual data element gets processed at any stage in the pipeline.
Windowing
Windowing enables grouping operations over unbounded collections by dividing the collection into windows of finite collections according to the timestamps of the individual elements. A windowing function tells the runner how to assign elements to an initial window, and how to merge windows of grouped elements. Apache Beam lets you define different kinds of windows or use the predefined windowing functions.
Watermarks
Apache Beam tracks a watermark, which is the system's notion of when all data in a certain window can be expected to have arrived in the pipeline. Apache Beam tracks a watermark because data is not guaranteed to arrive in a pipeline in time order or at predictable intervals. In addition, there are no guarantees that data events will appear in the pipeline in the same order that they were generated.
Trigger
Triggers determine when to emit aggregated results as data arrives. For bounded data, results are emitted after all of the input has been processed. For unbounded data, results are emitted when the watermark passes the end of the window, indicating that the system believes all input data for that window has been processed. Apache Beam provides several predefined triggers and lets you combine them.

What's next

For detailed explanations, see the Apache Beam Programming Guide on the Apache Beam website.

Apache Beam is a trademark of The Apache Software Foundation or its affiliates in the United States and/or other countries.
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