Structured Streaming is a new streaming API, introduced in spark 2.0, rethinks stream processing in spark land. It models stream as an infinite table, rather than discrete collection of data. It’s a radical departure from models of other stream processing frameworks like storm, beam, flink etc. Structured Streaming is the first API to build stream processing on top of SQL engine.
Structured Streaming was in alpha in 2.0 and 2.1. But with release 2.2 it has hit stable status. In next few releases, it’s going to be de facto way of doing stream processing in spark. So it will be right time to make ourselves familiarise with this new API.
In this series of posts, I will be discussing about the different aspects of the structured streaming API. I will be discussing about new API’s, patterns and abstractions to solve common stream processing tasks.
This is the second post in the series. In this post, we discuss about the source and sink abstractions. You can read all the posts in the series here.
TL;DR You can access code on github.
In spark 1.3, with introduction of DataFrame abstraction, spark has introduced an API to read structured data from variety of sources. This API is known as datasource API. Datasource API is an universal API to read structured data from different sources like databases, csv files etc. The data read from datasource API is represented as DataFrame in the program. So data source API has become de facto way of creating dataframes in spark batch API.
You can read more about datasource API in my post Introduction to Spark Data Source API.
Extending Datasource API for streams
With Structured Streaming, streaming is moving towards dataframe abstraction. So rather than creating a new API to create dataframe’s for streaming, spark has extended the datasource API to support stream. It has added a new method readStream which is similar to read method.
Having same abstraction for reading data in both batch and streaming makes code more consistent and easy to understand.
Reading from Socket Stream
As an example to show case the datasource API, let’s read from socket stream. The below is the code to do that
As you can observe from above code, reading a stream has become very similar to reading static data. In the code, readStream indicates we are reading continuous data rather than static data.
If you have done spark streaming before, you may have observed there is no mention of batch time. This is because in structured streaming, the rate of consumption of stream is determined by the sink not by the source. So when we create the source we don’t need to worry about the time information.
The result , socketStreamDf is a dataframe containing data from socket.
Schema Inference in Structured Streaming
One of the important feature of data source API is it’s support for the schema inference. This means it can go through the data to automatically understand the schema and fill that in for dataframe. This is better than specifying the schema manually. But how that works for streams?
For streams, currently schema inference is not supported. The schema of the data has to be provided by the user or the data source connector. In our socket stream example, the schema is provided by the socket connector. The schema contains single column named value of the type string. This column contains the data from socket in string format.
So in structured streaming we will be specifying the schema explicitly contrast to schema inference of batch API.
Writing to Sink
To complete a stream processing, we need both source and sink. We have created dataframe from socket source. Let’s write that to a sink.
To create a sink, we use writeStream. In our example, we are using console sink which just prints the data to the console. In the code, we have specified output mode, which is similar to save modes in batch API. We will talk more about them in future posts.
The result of writeStream is a DataStreamWriter. Now we have connected the source and sink.
Streaming Query Abstraction
Once we have connected the source and sink, next step is to create a streaming query. StreamingQuery is an abstraction for query that is executing continuously in the background as new data arrives. This abstraction is the entry point for starting the stream processing. All the steps before it was the setting up the stream computatiuons.
Above code creates a streaming query from the dataframe writer. Once we have the query object, we can run awaitTermination to keep it running.
You can access complete code on github.
Running the example
Before we can run the example, we need to start the socket stream. You can do that by running below command
Now you can enter data in stdin, you can observe the result in the console.
We have successfully ran our first structured streaming example.
After successfully running the example, one question immediately comes in to mind. How frequently socket data is processed?. Another way of asking question is, what’s the batch time and where we have specified in code?
In structured streaming, there is no batch time. Rather than batch time, we use trigger abstraction to indicate the frequency of processing. Triggers can be specified in variety of ways. One of the way of specifying is using processing time which is similar to batch time of earlier API.
By default, the trigger is ProcessingTime(0). Here 0 indicates asap. This means as and when data arrives from the source spark tries to process it. This is very similar to per message semantics of the other streaming systems like storm.
Structured Streaming defines source and sinks using data source API. This unification of API makes it easy to move from batch world to streaming world.