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:extension OverloadedStrings
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:load krapsh
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:load KrapshDisplays KrapshDagDisplay
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import Spark.Core.Dataset
import Spark.Core.Context
import Spark.Core.Functions
import Spark.Core.Column
import Spark.Core.ColumnFunctions
Krapsh communicates with Spark through a session object, called a SparkSession. For the purpose of interactive exploration, an implicit session can be created for a notebook. This pattern is not recommended for production cases, but it lets you try things quickly in a notebook.
Create a configuration objet. You can specify the location of the Spark endpoint. Calling createSparkSessionDef will allocate a default session. Note that all the xxxDef functions also have a xxx equivalent that takes or returns a session.
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let conf = defaultConf {
confEndPoint = "http://localhost",
confRequestedSessionName = "session00_introduction" }
print conf
createSparkSessionDef conf
Let us run our first program on Spark. We are going to create a tiny dataset and compute the number of elements in this dataset.
Creating a sequence of Spark operations does not require a session: at this point, you declare the operations that you want to do.
The command to create a dataset from existing elements is (surprise) dataset:
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let ds = dataset ([1 ,2, 3, 4]::[Int])
ds
In order to count the number of elements, we are just going to use the built-in count command.
Unlike Spark, this command is also declarative and lazy: no computation will happen when it is called. It will return an observable that we can combine with other nodes or evaluate.
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let c = count ds
c
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:type c
In order to query the value and execute the computation graph, you need to call one of the exec commands. This analyzes the computation graph for possible errors, sends it to Spark for execution, and returns the result.
In this notebook, we will use the default execution context, which is implicitly used when calling exec1Def. For production cases, you should pass your own context and use exec1.
You can only send observables. Dataframes cannot be evaluated directly.
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mycount <- exec1Def c
As expected, mycount is an integer with the value 4:
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:t mycount
mycount
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:t mycount
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_ <- exec1Def c
Computations in Krapsh are completely deterministic: the same computation graph will always return the same exact result. Thanks to this property, Krapsh can aggressively cache final and intermediate results, and reuse them when they can remove some chunks of computations. Furthermore, since the graph of computations fully describes the computation, it can be saved along the data as a proof of how the result got generated, guaranteeing reproducible results.
Because some operations in Spark are intrisincally non-deterministic, this may require some changes from existing code. For example:
collect always sort their results to maintain a result that is independent from the data layoutrandom is not available yet. Some strategies based on hashing are being considered.current_time will most probably never be available within Krapsh. However, the current time can be retrieved from the environment and passed as a constant.Note this is a preview, so the caching is not complete yet.
For example, when distributing and collecting a dataset, the order of the initial data does not matter:
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set = dataset ([1,2,3] :: [Int])
x = collect (asCol set)
exec1Def x
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set = dataset ([3,2,1] :: [Int]) -- Data is reversed, but the output is the same.
x = collect (asCol set)
exec1Def x
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