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A port of Apache Sparks to Haskell using distributed process
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README.md

Haskell Spark

This is an attempt to have functionality similar to Apache Spark using Cloud Haskell.

Apache Spark implements a generic cluster computing framework that can work with multiple types of backends, such as file system, in-memory, hadoop, mesos etc.

Intention of this project to mimic the Spark computation model, using Cloud Haskell

Dissecting Apache RDDs

Spark implements RDD (Resilient Distributed Data). This is resilient because it can recreate itself in case of an excepton. It does so by remembering the lineage, and recreating itself from the lineage, thus making it possible to recover a lost node.

The RDD is distributed because it is typically distributed over the participating nodes (called partitions). The distribution can by just partitioning, or by grouping the data using hash or range partitions.

The RDD is lazily calculated. It is possible to create RDD by mapping over base RDD. This way map-reduce process can be constructed using various mapping or grouping functions. Apache Spark provides many RDD types which can used to stack up RDDs.

When computed, Spark creates DAG of dependency RDDs and partitions, and schedules tasks on each partition. It also tries to pipeline the mapping operations on same node wherever possible.

Closure Serialization

RDDs in Spark are serializable, in addition to this, Spark implementation also serializes the closure of the RDDs (and functions there within) to slave nodes, and can execute there.

hspark

In hspark, an attempt is made to mimic computation model of Spark using cloud haskell. (distributed-process).

Scope

* Creation of simple DSL for creating map-reduce operations
* Porting computation model over a cluster

The aspects which are not considered here: * Resiliency - The resiliency of the data is not considered. However, since there is a inherent dependency between RDDs, it should be possible to have this ability. * Error Handling - Current implementation depicts happy scenario, and exceptions may not be proliferated properly.

Static Pointer

With GHC 7.10.x, GHC has *StaticPointers extension that allows fingerprinting the closed expression (without free variable). The closure can be constructed around static pointers. The closure then can be transferred over the wire.

Note that the static pointers can take care of monomorphic functions. By using Dict Trick, qualified types can also be serialized. Polymorphic types are not handled by StaticPointers.

distributed-static uses Rank1Types, and applies to Dynamic. This enables, type checking polymorphic types equality while constructing closures.

Since polymorphic types are not covered by StaticPointers extension, distributed-process constrcts "remote table* that contains static labels for various functions (including static pointers). This table is shared over network to help looking up the functions.

hspark leverages this closure to spawn processes over nodes.

Context

Context stores the information about slave nodes and master node. Context also stores other information such as remote table.

RDD Type class

Each RDD in hspark implements a type class called RDD. This type class enables flow of RDD in the computation.

RDD can be independent (such as SeedRDD) which is typically starting point of the computation. Or it can be constructed from existing RDD (dependent RDD).

Process as a computation and storage

Each RDD is further divided into blocks. Each block is numbered, and actually is a process. The process here depicts an actor that can do following things:

  • Computation - Carrying out a unit work by applying function or transformation over a data set.
  • Fetching - Fetching the data from parent RDD partition (also a process).
  • Delivery - Holding up the data till it is asked for.

The block process exits when the data is read. In a way, the hspark treats process as distributed MVar with input and output channels set (and exception that it might exit after delivery of the data to child process. Also note that during reduction step, this may not happen).

RDD Computation

Each RDD starts by triggering of "base" RDD. This usually returns asynchronously, and gives access to blocks. Each block is a process.

For each such process, the current RDD, may create another dependent process, and would wait for parent process to deliver the data. Upon delivery it starts working up computation that is is supposed to do.

After completion of the computation, the process holds the data until asked.

Work Distribution

The work distribution is done by dividing the data into multiple blocks. Each block is assgined (pushed) to a node. A node can host multiple processes.

The mapping operations are carried on the same node where parent operation is performed. This is currently fixed, and should change in the future to help resliency.

Reduction and Shuffle

Reduction step necessiates shuffling. Currently shuffling is handled in two stages.

In the first stage, the data is reduced locally using the combining functions.

Second stage involves grouping based on partitioning function provided by user (In future, this might be based on inherent property of input data, such as range etc.). Partitioning divides the data into independent (keys are localized), and further reduction is done.

Since second stage (actually partitioning function) ensures partitioning in such a way that across the group computation is eliminated. hspark does not check the validity of partitioning function.

Current Implementation

Current hspark implements following types of RDDs.

  • SeedRDD - For seeding the data.
  • MapRDD - For mapping the data.
  • MaoIORDD - For mapping an IO operation
  • ReduceRDD - For combining key value pair.

Tests

The simple tests for each RDD are in test folder.

Immediate enhancement

* Adding *FilterRDD*
* Adding range based partitioning
* Error handling and linking processes so that their lifespan can
  be controlled. 

References

/Note: This work is done as a part of CS240H coursework at stanford/

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