steampipes

Table of Contents generated with DocToc

• SteamPipes
  • Motivation
  • Notes
    • Ducts
      • Duct Configurations
    • Behavior for Ending Streams
    • Aborting Streams

SteamPipes

Fast, simple data pipelines built from first principles.

SteamPipes is the successor to PipeStreams and PipeDreams. PipeStreams was originally built on top of NodeJS streams and through; from version X███████████████ on, I switched to pull-streams.

Motivation

• Performance, X███████████████ insert benchmarks
• Simplicity of implementation, no recursion
• Observability, the data pipeline is an array of arrays that one may inspect

Notes

How to Construct Transforms

Sinks

Arbitrary objects can act as sinks provided they have a sink property; this property must be either set to true for a generic sink or else be an object that has push() method (such as a list). A sink may, furthermore, also have an on_end() method which, if set, must be a function that takes zero or one argument.

If the sink property is a list, then it will receive all data items that arrive through the pipeline (the resultant data of the pipeline); if it is true, then those data items will be discarded.

The on_end() method will be called when streaming has terminated (since the source was exhausted or a transform called aend.end()); if it takes one argument, then that will be the list of resultant data. If both the sink property has been set to a list and on_end() takes an argument, then that value will be the sink property (you probably only want the one or the other in most cases).

{ sink: true, }
{ sink: true, on_end: ( -> do_something() ), }
{ sink: true, on_end: ( ( result ) -> do_something result ), }
{ sink: x,    on_end: ( ( result ) -> do_something result ### NB result is x ### ), }

The only SteamPipes method that produces a sink is $drain() (it should really be called sink() but for compatibility with PipeStreams the name has been kept as a holdover from pull-stream). $drain() takes zero, one or two arguments:

$drain()                is equiv. to   { sink: true, }
$drain                     -> ...     is equiv. to   { sink: true, on_end: (       -> ... ), }
$drain               ( x ) -> ...     is equiv. to   { sink: true, on_end: ( ( x ) -> ... ), }
$drain { sink: x, },       -> ...     is equiv. to   { sink: x,    on_end: (       -> ... ), }
$drain { sink: x, }, ( x ) -> ...     is equiv. to   { sink: x,    on_end: ( ( x ) -> ... ), }

Asynchronous Sources and Transforms

Asynchronous transforms can be constructed using the 'asynchronous remit' method, $async(). The method passed into $async() must accept three arguments, namely d (the data item coming down the pipeline), send (the method to send data down the pipeline), and, in addition to synchronous transforms, done, which is a callback function used to signal completion (it is analogous to the resulve argument of promises, new Promise ( resulve, reject ) -> and indeed implemented as such). An example:

X███████████████
X███████████████
X███████████████
X███████████████

Ducts

Duct Configurations

I. Special Arities

There are two special duct arities, empty and single. An empty pipeline producers a duct marked with is_empty: true; it is always a no-op, hence discardable. The duct does not have a type property.

A pipeline with a single element produces a duct with the property is_single: true; it is always equivalent to its sole transform, and its type property is that of its sole element.

SHAPE OF PIPELINE                     SHAPE OF DUCT                   REMARKS
⋆ []                                  ⇨ { is_empty:  true,       } # equiv. to a no-op
⋆ [ x, ]                              ⇨ { is_single: true,       } # equiv. to its single member

II. Open Ducts

Open ducts may always take the place of a non-composite element of the same type; this is what makes pipelines composable. As one can always replace a sequence like ( x += a ); ( x += b ); by a non-composed equivalent ( x += a + b ), so can one replace a non-composite through (i.e. a single function that transforms values) with a composite one (i.e. a list of throughs), and so on:

SHAPE OF PIPELINE                     SHAPE OF DUCT                   REMARKS
⋆ [ source, transforms...,        ]   ⇨ { type:      'source',   } # equiv. to a non-composite source
⋆ [         transforms...,        ]   ⇨ { type:      'through',  } # equiv. to a non-composite transform
⋆ [         transforms..., sink,  ]   ⇨ { type:      'sink',     } # equiv. to a non-composite sink

III. Closed Ducts

Closed ducts are pipelines that have both a source and a sink (plus any number of throughs). They are like a closed electric circuit and will start running when being passed to the pull() method (but note that actual data flow may be indefinitely postponed in case the source does not start delivering immediately).

SHAPE OF PIPELINE                     SHAPE OF DUCT                   REMARKS
⋆ [ source, transforms..., sink,  ]   ⇨ { type:      'circuit',  } # ready to run

Behavior for Ending Streams

Two ways to end a stream from inside a transform: either

1. call send.end(), or
2. send SP.symbols.end.

The two methods are 100% identical. In SteamPipes, 'ending a stream' means 'to break from the loop that iterates over the data source'.

Note that when the pull method receives an end signal, it will not request any further data from the source, but it will allow all data that is already in the pipeline to reach the sink just as in regular operation, and it will also supply all transforms that have requested a last value with such a terminal value.

Any of these actions may cause any of the transforms to issue an unlimited number of further values, so that, in the general case, ending a stream is not guaranteed to actually stop processing at any point in time; this is only true for properly coöperating transforms.

Aborting Streams

There's no API to abort a stream—i.e. make the stream and all transforms stop processing immediately—but one can always wrap the pull pipeline... invocation into a try/catch clause and throw a custom symbolic value:

pipeline = []
...
pipeline.push $ ( d, send ) ->
  ...
  throw 'abort'
  ...
...
try
  pull pipeline...
catch error
  throw error if error isnt 'abort'
  warn "the stream was aborted"
...