Pipes

Joule pipes provide a protocol independent interface to data streams. This decouples module design from pipeline implementation. There are three different pipe implementations, joule.LocalPipe, joule.InputPipe, and joule.OutputPipe all of which derive from the abstract base class joule.Pipe. LocalPipe's are intended for intra-module communication (see Composite Modules). It is bidirectional meaning it supports both read and write operations. InputPipe's and OutputPipe are unidirectional supporting only read and write respectively. These are intended for inter-module communication with one module's OutputPipe connected to another's InputPipe. The figure below illustrates how pipes move stream data between modules.

_images/pipe_buffer.png

In the figure the pipe is initially empty at Time 0. At Time 1, the producer adds four rows of data which the consumer reads at Time 2. The consumer only consumes two rows so the last two rows of data remain in the pipe. At Time 4 the producer writes three more rows of data which are appended to the two unconsumed rows. The next call to read returns all five rows of data. This time the consumer consumes all the data and the pipe is empty. Conceptually the pipe is a buffer between the producer and consumer, however it is important to note that these are independent processes and may not even be running on the same machine.

Note

When designing modules care must be taken to ensure that they execute fast enough to handle streaming data. If a module’s memory usage increases over time this indicates the module cannot keep up with the input and the Joule Pipe buffers are accumulating data.

Intervals

Streams are divided into intervals of continuous time series data. The first write to a pipe starts a new stream interval. Subsequent writes append data to this interval. This indicates to data consumers that there are no missing samples in the stream. To indicate missing data the producer closes the

interval. A new interval is started on the next write. The plot below shows a stream

with three seperate intervals indicating two regions of missing data.

_images/intervals.png
Data Producers
The code snippet below shows how a data producer indicates missing samples using intervals. In normal operation the sensor output is a single continuous stream interval. If the sensor has an error the exception handler closes the current interval and logs the event. See IntermittentReader for a complete example.
while True:
    try:
        data = sensor.read()
        await pipe.write(data)
    except SensorException:
        log.error("sensor error!")
        await pipe.close_interval()
Data Consumers
The code snippet below shows how a data consumer detects interval breaks in a stream. At an interval boundary read will return data up to the end of the current interval and set the end_of_interval flag. The next call to read will return data from the new interval and clear the flag. Any unconsumed data will be returned with the next read even though it is from a previous interval. Therefore it is best practice to completely consume data on an interval break and reset any buffers to their initial state as suggested with the fir_filter logic below. See MedianFilter for a complete example on handling interval boundaries.
while True:
    data = await pipe.read()
    pipe.consume(len(data))
    output = fir_filter.run(data)
    if(pipe.end_of_interval):
        fir_filter.reset()

In general filters should propagate input interval breaks to their outputs. In other words unless a filter can restore missing input data, it should have at least as much missing output data. The snippet below shows the logic for propagating interval breaks.

data = await input_pipe.read()
input_pipe.consume(len(data))
await output_pipe.write(data)
if data.end_of_interval:
    await output_pipe.close_interval()

Caching

By default a call to write will immediately send the data to the transport layer (OS pipe, network socket, etc). In order to reduce the overhead associated with the transport layer data should be batched. Data may be batched manually (see high_bandwidth_reader) or you may use the pipe cache to perform data batching. When the cache is enabled the transport layer will only be executed when the specified number of rows have been written. This eliminates the performance penalty of frequent short writes. Timestamps should be linearly interpolated for high bandwidth data rather than individually timestamped. The cache should be sized to execute writes at about 1 Hz.

# sensor produces data at 1KHz
time = time.now()
pipe.enable_cache(1000)
while(1):
    data_point = sensor.read()
    ts += 1000 #1ms = 1000us
    await pipe.write([[ts,data_point]])

Subscriptions

A single input can be copied to multiple outputs using pipe subscriptions. Pipes that produce output (OutputPipe or LocalPipe) A LocalPipe can subscribe

to input end of a LocalPipe can be

subscribed to either an OutputPipe or the output end of another LocalPipe.

Note

Pipe subscriptions are only necessary for creating data paths within Composite Modules. Joule manages inter-module data paths automatically

p1.subscribe(p2)
await p1.write([1,2,3])
await p1.read() # 1,2,3
p1.consume(len(data))
# p2 still has the data
p2.read() # 1,2,3
p2.consume(2)
await p1.write([4,5,6])
p1.read() # 4,5,6
p2.read() # 3,4,5,6