Flat Buffers

https://google.github.io/flatbuffers/

Flashback to structs in c

struct contact
{
   char name[50];
   char phone[10];
   int age;
};

Data is stored as bytes, co-located in memory.  It's very small (compared to serialized formats like json), it's fast to read, but it's hard to transport, why?

* If I gave someone else a memory address to read, they'd need to know how long each field is (ex, first 50 bytes are the name field)

* They'd need to know the order of the bytes (big vs little endian)

* What if consumer is java and not c? what's a struct?

Typical deserialization (not just Java)

* Given a byte array

* (for non-schema formats like json) scan the array, look for tokens, parse out structure (i.e. look for {}, "", :, etc)

* Create a new object in memory

* Copy the data into the new memory object

[ A1, 00, B7, 23 ]

class Foo {
    int bar; // 161
    int baz; // 9143
}

needed to know byte order, had to copy data, data in memory twice, extra GC

Flat Buffer approach (simplified)

* Given a byte array

* Given a schema

* data "on demand" read out of the byte array according to schema

[ A1, 00, B7, 23 ]

class Foo(val byteArray) {

    fun getBar(): Int {
      // jump to offset 0, read 2 bytes as little endian int
    }
    fun getBaz(): Int {
      // jump to offset 2, read 2 bytes as little endian int
    }
}

Flat Buffer use (tl;dr)

* Write Schema file

* Generate Java classes (or c++/python/etc) from schema

* Use FlatBufferBuilder to build FBO

* Traverse your byte buffer in place

 

Flat Buffer (physIQ)

* Write Schema file

contracts/flatbuffers/schema/com.physiq.vitalink.sdk.flatbuffers.series.fbs

table Int8Channel {
    data:[byte];
}

union ChannelDataUnion {
    Int8Channel,
...
    StringChannel
}

table ChannelData {
    readings:ChannelDataUnion;
}

table SamplingSetData {
    channels:[ChannelData];
}

Flat Buffer (physIQ)

* Generate Java Classes

cloud/code/sdk/java-sdk/build/generated/source/

flatbuffers-generator/main/java/com/physiq/

vitalink/sdk/flatbuffers/series/Int8Channel.java

public final class Int8Channel extends Table {
  public static Int8Channel getRootAsInt8Channel(ByteBuffer _bb) { return getRootAsInt8Channel(_bb, new Int8Channel()); }
...

  public static int createInt8Channel(FlatBufferBuilder builder,
      int dataOffset) {
    builder.startObject(1);
    Int8Channel.addData(builder, dataOffset);
    return Int8Channel.endInt8Channel(builder);
  }

Flat Buffer (physIQ)

* Build FBO

sampling_sets:
- alias: min-avg
  channels:
  - alias: hr
    desc: Minute average of heart rate.
    name: Heart Rate
    type: INT16
    classification: HR
    units: BPM
table SeriesFrame {
    frameId:long;
    samplingSets:[SamplingSetData];
}
table SamplingSetData {
    id:byte;
    channels:[ChannelData];
}
cloud/code/flink/processor/sbm/src/test/kotlin/com/physiq/vitalink/timeseries/sbm/DataHelpers.kt
fun buildVitalHRSeriesFrame(frameNumber: Int, x: ShortArray): ByteArray {
    return FlatBufferBuilder().apply {
        finish(SeriesFrame.createSeriesFrame(
            this, // builder
            17460 + frameNumber.toLong(), // frame id
            -1, // ingested at micros
            SeriesFrame.createSamplingSetsVector(
                this, // builder
                intArrayOf(SamplingSetData.createSamplingSetData(
                    this, // builder
                    0.toByte(), // sampling set number
                    0, // start offset
                    SamplingSetData.createChannelsVector(
                        this, // builder
                        intArrayOf(ChannelData.createChannelData(
                            this, // builder
                            0.toByte(), // vector number
                            ReadingType.INT16,
                            Int16Channel.createInt16Channel(
                                this, // builder
                                Int16Channel.createDataVector(
                                    this, // builder
                                    x // shortArray
                                )
                            )
                        ))
                    )
                ))),
            -1,
            0
        ))
    }.sizedByteArray()
}
    val result = Int16Channel().apply { 
        flatBufferObject.obj.dataAsSeriesFrame().samplingSets(0)
                    .channels(0).readings(this)
    }
    val hr = result.data(0)

Read the data without copies

short hp = monster.hp();
Vec3 pos = monster.pos();

Compare to example code from google

Benchmarks and Benefits

also less GC = better performance for us jvm'ers

Further Reading

Flat buffers https://google.github.io/flatbuffers/

C structs https://www.geeksforgeeks.org/structures-c/

 

Flat Buffers

By Philip Doctor

Flat Buffers

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