tempo

Go implementation of Facebook's GorillaDB: an in-memory write-through cache for time-series data

import (
  "time"
  "github.com/s4ayub/tempo"
)

func main() {
  // A time series is the underlying data structure behind the time series map.
  // A user of the package will never interact with the TimeSeries struct, but the
  // example code highlights our implementation of the delta-of-delta encoding for time and XOR encoding for values.

  startTime := time.Now()
  ts := NewTimeSeries(startTime)

  t1 := startTime.Add(time.Second * time.Duration(60)).Unix() // store in seconds
  ts.timeEncode(t1)
  ts.dataEncode(2)

  t2 := t1.Add(time.Second * time.Duration(60)).Unix()
  ts.timeEncode(t2)
  ts.dataEncode(3)
  
  fmt.Println(ts)
}

Changes to the encoding scheme:

Tempo stores its timestamps and data values in a single byte-stream (for now). The encoding schemes used by GorillaDB takes advantage of bit-level granularity.

Here, we discuss how we changed the algorithms to be byte-friendly, and the ramifications for doing so.

Timeseries encoding scheme:

1. The tag bits are always stored in 8 bits instead of 4.

   • The compression ratio for timestamps is reduced

2. The value bits associated with each tag is changed:

   • [0] -> 0 bits (the same as before)

   • [-63, 64] -> 8 bits

   • [-255, 256] -> 16 bits

   • [-2047, 2048] -> 16 bits

   • [Otherwise] -> 32 bits (the same as before)

   • Again, the compression ratio drops as unused bits are added.

   • [-255, 256] for example, requires only 9 bits but 16 bits are reserved.

3. Example of an encoded timestamp entry

   • {Tag bits}{Value}
     • 'Tag bits' is the tag defining the number of bits the time value will use
     • 'Value' is the actual delta-of-delta time value
     • Each { } item is 1 byte, but '...' represents a variable amount of bytes

Data encoding scheme:

1. The dissimilar bit, the control bit, and the leading zeroes are stored in the first byte.

   • [xxxx xxxx]
   • b[7] is the dissimilar bit
   • b[6] is the control bit
   • b[5:0] is the number of leading zeroes of the meaningful xor
     • LZ is encoded in GorillaDB, but it is not encoded in Tempo
     • These 6 bits would have been here regardless of whether we used it to store LZ

2. The control bit represents whether the length of the meaningful xor is reused

   • In GorillaDB, this is an indicator for whether the LZ and the MXOR length is reusued
   • In Tempo, this is an indicator for whether the MXOR length is reused
     • The LZ is always in the first byte

3. Example of an encoded data entry

   • {Header}{MXOR length}{MXOR...}
     • 'Header' is the byte containing the dissimilar bit, control bit, and LZ
     • 'MXOR length' is the number of bits used to store the MXOR
     • 'MXOR' is the actual meaningful xor bits
     • Each { } item is 1 byte, but '...' represents a variable amount of bytes

To do:

• Lay out the TimeSeries struct to encase timestamps and data in a single byte stream
• Implement and test XOR encoding for data values
• Implement and test delta-of-delta encoding for timestamps
• Write a decoder to return decoded blocks of time series data
• Build the TimeSeriesMap structure which will abstract away the TimeSeries struct