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Go HLC(Hybrid Logic Clock)

Intro

To implement a mvcc manager for object storage, we usually design a version allocator usually a number that increases monotonically, but sometimes machine crashed and time drift back to the past, and version(clock) allocated re-allocated by mistake, so HLC is for solving this problem.

I didn't find any repo fit for my request, so I decided to designed/implemented my own hlc module.

Detail

I use a time(unix_nano) as part of the physical clock and allocate version from this time(clock) like this:

// pseudo-code for allocate one version
 
v := allocatedVersion
desire := v+1
cas(&allocatedVersion, v, desire)

In addition to the logic above , we have another thread to save version to persistent storage, it updated lastSavedTimestamp after save and signal all clients who wait for versions.

if var desire catch up the lastSavedTimestamp, the allocate thread wait quietly, after saving hybrid logic clock to persistent storage, the process will be signaled.

Allocator process need not wait for the persistence of timestamp under normal conditions, there is a timed task to do this periodically, they even not feel the existence of persistence process.

Architecture

  • persistence: developers can define a persistence.Interface by themselves, save the version to where the like: eg boltdb, disk, etcd, mysql...(I have implemented a disk one for you to use)
  • service: it implemented a hlc service for user to call by grpc.

Use

It is easy to run service as a gprc server, I'll provide the full code sample later.

Benchmark

If all goes well(persistent process), it can reach 1.5M QPS, if use batch get request, you can get 67M version if every request get 50 versions.

hlc_test.go:56: finished 20000000 query & get 20000000 clocks in 12.70 sec
hlc_test.go:77: finished 20000000 query & get 1000000000 clocks in 14.77 sec