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Published: Dec 8, 2022 License: Apache-2.0, MIT Imports: 10 Imported by: 10



Package flowid implements a filter used for identifying incoming requests through their complete lifecycle for logging and monitoring or else.

Flow Ids let you correlate router logs for a given request against the upstream application logs for that same request. If your upstream application makes other requests to other services it can provide the same Flow ID value so that all of those logs can be correlated.

How It Works

Skipper generates a unique Flow Id for every HTTP request that it receives. The Flow ID is then passed to your upstream application as an HTTP header called X-Flow-Id.

The filter takes 1 optional string parameter that, when set to "reuse", will make the filter check for the presence of another FlowID already set in the inbound request in a header with the same name. If the existing header value is not a valid flow id it is ignored and a new flow id is generated, overwriting the previous one. Any other string used for this parameter is ignored and trigger the same, default, behavior - to ignore any existing X-Flow-Id header.


The Flow ID generation can follow any format. Skipper provides two Generator implementations - Standard and ULID. They offer different performance and options and you can choose which one you prefer.

Standard Flow IDs

The Standard Generator uses a base 64 alphabet and can be configured to generate flow IDs with length between 8 and 64 chars. It is very fast for small length FlowIDs and uses a system shared entropy source. It is safe for concurrent use.


The ULID Generator relies on the great work from https://github.com/alizain/ulid and https://github.com/oklog/ulid. It generates 26 char long Universally Unique Lexicographically Sortable IDs. It is very fast and it's also safe for concurrent use.

Programmatic Usage

To create a specification of the FlowID filter you can either create the default specification, which uses the Standard Generator with a length of 16 chars as default or provide your own Generator instance.

Default spec with Standard Generator (16 char FlowIDs)


Custom spec with ULID Generator (26 char FlowIDs)

g := NewULIDGenerator()

Custom spec with your own Generator implementation

myCustomGenerator := newCustomGenerator(arg1, arg2)

Routing Usage

The filter can be used with many different combinations of parameters. It can also be used without any parameter, using defaults

Default parameters


Without any parameters, the filter doesn't reuse existing X-Flow-Id headers and generates new ones for every request.

Reuse existing flow id


With a single string parameter with the value "reuse", the filter will accept an existing X-Flow-Id header, if it's present in the request. If it's invalid, a new one is generated and the header is overwritten.

Some Benchmarks

Built-In Flow ID Generator

To decide upon which hashing mechanism to use we tested some versions of UUID v1 - v4 and some other implementations. The results are as follow:

Benchmark_uuidv1-4	            5000000	       281 ns/op
Benchmark_uuidv2-4	            5000000	       284 ns/op
Benchmark_uuidv3-4	            2000000	       605 ns/op
Benchmark_uuidv4-4	            1000000	      1903 ns/op
BenchmarkRndAndSprintf-4  	    500000	      3312 ns/op
BenchmarkSha1-4 	            1000000	      2188 ns/op
BenchmarkMd5-4  	            1000000	      2076 ns/op
BenchmarkFnv-4  	            500000	      2223 ns/op

The next approach was just to get len / 2 (hex.DecodedLen) bytes from the crypto/rand and hex encode them. The performance was only dependent on the length of the generated FlowId and it performed like to the following benchmarks:

BenchmarkFlowIdLen8-4 	 1000000	      1157 ns/op
BenchmarkFlowIdLen10-4	 1000000	      1162 ns/op
BenchmarkFlowIdLen12-4	 1000000	      1163 ns/op
BenchmarkFlowIdLen14-4	 1000000	      1171 ns/op
BenchmarkFlowIdLen16-4	 1000000	      1180 ns/op
BenchmarkFlowIdLen32-4	 1000000	      1957 ns/op
BenchmarkFlowIdLen64-4	  300000	      3520 ns/op

Starting at len = 32 (16 random bytes) the performance started to drop dramatically.

The current implementation defines a static alphabet and build the flowid using random indexes to get elements from that alphabet. The initial approach was to get a random index for each element. The performance was:

BenchmarkFlowIdLen8-4 	 5000000	       375 ns/op
BenchmarkFlowIdLen10-4	 3000000	       446 ns/op
BenchmarkFlowIdLen12-4	 3000000	       508 ns/op
BenchmarkFlowIdLen14-4	 3000000	       579 ns/op
BenchmarkFlowIdLen16-4	 2000000	       641 ns/op
BenchmarkFlowIdLen32-4	 1000000	      1179 ns/op
BenchmarkFlowIdLen64-4	 1000000	      2268 ns/op

It was possible to optimize this behavior by getting a 64 bit random value and use every 6 bits (a total of 10 usable random indexes) to get an element from the alphabet. This strategy improved the performance to the following results:

BenchmarkFlowIdStandardGenerator/8-8           	20000000	       104 ns/op	      16 B/op	       2 allocs/op
BenchmarkFlowIdStandardGenerator/10-8          	10000000	       121 ns/op	      32 B/op	       2 allocs/op
BenchmarkFlowIdStandardGenerator/12-8          	10000000	       155 ns/op	      32 B/op	       2 allocs/op
BenchmarkFlowIdStandardGenerator/14-8          	10000000	       158 ns/op	      32 B/op	       2 allocs/op
BenchmarkFlowIdStandardGenerator/16-8          	10000000	       168 ns/op	      32 B/op	       2 allocs/op
BenchmarkFlowIdStandardGenerator/26-8          	10000000	       224 ns/op	      64 B/op	       2 allocs/op
BenchmarkFlowIdStandardGenerator/32-8          	 5000000	       262 ns/op	      64 B/op	       2 allocs/op
BenchmarkFlowIdStandardGenerator/64-8          	 3000000	       444 ns/op	     128 B/op	       2 allocs/op
BenchmarkFlowIdStandardGeneratorInParallel/8-8 	 5000000	       239 ns/op	      16 B/op	       2 allocs/op
BenchmarkFlowIdStandardGeneratorInParallel/10-8         	 5000000	       244 ns/op	      32 B/op	       2 allocs/op
BenchmarkFlowIdStandardGeneratorInParallel/12-8         	 3000000	       469 ns/op	      32 B/op	       2 allocs/op
BenchmarkFlowIdStandardGeneratorInParallel/14-8         	 3000000	       473 ns/op	      32 B/op	       2 allocs/op
BenchmarkFlowIdStandardGeneratorInParallel/16-8         	 3000000	       473 ns/op	      32 B/op	       2 allocs/op
BenchmarkFlowIdStandardGeneratorInParallel/26-8         	 2000000	       701 ns/op	      64 B/op	       2 allocs/op
BenchmarkFlowIdStandardGeneratorInParallel/32-8         	 2000000	       918 ns/op	      64 B/op	       2 allocs/op
BenchmarkFlowIdStandardGeneratorInParallel/64-8         	 1000000	      1576 ns/op	     128 B/op	       2 allocs/op

ULID Flow ID Generator

BenchmarkFlowIdULIDGenerator/Std-8                      	10000000	       194 ns/op	      48 B/op	       2 allocs/op
BenchmarkFlowIdULIDGeneratorInParallel-8                	 5000000	       380 ns/op	      48 B/op	       2 allocs/op



View Source
const (
	// Deprecated, use filters.FlowIdName instead
	Name = filters.FlowIdName

	ReuseParameterValue = "reuse"
	HeaderName          = "X-Flow-Id"
View Source
const (
	MaxLength = 64
	MinLength = 8


View Source
var (
	ErrInvalidLen = fmt.Errorf("invalid length, must be between %d and %d", MinLength, MaxLength)


func New

func New() *flowIdSpec

New creates a new instance of the flowId filter spec which uses the StandardGenerator. To use another type of Generator use NewWithGenerator()

func NewFlowId deprecated

func NewFlowId(l int) (string, error)

NewFlowId creates a new standard generator with the defined length and returns a Flow ID.

Deprecated: For backward compatibility this exported function is still available but will removed in upcoming releases. Use the new Generator interface and respective implementations

func NewWithGenerator

func NewWithGenerator(g Generator) *flowIdSpec

New behaves like New but allows you to specify any other Generator.


type Generator

type Generator interface {
	// Generate returns a new Flow ID using the implementation specific format or an error in case of failure.
	Generate() (string, error)
	// MustGenerate behaves like Generate but panics on failure instead of returning an error.
	MustGenerate() string
	// IsValid asserts if a given flow ID follows an expected format
	IsValid(string) bool

Generator interface should be implemented by types that can generate request tracing Flow IDs.

func NewStandardGenerator

func NewStandardGenerator(l int) (Generator, error)

NewStandardGenerator creates a new FlowID generator that generates flow IDs with length l. The alphabet is limited to 64 elements and requires a random 6 bit value to index any of them. The cost to rnd.IntXX is not very relevant but the bit shifting operations are faster. For this reason a single call to rnd.Int63 is used and its bits are mapped up to 10 chunks of 6 bits each. The byte data type carries 2 additional bits for the next chunk which are cleared with the alphabet bit mask. It is safe for concurrent use.

func NewULIDGenerator

func NewULIDGenerator() Generator

NewULIDGenerator returns a flow ID generator that is able to generate Universally Unique Lexicographically Sortable Identifier (ULID) flow IDs. It uses a shared, pseudo-random source of entropy, seeded with the current timestamp. It is safe for concurrent usage.

func NewULIDGeneratorWithEntropyProvider

func NewULIDGeneratorWithEntropyProvider(r io.Reader) Generator

NewULIDGeneratorWithEntropyProvider behaves like NewULIDGenerator but allows you to specify your own source of entropy. Access to the entropy provider is safe for concurrent usage.

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