What is `<<` and `&` in yaml mean?

Question

When I review the cryptogen(a fabric command) config file . I saw there symbol.

Profiles:      SampleInsecureSolo:         Orderer:             <<: *OrdererDefaults  ## what is the `<<`             Organizations:                 - *ExampleCom     ## what is the `*`         Consortiums:             SampleConsortium:                 Organizations:                     - *Org1ExampleCom                     - *Org2ExampleCom 

Above there a two symbol << and *.

Application: &ApplicationDefaults  # what is the `&` mean      Organizations: 

As you can see there is another symbol &. I don't know what are there mean. I didn't get any information even by reviewing the source code (fabric/common/configtx/tool/configtxgen/main.go)

Answer 1

Well, those are elements of the YAML file format, which is used here to provide a configuration file for configtxgen. The "&" sign mean anchor and "*" reference to the anchor, this is basically used to avoid duplication, for example:

person: &person     name: "John Doe"  employee: &employee     << : *person     salary : 5000 

will reuse fields of person and has similar meaning as:

employee: &employee     name   : "John Doe"     salary : 5000 

another example is simply reusing value:

key1: &key some very common value  key2: *key 

equivalent to:

key1: some very common value  key2: some very common value 

Since abric/common/configtx/tool/configtxgen/main.go uses of the shelf YAML parser you won't find any reference to these symbols in configtxgen related code. I would suggest to read a bit more about YAML file format.