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

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.

Answer 2

in yaml if data is like
user: &userId '123'
username: *userId
equivalent yml is
user: '123'
username: '123'
or
equivalent json will is
{
"user": "123",
"username": "123"
}
so it basically allows to reuse data, you can also try with array instead of single value like 123
try converting below yml to json using any yml to json online converter
users: &users
k1: v1
k2: v2
usernames: *users