haproxy.conf

global
    daemon
	chroot /data/haproxy
	user vagrant
	group vagrant

defaults
    timeout connect 1000ms
    timeout server  1000ms
    timeout client  1000ms

frontend frontend_mongodb
    mode tcp
    bind *:27016
    default_backend backend_mongodb

backend backend_mongodb
    option tcp-check

    # @see https://blog.danman.eu/mongodb-haproxy/
    # Wire protocol
    tcp-check send-binary 3a000000 # Message Length (58)
    tcp-check send-binary EEEEEEEE # Request ID (random value)
    tcp-check send-binary 00000000 # Response To (nothing)
    tcp-check send-binary d4070000 # OpCode (Query)
    tcp-check send-binary 00000000 # Query Flags
    tcp-check send-binary 61646d696e2e # fullCollectionName (admin.$cmd)
    tcp-check send-binary 24636d6400 # continued
    tcp-check send-binary 00000000 # NumToSkip
    tcp-check send-binary FFFFFFFF # NumToReturn
    # Start of Document
    tcp-check send-binary 13000000 # Document Length (19)
    tcp-check send-binary 10 # Type (Int32)
    tcp-check send-binary 69736d617374657200 # ismaster:
    tcp-check send-binary 01000000 # Value : 1
    tcp-check send-binary 00 # Term
    tcp-check expect binary 69736d61737465720001 #ismaster True

    server mongodb_27017 127.0.0.1:27017 check inter 1s
    server mongodb_27018 127.0.0.1:27018 check inter 1s