Balance Algorithms

The Dataplane v3 API accepts exactly ten balance algorithms. HAProxy UI exposes all ten in the backend inspector, and exposes the argument fields for the six that take arguments.

Algorithms

Algorithm	Arguments	Inspector fields	Emitted as
`roundrobin`	none	—	`balance roundrobin`
`static-rr`	none	—	`balance static-rr`
`leastconn`	none	—	`balance leastconn`
`first`	none	—	`balance first`
`source`	none	—	`balance source`
`hash`	required	Hash sample expression	`balance hash req.uri`
`uri`	optional	Hash whole URI (toggle), URI length, URI depth	`balance uri whole len 5`
`hdr`	required	Header name	`balance hdr(host)`
`url_param`	required	Parameter name, Check on POST, Max wait (ms)	`balance url_param user_id check_post 1 max_wait 5`
`rdp-cookie`	required	Cookie name	`balance rdp-cookie sessid`
`random`	optional	Draws	`balance random draws 4`

Algorithms not in this list — alpha, cookie, map(<file>) — are not part of the v3 schema and are rejected by the API. They are deliberately absent from the dropdown rather than offered and then refused.

Hash type

When the algorithm is hash or uri, haproxy requires the backend to also declare a hash-type. Without it the apply is rejected with “balance hash requires a sample expression”-class errors.

The dropdown offers the seven values the v3 spec enumerates across hash_type.method, .function and .modifier:

Value	Spec slot	Notes
`map-based`	`method`	The haproxy default
`consistent`	`method`	Minimal remapping when servers join or leave — the usual choice for stickiness
`avalanche`	`modifier`	Defends the hash map against chosen-key attacks
`crc32`, `djb2`, `sdbm`, `wt6`	`function`	Hash function selection

How hash-type is stored. The v3 spec models it as an object ({method, function, modifier}). HAProxy UI keeps the whole directive as a string instead. This is deliberate: the string form round-trips byte-stable through the parser and serializer, and the API accepts it. Splitting it into three fields would add a shape conversion that changes nothing a user can act on. Upgrade this only if the Dataplane ever rejects the string form.

How the fields behave

Fields appear on demand. Each argument field is hidden unless its algorithm is selected, so the backend inspector shows only the arguments that are meaningful for the current choice.
Clearing a field removes the argument. Emptying the header name on a hdr backend drops hdr_name and the backend reverts to the bare balance hdr form — which haproxy will reject, deliberately, rather than silently invent a header.
Switching algorithm keeps other arguments. Changing from uri to hash preserves uri_whole in the model until it is explicitly cleared. This is intentional: the arguments are keyed independently of the algorithm, and dropping them silently on a dropdown change would lose work.

Troubleshooting

“balance hash requires a sample expression” — the hash expression field was left empty. Fill it in (for example req.uri).

“backend requires a hash-type” — set the hash type as well. Both fields are required together.

Apply succeeds but the config does not look right — check the raw-payload panel for a balance line carried verbatim from a paste-import; an unparsed directive will take precedence over the modelled one.