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CW4 Spec: Group Members

cw4 package source code: https://github.com/CosmWasm/cw-plus/blob/master/packages/cw4/README.md

cw4 is a spec for storing group membership, which can be combined with cw3 multisigs. The purpose is to store a set of members/voters that can be accessed to determine permissions in another section.

Since this is often deployed as a contract pair, we expect this contract to often be queried with QueryRaw and the internal layout of some of the data structures becomes part of the public API. Implementations may add more data structures, but at least the ones laid out here should be under the specified keys and in the same format.

In this case, a cw3 contract could read an external group contract with no significant cost more than reading local storage. However, updating that group contract (if allowed), would be an external message and charged the instantiation overhead for each contract.

Messages#

We define an InitMsg{admin, members} to make it easy to set up a group as part of another flow. Implementations should work with this setup, but may add extra Option<T> fields for non-essential extensions to configure in the init phase.

There are three messages supported by a group contract:

UpdateAdmin{admin} - changes (or clears) the admin for the contract

AddHook{addr} - adds a contract address to be called upon every UpdateMembers call. This can only be called by the admin, and care must be taken. A contract returning an error or running out of gas will revert the membership change (see more in Hooks section below).

RemoveHook{addr} - unregister a contract address that was previously set by AddHook.

Only the admin may execute any of these function. Thus, by omitting an admin, we end up with a similar functionality ad cw3-fixed-multisig. If we include one, it may often be desired to be a cw3 contract that uses this group contract as a group. This leads to a bit of chicken-and-egg problem, but we cover how to instantiate that in cw3-flexible-multisig

Queries#

Smart#

TotalWeight{} - Returns the total weight of all current members, this is very useful if some conditions are defined on a "percentage of members".

Member{addr, height} - Returns the weight of this voter if they are a member of the group (may be 0), or None if they are not a member of the group. If height is set and the cw4 implementation supports snapshots, this will return the weight of that member at the beginning of the block with the given height.

MemberList{start_after, limit} - Allows us to paginate over the list of all members. 0-weight members will be included. Removed members will not.

Admin{} - Returns the admin address, or None if unset.

Raw#

In addition to the above "SmartQueries", which make up the public API, we define two raw queries that are designed for more efficiency in contract-contract calls. These use keys exported by cw4

TOTAL_KEY - making a raw query with this key (b"total") will return a JSON-encoded u64

members_key() - takes a CanonicalAddr and returns a key that can be used for raw query ("\x00\x07members" || addr) . This will return empty bytes if the member is not inside the group, otherwise a JSON-encoded u64

Hooks#

One special feature of the cw4 contracts is they allow the admin to register multiple hooks. These are special contracts that need to react to changes in the group membership, and this allows them stay in sync. Again, note this is a powerful ability and you should only set hooks to contracts you fully trust, generally some contracts you deployed alongside the group.

If a contract is registered as a hook on a cw4 contract, then anytime UpdateMembers is successfully executed, the hook will receive a handle call with the following format:

{  "member_changed_hook": {    "diffs": [      {        "addr": "cosmos1y3x7q772u8s25c5zve949fhanrhvmtnu484l8z",        "old_weight": 20,        "new_weight": 24      }    ]  }}

See hook.rs for full details. Note that this example shows an update or an existing member. old_weight will be missing if the address was added for the first time. And new_weight will be missing if the address was removed.

The receiving contract must be able to handle the MemberChangedHookMsg and should only return an error if it wants to change the functionality of the group contract (eg. a multisig that wants to prevent membership changes while there is an open proposal). However, such cases are quite rare and often point to fragile code.

Note that the message sender will be the group contract that was updated. Make sure you check this when handling, so external actors cannot call this hook, only the trusted group.