validation-rules

concepts Holochain

Validation workflow

• Validation needs to be deterministic: same result no matter who runs it and when.
• After an action is committed, it is transformed into different DHT Operations (DHT Ops)
• Then, those DHT Ops are published to the different neighborhoods in the DHT
• When those nodes receive that publish, they add it to their validation queue
  • If the validation fails, those nodes produce warrants that get gossiped throughout the network
    • All other nodes can verify that the bad agent is behaving badly
    • They then disconnect from the bad agent
  • If the validation succeeds, the node starts to serve that record to DHT queries (get, get_links, etc.)

Validation callback function context

• The function validate is called in two contexts
  1. The author validates
  2. All prospective hosts of the DHT Op validate

DHT Ops

System validation

• Does not depend on your application code
• Holochain enforces some system level validations across all apps
  • Actions
    • Source chains can’t be forked
    • All actions must be signed by their author
    • Timestamp and sequence number for the actions must be incremental
    • If the action is accompanied by an entry, the entry hash in the action is correct
  • Entries
    • The entry byte array can be deserialized to the appropriate struct

App validation

• Validation rules for a given entry or link type must go in the same zome as the type definition
• Private entries cannot be validated
• Callback: validate(dht_op: Op)

#[hdk_extern]
pub fn validate(op: Op) -> ExternResult<ValidateCallbackResult> {
  // Right now there is bunch of boilerplate here that's going to be replaced in the next version 
}     

• Three results possible:
  1. Valid
  2. Invalid: it specifies the reason why it failed
  3. Unresolved dependencies: Not enough data to complete the validation

Validation challenges

• Non-deterministic queries can’t be considered during validation
  • get and get_details are forbidden
    • They attempt to return a current list of updates and deletes
  • get_links is forbidden
  • get_agent_activity is forbidden unless bounded by action hashes
• Deterministic alternatives:
  • must_get_entry
    • Only returns the entry, not its action
    • Never returns None, rather the validation has unresolved dependencies
  • must_get_action
    • Only returns the action
    • Never returns None, rather the validation has unresolved dependencies
  • must_get_valid_record
    • Returns the record
    • Never returns None, rather the validation has unresolved dependencies

Example code (HDK)

must_get_entry

let role = PublicationRole::new("editor", author);  
let role_entry_hash = hash_entry(&EntryTypes::PublicationRole(role))?;  
  
let entry = must_get_entry(role_entry_hash);  
match entry {  
  Ok(_) => Ok(ValidateCallbackResult::Valid),  
  Err(_) => Ok(ValidateCallbackResult::Invalid(String::from(  
    "Only editors can create posts",  
  ))),  
}

Genesis self-check

• Callback executed when the cell is instantiated
• If it fails, the app for that cell will fail to install

#[hdk_extern]
pub fn genesis_self_check(data: GenesisSelfCheckData) -> ExternResult<ValidateCallbackResult> {
    // Check data.agent_key, data.membrane_proof
}