Collator

Collators on Phala (Khala) are supposed to be Gatekeepers, who also run the pRuntime as the Gatekeeper role, helping manage the keys in the network. However, Gatekeeper is not open for election now.

Concepts

Phala Network is currently running its collators on Aura consensus. Aura is a simple block authoring engine that rotates the block producers among the registered collator candidates one by one.

            Host              Session
            Api               Manager
 Collator <===== [Session]  <--------- [CollatorSelection]
   node            pallet                     pallet

On the blockchain, the block authoring is handled collaboratively by Session and CollatorSelection pallets. The Session pallet controls how the blocks are produced and validated. Therefore it has a list of the selected block authors. However, the Session doesn’t decide the list on its own. Instead, it delegates that to the CollatorSelection pallet.

The CollatorSelection pallet implements the SessionManager trait and is injected as the manager of the Session pallet in the runtime.

The CollatorSelection pallet is a minimal PoS system. It has a configurable bound amount for all the collator candidates. Anyone can run a collator as long as they meet the minimal bound. It also kicks the unresponsive collator out after each session. Besides the public collators, it also holds a special group of collators called invulnerables, who are not vulnerable to kicks even if they are offline.

Prepare a new collator

1. Create an account with a minimum amount of bound available (16 PHA by default)

2. Deploy a collator full node with the --collator flag enabled

3. Generate a session key. Two options:

   1. Call RPC api.rpc.author.rotateKeys() to generate a new session key in the node, and note the public key it outputs

   2. Generate the session key externally (dangerous), calculate the public key, and note it

4. Set the session (public) key of the collator account by calling session.setKeys()

5. Start the collator node

Enable an invulnerable collator

This operation can only be done with the Sudo permission. To apply for a public collator, please refer to the next section.

Call collatorSelection.setInvulnerables(list), where list is the full list of all the invulnerables collators including the old ones. The current invulnerable collator list can be found from collatorSelection.invulnerables().

Apply for a public collator

To become a public collator, you will need to put a fixed amount of the deposit. The deposit can be found at collatorSelection.candidacyBond(). Up to a certain number of the public collators are allowed (the genesis collators are not included). The number can be found at collatorSelection.desiredCandidates(), and the current candidates can be found at collatorSelection.candidates(). As of writing, the bond is 1000 PHA, and the desired candidate number is 12.

After the collator account and the node is setup correctly, submit a proposal by calling collatorSelection.registerAsCandidate() from the account. Once it’s submitted, you will need to wait for two sessions to become a collator.

The blockchain will detect offline collators. If the produced blocks in a session is under a threshold, it will be kicked out from the collator candidate list, and the bond will be returned in full.

At any time, a collator account can signal the intention to leave the collator candidate list by calling collatorSelection.leaveIntent().