How is the 24 nodes picked for a session?

immunity20 · March 27, 2022, 1:54pm

Hello,
If my node is picked for a session 0040, (a) is it possible to get picked again (low chances) for the next session in a row(13:00-14:00 and 14:05-15:05) or (b) every unjailed Nodes has to get in a session before a Node get selected for a second time.

Also, is it possible for a Node to be selected in the same hour for 2 sessions of different chains?
Something like 13:00-14:00 session for 0040 and 13:03-14:03 session for 0021?

poktblade · March 29, 2022, 6:44am

From my understanding - a session key (seed) is generated by hashing (sha256) a json object composed of an application public key, the block hash, and the chain id.

This “seed” is then used to generate nodes that would be part of a session, pseudorandomly every 4 blocks per app. So, your node can be part of multiple sessions of different or same chains. It is not mutually exclusive.

Correct me if I’m wrong , anyone! Still learning the core code myself.

Here’s the code itself to figure this out:

github.com

pokt-network/pocket-core/blob/d2ae3f8e28acf7f829fd0bf38b8a19e3bae9e3a4/x/pocketcore/types/session.go#L104

      
        
            }
            
            
func (s Session) Key() ([]byte, error) {
            	return s.SessionHeader.Hash(), nil
            }
            
            
// "SessionNodes" - Service nodes in a session
            type SessionNodes []sdk.Address
            
            
// "NewSessionNodes" - Generates nodes for the session
            func NewSessionNodes(sessionCtx, ctx sdk.Ctx, keeper PosKeeper, chain string, sessionKey SessionKey, sessionNodesCount int) (sessionNodes SessionNodes, err sdk.Error) {
            	// all nodesAddrs at session genesis
            	nodesAddrs, totalNodes := keeper.GetValidatorsByChain(sessionCtx, chain)
            	// validate nodesAddrs
            	if totalNodes < sessionNodesCount {
            		return nil, NewInsufficientNodesError(ModuleName)
            	}
            	sessionNodes = make(SessionNodes, sessionNodesCount)
            	var node exported.ValidatorI
            	// only select the nodesAddrs if not jailed
            	for i, numOfNodes := 0, 0; ; i++ {

github.com

pokt-network/pocket-core/blob/d2ae3f8e28acf7f829fd0bf38b8a19e3bae9e3a4/x/pocketcore/types/session.go#L182

      
        
            type SessionKey []byte
            
            
// "sessionKey" - Used for custom json
            type sessionKey struct {
            	AppPublicKey   string `json:"app_pub_key"`
            	NonNativeChain string `json:"chain"`
            	BlockHash      string `json:"blockchain"`
            }
            
            
// "NewSessionKey" - generates the session key from metadata
            func NewSessionKey(appPubKey string, chain string, blockHash string) (SessionKey, sdk.Error) {
            	// validate appPubKey
            	if err := PubKeyVerification(appPubKey); err != nil {
            		return nil, err
            	}
            	// validate chain
            	if err := NetworkIdentifierVerification(chain); err != nil {
            		return nil, NewEmptyChainError(ModuleName)
            	}
            	// validate block addr
            	if err := HashVerification(blockHash); err != nil {

immunity20 · March 29, 2022, 8:22am

Shame on me xD.
Didn’t thought to check the github repository.

Thanks poktBlade!

I mainly asked for the array of nodes from which the 24 are picked randomly

nodesAddrs, totalNodes := keeper.GetValidatorsByChain(sessionCtx, chain)

pokt-network/pocket-core/blob/d2ae3f8e28acf7f829fd0bf38b8a19e3bae9e3a4/x/pocketcore/types/session.go#L106

JackALaing · March 29, 2022, 2:51pm

All nodes staked with at least 15,000 POKT are qualified as servicers and have equal chance of being selected for sessions.

Within sessions, the Portal does its own cherry picking based on node quality, to ensure quality service for apps.

poktblade · March 29, 2022, 5:23pm

Those nodes are filtered out based off if they’re jailed or not as well. As Jackal also mentioned, the service quality is then filtered out from these session nodes, so possible you are part of a session and still don’t service any rewards if you aren’t synced up. The cherry picker does do sync checks and some other shenanigans. Cherry picker is also open source if you’re up for learning how it actually works.

immunity20 · March 29, 2022, 6:03pm

Ye I had read the cherry picker code (calculating median latency and sort the 24 nodes by latency