# Engine API sequence

## Block validation

### Canonical chain

Note over CL: Block B arrives
Note over CL: state_transition(state: pre_state, block: B)
CL->EL: engine_newPayload(B.payload)
Note over EL: runs full payload validation
EL-->CL: {status: VALID}
CL->EL: engine_forkchoiceUpdated(head: B.payload.hash)
EL-->CL: {payloadStatus: {status: VALID}}

### Non-canonical chain

#### Deep-state EL client

Note over CL: Block B arrives
Note over CL: state_transition(state: pre_state, block: B)
CL->EL: engine_newPayload(B.payload)
Note over EL: runs full payload validation
EL-->CL: {status: VALID}
Note over CL: Chain re-orgs to B
CL->EL: engine_forkchoiceUpdated(head: B.payload.hash)
EL-->CL: {payloadStatus: {status: VALID}}

#### Shallow-state EL client

Note over CL: Block B arrives
Note over CL: state_transition(state: pre_state, block: B)
CL->EL: engine_newPayload(B.payload)
Note over EL: validates payload is from a well-formed chain
EL-->CL: {status: ACCEPTED}
Note over CL: Chain re-orgs to B
CL->EL: engine_forkchoiceUpdated(head: B.payload.hash)
Note over EL: runs full payload validation
EL-->CL: {payloadStatus: {status: VALID}}

## Block proposal

Note over CL: Block B arrives
Note over CL: state_transition(state: pre_state, block: B)
CL->EL: engine_newPayload(B.payload)
Note over EL: runs full payload validation
EL-->CL: {status: VALID}
CL->EL: engine_forkchoiceUpdated(head: B.payload.hash, payloadAttributes: {...})
EL-->CL: {payloadStatus: {status: VALID}, payloadId: buildProcessId}
Note over CL: slot ticks
CL->EL: engine_getPayload(payloadId: buildProcessId)
EL-->CL: builtExecutionPayload
Note over CL: proposes B', a child of B with builtExecutionPayload payload