Request handling

To handler requests sent by client, the node has a several managers:

• write_manager
• read_manager
• action_manager All of this managers have a 2 type of handlers:
• request handlers (WriteRequestHandler, ReadRequestHandler and ActionRequestHandler)
• batch handlers (Pool/Domain/Config/Audit BatchHandler) Request handler needs to making static and dynamic validation and updating state. Batch handlers perform a batch-related functions, like apply_batch, commit_batch and reject_batch. All of managers have an API method for registering request and batch handlers, that's called register_req_handler and register_batch_handler correspondingly. During static or dynamic validation all of handlers which associated with required transaction type will be called and performed. It means, that we can divide some specific validations into different request handlers. Also, this logic is suitable for batch's handlers too.

There are 3 types of requests a client can send:

• Query: Here the client is requesting transactions or some state variables from the node(s). The client can either send a GET_TXN to get any transaction with a sequence number from any ledger. Or it can send specific GET_* transactions for queries. read_manager will be used for this request type.
• Write: Here the client is asking the nodes to write a new transaction to the ledger and change some state variable. This requires the nodes to run a consensus protocol (currently RBFT). If the protocol run is successful, then the client's proposed changes are written. write_manager will be used for this request type.

Request handling

Below is a description of how a request is processed. A node on receiving a client request in validateClientMsg:

• The node performs static validation checks (validation that does not require any state, like mandatory fields are present, etc), it uses ClientMessageValidator and static_validation from associated manager.
• If the static validation passes, it checks if the signature check is required (not required for queries) and does that if needed in verifySignature. More on this later.
• Checks if it's a generic transaction query (GET_TXN). If it is then query the ledger for that particular sequence number and return the result. A REQNACK might be sent if the query is not correctly constructed.
• Checks if it's a specific query, then corresponded request_handler from read_manager return a result. A REQNACK might be sent if the query is not correctly constructed.
• If it is a write, then node checks if it has already processed the request before by checking the uniqueness of identifier and reqId fields of the Request.
  • If it has already processed the request, then it sends the corresponding Reply to the client
  • If the Request is already in process, then it sends an acknowledgement to the client as a REQACK
  • If the node has not seen the Request before it broadcasts the Request to all nodes in a PROPAGATE.
  • Once a node receives sufficient (Quorums.propagate) PROPAGATEs for a request, it forwards the request to its replicas.
  • A primary replica on receiving a forwarded request does dynamic validation (requiring state, like if violating some unique constraint or doing an unauthorised action) on the request calling dynamic_validation of write_manager which choose a specific request handlers. If the validation succeeds then apply_request method of write_manager is called which optimistically applies the changes to ledger and state. If the validation fails, the primary sends a REJECT to the client. Then the primary sends a PRE-PREPARE to other nodes which contains the merkle roots and some other information for all such requests.
  • The non-primary replicas on receiving the PRE-PREPARE performs the same dynamic validation that the primary performed on each request. It also checks if the merkle roots match. If all checks pass the replicas send a PREPARE otherwise they reject the PRE-PREPARE
  • Once the consensus protocol is successfully executed on the request, the replicas send ORDERED message to its node and the node updates the monitor.
  • If the ORDERED message above was sent by the master replica then the node executes the request; meaning they commit any changes made to the ledger and state by that request by calling commit_batch method of write_manager and send a Reply back to client.
  • The node also tracks the request's identifier and reqId in a key value database in updateSeqNoMap.

Signature verification

Each node has a ReqAuthenticator object which allows to register ClientAuthNr objects using register_authenticator method. During signature verification, a node runs each registered authenticator over the request and if any authenticator results in an exception then signature verification is considered failed. A node has atleast 1 authenticator called CoreAuthNr whose authenticate method is called over the serialised request data to verify signature.

Relevant code:

• Node: plenum/server/node.py
• Replica: plenum/server/replica.py
• Propagator: plenum/server/propagator.py
• Request: plenum/common/request.py
• Request structure validation: plenum/common/messages/client_request.py
• WriteRequestManager: plenum/server/request_managers/write_request_manager.py
• ReadRequestManager: plenum/server/request_managers/read_request_manager.py
• ActionRequestManager: plenum/server/request_managers/action_request_manager.py
• WriteRequestHandler: plenum/server/request_handlers/handler_interfaces/write_request_handler.py
• ReadRequestHandler: plenum/server/request_handlers/handler_interfaces/read_request_handler.py
• ActionRequestHandler: plenum/server/request_handlers/handler_interfaces/action_request_handler.py
• Base RequestHandlers: plenum/server/request_handlers/
• Request Authenticator: plenum/server/req_authenticator.py
• Core Authenticator: plenum/server/client_authn.py
• Quorums: plenum/server/quorums.py