Allow TP to read by address prefix

text/0000-read-by-prefix.md

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+- Feature Name: read_by_prefix
+- Start Date: 2018-12-19
+- RFC PR:
+- Sawtooth Issue:
+
+# Summary
+[summary]: #summary
+
+This RFC proposes an additional API to transaction context,
+context.listAddresses(address_prefix), which will get address prefix as input
+and return list of all existing addresses that starts with the prefix.
+
+This capability exists when reading state from rest-api but is missing when 
+transaction processor reads state.
+
+# Motivation
+[motivation]: #motivation
+
+The motivation came when looking for a way to improve processing transaction 
+logic by making better use optimization that comes from using the radix tree. 
+
+Since addresses are stored in a radix tree, reading many addresses by 
+one prefix is much faster than reading the same amount of addresses one by one.
+Further more, read by prefix doesn't require transaction processor to know 
+which addresses exist, removing the need to store list of existing addresses 
+in a separate address.
+
+Having this capability allows a better and smarter address mapping this a 
+better transaction processor logic.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Get state by prefix is a feature available for reading via sawtooth rest-api.
+This RFC adds method to expose this capability of reading address prefix 
+to transaction processor.
+
+No change is recommended to the existing methods, so the implementation of this 
+RFC should not cause any breakage for pre-existing transaction processors.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Example of implementation can be found here: 
+Sawtooth-core + Python SDK changes: 
+https://github.com/arsulegai/sawtooth-core/tree/jira_1375 
+(commits: 4225dc7 and 2c3a223)
+
+Sawtooth-sdk-cxx changes: 
+https://github.com/arsulegai/sawtooth-sdk-cxx/tree/jira_1375 (commit: 23697e8)
+
+In the suggested implementation there are two proto changes:
+
+Addition to state_context.proto:
+
+
+	// A request from a handler/tp for list of addresses with valid data
+	message TpStateAddressesListRequest {
+		// The context id that references a context in the contextmanager
+		string context_id = 1;
+		repeated string addresses = 2;
+	}
+
+	// A response from the contextmanager/validator with a series of State 
+	// entries having valid data
+	message TpStateAddressesListResponse {
+		enum Status {
+			STATUS_UNSET = 0;
+			OK = 1;
+			AUTHORIZATION_ERROR = 2;
+		}
+
+		repeated string addresses = 1;
+		Status status = 2;
+	}
+
+Addition to validator.proto:
+
+	// State list request to get all addresses having non empty data
+	TP_STATE_LIST_REQUEST = 17;
+	// State list response to get all addresses having non empty data
+	TP_STATE_LIST_RESPONSE = 18;
+
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+This will need to be implemented in every sawtooth SDK and will increase 
+the complexity of the validator slightly.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+An alternative could be in the transaction processor logic, store an address
+that contains list of existing addresses. When need to manipulate multiple 
+addresses transaction processor will first read the special address, then 
+read/write/delete each address in the list.
+This has a performance downside that each time transaction processor needs to 
+write a new address, it will need to modify the special address containing 
+list of all addresses. This address could be huge, when the need to actually 
+access all the addresses in the list can be rare.
+
+# Prior art
+[prior-art]: #prior-art
+
+Read state by address prefix is available from sawtooth-rest-api, and is
+one of the key capability that comes from using the radix tree.
+
+
+# Unresolved questions
+[unresolved]: #unresolved-questions