1/* File: paxos/http_handlers.pl 2 Author: Roy Ratcliffe 3 WWW: https://github.com/royratcliffe 4 Created: Jun 12 2021 5 Purpose: Paxos HTTP Handlers 6 7Copyright (c) 2021, Roy Ratcliffe, United Kingdom 8 9Permission is hereby granted, free of charge, to any person obtaining a 10copy of this software and associated documentation files (the 11"Software"), to deal in the Software without restriction, including 12without limitation the rights to use, copy, modify, merge, publish, 13distribute, sublicense, and/or sell copies of the Software, and to 14permit persons to whom the Software is furnished to do so, subject to 15the following conditions: 16 17 The above copyright notice and this permission notice shall be 18 included in all copies or substantial portions of the Software. 19 20THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS 21OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 22MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 23IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY 24CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, 25TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE 26SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. 27 28*/ 29 30:- module(paxos_http_handlers, ). 31:- autoload(library(paxos), 32 [ paxos_property/1, 33 paxos_get/3, 34 paxos_set/3 35 ]). 36:- autoload(library(http/http_dispatch), [http_handler/3]). 37:- autoload(library(http/http_json), 38 [ reply_json/1, 39 reply_json/2, 40 http_read_json/2 41 ]). 42:- autoload(library(http/http_parameters), [http_parameters/2]). 43:- autoload(library(lists), [nth1/3]). 44:- use_module(library(swi/paxos)).
109:- http_handler(root(paxos/properties), properties, ). 110:- http_handler(root(paxos/Key), key(Method, Key), 111 [ method(Method), 112 methods([get, post]) 113 ]). 114:- http_handler(root(paxos/quorum), quorum, ).
124properties(_) :- 125 findall(Property, paxos_property(Property), Properties), 126 reply_json(json(Properties)).
?- http_post('http://localhost:8080/paxos/hello', json(@false), A, [status_code(B)]). A = @true, B = 200.
The POST method simply replies true or false in JSON when paxos_set/2 succeeds or fails respectively.
Also note that Paxos gets and sets are not instantaneous. Getting a key's value involves communication with the quorum since the enquiring node does not necessarily carry the key at first.
149key(get, Key, Request) :- 150 request_options(Request, Options), 151 ( paxos_get(Key, Data, Options) 152 -> reply_json(Data, [serialize_unknown(true)]) 153 ; throw(http_reply(no_content)) 154 ). 155key(post, Key, Request) :- 156 http_read_json(Request, Data), 157 request_options(Request, Options), 158 ( paxos_set(Key, Data, Options) 159 -> Reply = true 160 ; Reply = false 161 ), 162 reply_json(@(Reply)).
Seconds must be in-between one millisecond and 10 seconds. These arbitrary limits intend to strike a reasonable balance between resource usages.
175request_options(Request, Options) :- 176 findall(Option, request_option(Request, Option), Options). 177 178request_option(Request, retry(Retries)) :- 179 http_parameters(Request, 180 [retry(Retries, [nonneg, optional(true)])]), 181 nonvar(Retries). 182request_option(Request, timeout(Seconds)) :- 183 http_parameters(Request, 184 [ timeout(Seconds, 185 [ between(0.001, 10), 186 optional(true) 187 ]) 188 ]), 189 nonvar(Seconds). 190 191quorum(_) :- 192 paxos_property(node(Node)), 193 paxos_quorum_nodes(Nodes), 194 once(nth1(Nth1, Nodes, Node)), 195 reply_json(json([nodes=Nodes, nth1=Nth1]))
Paxos HTTP Handlers
These handlers spool up a JSON-based HTTP interface to the Paxos predicates, namely
Take the example below. Uses http_server/1 to start a HTTP server on some given port.
Getting and setting using JSON encoding works as follows.
Note that the initial GET fails. It replies with the empty atom since no content exists. Predicate paxos_get/2 is semi-deterministic; it can fail. Empty atom is not valid Prolog-encoding for JSON. Status code of 204 indicates no content. The Paxos ledger does not contain data for that key.
Thereafter, POST writes a string value for the key and a repeated GET attempt now answers the new consensus data. Status code 200 indicates a successful ledger concensus.
Serialises unknowns. Paxos ledgers may contain non-JSON compatible data. Anything that does not correctly serialise as JSON becomes an atomicly rendered Prolog term. Take a consensus value of term
a(1)for example; GET requests see "
a(1)" as a rendered Prolog string. The ledger comprises Prolog terms, fundamentally, rather than JSON-encoded strings.
Setting a Paxos value reads JSON from the POST request body. It can be any valid JSON value including atomic values as well as objects and arrays.