Redis concurrency control

redis-concurrency-control.js

const redis = require('redis');
const bluebird = require('bluebird');

const redisClient = redis.createClient({});
bluebird.promisifyAll(Object.getPrototypeOf(redisClient));

const luaScript = `
local newPayload = ARGV[1]
local newVersionStr, newData = ARGV[1]:match("^([0-9]+)|(.+)$")
local prevVal = redis.call("get", KEYS[1]) or nil

if prevVal == nil then
  return redis.call("set", KEYS[1], "1|" .. newData)
end

local oldVersionStr, oldData = prevVal:match("^([0-9]+)|(.+)$")
local newVersion = tonumber(newVersionStr)
local oldVersion = tonumber(oldVersionStr)

-- check if version matches before writing
if oldVersion == (newVersion - 1) then
  return redis.call('set', KEYS[1], newPayload)
else
  return nil
end
`;

(async () => {
  const results = await Promise.all([
    redisClient.evalAsync(luaScript, 1, 'cc', '1|{a: 1}'),
    redisClient.evalAsync(luaScript, 1, 'cc', '1|{b: 2}'),
    redisClient.evalAsync(luaScript, 1, 'cc', '2|{b: 2}'),
  ]);
  console.log(results); // potential output [ 'OK', null, 'OK' ]
  console.log(await redisClient.getAsync('cc'));
  console.log(await redisClient.delAsync('cc'));
})();

What should we do when we get null?

When you get back null, either show error to user

OR if automatic merge it's possible,
fetch data again, merge new data with old, increase the version number and try to write again.