zmts

Advisory Lock в PostgreSQL

Проблема: два параллельных запроса могут одновременно проверить данные и принять несовместимые решения. Например, каждый активирует свой кошелек — в результате активных окажется два. Одна транзакция сама по себе такую гонку не исключает.

Решение: перед изменением оба запроса берут один и тот же именованный lock. Первый выполняет операцию, второй ждёт. Разные ключи позволяют выполнять независимые операции параллельно.

await db.transaction(async tx => {
  const lockKey = `wallets:trader:${chainKey}`;

zmts

Docker compose kafka local config

services:
  zookeeper:
    container_name: zookeeper
    image: confluentinc/cp-zookeeper:latest
    environment:
      ZOOKEEPER_CLIENT_PORT: 2181
    ports:

zmts

Nest.js Dynamic module

Implementation example

.
├── index.ts
├── sender.module.ts
├── sender.service.ts
└── types.ts

zmts

SQL: Aggregation of minute candles into 5-minute candles

CREATE "ohlc_minutes" (
    "id" int4 NOT NULL DEFAULT nextval('ohlc_minutes_id_seq'::regclass),
    "ticker" varchar NOT NULL,
    "date" timestamptz NOT NULL,
    "open" numeric(20,8) NOT NULL,
    "high" numeric(20,8) NOT NULL,
 "low" numeric(20,8) NOT NULL,

zmts

Merge objects in JavaScript (lodash)

const mergeWith = require('lodash/mergeWith')
const isArray = require('lodash/isArray')

function mergeObjects(obj1, obj2) {
  return mergeWith(obj1, obj2, (objValue, srcValue) => {
    if (isArray(objValue) && isArray(srcValue)) {
 return objValue.concat(srcValue);

zmts

Convert .env file to object

const isNumber = (value) => {
  return !isNaN(Number(value))
}

const getValue = (value) => {
  if (value === '') return value
  if (['true', 'false'].includes(value)) return JSON.parse(value)

zmts

Too slow response .local domains Mac OS

The Multicast DNS feature of Bonjour technology allows devices on a local network to connect to each other by name without a separate DNS server. By default, any hostname ending in .local is treated as a Bonjour host rather than by querying the DNS server entries in Network preferences.

Though the .local domain is not defined as a valid top-level domain on the Internet, some private networks have DNS servers that assign hostnames in the .local domain. By default Mac OS X does not use the DNS server to resolve these names. This may result in unexpected failures to connect to .local hostnames defined by your server.

To fix this problem you need to add IPv6 entries for each of your vhosts in your /etc/hosts file:

127.0.0.1       localhost
127.0.0.1 myproject.local

zmts

Build tree from flat array

function buildTree(nodes, parentId = null) {
  const tree = [];
  for (const node of nodes) {
    if (node.parentId === parentId) {
      const children = buildTree(nodes, node.id);
      if (children.length) {
        node.children = children;

zmts

SQL: get item level (aka generation) from parent child hierarchy

CREATE TABLE "public"."parent_child" (
    "id" int4 NOT NULL DEFAULT nextval('people_id_seq'::regclass),
    "first_name" varchar(100),
    "parent_id" int4,
    "city" varchar(100),
    PRIMARY KEY ("id")
);

zmts

SQL: select rows where some id in array

CREATE TABLE "public"."items" (
    "id" int4 NOT NULL DEFAULT nextval('item_id_seq'::regclass),
    "name" text,
    "has_access" _int4 NOT NULL DEFAULT '{}'::integer[],
    PRIMARY KEY ("id")
);