Julien Cortesi ju-c

System Design Cheatsheet

Picking the right architecture = Picking the right battles + Managing trade-offs

Basic Steps

Clarify and agree on the scope of the system

User cases (description of sequences of events that, taken together, lead to a system doing something useful)
- Who is going to use it?
- How are they going to use it?

Got the dreaded, only-macOs documented "No such file or directory Is the server running locally and accepting connections on Unix domain socket "/tmp/.s.PGSQL.5432" but you are using wither Mint or Ubuntu?

Here's how to fix it.

verify the contents of /etc/postgresql/9.5/main/pg_hba.conf
if the file is empty, add this conf

Previously described at: ERC20 critical problems medium article.

Description.

ERC20 is the most common Ethereum token standard. It should be noted that it is also the first Ethereum's token standard as well.

It is also important that the original ERC20 proposal is a definition of token interface. EIP20 does not define a reference implementation for this token standard. Here is OpenZeppelin implementation of ERC20 token: https://github.com/OpenZeppelin/zeppelin-solidity/tree/master/contracts/token/ERC20

ERC20 token standard implementation assumes two ways of token transferring: (1) transfer function and (2) approve + transferFrom pattern.

Stream Mastodon activity into a SQLite database

This script subscribes to the live HTTP feed of public activity on my Mastodon instance and writes the results into SQLite database tables.

It needs sqlite-utils and httpx:

pip install sqlite-utils httpx

Then run:

	# Hello, and welcome to makefile basics.
	#
	# You will learn why `make` is so great, and why, despite its "weird" syntax,
	# it is actually a highly expressive, efficient, and powerful way to build
	# programs.
	#
	# Once you're done here, go to
	# http://www.gnu.org/software/make/manual/make.html
	# to learn SOOOO much more.

	static bool ConnectBlock(const Config &config, const CBlock &block,
	CValidationState &state, CBlockIndex *pindex,
	CCoinsViewCache &view, bool fJustCheck = false) {
	==> AssertLockHeld(cs_main);


	// ...

	==> if (!control.Wait()) {
	return state.DoS(100, false, REJECT_INVALID, "blk-bad-inputs", false,