MIT OpenCourseWareLecture 2: Blockchain as a Database
MIT OpenCourseWare
Blockchain as a Database Overview 0:00
You are introduced to the lecture's outline: treating blockchains and financial accounts as related database structures, exploring the tension between individual and community perspectives, and looking at technical limitations of blockchains along with progress in mapping economies. A blockchain is defined simply as a data structure used to store information about transactions, meaning it can be treated as a database and nothing more or less. Data sits in signed blocks that link to each other, forming an immutable chain. Each Bitcoin block contains a timestamp, a nonce used for proof of work, a reference hash to the previous block, and a list of transactions. Oddly, the Bitcoin blockchain does not hand you the current state directly; you must replay the entire history since the genesis block to know where things stand. Messages are encoded with public and private keys so no one can impersonate another node, and a node is described as being like a computer account rather than a personal identity.
Bitcoin's Creation and Origins 5:01
Bitcoin was registered on August 18, 2008, through a white paper by the pseudonymous Satoshi Nakamoto, titled "A Peer-to-Peer Electronic Cash System." Open-source code followed in January 2009, and the genesis block was mined on January 3, 2009. Nine days later, Hal Finney received the first Bitcoin transaction from Nakamoto, and in May a programmer used Bitcoin to buy pizza, now called Bitcoin Pizza Day. Computer scientist Arvind Narayanan notes that every individual component of Bitcoin existed in earlier literature; what made it new was the complex interplay producing the first decentralized, Sybil-resistant, Byzantine fault-tolerant digital cash system.
Babysitting Certificates as Money 7:05
To question what counts as money, the lecture brings in Paul Krugman's babysitting co-op story from the 1970s, where congressional staffer couples traded babysitting certificates instead of cash. Couples earned certificates by babysitting and spent them to get babysitting later. Because couples grew anxious to hoard certificates rather than spend them, the whole system collapsed into a recession, and issuing more certificates to fix it caused inflation instead. Krugman has said this experience shaped his thinking on monetary theory and business cycles, and it is used here to provoke thought about what money really is.
Central Banks and Digital Currency 10:32
Central banks reacted warily to Bitcoin, and Facebook's Meta proposal was seen as an even bigger threat. A 2018 quote from Hyun Shin dismisses cryptocurrencies as unable to scale, prone to congestion, and a poor substitute for institutionally backed money, yet by then about 80 percent of central banks were exploring their own digital currencies. A 2023 BIS paper by Augustin Carstens describes existing financial systems as fragmented into separate proprietary databases stitched together by messaging systems, proposing instead a shared programmable platform called a unified ledger, while still insisting on the "singleness" of central bank money, a claim questioned given that many different fiat currencies already coexist worldwide.
Financial Accounts as Twin Database 15:31
Financial accounts are presented as a parallel database to blockchains, built from cash flow statements, income statements, and balance sheets, applied even to households as if they were firms. A monthly survey of Thai households traced how a single transaction, such as receiving wage income, flows into the balance sheet as increased cash and into the income statement as revenue. Cash flow gets split into flows from production, from consumption and investment, and from financing, with the total checked against balance sheet changes in the spirit of double-entry bookkeeping. A wage-earning household in Lopburi, Thailand serves as a concrete example, showing income mostly from labor and a balanced sheet of assets against liabilities and net worth.
Multi-Dimensional Money and Velocity 23:02
The lecture stresses that blockchains and financial accounts are multi-dimensional databases, not just about money, and introduces payment matrices to define money by velocity: an object with high turnover relative to its stock, meaning it changes hands frequently. Village India data with co-author Yung Jae Lim shows goods, services, IOUs, and grain used interchangeably, even grain paid as labor compensation, illustrating a dual monetary economy where barter-like exchange coexists with currency. A parallel account system built with the Boston Fed for US households distinguishes flows among currency, demand deposits, debit accounts, and foreign currency, again multiple monies operating alongside each other.
Discrepancies and Reconciliation Problems 31:34
A student's question about mismatches between account flows and balance sheets leads to acknowledgment that the Thai survey data were never designed for perfectly consistent accounts, so gaps appear. In the Thai village data, transactions recorded by household i buying from household j often fail to show up symmetrically on both sides, creating discrepancies that a shared, distributed ledger could have caught immediately rather than after the fact. US financial markets face similar problems: State Street and other institutions spend hours at the end of each trading day reconciling mismatched transaction records between parties, with prices having already shifted in the meantime.
Distributed Ledgers Defined 38:00
A distributed ledger is defined as infrastructure and protocols allowing simultaneous access, validation, and record-keeping across a networked database, with blockchain being one technology built from this concept. The idea is not entirely new, since parallel computing across multiple machines has existed for decades, but advances in data science and hardware have made ledgers far more capable. Every blockchain is a distributed ledger, but not every distributed ledger is a blockchain, since blockchains specifically require a validation algorithm linking blocks together.
Central Bank Payment Timing Games 39:31
Moving into tensions between individual and community perspectives, the lecture cites Bech and Garratt's 2003 paper on intraday liquidity management, showing that banks control when they report payment obligations to real-time gross settlement systems. Because reporting early means reserves leave the bank's account sooner, banks prefer to delay, producing a Nash equilibrium where payments bunch at the end of the day, a potentially inefficient outcome. A related classic paper on payments and financial fragility argues that payments inherently involve a conflict between the medium of exchange role and the store of value role, since settling a payment requires transferring a scarce reserve asset that also holds value for other purposes.
Money as Store of Value or Medium 42:01
The theory shows payments happen only when reserves are abundant enough, and otherwise you get history dependent equilibriums, building on global games and echoing the tension seen earlier in Krugman's babysitting co-op story, where people hoarded coupons instead of spending them. This is a universal conflict built into money itself, since it is expected to serve as both a store of value and a medium of exchange, and those two roles can pull against each other with real consequences.
Pacioli and Negative Numbers 43:07
Luca Pacioli is credited with double entry bookkeeping, though he likely consolidated earlier work into the first published manuscript on it. Even in the Renaissance, negative numbers seemed absurd, since a physical object either exists or it does not, even though Chinese and Arab scholars already understood the concept. Pacioli treated money not as an asset but as a liability, an obligation owed to the community rather than something simply owned. Tomaž Fleischman's research into Venetian journals found two terms, per for debtor and A for creditor, confirming that cash entries were recorded as obligations, per, rather than possessions.
Community Currencies and Sarafu 46:30
Community inclusion currencies, or mutual credit networks, work only within a specific community, much like the peer to peer babysitting scheme. Selling goods earns you the currency rather than something you directly value, and you are then obligated to spend it on others' production, with incentives built in to discourage hoarding. Sarafu, the best known example, runs in informal settlements in Nairobi and South Africa, where vouchers are promissory notes against future production, designed to keep value circulating locally rather than leaking out to places like Walmart. Sarafu itself runs on the Ethereum blockchain.
The CAP Theorem Tradeoff 50:05
The CAP theorem from computer science says a system cannot fully guarantee consistency, availability, and partition tolerance at once. Bitcoin faces this because competing blocks are temporarily inconsistent until validation settles things. Historically, economies handled this by creating intermediaries, then centralized ledgers like securities depositories, and now techniques like sharding or layer two systems that only settle asset transfers on-chain. The BIS's idea of a unified ledger aims to merge partitioned systems into something more consistent, though this raises questions about whether smart contracts stay coherent across ledgers.
Legacy Systems and Fraud Risk 58:30
The Fed's instant payment system, FedNow, requires banks to modernize outdated infrastructure, and many rely on a highly concentrated aggregator industry to cope. Sweden's Riksbank worried that cash was disappearing in favor of the private instant payment app Swish, prompting talk of a digital currency that was ultimately dropped. Kenya's mobile money system, built on phone credits from Safaricom, works well but is not on a blockchain and carries bank failure risk. FTX exemplifies the trust problem, having diverted customer funds while operating as a centralized exchange rather than a blockchain.
Measurement Gaps in Currency Data 1:06:03
Research on Thailand uncovered major welfare losses tied to heavy reliance on paper currency. The Hamilton project, MIT's digital dollar design with the Boston Fed, prioritized transaction privacy so strongly that it sacrificed contract functionality. Studies of Indonesian payment apps found separate databases for each service with no unified household or business identifier. Work with the Boston Fed on the Panel Study of Income Dynamics and related US surveys found large, unexplained discrepancies between reported cash flows and balance sheet changes, since these surveys were never designed for financial consistency, a problem that shows up at both the micro and macro level in US data.
Building Consistent Household Accounts 1:11:01
A project in West Virginia is developing templates that let households share electronic banking and credit card data directly, merging it with recorded cash transactions to build consistent financial accounts, extending the approach used successfully in Thailand. Related lecture material exists in a parallel course, 193, covering measurement of cash inefficiency, including findings from Indonesia showing households operate in a dual economy, juggling digital and cash payments with real transaction costs and fraud risk from informal agents. Students are invited to use the syllabus readings to develop a short paper or research proposal building on these themes.
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