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Blockchain of blockchains

tip

Terms 'smart contract', 'account', and 'actor' are used interchangeably in this document to describe a blockchain entity.

Single actor​

Let's consider one smart contract.

In TON, it is a thing with properties like address, code, data, balance and others. In other words, it is an object with some storage and behavior. That behavior has the following pattern:

  • contract receives a message
  • contract handles that event according to its properties by executing its code in TON Virtual Machine
  • contract modifies its properties consisting of code, data, and others
  • contract optionally generates outgoing messages
  • contract goes into standby mode until the next event occurs

A combination of these steps is called a transaction. Since it is essential to handle events one by one, transactions follow a strict order and cannot interrupt each other.

This behavior pattern is well known and called actor.

The lowest level: AccountChain​

A sequence of transactions Tx1 -> Tx2 -> Tx3 -> .... may be called a chain. When considering a single account, we call the chain of transactions an AccountChain.

Now, since nodes that process transactions need from time to time to coordinate the state of the smart contract to reach a consensus about the state, those transactions are batched: [Tx1 -> Tx2] -> [Tx3 -> Tx4 -> Tx5] -> [] -> [Tx6]. Batching does not intervene in sequencing; each transaction still has only one 'prev tx' and at most one 'next tx', but now this sequence is cut into the blocks.

It is also expedient to include queues of incoming and outgoing messages in blocks. In that case, the block will contain a full set of information that determines and describes what happened to the smart contract during that block.

Many AccountChains: Shards​

Now let's consider many accounts. We can get a few AccountChains and store them together; such a set of AccountChains is called a ShardChain. In the same way, we can cut ShardChain into ShardBlocks, which are an aggregation of individual AccountBlocks.

Dynamic splitting and merging of ShardChains​

Note that since a ShardChain consists of easily distinguished AccountChains, we can easily split it. That way, if we have one ShardChain that describes events that happen with one million accounts and there are too many transactions per second to be processed and stored in one node, so we just split that chain into two smaller ShardChains with each chain accounting for half a million accounts and each chain processed on a separate subset of nodes.

Analogously, if some shards become too unoccupied, they can be merged into one more enormous shard.

There are two limiting cases: when the shard contains only one account (and thus cannot be split further) and when the shard contains all accounts.

Accounts can interact with each other by sending messages. A unique routing mechanism moves messages from outgoing queues to corresponding incoming queues and ensures:

  1. The delivery of all messages
  2. Consecutive delivery of messages — a message sent earlier will reach the destination earlier
SIDE NOTE

An aggregation of AccountChains into shards is based on the bit-representation of account addresses to make splitting and merging deterministic. For example, an address looks like (shard prefix, address). That way, all accounts in the ShardChain will have the same binary prefix (for instance, all addresses will start with 0b00101).

Blockchain​

An aggregation of all shards, which contains all accounts behaving according to one set of rules, is called a Blockchain.

In TON, there can be many sets of rules, and thus, many blockchains operate simultaneously and can interact with each other by sending messages cross-chain in the same way that accounts of one chain can interact with each other.

WorkChain: a blockchain with your own rules​

If you want to customize the rules of the ShardChains group, you could create a WorkChain. A good example is to make a workchain that works on the base of EVM to run Solidity smart contracts on it.

Theoretically, everyone in the community can create their own WorkChain. Building it isn't very easy, and then you have to pay a high price and receive 2/3 of votes from validators to approve it.

TON allows creating up to 2^32 workchains, subdivided into 2^60 shards.

Nowadays, there are only two workchains in TON: MasterChain and BaseChain.

BaseChain is used for everyday transactions between actors because it's cheap, while MasterChain has a crucial function for TON.

MasterChain: blockchain of blockchains​

There is a necessity for the synchronization of message routing and transaction execution. In other words, nodes in the network need a way to fix some 'point' in a multichain state and reach a consensus about that state. In TON, a special chain called MasterChain is used for that purpose. Blocks of MasterChain contain additional information, like the latest block hashes, about all other chains in the system, thus any observer unambiguously determines the state of all multichain systems at a single MasterChain block.

See also​

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