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Blockchain of Blockchains

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

  • something happens (the most common situation is that a contract gets a message)
  • contract handles that event according to its own properties by executing its code in TOS Virtual Machine.
  • contract modifies its own properties (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. It is important that events are handled one by one, thus transactions are strictly ordered and cannot interrupt each other.

This behavior pattern is well known and called 'Actor'.

The lowest level: Account Chain​

A sequence of transactions Tx1 -> Tx2 -> Tx3 -> .... may be called a chain. And in the considered case it is called AccountChain to emphasize that it is the chain of a single account of transactions.

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 to blocks. In that case, a block will contain a full set of information which 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 1 ShardChain which describes events that happen with 1 million accounts and there are too many transactions per second to be processed and stored in one node, so we just divide (or 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 became too unoccupied they can be merged into one bigger shard.

There are obviously 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. There is a special mechanism of routing which move messages from outgoing queues to corresponding incoming queues and ensures that 1) all messages will be delivered 2) messages will be delivered consecutively (the message sent earlier will reach the destination earlier).

SIDE NOTE

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

Blockchain​

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

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

Workchain: Blockchain with your own rules​

If you want to customize rules of the group of Shardchains, 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 community can create own workchain. In fact, it's pretty complicated task to build it, after that to pay (expensive) price of creating it and receive 2/3 of votes from validators to approve creation of your Workchain.

TOS allows creating up to 2^30 workchains, each subdivided to up to 2^60 shards.

Nowadays, there are only 2 workchains in TOS: MasterChain and BaseChain.

BaseChain is used for everyday transactions between actors because it's pretty cheap, while MasterChain have a crucial function for TOS, so let's cover what does it do!

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 TOS, a special chain called MasterChain is used for that purpose. Blocks of masterchain contain additional information (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.