Extrinsic is a Substrate specific generalization which includes transactions (a normal term from the blockchain world) as well as inherents (also Substrate-specific).
In classic blockchains, blocks usually contain pieces of information called transactions that give instructions from “the outside world” (i.e. you and me) to the blockchain logic. For crypto-currency blockchains such instructions are generally confined to variations on the theme of transfering funds from one account to another. Substrate generalises this by removing the assumption that this piece of information be signed, instead merely characterising it on the fact that it “comes from the outside world” and is thus “extrinsic” to the blockchain and its state.
For pieces of information that do nonetheless contain a signature to authenticate whatever data it contains, then we reuse the classic term “transaction”. Conversely for those that do not contain a signature, we say they are “inherents”, since the chain assumes that by the fact they are finalised means that what they contain is deemed “inherently true”. This is reminiscent of the timestamp field in the header of classical chains, which is true simply because enough of the network maintainers (miners or validators) accepted it as being reasonable.
Substrate makes almost no assumptions about the extrinsic format: rather it is up to the runtime to determine whether an extrinsic is valid, and if so what to do about it. The only requirement is that it is prefixed with a 4-byte LE-encoded length of the extrinsic (not including the 4-bytes itself) (making it compatible with
Vec<u8> in terms of the general Parity Codec). This allows it to be opaquely encoded and decoded and thus be sent around the network and included in transaction pools easily.
Despite Substrate not caring about the extrinsic format at its core, the Substrate Runtime Module Library (SRML) does provide generic implementations of extrinsics which imply their transaction format. Currently two are provided:
UncheckedMortalExtrinsic. The Substrate Node implementation uses the latter. Do note: it’s not the only possible option. Other chains, for example a UTXO chain, may define a wildly different format more suitable for their needs.
Extrinsics are expected to be encodable and decodable. See the
Note: It may seem strange, but currently, Substrate does not define how extrinsics should be encoded. In case of a Polkadot, extrinsics are formed and encoded in the JS part of the code, so you would not find this in the Rust codebase.
In Polkadot runtime, extrinsics are said to be future-proof. Basically, that means that the actual details of the extrinsic are defined by the runtime, whereas external logic, such as network or storage, treats and handles them as opaque byte vectors.
This allows us to update or extend extrinsic definition by changing the runtime only, without forcing clients to update their software.
Extrinsics fall into two broad categories of which only one is conventional transactions. The other is known as inherents. The difference between these two is that transactions are signed and gossipped on the network and can be deemed useful per se. This fits the mould of what you would call transactions in Bitcoin or Ethereum.
Inherents, meanwhile, are not passed on the network and are not signed. They represent data which describes the environment but which cannot call upon anything to prove it per se such as a signature. Rather they are assumed to be “true” simply because a sufficiently large number of validators have agreed on them being reasonable.
To give an example, there is the timestamp inherent which sets the current timestamp of the block. This is not a fixed part of Substrate, but does come as part of the Substrate Runtime Module Library to be used as desired. No signature could fundamentally prove that a block were authored at a given time in quite the same way that a signature can “prove” the desire to spend some particular funds. Rather, it is the business of each validator to ensure that they believe the timestamp is set to something reasonable before they agree that the block candidate is valid.
Other examples include the parachain-heads extrinsic in Polkadot and the “note-missed-proposal” extrinsic used in the Substrate Runtime Module Library to determine and punish or deactivate offline validators.
Substrate Node, as of genesis, defines a specific Extrinsic format, based around the generic
UncheckedMortalExtrinsic with a standard
Era format, a Node-specific function format, a 64-bit block number, 64-bit transaction index, a standard
Address format combining 256-bit
AccountId with a variable-width 64-bit enumerated
AccountIndex, and Ed25519 crypto:
An account in Node may be identified in several ways: either directly through its 32-byte public key or
AccountId; or alternatively through a 64-bit index into an account lookup table stored on-chain known as an
AccountIndex. If either may be used, then there is an
Address type which can encode to either of them. This is the type that is used in transactions.
AccountId should be considered the canonical means to identify an account. However, at a difficult-to-compress 32-bytes, it is very heavyweight for a system that has well-under a million accounts. The idea of account indices lets us use a much smaller number to identify accounts provided we are happy that the account will still be in existence when the index comes to be used.
To get maximum efficiency with on-chain storage,
Addresses are encoded as variable length quantities: if it’s an index whose value is small enough, the value is encoded as a single byte. If it’s an index that fits into 2, 4 or 8 bytes, then it’ll be encoded into 3, 5 and 9 bytes respectively. If it’s a straight 32-byte identifier, then it’ll be encoded into 33 bytes.
The translation from AccountIndex to AccountId, if necessary, is done through a on-chain query provided by the Balances module. Account Indexes can be reused if the account becomes dead, implying that any transaction which refers to the account using an Index must be executed in a timely fashion, i.e. while the account is still alive. If the account holder keeps ample funds in it, then it’ll be fine, but if they spend it all, then the transaction could become invalid since the Account ID that it is signed with could no longer match the account to which the index later refers.
The signing payload is a simple concatenation of four fields, three from the transaction and one derived:
The Transaction Era is a data structure that, with the help of the Signing Payload, restricts a transaction’s validity to a particular period of blocks in a specific blockchain.
Given a Transaction Era instance and a block number from a time recent to when the transaction was authored, it can be used to calculate the exact block number at which the transaction was authored. Crucially, if the block number provided is not sufficiently recent (or is before the block that the transaction was authored), then it will provide the wrong answer, which helps make an “out of date” transaction invalid. The level of recent-ness required before the transaction becomes invalid can be configured from 4 blocks through to 65536 blocks, or can be disabled entirely.
It is defined as a 16-bit quantity:
More formally, given
hi12, we can define
enabled := low4 > 0
enabled then we can define:
period := 2 ** (low4 + 1)
phase := hi12 * factor
factor := MAX(1, period / (2 ** 12))
phase, given an authoring block number
calculate_phase(period, current) := Q(current, factor) % period
Q(n, d) := FLOOR(n / d) * d
And, given a sufficiently recent block number
recent, then we can reverse it and get the original authoring block as:
calculate_current(period, phase, recent) := Q(current - phase, period) + phase