State Object Ranges¶
Our generalized state system introduces the idea of state objects, which can be used to control the ownership of assets deposited into the plasma chain. Plasma Cash describes a version of the generalized state system in which each state object represents an indivisible asset of fixed value. This system significantly reduces the total amount of data each user of the plasma client must store.
Arbitrary value payments become difficult to make within Plasma Cash. This is primarily because state objects in Plasma cash may not be split apart or combined. A user may only make payments of a given amount if they are in possession of a set of state objects such that the value of these state objects is exactly the payment amount. Compare this, for example, to the simplicity of making payments in Bitcoin-like systems where a UTXO can be arbitrarily broken apart into outputs with different values.
The following diagram shows the basic idea behind transactions in Plasma Cash:
Each of the eight shown transactions can only operate on one of the eight state objects. As a result, they have to be treated as separate objects that a user might have to keep track of.
One method of simplifying payments within such a system is to require that each state object to have an identical small value. Payments of any amount can then be made by sending many state objects simultaneously. In order to support simultaneous transfer of state object, we introduce mechanisms that allow users to reference ranges of state objects within transactions. This was the primary advancement of Plasma Cashflow.
Here’s a diagram of what this looks like in practice:
As you can see, we can manipulate all eight of the state objects with a single transaction. This means we can imagine the state objects as a single larger object, which reduces total storage requirements.
The transaction format described in our generalized state system specifies that a transaction MUST provide a reference to the state object, or set of state objects, from which it spends. Conveniently, this allows us to create transactions over ranges of state objects.
The specification for a transaction over a range is very simple. Remember that each
objectId in our system is a unique 32 byte identifier. We therefore require that the
objectIds field be a 64 byte value. The first 32 bytes of this value represents the start of the transacted range and the last 32 bytes represents the end of the transacted range. Transactions over ranges are, in effect, transactions on each individual state object where
object.id falls within the specified range.
As described in the background section above, we need to be able to efficiently transact many state objects simultaneously. By allowing transactions to refer to a set of state objects with a range, it’s no longer necessary to submit a transaction for each individual state object.
objectIdsfield of every transaction:
- MUST be a 64 byte value.
- MUST begin with a 32 byte value that represents that start of the transacted range.
- MUST end with a 32 byte value that represents the end of the transacted range.