# Exceptions¶

The introduction of exceptions, a new Daml feature, has many implications for the ledger model. This page describes the changes to the ledger model introduced as part of this new feature.

## Structure¶

Under the new feature, Daml programs can raise and catch exceptions. When an exception is caught in a catch block, the subtransaction starting at the corresponding try block is rolled back.

To support this in our ledger model, we need to modify the transaction structure to indicate which subtransactions were rolled back. We do this by introducing rollback nodes in the transaction. Each rollback node contains a rolled back subtransaction. Rollback nodes are not considered ledger actions.

Therefore we define transactions as a list of nodes, where each node is either a ledger action or a rollback node. This is reflected in the updated EBNF grammar for the transaction structure:

Transaction  ::= Node*
Node         ::= Action | Rollback
Rollback     ::= 'Rollback' Transaction
Action       ::= 'Create' contract
| 'Exercise' party* contract Kind Transaction
| 'Fetch' party* contract
| 'NoSuchKey' key
Kind         ::= 'Consuming' | 'NonConsuming'


Note that Action and Kind have the same definition as before, but since Transaction may now contain rollback nodes, this means that an Exercise action may have a rollback node as one of its consequences.

For example, the following transaction contains a rollback node inside an exercise. It represents a paint offer involving multiple banks. The painter P is offering to paint A’s house as long as they receive an Iou from Bank1 or, failing that, from Bank2. When A accepts, they confirm that transfer of an Iou via Bank1 has failed for some reason, so they roll it back and proceed with a transfer via Bank2:

Note also that rollback nodes may be nested, which represents a situation where multiple exceptions are raised and caught within the same transaction.

For example, the following transaction contains the previous one under a rollback node. It represents a case where the “accept” has failed at the last moment, for some reason, and a “cancel” exercise has been issued in response.

## Consistency¶

In the previous section on consistency, we defined a “before-after” relation on ledger actions. This notion needs to be revised in the presence of rollback nodes. It is no longer enough to perform a preorder traversal of the transaction tree, because the actions under a rollback node cannot affect actions that appear later in the transaction tree.

For example, a contract may be consumed by an exercise under a rollback node, and immediately again after the rollback node. This is allowed because the exercise was rolled back, and this does not represent a “double spend” of the same contract. You can see this in the nested example above, where the PaintOffer contract is consumed by an “agree” exercise, which is rolled back, and then by a “cancel” exercise.

So, we now define the “before-after” relation as a partial order, rather than a total order, on all the actions of a transaction. This relation is defined as follows: act1 comes before act2 (equivalently, act2 comes after act1) if and only if act1 appears before act2 in a preorder traversal of the transaction tree, and any rollback nodes that are ancestors of act1 are also ancestors of act2.

With this modified “before-after” relation, the notion of internal consistency remains the same. Meaning that, for example, for any contract c, we still forbid the creation of c coming after any action on c, and we forbid any action on c coming after a consuming exercise on c.

In the example above, neither consuming exercise comes “after” the other. They are part of separate “continuities”, so they don’t introduce inconsistency. Here are three continuities implied by the “before-after” relation. The first:

The second:

And the third:

As you can see, in each of these continuities, no contract was consumed twice.

## Transaction Normalization¶

The same “before-after” relation can be represented in more than one way using rollback nodes. For example, the following three transactions have the same “before-after” relation among their ledger actions (act1, act2, and act3):

Because of this, these three transactions are equivalent. More generally, two transactions are equivalent if:

• The transactions are the same when you ignore all rollback nodes. That is, if you remove every rollback node and absorb its children into its parent, then two transactions are the same. Equivalently, the transactions have the same ledger actions with the same preorder traversal and subaction relation.
• The transactions have the same “before-after” relation between their actions.
• The transactions have the same set of “rollback children”. A “rollback child” is an action whose direct parent is a rollback node.

For all three transactions above, the “transaction tree ignoring rollbacks” consists only of top-level actions (act1, act2, and act3), the “before-after” relation only says that act2 comes before act3, and all three actions are rollback children. Thus all three transactions are equivalent.

Transaction normalization is the process by which equivalent transactions are converted into the same transaction. In the case above, all three transactions become the transaction in the middle when normalized.

To normalize a transaction, we apply three rules repeatedly across the whole transaction:

1. If a rollback node is empty, we drop it.

2. If a rollback node starts with another rollback node, for instance:

'Rollback' [ 'Rollback' tx , node1, ..., nodeN ]


Then we re-associate the rollback nodes, bringing the inner rollback node out:

'Rollback' tx, 'Rollback' [ node1, ..., nodeN ]

3. If a rollback node ends with another rollback node, for instance:

'Rollback' [ node1, ..., nodeN, 'Rollback' [ node1', ..., nodeM' ] ]


Then we flatten the inner rollback node into its parent:

'Rollback' [ node1, ..., nodeN, node1', ..., nodeM' ]


In the example above, using rule 3 we can turn the left transaction into the middle transaction, and using rule 2 we can turn the right transaction into the middle transaction. None of these rules apply to the middle transaction, so it is already normalized.

In the end, a normalized transaction cannot contain any rollback node that starts or ends with another rollback node, nor may it contain any empty rollback nodes. The normalization process minimizes the number of rollback nodes and their depth needed to represent the transaction.

To reduce the potential for information leaks, the ledger model must only contain normalized transactions. This also applies to projected transactions. An unnormalized transaction is always invalid.

## Authorization¶

Since they are not ledger actions, rollback nodes do not have authorizers directly. Instead, a ledger is well-authorized exactly when the same ledger with rollback nodes removed (that is, replacing the rollback nodes with their children) is well-authorized, according to the old definition.

This is captured in the following rules:

• When a rollback node is authorized by p, then all of its children are authorized by p. In particular:
• Top-level rollback nodes share the authorization of the requestors of the commit with all of its children.
• Rollback nodes that are a consequence of an exercise action act on a contract c share the authorization of the signatories of c and the actors of act with all of its children.
• A nested rollback node shares the authorization it got from its parent with all of its children.
• The required authorizers of a rollback node are the union of all the required authorizers of its children.

## Privacy¶

Rollback nodes also have an interesting effect on the notion of privacy in the ledger model. When projecting a transaction for a party p, it’s necessary to preserve some of the rollback structure of the transaction, even if p does not have the right to observe every action under it. For example, we need p to be able to verify that a rolled back exercise (to which they are an informee) is conformant, but we also need p to know that the exercise was rolled back.

We adjust the definition of projection as follows:

1. For a ledger action, the projection for p is the same as it was before. That is, if p is an informee of the action, then the entire subtree is preserved. Otherwise the action is dropped, and the action’s consequences are projected for p.
2. For a rollback node, the projection for p consists of the projection for p of its children, wrapped up in a new rollback node. In other words, projection happens under the rollback node, but the node is preserved.

After applying this process, the transaction must be normalized.

Consider the deeply nested example from before. To calculate the projection for Bank1, we note that the only visible action is the bottom left exercise. Removing the actions that Bank1 isn’t an informee of, this results in a transaction containing a rollback node, containing a rollback node, containing an exercise. After normalization, this becomes a simple rollback node containing an exercise. See below:

The privacy section of the ledger model makes a point of saying that a contract model should be subaction-closed to support projections. But this requirement is not necessarily true once we introduce rollbacks. Rollback nodes may contain actions that are not valid as standalone actions, since they may have been interrupted prematurely by an exception.

Instead, we require that the contract model be projection-closed, i.e. closed under projections for any party ‘p’. This is a weaker requirement that matches what we actually need.

## Relation to Daml Exceptions¶

Rollback nodes are created when an exception is thrown and caught within the same transaction. In particular, any exception that is caught within a try-catch will generate a rollback node if there are any ledger actions to roll back. For example:

try do
cid <- create MyContract { ... }
exercise cid MyChoice { ... }
throw MyException
catch
MyException ->
create MyOtherContract { ... }


This Daml code will try to create a contract, and exercise a choice on this contract, before throwing an exception. That exception is caught immediately, and then another contract is created.

Thus a rollback node is created, to reset the ledger to the state it had at the start of the “try” block. The rollback node contains the create and exercise nodes. After the rollback node, another contract is created. Thus the final transaction looks like this:

Note that rollback nodes are only created if an exception is caught. An uncaught exception will result in an error, not a transaction.

After execution of the Daml code, the generated transaction is normalized.