ADR 4: PAB and indexing component integration
Let’s start with the problematic example (copy-paste of the current PubKey contract in plutus-use-cases).
-- | Lock some funds in a 'PayToPubKey' contract, returning the output's address
-- and a 'TxIn' transaction input that can spend it.
:: forall w s e.
( AsPubKeyError e
-> Contract w s e (TxOutRef, Maybe ChainIndexTxOut, TypedValidator PubKeyContract)
pubKeyContract pk vl = mapError (review _PubKeyError ) $ do
-- Step 1
let inst = typedValidator pk
address = Scripts.validatorAddress inst
tx = Constraints.mustPayToTheScriptWithDatumHash () vl
ledgerTx <- mkTxConstraints (Constraints.typedValidatorLookups inst) tx
>>= submitUnbalancedTx . Constraints.adjustUnbalancedTx
-- Step 2
_ <- awaitTxConfirmed (getCardanoTxId ledgerTx)
-- Step 3
let refs = Map.keys
$ Map.filter ((==) address . txOutAddress)
$ getCardanoTxProducedOutputs ledgerTx
case refs of
 -> throwing _ScriptOutputMissing pk
[outRef] -> do
-- Step 4
ciTxOut <- unspentTxOutFromRef outRef
pure (outRef, ciTxOut, inst)
_ -> throwing _MultipleScriptOutputs pk
Here’s an outline of the contract’s steps:
Creates a transaction and submits it to the node
Waits for transaction to be confirmed
Finds the first UTXO of that transaction (return type TxOutRef)
Queries the plutus-chain-index to get the ChainIndexTxOut out of that TxOutRef
The problem is that the ciTxOut variable in step 4 will almost always result in Nothing.
Why? Here’s some context.
The PAB listens to the local node and stores blockchain information in memory such as the status of transactions, the status of transaction outputs, the last synced slot, the current slot, etc., in a variable of type BlockchainEnv. The awaitTxConfirmed is actually querying the state of BlockchainEnv and waits until the status of the transaction transitions to Confirmed.
Meanwhile, plutus-chain-index (our main indexing component at the time of this writing) is also listening to incoming blocks from the local node and indexes them into a database. The indexed data can be queried using the REST API interface.
This brings up the main issue: the PAB and plutus-chain-index each listen to the same source of information (a local Cardano node), but each index the information at different speeds. For a dApp developer writing off-chain code using the Contract API, there is no abstraction for handling multiple sources of truth.
Currently, in the best case scenario (fully synced PAB and plutus-chain-index), plutus-chain-index will always trail behind the in-memory storage of the PAB by a few seconds. Therefore, even in this scenario, querying the plutus-chain-index with unspentTxOutFromRef in the above contract has a high probability of returning Nothing.
The best solution is probably a combination of the Alternative solutions described below. However, we will mainly choose the Query functions should interact with a single source of truth solution.
We will replace plutus-chain-index with Marconi as PAB’s indexing component
We will move out the blockchain information indexed by PAB in Marconi
We will add new indexers in Marconi in order to replicate the information indexed by plutus-chain-index
We will adapt the architecture of Marconi (which will become our new indexing component) to support waiting queries
Since we suppose that indexing component should be in the same machine as the PAB, then we will use Marconi as a library to index and query the indexed blockchain information without relying on an HTTP API
In this section, we describe all the ways to deal with the problem. Note that final decision might include one or more of these alternate solutions.
Make sure all components are in sync
We can make sure that all indexing components are syncing at the same speed. For example, if PAB syncs from the local node and arrives at slot 100, then it needs to wait for the chain-index to also arrive at slot 100. Only then can it respond to a Contract request.
A simply way to achieve this behavior is to change the implementation of Contract API handler functions (like utxosAt, unspentTxOutFromRef, etc.) by waiting for the component to be in sync with all other components. For example, let’s take the utxosAt request which basically queries UTXOs from a given address using plutus-chain-index. We could, before querying, wait for plutus-chain-index to be in sync with all components which index blockchain data (PAB, Blockfrost).
All the indexed information is consistent with each other
As fast as the slowest indexing component
Tight coupling between the indexing components meaning that if the Contract only uses chain-index requests without using requests from other indexing components, the chain-index will still have to wait for all other components to be in sync with each other
Add indexing specific functions in the Contract API
In this scenario, we would need to split Contract API requests which interact with an external indexing component to the ones that use the PAB. Currently, we have awaitTxConfirmed which uses the indexed information in the PAB to wait for a transaction status to change to Confirmed. On top of that, we can have awaitTxIndexed or awaitTxOutIndexed which will wait for the information to be indexed in the external indexing component.
Limits design change on the PAB
More control given to the user of the Contract API
Adds an undesired complexity to the Contract API
We’ll need to add a bunch of functions (e.g., currentNodeSlot, currentMarconiSlot, awaitMarconiTxConfirmed, `awaitScrollsTxConfirmed, etc.) for each new indexing component we want to support
Query functions should interact with a single source of truth
In this scenario, we make the design decision that the Contract API should only interact with a single indexing component. Thus, any blockchain information currently stored in the PAB should be moved to the indexing component. Also, combining indexing components would need to be integrated in the single indexing solution that’s connected to the PAB.
Simplest in design to implement (other than manual work to move code)
No modification to the Contract API
Augments PAB’s cohesion, because it’s responsability will be limited to contract instance management
The design of the indexing component will need to be changed to support waiting queries (like the awaitTxConfirmed from PAB)
Still under the assumption that the indexing component is in sync with the PAB in order to use some querying functions
Having a single source of truth indexer will augment the PAB’s cohesion and greatly simpy the plutus-pab codebase. Also, we will not encounter the situation where we have multiple indexers that index at different speeds.
However, there is still the problem that the indexing solution might not be in sync with the local node. Therefore, we need to make another decision on how to deal with it.
This ADR has been discussed here: #550.