Hyperledger fabric - Concurrent transactions - hyperledger-fabric

Question

i'm wondering how is possible to execute concurrent transactions in hyperledger fabric using hyperledger composer.
When i try to submit two transactions at the same time against the same resource I get this error:
Error trying invoke business network. Error: Peer has rejected transaction \'transaction-number\' with code MVCC_READ_CONFLICT
Does anyone know if exist a workaround or a design pattern to avoid this?

Answer 1

Though I may not be providing the best solution, I hope to share some ideas and possible workarounds to this question.
First let's briefly explain why you are getting this error. The underlying DB of Hyperledger Fabric employs a MVCC-like (Multi-Version Concurrency Control) model. An example to this would be two clients trying to update an asset of version 0 to a certain value. One would succeed (updated the value and incremented the version number in the stateDB to 1), while another would fail with this error (MVCC_READ_CONFLICT) due to version mismatch.
One possible solution discussed here (https://medium.com/wearetheledger/hyperledger-fabric-concurrency-really-eccd901e4040) would be to implement a FIFO queue on your own between the business logic and Fabric SDK. Retry logic could also be added in this case.
Another way would be using the delta-concept. Suppose there is an asset A with value 10 (maybe it's representing account balance). This asset is being updated frequently (say being updated in this set of value 12 -> 19 -> 16) by multiple concurrent transactions and the above mentioned error would easily be triggered. Instead, we store the value as deltas (+2 -> +7 -> -3) and the final aggregated value would be the same in the ledger. But keep in mind this trick MAY NOT suit every case and in this example, you may also need to closely monitor the running total to avoid giving money if you got empty in your account. So it depends heavily on the data type and use case.
For more information, you can take a look at this: https://github.com/hyperledger/fabric-samples/tree/release-1.1/high-throughput

Answer 2

I recently ran into this problem and solved it by creating an array of promises of calls to async functions, then resolving one at a time.
My transactions add items from arrays of asset2Ids and asset3Ids to an array field on asset1. My transactions are all acting on the same asset so I was getting an MVCC_READ_CONFLICT error as the read/write set is changes before each transaction is committed. By forcing the transactions to resolve in a synchronous way, this conflict is fixed:
// Create a function array
let funcArray = [];
for (const i of asset2Ids) {
// Add this transaction to array of promises to be resolved
funcArray.push(()=>transactionFunctionThatAddsAsset2IdToAsset1(i).toPromise());
}
for (const j of asset3Ids) {
// Add this transaction to array of promises to be resolved
funcArray.push(()=>transactionFunctionThatAddsAsset3IdToAsset1(j).toPromise());
}
// Resolve all transaction promises against asset in a synchronous way
funcArray.reduce((p,fn) => p.then(fn), Promise.resolve());