Hyperledger Fabric FAQ¶
- How many peers in the network need to endorse a transaction?
A. The number of peers required to endorse a transaction is driven by the endorsement policy that is specified at chaincode deployment time.
- Does an application client need to connect to all peers?
A. Clients only need to connect to as many peers as are required by the endorsement policy for the chaincode.
Security & Access Control¶
Data Privacy and Access Control:
- How do I ensure data privacy?
A. There are various aspects to data privacy. First, you can segregate your network into channels, where each channel represents a subset of participants that are authorized to see the data for the chaincodes that are deployed to that channel. Second, within a channel you can restrict the input data to chaincode to the set of endorsers only, by using visibility settings. The visibility setting will determine whether input and output chaincode data is included in the submitted transaction, versus just output data. Third, you can hash or encrypt the data before calling chaincode. If you hash the data then you will need to provide a means to share the source data. If you encrypt the data then you will need to provide a means to share the decryption keys. Fourth, you can restrict data access to certain roles in your organization, by building access control into the chaincode logic. Fifth, ledger data at rest can be encrypted via file system encryption on the peer, and data in-transit is encrypted via TLS.
- Do the orderers see the transaction data?
A. No, the orderers only order transactions, they do not open the transactions. If you do not want the data to go through the orderers at all, and you are only concerned about the input data, then you can use visibility settings. The visibility setting will determine whether input and output chaincode data is included in the submitted transaction, versus just output data. Therefore, the input data can be private to the endorsers only. If you do not want the orderers to see chaincode output, then you can hash or encrypt the data before calling chaincode. If you hash the data then you will need to provide a meansto share the source data. If you encrypt the data then you will need to provide a means to share the decryption keys.
Application-side Programming Model¶
Transaction execution result:
- How do application clients know the outcome of a transaction?
A. The transaction simulation results are returned to the client by the endorser in the proposal response. If there are multiple endorsers, the client can check that the responses are all the same, and submit the results and endorsements for ordering and commitment. Ultimately the committing peers will validate or invalidate the transaction, and the client becomes aware of the outcome via an event, that the SDK makes available to the application client.
- How do I query the ledger data?
A. Within chaincode you can query based on keys. Keys can be queried by range, and composite keys can be modeled to enable equivalence queries against multiple parameters. For example a composite key of (owner,asset_id) can be used to query all assets owned by a certain entity. These key-based queries can be used for read-only queries against the ledger, as well as in transactions that update the ledger.
If you model asset data as JSON in chaincode and use CouchDB as the state database, you can also perform complex rich queries against the chaincode data values, using the CouchDB JSON query language within chaincode. The application client can perform read-only queries, but these responses are not typically submitted as part of transactions to the ordering service.
- How do I query the historical data to understand data provenance?
A. The chaincode API
GetHistoryForKey() will return history of
values for a key.
Q. How to guarantee the query result is correct, especially when the peer being queried may be recovering and catching up on block processing?
A. The client can query multiple peers, compare their block heights, compare their query results, and favor the peers at the higher block heights.
Chaincode (Smart Contracts and Digital Assets)¶
- Does Hyperledger Fabric support smart contract logic?
A. Yes. We call this feature Chaincode. It is our interpretation of the smart contract method/algorithm, with additional features.
A chaincode is programmatic code deployed on the network, where it is executed and validated by chain validators together during the consensus process. Developers can use chaincodes to develop business contracts, asset definitions, and collectively-managed decentralized applications.
- How do I create a business contract?
A. There are generally two ways to develop business contracts: the first way is to code individual contracts into standalone instances of chaincode; the second way, and probably the more efficient way, is to use chaincode to create decentralized applications that manage the life cycle of one or multiple types of business contracts, and let end users instantiate instances of contracts within these applications.
- How do I create assets?
A. Users can use chaincode (for business rules) and membership service (for digital tokens) to design assets, as well as the logic that manages them.
There are two popular approaches to defining assets in most blockchain solutions: the stateless UTXO model, where account balances are encoded into past transaction records; and the account model, where account balances are kept in state storage space on the ledger.
Each approach carries its own benefits and drawbacks. This blockchain technology does not advocate either one over the other. Instead, one of our first requirements was to ensure that both approaches can be easily implemented.
- Which languages are supported for writing chaincode?
A. Chaincode can be written in any programming language and executed in containers. The first fully supported chaincode language is Golang.
Support for additional languages and the development of a templating language have been discussed, and more details will be released in the near future.
It is also possible to build Hyperledger Fabric applications using Hyperledger Composer.
- Does the Hyperledger Fabric have native currency?
A. No. However, if you really need a native currency for your chain network, you can develop your own native currency with chaincode. One common attribute of native currency is that some amount will get transacted (the chaincode defining that currency will get called) every time a transaction is processed on its chain.