Hyperledger Fabric 1.4 Private data collection

Question

Hyperledger fabric provides inbuilt support storing offchain data with the help of private collections. For this we need to specify the collection config which contains various collection names along with the participants that has access to data present in those collections. There is a setting called "BlockToLive" using which we can specify for how many blocks the peers should store the private data they have access to. The peers will automatically purge the private data after the ledger block height reaches to the mentioned threshold.

We have a requirement in which we need to use the private data collections but the data should be removed (automatically/manually) after exactly 30 days. Is there any possibility to achieve the same?

1. timeToLive: Is there any implementation for specifying the timeToLive or similar configuration? Using this the peers will automatically purge the data after mentioned duration.
2. If there is no automatic way possible currently, how can the data present in private collection be removed manually? Is there any way by which the data in private collections be removed directly using external scripts/code? We don't want to to create chaincode methods that will be used to invoke as transactions to delete private data as even the deletion of private data will need to be endorsed and sent to the orderer and needs to be added to the ledger. How can the private data be removed directly?

Answer 1

First, everything that you put on the blockchain is permanent and supposed to be decentralized. So, having unilateral control over when to delete the private data goes against the virtue of decentralization and you should avoid it (answer to point 2). Endorsers endorse every change or transactions. (including the BlockToLive), so it does not make sense to deviate from the agreed period.

Second, time in distributed systems is subjective and it impossible to have a global clock ⏰ (say 30 days for one node can be 29.99 for another or 29.80 days for another). Hence, time is measured in blocks which is objective for all nodes. So, it is recommended that you use BlockToLive. It can be difficult first, but you can calculate backwards. Say you have BlockSize as 10 (no. of transactions in a block) and expect around 100 transactions per day, then you can set BlockToLive = 300. (Of course, this is a ballpark number).

Finally, if you still want to delete private data at will, I would recommend manual off-chain storage mechanisms.