BigchainDB lets users control what other users can do, to some extent. That ability resembles "permissions" in the *nix world, "privileges" in the SQL world, and "access control" in the security world.
In BigchainDB, every output has an associated condition (crypto-condition).
To spend/transfer an unspent output, a user (or group of users) must fulfill the condition. Another way to say that is that only certain users have permission to spend the output. The simplest condition is of the form, "Only someone with the private key corresponding to this public key can spend this output." Much more elaborate conditions are possible, e.g. "To spend this output, …"
- "…anyone in the Accounting Group can sign."
- "…three of these four people must sign."
- "…either Bob must sign, or both Tom and Sylvia must sign."
For details, see the section titled Transaction Components: Conditions in the relevant BigchainDB Transactions Spec.
Once an output has been spent, it can't be spent again: nobody has permission to do that. That is, BigchainDB doesn't permit anyone to "double spend" an output.
When someone builds a TRANSFER transaction, they can put an arbitrary JSON object in the metadata field (within reason; real BigchainDB networks put a limit on the size of transactions). That is, they can write just about anything they want in a TRANSFER transaction.
Does that mean there are no "write permissions" in BigchainDB? Not at all!
A TRANSFER transaction will only be valid (allowed) if its inputs fulfill some previous outputs. The conditions on those outputs will control who can build valid TRANSFER transactions. In other words, one can interpret the condition on an output as giving "write permissions" to certain users to write something into the history of the associated asset.
As a concrete example, you could use BigchainDB to write a public journal where only you have write permissions. Here's how: First you'd build a CREATE transaction with the asset.data being something like {"title": "The Journal of John Doe"}, with one output. That output would have an amount 1 and a condition that only you (who has your private key) can spend that output.
Each time you want to append something to your journal, you'd build a new TRANSFER transaction with your latest entry in the metadata field, e.g.
{"timestamp": "1508319582",
"entry": "I visited Marmot Lake with Jane."}The TRANSFER transaction would have one output. That output would have an amount 1 and a condition that only you (who has your private key) can spend that output. And so on. Only you would be able to append to the history of that asset (your journal).
The same technique could be used for scientific notebooks, supply-chain records, government meeting minutes, and so on.
You could do more elaborate things too. As one example, each time someone writes a TRANSFER transaction, they give someone else permission to spend it, setting up a sort of writers-relay or chain letter.
Note
Anyone can write any JSON (again, within reason) in the asset.data field of a CREATE transaction. They don't need permission.
See the page titled, :doc:`BigchainDB, Privacy and Private Data <private-data>`.
In September 2017, we published a blog post about how one can define an RBAC sub-system on top of BigchainDB. At the time of writing (January 2018), doing so required the use of a plugin, so it's not possible using standard BigchainDB (which is what's available on the BigchainDB Testnet). That may change in the future. If you're interested, contact BigchainDB.