This topic assumes an understanding of the conceptual material in the documentation on private data.
Private data collection definition¶
A collection definition contains one or more collections, each having a policy definition listing the organizations in the collection, as well as properties used to control dissemination of private data at endorsement time and, optionally, whether the data will be purged.
Beginning with the Fabric chaincode lifecycle introduced with Fabric v2.0, the
collection definition is part of the chaincode definition. The collection is
approved by channel members, and then deployed when the chaincode definition
is committed to the channel. The collection file needs to be the same for all
channel members. If you are using the peer CLI to approve and commit the
chaincode definition, use the
--collections-config flag to specify the path
to the collection definition file. If you are using the Fabric SDK for Node.js,
visit How to install and start your chaincode.
To use the previous lifecycle process to deploy a private data collection,
--collections-config flag when instantiating your chaincode.
Collection definitions are composed of the following properties:
name: Name of the collection.
policy: The private data collection distribution policy defines which organizations' peers are allowed to persist the collection data expressed using the
Signaturepolicy syntax, with each member being included in an
ORsignature policy list. To support read/write transactions, the private data distribution policy must define a broader set of organizations than the chaincode endorsement policy, as peers must have the private data in order to endorse proposed transactions. For example, in a channel with ten organizations, five of the organizations might be included in a private data collection distribution policy, but the endorsement policy might call for any three of the organizations to endorse.
requiredPeerCount: Minimum number of peers (across authorized organizations) that each endorsing peer must successfully disseminate private data to before the peer signs the endorsement and returns the proposal response back to the client. Requiring dissemination as a condition of endorsement will ensure that private data is available in the network even if the endorsing peer(s) become unavailable. When
0, it means that no distribution is required, but there may be some distribution if
maxPeerCountis greater than zero. A
0would typically not be recommended, as it could lead to loss of private data in the network if the endorsing peer(s) becomes unavailable. Typically you would want to require at least some distribution of the private data at endorsement time to ensure redundancy of the private data on multiple peers in the network.
maxPeerCount: For data redundancy purposes, the maximum number of other peers (across authorized organizations) that each endorsing peer will attempt to distribute the private data to. If an endorsing peer becomes unavailable between endorsement time and commit time, other peers that are collection members but who did not yet receive the private data at endorsement time, will be able to pull the private data from peers the private data was disseminated to. If this value is set to
0, the private data is not disseminated at endorsement time, forcing private data pulls against endorsing peers on all authorized peers at commit time.
blockToLive: Represents how long the data should live on the private database in terms of blocks. The data will live for this specified number of blocks on the private database and after that it will get purged, making this data obsolete from the network so that it cannot be queried from chaincode, and cannot be made available to requesting peers. To keep private data indefinitely, that is, to never purge private data, set the
memberOnlyRead: a value of
trueindicates that peers automatically enforce that only clients belonging to one of the collection member organizations are allowed read access to private data. If a client from a non-member org attempts to execute a chaincode function that performs a read of a private data key, the chaincode invocation is terminated with an error. Utilize a value of
falseif you would like to encode more granular access control within individual chaincode functions.
memberOnlyWrite: a value of
trueindicates that peers automatically enforce that only clients belonging to one of the collection member organizations are allowed to write private data from chaincode. If a client from a non-member org attempts to execute a chaincode function that performs a write on a private data key, the chaincode invocation is terminated with an error. Utilize a value of
falseif you would like to encode more granular access control within individual chaincode functions, for example you may want certain clients from non-member organization to be able to create private data in a certain collection.
endorsementPolicy: An optional endorsement policy to utilize for the collection that overrides the chaincode level endorsement policy. A collection level endorsement policy may be specified in the form of a
signaturePolicyor may be a
channelConfigPolicyreference to an existing policy from the channel configuration. The
endorsementPolicymay be the same as the collection distribution
policy, or may require fewer or additional organization peers.
Here is a sample collection definition JSON file, containing an array of two collection definitions:
"policy": "OR('Org1MSP.member', 'Org2MSP.member')",
This example uses the organizations from the Fabric test network,
Org2. The policy in the
collectionMarbles definition authorizes both
organizations to the private data. This is a typical configuration when the
chaincode data needs to remain private from the ordering service nodes. However,
the policy in the
collectionMarblePrivateDetails definition restricts access
to a subset of organizations in the channel (in this case
Org1 ). Additionally,
writing to this collection requires endorsement from an
Org1 peer, even
though the chaincode level endorsement policy may require endorsement from
Org2. And since "memberOnlyWrite" is true, only clients from
Org1 may invoke chaincode that writes to the private data collection.
In this way you can control which organizations are entrusted to write to certain
private data collections.
Private data dissemination¶
Since private data is not included in the transactions that get submitted to
the ordering service, and therefore not included in the blocks that get distributed
to all peers in a channel, the endorsing peer plays an important role in
disseminating private data to other peers of authorized organizations. This ensures
the availability of private data in the channel's collection, even if endorsing
peers become unavailable after their endorsement. To assist with this dissemination,
requiredPeerCount properties in the collection definition
control the degree of dissemination at endorsement time.
If the endorsing peer cannot successfully disseminate the private data to at least
requiredPeerCount, it will return an error back to the client. The endorsing
peer will attempt to disseminate the private data to peers of different organizations,
in an effort to ensure that each authorized organization has a copy of the private
data. Since transactions are not committed at chaincode execution time, the endorsing
peer and recipient peers store a copy of the private data in a local
alongside their blockchain until the transaction is committed.
When authorized peers do not have a copy of the private data in their transient
data store at commit time (either because they were not an endorsing peer or because
they did not receive the private data via dissemination at endorsement time),
they will attempt to pull the private data from another authorized
peer, for a configurable amount of time based on the peer property
peer.gossip.pvtData.pullRetryThreshold in the peer configuration
The peers being asked for private data will only return the private data if the requesting peer is a member of the collection as defined by the private data dissemination policy.
Considerations when using
- If the requesting peer is able to retrieve the private data within the
pullRetryThreshold, it will commit the transaction to its ledger (including the private data hash), and store the private data in its state database, logically separated from other channel state data.
- If the requesting peer is not able to retrieve the private data within
pullRetryThreshold, it will commit the transaction to it’s blockchain (including the private data hash), without the private data.
- If the peer was entitled to the private data but it is missing, then that peer will not be able to endorse future transactions that reference the missing private data - a chaincode query for a key that is missing will be detected (based on the presence of the key’s hash in the state database), and the chaincode will receive an error.
Therefore, it is important to set the
properties large enough to ensure the availability of private data in your
channel. For example, if each of the endorsing peers become unavailable
before the transaction commits, the
properties will have ensured the private data is available on other peers.
For collections to work, it is important to have cross organizational gossip configured correctly. Refer to our documentation on Gossip data dissemination protocol, paying particular attention to the "anchor peers" and "external endpoint" configuration.
Referencing collections from chaincode¶
A set of shim APIs are available for setting and retrieving private data.
The same chaincode data operations can be applied to channel state data and
private data, but in the case of private data, a collection name is specified
along with the data in the chaincode APIs, for example
A single chaincode can reference multiple collections.
Referencing implicit collections from chaincode¶
Starting in v2.0, an implicit private data collection can be used for each organization in a channel, so that you don't have to define collections if you'd like to utilize per-organization collections. Each org-specific implicit collection has a distribution policy and endorsement policy of the matching organization. You can therefore utilize implicit collections for use cases where you'd like to ensure that a specific organization has written to a collection key namespace. The v2.0 chaincode lifecycle uses implicit collections to track which organizations have approved a chaincode definition. Similarly, you can use implicit collections in application chaincode to track which organizations have approved or voted for some change in state.
To write and read an implicit private data collection key, in the
GetPrivateData chaincode APIs, specify the collection parameter as
"_implicit_org_<MSPID>", for example
Application defined collection names are not allowed to start with an underscore, therefore there is no chance for an implicit collection name to collide with an application defined collection name.
How to pass private data in a chaincode proposal¶
Since the chaincode proposal gets stored on the blockchain, it is also important
not to include private data in the main part of the chaincode proposal. A special
field in the chaincode proposal called the
transient field can be used to pass
private data from the client (or data that chaincode will use to generate private
data), to chaincode invocation on the peer. The chaincode can retrieve the
transient field by calling the GetTransient() API.
transient field gets excluded from the channel transaction.
Protecting private data content¶
If the private data is relatively simple and predictable (e.g. transaction dollar amount), channel members who are not authorized to the private data collection could try to guess the content of the private data via brute force hashing of the domain space, in hopes of finding a match with the private data hash on the chain. Private data that is predictable should therefore include a random "salt" that is concatenated with the private data key and included in the private data value, so that a matching hash cannot realistically be found via brute force. The random "salt" can be generated at the client side (e.g. by sampling a secure psuedo-random source) and then passed along with the private data in the transient field at the time of chaincode invocation.
Access control for private data¶
Until version 1.3, access control to private data based on collection membership
was enforced for peers only. Access control based on the organization of the
chaincode proposal submitter was required to be encoded in chaincode logic.
Collection configuration options
memberOnlyRead (since version v1.4) and
memberOnlyWrite (since version v2.0) can automatically enforce that the chaincode
proposal submitter must be from a collection member in order to read or write
private data keys. For more information about collection
configuration definitions and how to set them, refer back to the
Private data collection definition section of this topic.
If you would like more granular access control, you can set
memberOnlyWrite to false. You can then apply your
own access control logic in chaincode, for example by calling the GetCreator()
chaincode API or using the client identity
chaincode library .
Querying Private Data¶
Private data collection can be queried just like normal channel data, using shim APIs:
GetPrivateDataByRange(collection, startKey, endKey string)
GetPrivateDataByPartialCompositeKey(collection, objectType string, keys string)
And for the CouchDB state database, JSON content queries can be passed using the shim API:
GetPrivateDataQueryResult(collection, query string)
- Clients that call chaincode that executes range or rich JSON queries should be aware that they may receive a subset of the result set, if the peer they query has missing private data, based on the explanation in Private Data Dissemination section above. Clients can query multiple peers and compare the results to determine if a peer may be missing some of the result set.
- Chaincode that executes range or rich JSON queries and updates data in a single transaction is not supported, as the query results cannot be validated on the peers that don’t have access to the private data, or on peers that are missing the private data that they have access to. If a chaincode invocation both queries and updates private data, the proposal request will return an error. If your application can tolerate result set changes between chaincode execution and validation/commit time, then you could call one chaincode function to perform the query, and then call a second chaincode function to make the updates. Note that calls to GetPrivateData() to retrieve individual keys can be made in the same transaction as PutPrivateData() calls, since all peers can validate key reads based on the hashed key version.
Using Indexes with collections¶
The topic CouchDB as the State Database describes indexes that can be
applied to the channel’s state database to enable JSON content queries, by
packaging indexes in a
META-INF/statedb/couchdb/indexes directory at chaincode
installation time. Similarly, indexes can also be applied to private data
collections, by packaging indexes in a
directory. An example index is available here.
Considerations when using private data¶
Private data purging¶
Private data can be periodically purged from peers. For more details,
blockToLive collection definition property above.
Additionally, recall that prior to commit, peers store private data in a local
transient data store. This data automatically gets purged when the transaction
commits. But if a transaction was never submitted to the channel and
therefore never committed, the private data would remain in each peer’s
transient store. This data is purged from the transient store after a
configurable number blocks by using the peer’s
peer.gossip.pvtData.transientstoreMaxBlockRetention property in the peer
Updating a collection definition¶
To update a collection definition or add a new collection, you can update
the chaincode definition and pass the new collection configuration
in the chaincode approve and commit transactions, for example using the
flag if using the CLI. If a collection configuration is specified when updating
the chaincode definition, a definition for each of the existing collections must be
When updating a chaincode definition, you can add new private data collections, and update existing private data collections, for example to add new members to an existing collection or change one of the collection definition properties. Note that you cannot update the collection name or the blockToLive property, since a consistent blockToLive is required regardless of a peer's block height.
Collection updates becomes effective when a peer commits the block with the updated chaincode definition. Note that collections cannot be deleted, as there may be prior private data hashes on the channel’s blockchain that cannot be removed.
Private data reconciliation¶
Starting in v1.4, peers of organizations that are added to an existing collection will automatically fetch private data that was committed to the collection before they joined the collection.
This private data "reconciliation" also applies to peers that were entitled to receive private data but did not yet receive it --- because of a network failure, for example --- by keeping track of private data that was "missing" at the time of block commit.
Private data reconciliation occurs periodically based on the
properties in core.yaml. The peer will periodically attempt to fetch the private
data from other collection member peers that are expected to have it.
Note that this private data reconciliation feature only works on peers running v1.4 or later of Fabric.