CouchDB as the State Database

State Database options

State database options include LevelDB and CouchDB. LevelDB is the default key/value state database embedded in the peer process. CouchDB is an optional alternative external state database. Like the LevelDB key/value store, CouchDB can store any binary data that is modeled in chaincode (CouchDB attachment functionality is used internally for non-JSON binary data). But as a JSON document store, CouchDB additionally enables rich query against the chaincode data, when chaincode values (e.g. assets) are modeled as JSON data.

Both LevelDB and CouchDB support core chaincode operations such as getting and setting a key (asset), and querying 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 assets as JSON and use CouchDB, you can also perform complex rich queries against the chaincode data values, using the CouchDB JSON query language within chaincode. These types of queries are excellent for understanding what is on the ledger. Proposal responses for these types of queries are typically useful to the client application, but are not typically submitted as transactions to the ordering service. In fact, there is no guarantee the result set is stable between chaincode execution and commit time for rich queries, and therefore rich queries are not appropriate for use in update transactions, unless your application can guarantee the result set is stable between chaincode execution time and commit time, or can handle potential changes in subsequent transactions. For example, if you perform a rich query for all assets owned by Alice and transfer them to Bob, a new asset may be assigned to Alice by another transaction between chaincode execution time and commit time, and you would miss this “phantom” item.

CouchDB runs as a separate database process alongside the peer, therefore there are additional considerations in terms of setup, management, and operations. You may consider starting with the default embedded LevelDB, and move to CouchDB if you require the additional complex rich queries. It is a good practice to model chaincode asset data as JSON, so that you have the option to perform complex rich queries if needed in the future.

Using CouchDB from Chaincode

Most of the chaincode shim APIs can be utilized with either LevelDB or CouchDB state database, e.g. GetState, PutState, GetStateByRange, GetStateByPartialCompositeKey. Additionally when you utilize CouchDB as the state database and model assets as JSON in chaincode, you can perform rich queries against the JSON in the state database by using the GetQueryResult API and passing a CouchDB query string. The query string follows the CouchDB JSON query syntax.

The marbles02 fabric sample demonstrates use of CouchDB queries from chaincode. It includes a queryMarblesByOwner() function that demonstrates parameterized queries by passing an owner id into chaincode. It then queries the state data for JSON documents matching the docType of “marble” and the owner id using the JSON query syntax:

{"selector":{"docType":"marble","owner":<OWNER_ID>}}

Indexes in CouchDB are required in order to make JSON queries efficient and are required for any JSON query with a sort. Indexes can be packaged alongside chaincode in a /META-INF/statedb/couchdb/indexes directory. Each index must be defined in its own text file with extension *.json with the index definition formatted in JSON following the CouchDB index JSON syntax. For example, to support the above marble query, a sample index on the docType and owner fields is provided:

{"index":{"fields":["data.docType","data.owner"]},"ddoc":"indexOwnerDoc", "name":"indexOwner","type":"json"}

The sample index can be found here.

Note

In 1.1 alpha, the “data” wrapper must be specified for each field referenced in the index definition. In subsequent releases the requirement to specify the “data” wrapper may be lifted.

Any index in the chaincode’s META-INF/statedb/couchdb/indexes directory will be packaged up with the chaincode for deployment. When the chaincode is both installed on a peer and instantiated on one of the peer’s channels, the index will automatically be deployed to the peer’s channel and chaincode specific state database (if it has been configured to use CouchDB). If you install the chaincode first and then instantiate the chaincode on the channel, the index will be deployed at chaincode instantiation time. If the chaincode is already instantiated on a channel and you later install the chaincode on a peer, the index will be deployed at chaincode installation time.

Upon deployment, the index will automatically be utilized by chaincode queries. CouchDB can automatically determine which index to use based on the fields being used in a query. Alternatively, in the selector query the index can be specified using the use_index keyword.

The same index may exist in subsequent versions of the chaincode that gets installed. To change the index, use the same index name but alter the index definition. Upon installation/instantiation, the index definition will get re-deployed to the peer’s state database.

If you have a large volume of data already, and later install the chaincode, the index creation upon installation may take some time. Similarly, if you have a large volume of data already and instantiate a subsequent version of the chaincode, the index creation may take some time. Avoid calling chaincode functions that query the state database at these times as the chaincode query may time out while the index is getting initialized. During transaction processing, the indexes will automatically get refreshed as blocks are committed to the ledger.

CouchDB Configuration

CouchDB is enabled as the state database by changing the stateDatabase configuration option from goleveldb to CouchDB. Additionally, the couchDBAddress needs to configured to point to the CouchDB to be used by the peer. The username and password properties should be populated with an admin username and password if CouchDB is configured with a username and password. Additional options are provided in the couchDBConfig section and are documented in place. Changes to the core.yaml will be effective immediately after restarting the peer.

You can also pass in docker environment variables to override core.yaml values, for example CORE_LEDGER_STATE_STATEDATABASE and CORE_LEDGER_STATE_COUCHDBCONFIG_COUCHDBADDRESS.

Below is the stateDatabase section from core.yaml:

state:
  # stateDatabase - options are "goleveldb", "CouchDB"
  # goleveldb - default state database stored in goleveldb.
  # CouchDB - store state database in CouchDB
  stateDatabase: goleveldb
  couchDBConfig:
     # It is recommended to run CouchDB on the same server as the peer, and
     # not map the CouchDB container port to a server port in docker-compose.
     # Otherwise proper security must be provided on the connection between
     # CouchDB client (on the peer) and server.
     couchDBAddress: couchdb:5984
     # This username must have read and write authority on CouchDB
     username:
     # The password is recommended to pass as an environment variable
     # during start up (e.g. LEDGER_COUCHDBCONFIG_PASSWORD).
     # If it is stored here, the file must be access control protected
     # to prevent unintended users from discovering the password.
     password:
     # Number of retries for CouchDB errors
     maxRetries: 3
     # Number of retries for CouchDB errors during peer startup
     maxRetriesOnStartup: 10
     # CouchDB request timeout (unit: duration, e.g. 20s)
     requestTimeout: 35s
     # Limit on the number of records to return per query
     queryLimit: 10000

CouchDB hosted in docker containers supplied with Hyperledger Fabric have the capability of setting the CouchDB username and password with environment variables passed in with the COUCHDB_USER and COUCHDB_PASSWORD environment variables using Docker Compose scripting.

For CouchDB installations outside of the docker images supplied with Fabric, the local.ini file of that installation must be edited to set the admin username and password.

Docker compose scripts only set the username and password at the creation of the container. The local.ini file must be edited if the username or password is to be changed after creation of the container.

Note

CouchDB peer options are read on each peer startup.