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Keta - A Transactional Metadata Store Backed by Apache Kafka

Keta is a transactional metadata store backed by Apache Kafka.

Maven

Releases of Keta are deployed to Maven Central.

<dependency>
    <groupId>io.kcache</groupId>
    <artifactId>keta-core</artifactId>
    <version>1.0.0</version>
</dependency>

Getting Started

To run Keta, download a release, unpack it, and then modify config/keta.properties to point to an existing Kafka broker. Then run the following:

$ bin/keta-start config/keta.properties

Keta can be used with any client that supports the etcd v3 APIs. To use Keta with the etcdctl command line client, first download etcd here. Then at a separate terminal, start etcdctl.

$ etcdctl put mykey "this is awesome"
$ etcdctl get mykey

The etcd APIs have a concise way for expressioning transactions.

$ etcdctl put user1 bad
$ etcdctl txn --interactive

compares:
value("user1") = "bad"      

success requests (get, put, delete):
del user1  

failure requests (get, put, delete):
put user1 good

To expire key-values, use a lease.

$ etcdctl lease grant 300
# lease 2be7547fbc6a5afa granted with TTL(300s)

$ etcdctl put sample value --lease=2be7547fbc6a5afa
$ etcdctl get sample

$ etcdctl lease keep-alive 2be7547fbc6a5afa
$ etcdctl lease revoke 2be7547fbc6a5afa
# or after 300 seconds
$ etcdctl get sample

To receive change notifications, use a watch.

$ etcdctl watch stock --prefix

Then at a separate terminal, enter the following:

$ etcdctl put stock1 10
$ etcdctl put stock2 20

If multiple Keta servers are configured with the same cluster group ID (see Basic Configuration), then they will form a cluster and one of them will be elected as leader, while the others will become followers (replicas). If a follower receives a request, it will be forwarded to the leader. If the leader fails, one of the followers will be elected as the new leader.

Basic Configuration

Keta has a number of configuration properties that can be specified.

• listeners - List of listener URLs that include the scheme, host, and port. Defaults to http://0.0.0.0:2379.
• cluster.group.id - The group ID to be used for leader election. Defaults to keta.
• leader.eligibility - Whether this node can participate in leader election. Defaults to true.
• kafkacache.backing.cache - The backing cache for KCache, one of memory (default), bdbje, lmdb, mapdb, or rocksdb.
• kafkacache.data.dir - The root directory for backing cache storage. Defaults to /tmp.
• kafkacache.bootstrap.servers - A list of host and port pairs to use for establishing the initial connection to Kafka.
• kafkacache.group.id - The group ID to use for the internal consumers, which needs to be unique for each node. Defaults to keta-1.
• kafkacache.topic.replication.factor - The replication factor for the internal topics created by Keta. Defaults to 3.
• kafkacache.init.timeout.ms - The timeout for initialization of the Kafka cache, including creation of internal topics. Defaults to 300 seconds.
• kafkacache.timeout.ms - The timeout for an operation on the Kafka cache. Defaults to 60 seconds.

Security

HTTPS

To use HTTPS, first configure the listeners with an https prefix, then specify the following properties with the appropriate values.

ssl.keystore.location=/var/private/ssl/custom.keystore
ssl.keystore.password=changeme
ssl.key.password=changeme
ssl.truststore.location=/var/private/ssl/custom.truststore
ssl.truststore.password=changeme

Authentication and Role-Based Access Control

Keta supports the same authentication and role-based access control (RBAC) APIs as etcd. For more info, see the etcd documentation here.

Kafka Authentication

Authentication to a secure Kafka cluster is described here.

Implementation Notes

Keta uses seven topics to hold metadata:

• _keta_kv - A topic that holds the key-values.
• _keta_leases - A topic that holds leases.
• _keta_auth - A topic that holds auth configuration.
• _keta_auth_users - A topic that holds auth users.
• _keta_auth_roles - A topic that holds auth roles.
• _keta_commits - A topic that holds the list of committed transactions.
• _keta_timestamps - A topic that stores the maximum timestamp that the transaction manager is allowed to return to clients.

For more info on Keta, see this blog post.