What is XTDB?
XTDB—to use Martin Kleppmann’s phrase—is an unbundled database.
What do we have to gain from turning the database inside out? Simpler code, better scalability, better robustness, lower latency, and more flexibility for doing interesting things with data.
XTDB embodies this principle using a combination of:
Apache Kafka for the primary (default) storage of transactions and documents as semi-immutable logs
RocksDB or LMDB to host indexes for rich query support
Clojure protocols for pluggablity (e.g. swap Kafka for SQLite)
This decoupled design enables XTDB to be maintained as a small and efficient core that can be scaled and evolved to support a large variety of use-cases.
XTDB is an open source document database with bitemporal graph queries.
XTDB is a bitemporal database that stores transaction time and valid time histories for all data:
Transaction time provides essential auditability and horizontal scaling
Valid time provides an optional ability to model temporal domain concepts and integrate upstream sources of temporal data
Many databases can support various levels of "time travel" queries across transaction time (i.e. the transactional sequence of database states from the moment of database creation to its current state), however such capabilities are typically complex to use and have practical limitations. By contrast, XTDB provides an always-on capability for point-in-time querying of past transactional states and across the valid time axis.
Bitemporal modelling is broadly useful for event-based architectures and is a critical requirement for systems in any industry with strong auditing regulations, where you need to be able to answer the question "what did you know and when did you know it?".
XTDB supports a Datalog query interface for traversing graph relationships across your documents. Query results are lazily streamed from the underlying Key-Value indexes.
XTDB is ultimately a store of versioned EDN documents. The fields within these documents are automatically indexed as Entity-Attribute-Value triples to support efficient graph queries.
XTDB does not enforce any schema for the documents it stores. One reason for this is that data might come from many different places, and may not ultimately be owned by the service using XTDB to query the data. This design enables schema-on-write and/or schema-on-read to be achieved outside of the core of XTDB, to meet the exact application requirements.
Nodes can come and go, with local indexes stored in a Key/Value store such as RocksDB, whilst reading and writing master data to central log topics (hosted by Kafka or a JDBC data store such as Postgres). Queries are not distributed and there is no sharding of data or indexes across nodes.
XTDB can also run in a non-distributed "standalone" mode, where the transaction and document logs exist only inside of a local Key/Value store such as RocksDB or SQLite. This is appropriate for non-critical usage where availability and durability requirements do not warrant additional infrastructure services like Kafka or Postgres.
XTDB supports eviction of active and historical data to assist with technical compliance for information privacy regulations.
The main transaction log contains only hashes and is immutable. All document content is stored in a dedicated document store which supports eviction, such as a Kafka topic with compacted tombstones.