Storage and consistency

This document provides an overview on how CrateDB stores and distributes state across the cluster and what consistency and durability guarantees are provided.

Note

Since CrateDB heavily relies on Elasticsearch and Lucene for storage and cluster consensus, concepts shown here might look familiar to Elasticsearch users, since the implementation is actually reused from the Elasticsearch code.

Table of contents

Data storage

Every table in CrateDB is sharded, which means that tables are divided and distributed across the nodes of a cluster. Each shard in CrateDB is a Lucene index broken down into segments getting stored on the filesystem. Physically the files reside under one of the configured data directories of a node.

Lucene only appends data to segment files, which means that data written to the disc will never be mutated. This makes it easy for replication and recovery, since syncing a shard is simply a matter of fetching data from a specific marker.

An arbitrary number of replica shards can be configured per table. Every operational replica holds a full synchronized copy of the primary shard.

With read operations, there is no difference between executing the operation on the primary shard or on any of the replicas. CrateDB randomly assigns a shard when routing an operation. It is possible to configure this behavior if required, see our best practice guide on multi zone setups for more details.

Write operations are handled differently than reads. Such operations are synchronous over all active replicas with the following flow:

  1. The primary shard and the active replicas are looked up in the cluster state for the given operation. The primary shard and a quorum of the configured replicas need to be available for this step to succeed.

  2. The operation is routed to the according primary shard for execution.

  3. The operation gets executed on the primary shard

  4. If the operation succeeds on the primary, the operation gets executed on all replicas in parallel.

  5. After all replica operations finish the operation result gets returned to the caller.

Should any replica shard fail to write the data or times out in step 5, it’s immediately considered as unavailable.

Atomicity at document level

Each row of a table in CrateDB is a semi structured document which can be nested arbitrarily deep through the use of object and array types.

Operations on documents are atomic. Meaning that a write operation on a document either succeeds as a whole or has no effect at all. This is always the case, regardless of the nesting depth or size of the document.

CrateDB does not provide transactions. Since every document in CrateDB has a version number assigned, which gets increased every time a change occurs, patterns like Optimistic Concurrency Control can help to work around that limitation.

Durability

Each shard has a WAL also known as translog. It guarantees that operations on documents are persisted to disk without having to issue a Lucene-Commit for every write operation. When the translog gets flushed all data is written to the persistent index storage of Lucene and the translog gets cleared.

In case of an unclean shutdown of a shard, the transactions in the translog are getting replayed upon startup to ensure that all executed operations are permanent.

The translog is also directly transferred when a newly allocated replica initializes itself from the primary shard. There is no need to flush segments to disc just for replica recovery purposes.

Addressing documents

Every document has an internal identifier. By default this identifier is derived from the primary key. Documents living in tables without a primary key are assigned a unique auto-generated ID automatically when created.

Each document is routed to one specific shard according to the routing column. All rows that have the same routing column row value are stored in the same shard. The routing column can be specified with the CLUSTERED clause when creating the table. If a primary key has been defined, it will be used as the default routing column, otherwise the internal document ID is used.

While transparent to the user, internally there are two ways how CrateDB accesses documents:

get

Direct access by identifier. Only applicable if the routing key and the identifier can be computed from the given query specification. (e.g: the full primary key is defined in the where clause).

This is the most efficient way to access a document, since only a single shard gets accessed and only a simple index lookup on the _id field has to be done.

search

Query by matching against fields of documents across all candidate shards of the table.

Consistency

CrateDB is eventual consistent for search operations. Search operations are performed on shared IndexReaders which besides other functionality, provide caching and reverse lookup capabilities for shards. An IndexReader is always bound to the Lucene segment it was started from, which means it has to be refreshed in order to see new changes, this is done on a time based manner, but can also be done manually (see refresh). Therefore a search only sees a change if the according IndexReader was refreshed after that change occurred.

If a query specification results in a get operation, changes are visible immediately. This is achieved by looking up the document in the translog first, which will always have the most recent version of the document. The common update and fetch use-case is therefore possible. If a client updates a row and that row is looked up by its primary key after that update the changes will always be visible, since the information will be retrieved directly from the translog.

Note

Dirty reads can occur if the primary shard becomes isolated. The primary will only realize it is isolated once it tries to communicate with its replicas or the master. At that point, a write operation is already committed into the primary and can be read by a concurrent read operation. In order to minimise the window of opportunity for this phenomena, the CrateDB nodes communicate with the master every second (by default) and once they realise no master is known, they will start rejecting write operations.

Every replica shard is updated synchronously with its primary and always carries the same information. Therefore it does not matter if the primary or a replica shard is accessed in terms of consistency. Only the refresh of the IndexReader affects consistency.

Caution

Some outage conditions can affect these consistency claims. See the resiliency documentation for details.

Cluster meta data

Cluster meta data is held in the so called “Cluster State”, which contains the following information:

  • Tables schemas.

  • Primary and replica shard locations. Basically just a mapping from shard number to the storage node.

  • Status of each shard, which tells if a shard is currently ready for use or has any other state like “initializing”, “recovering” or cannot be assigned at all.

  • Information about discovered nodes and their status.

  • Configuration information.

Every node has its own copy of the cluster state. However there is only one node allowed to change the cluster state at runtime. This node is called the “master” node and gets auto-elected. The “master” node has no special configuration at all, all nodes are master-eligible by default, and any master-eligible node can be elected as the master. There is also an automatic re-election if the current master node goes down for some reason.

Note

To avoid a scenario where two masters could be elected due to network partitioning, CrateDB automatically defines a quorum of nodes with which it is possible to elect a master. For details on how this works and further information see Master Node Election.

To explain the flow of events for any cluster state change, here is an example flow for an ALTER TABLE statement which changes the schema of a table:

  1. A node in the cluster receives the ALTER TABLE request.

  2. The node sends out a request to the current master node to change the table definition.

  3. The master node applies the changes locally to the cluster state and sends out a notification to all affected nodes about the change.

  4. The nodes apply the change, so that they are now in sync with the master.

  5. Every node might take some local action depending on the type of cluster state change.