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Cluster of replicas

A TigerBeetle cluster is 6 machines each running the TigerBeetle server for strict serializability, high availability and durability. The TigerBeetle server is a single binary.

Each server operates on a single local data file for a total of 6 data files across the cluster.

The TigerBeetle server (the single binary) plus its single data file is called a replica.

It is important to ensure independent fault domains for each replica's data file, that each replica's data file is stored on a separate disk (required), machine (required), rack (recommended), data center (recommended) etc.

Provisioning a cluster

A cluster guarantees strict serializability, the highest level of consistency, by automatically electing a primary replica to order and backup transactions across replicas in the cluster.

The optimal, recommended size for any production cluster is 6 replicas, recognizing advances in flexible consensus quorums.

4/6 replicas are required to elect a new primary if the old primary fails.

A cluster remains highly available (able to process transactions), preserving strict serializability, provided that at least 3/6 machines have not failed (provided that the primary has not also failed) or provided that at least 4/6 machines have not failed (if the primary also failed and a new primary needs to be elected).

A cluster preserves durability (surviving, detecting and repairing corruption of any data file) provided that the cluster remains available.

A cluster will correctly remain unavailable if too many machine failures have occurred to preserve data. In other words, TigerBeetle is designed to operate correctly or else to shut down safely if safe operation with respect to strict serializability is no longer possible due to permanent data loss.

All 6 replicas may be within the same data center (zero geographic fault tolerance), or spread across 2 or more data centers, availability zones or regions (“sites”) for geographic fault tolerance.

For mission critical availability, the optimal number of sites is 3, since each site would then contain 2 replicas so that the loss of an entire site would not impair the availability of the cluster.

If only 2 sites are able to be provisioned, it is recommended to tune the cluster so that at least 4/6 replicas (instead of 3/6 replicas) acknowledge each transaction before commit. This would ensure that at least 1 replica in the other site will preserve data durability in the event that an entire site is permanently lost. Furthermore, this would ensure that the entire cluster would be able to be repaired once the lost site is restored.

Sites should preferably be within a few milliseconds of each other, since each transaction must be replicated across sites before being committed.

Provisioning a replica

TigerBeetle is designed to operate and provide more than adequate performance even on commodity hardware.

NVMe is preferred to SSD for high performance deployments.

However, spinning rust is perfectly acceptable, especially where a cluster is expected to be long lived, and the data file is expected to be large. There is no requirement for NVMe or SSD.

A 20 TiB disk containing a replica's data file is enough to address on the order of 50 billion accounts or transfers. It is more important to provision sufficient storage space for a replica’s data file, than to provision high performance storage.

A replica's data file may reside on local storage or else on remote storage. The most important concern is to ensure independent fault domains across replicas.

The operator may consider the use of RAID 10 to reduce the need for remote recovery if a replica's disk fails.

ECC memory is recommended for production deployments.

TigerBeetle requires only a single core per replica machine. TigerBeetle at present does not utilize more cores, but may in future.

There are no restrictions on sharing a server with other tenant processes.

A replica requires at least 2 GiB RAM per machine. Between 16 GiB and 32 GiB or more (depending on budget) is recommended to be allocated to each replica for caching. TigerBeetle uses static allocation and will use exactly how much memory is explicitly allocated to it for caching via command line argument.