Message Queue

All ThingsBoard services communicate through a message queue. Every message is partitioned by entity ID, so all messages for a given device always land on the same partition — preserving order and enabling stateful processing in the Actor System.

Queue Backends

ThingsBoard supports two queue implementations. Set the backend with the TB_QUEUE_TYPE environment variable:

Backend	`TB_QUEUE_TYPE`	Persistence	Use case
In-Memory	`in-memory`	None — messages lost on restart	Development, single-node demos
Apache Kafka	`kafka`	Disk-based, survives restarts	Production, any cluster deployment

Message Queue vs Rule Engine Queues

The message queue described on this page is the infrastructure-level transport — Kafka or in-memory — that carries messages between ThingsBoard services (transports, core, rule engine, JS executors).

This is different from Rule Engine Queues, which are a higher-level abstraction configured in the ThingsBoard UI. Rule Engine Queues control how messages are submitted to and processed by rule chains — submit strategy (burst, batch, sequential), retry policy, polling interval, and parallelism. They run on top of the message queue backend: when you configure a Rule Engine Queue named “HighPriority”, its messages still flow through Kafka (or in-memory) under the hood.

Message Flow

The diagram below shows how the main services communicate through Kafka topics:

Kafka Topic Topology

When running with Kafka, ThingsBoard creates the following topics. Each topic has a specific role in the data pipeline:

Core Topics

Topic	Direction	Purpose
`tb_rule_engine`	Transport → Rule Engine	Telemetry, attributes, RPC requests, device lifecycle events
`tb_core`	Rule Engine → Core	Entity lifecycle events, connectivity updates, subscription management
`tb_core.notifications`	Any → Core	Lightweight notifications (WebSocket pushes, entity invalidations)
`tb_transport.api.requests`	Transport → Core	Credential validation, attribute fetch, claim requests
`tb_transport.api.responses`	Core → Transport	Authentication results, attribute values

Script Execution Topics

Topic	Direction	Purpose
`js.eval.requests`	TB Node → JS Executor	JavaScript evaluation requests from rule engine script nodes
`js.eval.responses`	JS Executor → TB Node	Script evaluation results

Operational Topics

Topic	Direction	Purpose
`tb_ota_package`	Core → Transport	OTA firmware/software update distribution
`tb_housekeeper`	Internal	Cleanup tasks — orphaned entities, expired sessions, stale data
`tb_housekeeper.reprocessing`	Internal	Failed housekeeper tasks queued for retry (90-day retention)

Edge Topics

Topic	Direction	Purpose
`tb_edge`	Core ↔ Edge	Edge synchronization commands and responses
`tb_edge.notifications`	Core → Edge	Edge event notifications

Calculated Fields Topics

Topic	Direction	Purpose
`tb_cf_event`	Core → CF Engine	Calculated field trigger events
`tb_cf_state`	CF Engine	Calculated field state snapshots (compacted topic, infinite retention)

EDQS Topics (Entity Data Query Service)

Topic	Direction	Purpose
`edqs.events`	Core → EDQS	Entity change events for query indexing
`edqs.requests`	API → EDQS	Query requests (3-minute retention)
`edqs.state`	EDQS	Index state snapshots (compacted topic, infinite retention)

Partitioning and Ordering

Messages are assigned to Kafka partitions using a hash of the entity ID. ThingsBoard supports three hash functions, configured via TB_QUEUE_PARTITIONS_HASH_FUNCTION_NAME:

Hash Function	Value	Notes
MurmurHash3 128-bit	`murmur3_128`	Default — good distribution, fast
MurmurHash3 32-bit	`murmur3_32`	Lighter, slightly less uniform
SHA-256	`sha256`	Cryptographic, slower but uniform

Because the same entity always maps to the same partition, a single consumer thread processes all messages for that entity in order. This is critical for correctness — device state updates, rule chain processing, and attribute writes must happen sequentially.

Partition count controls parallelism. More partitions allow more consumer threads, increasing throughput. The number of partitions per topic is configured via topic property strings (e.g., TB_QUEUE_KAFKA_RE_TOPIC_PROPERTIES includes partitions:1 by default for the rule engine topic).

Producer Tuning

These environment variables control how ThingsBoard produces messages to Kafka:

Variable	Default	Description
`TB_KAFKA_ACKS`	`all`	Acknowledgment level. `all` = wait for all in-sync replicas (safest). `1` = leader only (faster). `0` = fire-and-forget (not recommended).
`TB_KAFKA_BATCH_SIZE`	`16384`	Maximum batch size in bytes before sending to broker
`TB_KAFKA_LINGER_MS`	`1`	Milliseconds to wait for more records before sending a batch. Higher values increase throughput at the cost of latency.
`TB_KAFKA_COMPRESSION_TYPE`	`none`	Compression algorithm: `none`, `gzip`. Use `gzip` to reduce network bandwidth in high-throughput deployments.
`TB_KAFKA_RETRIES`	`1`	Retry count for transient send failures
`TB_KAFKA_MAX_REQUEST_SIZE`	`1048576`	Maximum request size in bytes (1 MB default)

Consumer Tuning

These variables control how ThingsBoard consumes messages from Kafka:

Variable	Default	Description
`TB_QUEUE_KAFKA_MAX_POLL_RECORDS`	`8192`	Maximum records returned per `poll()` call. Lower values reduce processing latency; higher values increase throughput.
`TB_QUEUE_KAFKA_MAX_POLL_INTERVAL_MS`	`300000`	Maximum time between `poll()` calls before the consumer is considered dead and rebalanced (5 minutes).
`TB_QUEUE_KAFKA_SESSION_TIMEOUT_MS`	`10000`	Heartbeat session timeout. If Kafka doesn’t receive a heartbeat within this window, the consumer is evicted.
`TB_QUEUE_KAFKA_AUTO_OFFSET_RESET`	`earliest`	Where to start reading when no committed offset exists: `earliest` (beginning) or `latest` (end).
`TB_QUEUE_KAFKA_MAX_PARTITION_FETCH_BYTES`	`16777216`	Maximum data per partition per fetch request (16 MB)
`TB_QUEUE_KAFKA_FETCH_MAX_BYTES`	`134217728`	Maximum total data per fetch request across all partitions (128 MB)

Topic Retention and Compaction

Most topics use time-based retention (default 7 days / 604800000 ms). Notable exceptions:

Topic	Retention	Cleanup Policy	Reason
JS Executor (`js.eval.*`)	1 day	Delete	Short-lived request/response pairs
Housekeeper reprocessing	90 days	Delete	Failed tasks need longer retry window
CF State (`tb_cf_state`)	Infinite	Compact	State snapshots — only latest value per key matters
EDQS State (`edqs.state`)	Infinite	Compact	Index state — compacted to latest per key
EDQS Requests (`edqs.requests`)	3 minutes	Delete	Ephemeral query requests

Topic properties (retention, segment size, partition count, min.insync.replicas) are configurable per topic via environment variables like TB_QUEUE_KAFKA_RE_TOPIC_PROPERTIES. The format is semicolon-separated key:value pairs:

retention.ms:604800000;segment.bytes:52428800;retention.bytes:1048576000;partitions:1;min.insync.replicas:1

Replication Factor

Set TB_QUEUE_KAFKA_REPLICATION_FACTOR (default 1) to control how many Kafka brokers hold a copy of each partition. For production clusters, set this to 3 with at least 3 Kafka brokers.

Consumer Statistics

ThingsBoard can log consumer group lag — the difference between the latest message offset and the consumer’s committed offset. Enable with TB_QUEUE_KAFKA_CONSUMER_STATS_ENABLED (default true). Lag statistics print every TB_QUEUE_KAFKA_CONSUMER_STATS_MIN_PRINT_INTERVAL_MS (default 60 seconds).

High consumer lag indicates the service cannot keep up with incoming message volume — scale the service or increase partition count.

For the complete list of queue and Kafka environment variables, see Configuration Reference.