Collect OPC-UA data with Telegraf in dense format

Configure Telegraf to collect OPC-UA industrial automation data and insert it into QuestDB in a dense format. By default, Telegraf creates one row per metric with sparse columns, but for QuestDB it's more efficient to merge all metrics from the same timestamp into a single dense row.

Problem: Sparse data format

When using Telegraf's OPC-UA input plugin with the default configuration, each metric value generates a separate row. Even when multiple metrics are collected at the same timestamp, they arrive as individual sparse rows:

Sparse format (inefficient):

This wastes storage space and makes queries more complex.

Dense format (efficient):

Solution: Use Telegraf's merge aggregator

Configure Telegraf to merge metrics with matching timestamps and tags before sending to QuestDB. This requires two key changes:

1. Add a common tag to all metrics
2. Use the merge aggregator to combine rows

Complete configuration

[agent]
  omit_hostname = true

# OPC-UA Input Plugin
[[inputs.opcua]]
  endpoint = "${OPCUA_ENDPOINT}"
  connect_timeout = "30s"
  request_timeout = "30s"
  security_policy = "None"
  security_mode = "None"
  auth_method = "Anonymous"
  name_override = "${METRICS_TABLE_NAME}"

  [[inputs.opcua.nodes]]
    name = "ServerLoad"
    namespace = "2"
    identifier_type = "s"
    identifier = "Server/Load"
    default_tags = { source="opcua_merge" }

  [[inputs.opcua.nodes]]
    name = "ServerRAM"
    namespace = "2"
    identifier_type = "s"
    identifier = "Server/RAM"
    default_tags = { source="opcua_merge" }

  [[inputs.opcua.nodes]]
    name = "ServerIO"
    namespace = "2"
    identifier_type = "s"
    identifier = "Server/IO"
    default_tags = { source="opcua_merge" }

# Merge Aggregator
[[aggregators.merge]]
  drop_original = true
  tags = ["source"]

# QuestDB Output via ILP
[[outputs.influxdb_v2]]
  urls = ["${QUESTDB_HTTP_ENDPOINT}"]
  token = "${QUESTDB_HTTP_TOKEN}"
  content_encoding = "identity"

Key configuration elements

1. Common tag

default_tags = { source="opcua_merge" }

Adds the same tag value (source="opcua_merge") to all metrics. The merge aggregator uses this to identify which metrics should be combined.

2. Merge aggregator

[[aggregators.merge]]
  drop_original = true
  tags = ["source"]

• drop_original = true: Discards the original sparse rows after merging
• tags = ["source"]: Merges metrics with matching source tag values and the same timestamp

3. QuestDB output

[[outputs.influxdb_v2]]
  urls = ["${QUESTDB_HTTP_ENDPOINT}"]
  content_encoding = "identity"

• Uses the InfluxDB Line Protocol (ILP) over HTTP
• content_encoding = "identity": Disables gzip compression (QuestDB doesn't require it)

How it works

The data flow is:

1. OPC-UA server → Telegraf collects metrics
2. Telegraf input → Creates separate rows for each metric with the source="opcua_merge" tag
3. Merge aggregator → Combines rows with matching timestamp + source tag
4. QuestDB output → Sends merged dense rows via ILP

Merging logic

The merge aggregator combines metrics when:

• Timestamps match: Metrics collected at the same moment
• Tags match: All specified tags (in this case, source) have the same values

If metrics have different timestamps or tag values, they won't be merged.

Handling tag conflicts

If your OPC-UA nodes have additional tags with different values, those tags will prevent merging. Solutions:

Remove conflicting tags

Use the override processor to remove unwanted tags:

[[processors.override]]
  [processors.override.tags]
    node_id = ""  # Removes the 'node_id' tag
    namespace = ""  # Removes the 'namespace' tag

Convert tags to fields

Use the converter processor to convert tags to fields (fields don't affect merging):

[[processors.converter]]
  [processors.converter.tags]
    string = ["node_id", "namespace"]

This converts the tags to string fields, which won't interfere with the merge aggregator.

Remove the common tag after merging

If you don't want the source tag in your final QuestDB table:

# Place this AFTER the merge aggregator
[[processors.override]]
  [processors.override.tags]
    source = ""  # Removes the 'source' tag

Environment variables

Use environment variables for sensitive configuration:

export OPCUA_ENDPOINT="opc.tcp://your-opcua-server:4840"
export METRICS_TABLE_NAME="industrial_metrics"
export QUESTDB_HTTP_ENDPOINT="http://questdb-host:9000"
export QUESTDB_HTTP_TOKEN="your_token_here"

Alternatively, use a .env file:

# .env file
OPCUA_ENDPOINT=opc.tcp://localhost:4840
METRICS_TABLE_NAME=opcua_metrics
QUESTDB_HTTP_ENDPOINT=http://localhost:9000
QUESTDB_HTTP_TOKEN=

Then start Telegraf with:

telegraf --config telegraf.conf

Verification

Query QuestDB to verify the data format:

SELECT * FROM opcua_metrics
ORDER BY timestamp DESC
LIMIT 10;

Expected: Dense rows with all metrics populated:

Problem: Sparse rows with NULL values:

If you see sparse rows, check:

• All nodes have the same default_tags
• The merge aggregator is configured correctly
• Timestamps are identical (not just close)

Alternative: TCP output

For higher throughput, use TCP instead of HTTP:

[[outputs.socket_writer]]
  address = "tcp://questdb-host:9009"

Differences:

• TCP: Higher throughput, no acknowledgments, potential data loss on connection failure
• HTTP: Reliable delivery, acknowledgments, slightly lower throughput

Choose TCP when:

• You need maximum performance
• Occasional data loss is acceptable
• You're on a reliable local network

Choose HTTP when:

• Data integrity is critical
• You need error feedback
• You're sending over the internet

Multiple OPC-UA sources

To collect from multiple OPC-UA servers into separate tables:

# Server 1
[[inputs.opcua]]
  endpoint = "opc.tcp://server1:4840"
  name_override = "server1_metrics"
  [[inputs.opcua.nodes]]
    name = "Temperature"
    namespace = "2"
    identifier_type = "s"
    identifier = "Sensor/Temp"
    default_tags = { source="server1" }

# Server 2
[[inputs.opcua]]
  endpoint = "opc.tcp://server2:4840"
  name_override = "server2_metrics"
  [[inputs.opcua.nodes]]
    name = "Pressure"
    namespace = "2"
    identifier_type = "s"
    identifier = "Sensor/Press"
    default_tags = { source="server2" }

# Merge by source tag
[[aggregators.merge]]
  drop_original = true
  tags = ["source"]

This creates two tables (server1_metrics, server2_metrics) with merged metrics from each server.

:::tip Performance Tuning For high-frequency OPC-UA data:

• Increase Telegraf's flush_interval to batch more data
• Use aggregators.merge.period to specify merge window duration
• Monitor QuestDB's ingestion rate and adjust accordingly :::

:::warning Timestamp Precision OPC-UA timestamps may have different precision than QuestDB expects. Ensure:

• Telegraf agent precision matches your requirements (default: nanoseconds)
• OPC-UA server timestamps are synchronized (use NTP)
• Clock drift between systems is minimal :::

:::info Related Documentation

• Telegraf OPC-UA plugin
• Telegraf merge aggregator
• QuestDB ILP reference
• InfluxDB Line Protocol :::