Connectivity

An Orchester instance rarely runs alone. It replicates to and from other instances, accepts data from third-party systems, and polls or answers industrial field devices. Five mechanisms cover all of it, and this guide walks through each: what it's for, how to configure it, and what to watch out for.

Method	Who calls whom	Typical use
DO-to-DO Peer Link	Either side, signed, one-shot	Replicating variables or history between two Orchester instances
External REST API	External system calls in	A third-party system pushing data into Orchester
Modbus Master	Orchester polls out	Reading from and writing to a PLC, meter or drive
Modbus Slave	A remote master calls in	Exposing Orchester's own variables to a SCADA/HMI
MQTT	Orchester dials a broker	Publishing telemetry and subscribing to commands

DO-to-DO Peer Link

Two Orchester instances talk to each other through a single, generic endpoint: POST /orchester. Every call carries one action — variables.push, variables.get, variables.set, script.execute, logger.head, logger.append, or a ping — in a signed envelope, and gets a signed reply. There is no persistent connection: each call opens a fresh HTTP request and closes it once the response arrives.

The request is authenticated with four headers — X-DO-Peer, X-DO-Timestamp, X-DO-Nonce, X-DO-Signature — built from an HMAC-SHA256 signature over the method, path, peer code, timestamp, nonce and a hash of the body, using a secret both sides already share. A nonce cache rejects any request replayed inside the clock-skew window, and repeated authentication failures from the same source trip a rate limit.

You don't configure this exchange directly — you configure the peer it runs against, and the modules that use it:

Field	Meaning
enabled	Whether this peer is usable at all
code	The identity the far side signs with; also this instance's own identity if flagged self
url	Where to reach the peer — only needed if this instance calls out to it
secret	The shared HMAC key, or env:VAR_NAME to read it from an environment variable instead
timeout	How long an outbound call waits before giving up (ms)
skew	Maximum allowed clock drift between the two sides (ms), default 5 minutes
actions	The allow-list — which actions this peer may invoke here. Empty means none: a peer entity saved without actions is a misconfiguration, not a superuser

Two modules ride on top of a peer: DOPUSH sends variables.push whenever a watched variable changes; DOPULL calls variables.get on a cron schedule. Both point at the same peer entity, so the URL, secret and allow-list only need to be set once no matter how many modules use that link.

Example. Instance plant-a pushes two variable values to instance plant-b, which has granted it variables.push:

POST /orchester HTTP/1.1
X-DO-Peer: plant-a
X-DO-Timestamp: 1755000000000
X-DO-Nonce: 7c9e6679-7425-40de-944b-e07fc1f90ae7
X-DO-Signature: 3q2+7wYAAAA9CGFP...

{"v":1,"action":"variables.push","code":"plant-a","id":"7c9e6679-7425-40de-944b-e07fc1f90ae7",
 "data":{"values":{"TEMP_C":21.4,"PUMP_RUNNING":true}}}

plant-b answers with a signed {"status":"OK","data":{"applied":2}} — or FORBIDDEN if plant-a's peer entity on plant-b doesn't have variables.push in its actions list.

External REST API

The DO-to-DO link assumes the far side is another Orchester instance, speaking the same internal wire format. For everything else — a customer's own backend, an integration platform, a script — there's a second endpoint, POST /api/external/v1/data, built on exactly the same peer, signing, allow-list and licence model, but JSON-only and reachable by any system that can compute an HMAC-SHA256 signature.

The request shape is the same one-action-per-call envelope: a JSON body naming the action and carrying its data, the same four X-DO-* signature headers, the same peer allow-list deciding what a given external system may actually call. It's unidirectional by design — the external system always calls in, and Orchester never calls back out to it, so there's nothing to keep open between requests.

One difference from the internal link: when this instance's licence is in a read-only state (expired, invalid, or a tampered clock), the external endpoint answers ping only, and refuses everything else. The internal peer-to-peer channel doesn't carry that restriction, so replication between your own instances keeps working even while a licence issue is being sorted out — only the door held open to outside systems narrows.

Set this up exactly like a DO-to-DO peer — the same editor, the same actions allow-list shown above — since it's the same entity either way. Grant only the actions that integration actually needs: script.execute in particular hands the caller full control of the instance, so reserve it for peers you trust completely.

Example. An ERP system, registered as peer erp-integration with only variables.push granted, records a completed order count:

curl -X POST https://plant.example.com/api/external/v1/data \
  -H "Content-Type: application/json" \
  -H "X-DO-Peer: erp-integration" \
  -H "X-DO-Timestamp: 1755000000000" \
  -H "X-DO-Nonce: 3fa85f64-5717-4562-b3fc-2c963f66afa6" \
  -H "X-DO-Signature: <base64 HMAC-SHA256 over method, path, peer, timestamp, nonce and body>" \
  -d '{"v":1,"action":"variables.push","data":{"values":{"ORDER_COUNT":128}}}'

The signature is computed the same way on any platform: HMAC-SHA256 over POST\n/api/external/v1/data\n<peer>\n<timestamp>\n<nonce>\n<sha256 of the body>, using the peer's shared secret — there's no DataOrchester-specific SDK required, just a standard crypto library.

Modbus Master

As a Modbus master, Orchester is the one opening the connection — to a PLC, a power meter, a variable-speed drive, anything that answers Modbus requests. Two module types exist: MB-TCP-Master for Modbus TCP, and MB-SER-Master for serial RTU over an RS-485/RS-232 line, each configured with the reachability details for that transport (host/port for TCP; serial device, baud rate, parity and stop bits for RTU) plus a default unit ID.

Reading and writing are configured as two independent lists:

• Collectors read from the device on a cron schedule and land the result in a variable. Each row names the variable, the remote address, and a remoteType telling the module how to decode it: bool/input_bool (coil / discrete input, one bit), int/input_float and friends (one or two holding/input registers, reassembled into an integer or IEEE-754 float), or an array_* variant reading quantity words or coils at once.
• Forewarders write to the device whenever their variable changes. The write side is narrower than the read side — only bool (a single coil, function code 5) and int (a single register, function code 6) are supported.

Every request retries with a jittered backoff on failure rather than giving up on the first dropped packet or timeout, which matters on a noisy serial line shared by several devices.

Example. Reading a line's temperature (a 32-bit float split across two holding registers) every ten seconds, and writing an operator-adjustable setpoint back when it changes:

Collector    field=LINE1_TEMP_C   remoteType=float  remoteAddress=40010  cron=*/10 * * * * ?
Forewarder   field=LINE1_SETPOINT remoteType=int    remoteAddress=40020

LINE1_TEMP_C now updates itself every ten seconds from the device; writing to LINE1_SETPOINT anywhere in Orchester — a dashboard, a formula, another peer — sends function code 6 to register 40020 on the PLC.

Modbus Slave

Flip the direction, and Orchester becomes the thing being polled: MB-TCP-Slave and MB-SER-Slave turn an instance into a Modbus server that a SCADA system, an HMI, or another PLC can read from and write to. Four list-valued variables back the four Modbus tables:

Table	Backing variable holds	Function codes
Coils	List<Boolean>	FC1 read, FC5/FC15 write
Discrete inputs	List<Boolean>	FC2 read
Holding registers	List<Integer>	FC3 read, FC6/FC16 write
Input registers	List<Integer>	FC4 read

Discrete inputs and input registers are read-only from the remote master's side — only coils and holding registers accept writes, matching their table names.

An unsupported function code comes back as Modbus exception 1; an out-of-range address or a request past the end of the backing list comes back as exception 2. A request tagged with a unit ID that doesn't match this slave's configured unit gets no response at all, rather than an exception — indistinguishable, on the wire, from the slave not existing. And because these are ordinary Orchester variables, they hold their last written value across an engine restart, so a master reconnecting after a deploy sees continuity, not a gap.

Example. Exposing eight alarm bits and four analog readings to a plant SCADA, unit ID 3:

coils            -> SLAVE_COILS      (List<Boolean>, length 8)
holdingRegisters -> SLAVE_HOLD_REG   (List<Integer>, length 4)

The SCADA reads FC3 at address 2, quantity 1, and gets SLAVE_HOLD_REG[2]. If it writes FC5 to coil 3 to force an alarm, that write lands in SLAVE_COILS[3], exactly as the table above suggests.

MQTT

The MQTT module makes Orchester an MQTT v5 client — never a broker. It opens one connection to an external broker and, independently in each direction, publishes variable values out to topics and subscribes to topics that update variables.

Field	Meaning
url	Broker address, including scheme (tcp:// or ssl://) and port
client	The MQTT client ID this instance presents
user / password	Broker credentials, if required
cleanStart	Whether to start a fresh session on connect (default on)
keepAlive	Keep-alive interval in seconds (default 60)

Publish and subscribe are both configured as maps, one row per topic:

• Publish: variable to topic, with a QoS level, a retain flag, and a payload format — plain TEXT or the internal ASR3 encoding, which round-trips richer types than a plain string can.
• Subscribe: topic to variable, with the same QoS and format choice.

Topic matching is an exact string comparison — there's no #/+ wildcard support, so a subscription needs one row per concrete topic it should react to.

Example. A line records its power draw in the POWER_KW variable and needs it visible to a plant-wide dashboard subscribed to the site broker, while a remote setpoint arrives over the same broker:

Publish     POWER_KW -> plant/line1/power        QoS 1  retain=false  format=TEXT
Subscribe   plant/line1/setpoint -> TEMP_SETPOINT QoS 1  format=TEXT

Choosing between them

• Talking to another Orchester instance: use the DO-to-DO peer link — it's already there, and DOPUSH/ DOPULL cover the common push/pull patterns without any custom code.
• Talking to a third-party system that can call you: use the external REST API. It reuses the same peer and licence model as DO-to-DO, so a customer's integration is one more peer entity, not a new trust boundary.
• Talking to a field device that speaks Modbus: use Modbus Master if Orchester should poll the device, or Modbus Slave if the device (or a SCADA layer above it) needs to poll Orchester instead. Both can run at once for the same device family, if some points are read there and others are written here.
• Talking to a message broker, or integrating with anything already built around publish/subscribe: use the MQTT module.

All five share the same underlying safety property: none of them can be configured into pulling more than a licensed instance is entitled to, and none of them de-energizes a running engine on their own — a licence issue narrows what a peer or the external API will answer, it never stops a module that's already driving a device.

Next steps

• Configuration reference
• Licence activation