OpcDaToHttp

An OPC DA → HTTP bridge: the application reads tag values from an OPC DA server and serves them over HTTP by parameter name. The OPC part is built with OPCClientToolKit library (https://github.com/mkib002/OPC-Client-X64).

Requirements

• Windows, MSVC (Visual Studio 2019 / MSVC 14.29 was used) + CMake.
• OPC Core Components of the matching bitness — the standard redistributable package of libraries and services from the OPC Foundation (https://opcfoundation.org/developer-tools/samples-and-tools-classic/core-components/) required by all OPC Classic applications. Installed files: C:\Program Files\Common Files\OPC Foundation\Bin\.
• An OPC DA server. For testing, "OPC Server Simulator AggSoft" (https://www.aggsoft.ru/opc-server-sim/index.htm) will do. COM launches it automatically on connect.
• For a remote host (--host <name>) you additionally need the OpcEnum service, the RemoteRegistry service, and DCOM settings on the server machine.

Usage

OpcDaToHttp.exe --server OPC.OpcTestServer_x64.1 --port 8080
OpcDaToHttp.exe                                  :: first local DA server found
OpcDaToHttp.exe --list-servers                   :: list local OPC DA 2.0 servers
OpcDaToHttp.exe --help

Option	Default	Purpose
--server <ProgID>	first found	ProgID of the OPC DA server (e.g. OPC.OpcTestServer_x64.1)
--host <name>	localhost	host; '', localhost, 127.0.0.1 → selects the machine that runs the OPC DA server
--port <N>	8080	HTTP port; 0 — any free port
--rate <ms>	1000	OPC group update period
--deadband <value>	0	group deadband
--refresh-sec <sec>	0	extra periodic group refresh (0 — subscription only)
--items-file <file>	—	newline-separated tag list (like OPCPerformance\items.txt) instead of browsing the namespace
--filter <substring>	—	take only namespace names containing the substring
--max-items <N>	0	limit the number of tags
--reconnect-sec <sec>	10	reconnect on disconnect (0 — do not reconnect)
--max-conn <N>	64	limit of simultaneous HTTP connections
--stale-ms <ms>	0 (auto)	value "staleness" threshold
--timeout-ms <ms>	5000	wait for refresh for ?refresh=1
--log-level <level>	info	debug, info, warn, error
--list-servers	—	print local DA servers and exit

Stop with Ctrl+C (the HTTP server shuts down and the OPC session is released cleanly).

HTTP API

Request	Response
GET /	text/plain — help and current state
GET /health	JSON state; 503 if there is no connection to OPC
GET /items	JSON: {"count":N,"items":[...]} — known tags
GET /value?name=<TAG>	JSON value of the tag
GET /value/<TAG>	same, name in the path (can be URL-encoded: %2F, %20)
GET /raw?name=<TAG>	value only, text/plain (details in the X-Opc-* headers)
GET /values?name=A&name=B	several tags in one request
GET /dump	values of all known tags

The ?refresh=1 parameter (for /value, /raw, /values) makes the OPC thread perform a refresh and wait for it to complete — the HTTP thread does not read data from OPC in the meantime.

Example /value response:

{"name":"Test/Int32","value":42,"type":"VT_I4","quality":192,"qualityText":"Good",
 "timestamp":"2026-09-24T17:21:46.937Z","ageMs":11531,"stale":false,"hasValue":true,"error":null}

Fields: value — a ready-made JSON literal (number, string, true/false, null, array; a byte array is a hex string); type — the OPC type (VT_R4, VT_ARRAY|VT_UI2, …); quality/qualityText — OPC quality; timestamp — value time from the server (UTC, ISO-8601); ageMs — how many ms ago the value was received from the server; stale — the --stale-ms threshold was exceeded; hasValue — read without an error; error — the HRESULT of the error or null. For arrays, length and truncated are added (no more than 1024 elements are included in the response).

Example /health:

{"status":"ok","opcConnected":true,"server":"OPC.OpcTestServer_x64.1","host":"localhost","port":8080,
 "serverState":1,"tags":3,"values":3,"items":3,"lastUpdateAgeMs":11515,"serverContactAgeMs":1500,
 "snapshotVersion":6,"refreshAckId":0,"uptimeSec":11,"lastError":null,"connections":1,"version":"1.0.0"}

status: ok (connected and values present) / degraded (connected, but no values yet) / down (no connection, HTTP 503).

Response codes:

Code	When
200	success
400	missing name parameter / request body not supported
404	tag not found (suggestions with similar names in the body)
405	method is not GET/HEAD
431	header block too large
503	no connection to OPC (or --max-conn exceeded)
504	?refresh=1 did not get a refresh within --timeout-ms

Tag names are looked up as follows: exact match → case-insensitive → separator substitution (Test/Int32 ↔ Test.Int32). The name can be passed in the query (?name=) or in the path (/value/...), URL-encoded. Connections: HTTP/1.1 keep-alive (up to 100 requests per connection), the client's Connection: close is honored, read timeout is 5 s.

Limits

• Values are served from the server-side cache filled by the subscription: the rate is the group period (--rate). For a "snapshot as of the request", use ?refresh=1.
• OPC tag browsing is for servers whose tags live in branches, e.g. (Mera.Recorder.1). If the server does not support browsing, set the tag list with the --items-file option.
• For a 32-bit OPC server, it is recommended to use the 32-bit OpcDaToHttp, and likewise for 64-bit.
• There is no authentication or HTTPS; it is recommended to use it on a local network.