[Monitor Exporter] Make the connection string optional when multi-endpoint routing is enabled

[Rajkumar Rangaraj (rajkumar-rangaraj)]

Why

Multi-endpoint routing takes every destination from the telemetry itself: each item carries microsoft.instrumentation_key and microsoft.ingestion_endpoint, and is sent to the endpoint it names. A process that only routes therefore has no Application Insights component of its own.

It still had to supply a connection string. That forced the operator to nominate a component that receives nothing — and worse, the nominated instrumentation key was stamped on SDK statistics as though it owned all the routed traffic, misattributing every customer's volume to whichever component happened to be named.

What changes

A connection string is now optional, but only when the Azure.Monitor.OpenTelemetry.EnableMultiEndpointRouting AppContext switch is on. In that configuration ConnectionVars.CreateUnconfigured() supplies an empty key and placeholder endpoints that seed the REST client, and:

Behaviour	Without a connection string
Routed telemetry	Sent to the endpoint the item names, unchanged
Unrouted telemetry	Dropped — TrackAsync refuses and logs event 78
Statsbeat	Not created; it has no key to attribute to and no endpoint to pick a region from
Customer SDK Stats	Not registered; they identify the customer's own component
FileBlobProvider / drain handler	Not created — nothing writes to that storage and nothing may drain it
Per-destination routed storage	Unchanged; this is the one storage path that must survive
Startup diagnostic	Event 77 at Warning, explaining the process can send nothing of its own

Telemetry without routing attributes was already dropped under this switch, so nothing that previously reached a destination stops doing so.

What does not change

Nothing, for anyone who has a connection string or who has routing switched off. A missing connection string is still an InvalidOperationException; a malformed one still throws; routing is not a fallback for a typo. No public API changed — api/*.cs is untouched.

The one change that could have hurt existing users was the offline storage directory name: it is a hash seeded with the instrumentation key, so moving it would strand any persisted backlog. The seed now branches on an explicit omitInstrumentationKey flag passed by the caller, never on the key being empty — because a configured connection string can legitimately parse to an empty key, and such a process already has a directory. The tests pin all three directory names to literal SHA-256 values rather than recomputing them, so a change to the hash itself cannot satisfy them.

Testing

1,181 tests pass on net8.0, net9.0, net10.0 and net462. The new OptionalConnectionStringTests covers the unconfigured path, the unchanged routing-off path, the configured-connection-string path in both modes, the storage seed, and the refusal of the unrouted send.

Review notes

This was reviewed over three rounds by two independent models, which converged on no P0 or P1 findings. Their P2 findings are all addressed in the second commit: the send guard is now structural in TrackAsync rather than relying on callers being gated in two other files; HasConnectionString takes an IPlatform so it reads exactly what the transmitter reads; and three tests that would have passed with their production guard deleted were strengthened.

Two P3 observations were left deliberately. There is no end-to-end test asserting Customer SDK Stats still register for a normal user, because running the real registration leaves an undisposable MeterProvider and a cached transmitter behind for the rest of the test run — the guard is asserted directly instead. And the TransmitterFactory cache keys an absent connection string to "", which is pre-existing and introduces no new collision, since DefaultPlatform snapshots the environment at construction.

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).
12 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

🟡 Changes recommended

The primary successful routed-send path without a connection string lacks integration coverage.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

Enables connection-string-free operation for multi-endpoint routing while preventing unrouted telemetry and SDK statistics from using placeholder destinations.

Changes:

• Adds unconfigured connection variables and guarded transmission behavior.
• Preserves routed storage while disabling component-specific storage and statistics.
• Adds diagnostics and focused tests.

File summaries

File	Description
CHANGELOG.md	Documents optional connection strings for routing.
AzureMonitorTransmitter.cs	Implements unconfigured routing mode and send guards.
ConnectionVars.cs	Represents an unconfigured connection.
CustomerSdkStatsRegistration.cs	Skips customer statistics without a connection string.
AzureMonitorExporterEventSource.cs	Adds startup and dropped-telemetry diagnostics.
StorageHelper.cs	Supports connection-string-free storage hashing.
OptionalConnectionStringTests.cs	Tests configuration, storage, diagnostics, and guards.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Balanced

---

💡 Add a code-review agent skill for context-aware, tailored reviews. Learn more in the docs.

[Copilot]

🟡 Changes recommended

Removing an existing host connection string can strand routed retry data, and the demo and integration test do not reliably guarantee an unconfigured environment.

Get a fresh assessment by requesting another Copilot review.

Review details

• Files reviewed: 12/12 changed files
• Comments generated: 3
• Review effort level: Balanced