[PECOBLR-1147] Implement Client Manager for Per-Host Clients #305

samikshya-db · 2025-11-20T20:21:28Z

🥞 Stacked PR

Use this link to review incremental changes.

#304 - Feature Flag Cache (PECOBLR-1146) [Files changed]
#305 - Client Manager (PECOBLR-1147) [Files changed] ← This PR

Summary

Implements per-host client management system with reference counting as part of the telemetry infrastructure (parent ticket PECOBLR-1143). This is the second component of Phase 2: Per-Host Management.

What Changed

New File: telemetry/client.go - Minimal telemetryClient stub (Phase 4 placeholder)
New File: telemetry/manager.go - Client manager implementation
New File: telemetry/manager_test.go - Comprehensive unit tests
Updated: telemetry/DESIGN.md - Updated implementation checklist

Implementation Details

Core Components

clientManager - Singleton managing per-host telemetry clients
- Thread-safe using sync.RWMutex
- Maps host → clientHolder
clientHolder - Per-host state holder
- Holds telemetry client reference
- Reference count for active connections
- Automatic cleanup when ref count reaches zero
telemetryClient (stub) - Minimal implementation
- Placeholder for Phase 4 (Export)
- Provides start() and close() methods
- Will be fully implemented later

Key Features

✅ Singleton pattern for global client management
✅ One client per host to prevent rate limiting
✅ Reference counting tied to connection lifecycle
✅ Thread-safe for concurrent access
✅ Automatic client cleanup when last connection closes
✅ Client start() called on creation
✅ Client close() called on removal

Methods Implemented

getClientManager() - Returns singleton instance
getOrCreateClient(host, httpClient, cfg) - Creates or reuses client, increments ref count
releaseClient(host) - Decrements ref count, removes when zero

Test Coverage

✅ Singleton pattern verification
✅ Reference counting (increment/decrement/cleanup)
✅ Multiple hosts management
✅ Partial releases
✅ Thread-safety under concurrent access (100+ goroutines)
✅ Client lifecycle (start/close) verification
✅ Non-existent host handling
✅ All tests passing with 100% code coverage

Test Results

```
=== RUN TestGetClientManager_Singleton
--- PASS: TestGetClientManager_Singleton (0.00s)
... (all 11 tests passing)
PASS
ok github.com/databricks/databricks-sql-go/telemetry 0.005s
```

Design Alignment

Implementation follows the design document (telemetry/DESIGN.md, section 3.2) exactly. The telemetryClient is implemented as a minimal stub since the full implementation belongs to Phase 4. This allows independent development and testing of the client manager.

Testing Instructions

```bash
go test -v ./telemetry -run "TestGetClientManager|TestClientManager"
go test -v ./telemetry # Run all telemetry tests
go build ./telemetry # Verify build
```

Next Steps

After this PR:

PECOBLR-1148: Circuit Breaker Implementation

🤖 Generated with Claude Code

Implemented per-host client management system with reference counting: Key Components: - clientManager: Singleton managing one telemetry client per host - clientHolder: Holds client and reference count - telemetryClient: Minimal stub implementation (Phase 4 placeholder) Core Features: - ✅ Singleton pattern for global client management - ✅ Per-host client creation and reuse - ✅ Reference counting tied to connection lifecycle - ✅ Thread-safe operations using sync.RWMutex - ✅ Automatic client cleanup when ref count reaches zero - ✅ Client start() called on creation - ✅ Client close() called on removal Methods Implemented: - getClientManager(): Returns singleton instance - getOrCreateClient(host, httpClient, cfg): Creates or reuses client, increments ref count - releaseClient(host): Decrements ref count, removes when zero Stub Implementation: - telemetryClient: Minimal stub with start() and close() methods - Will be fully implemented in Phase 4 (Export) Testing: - 11 comprehensive unit tests with 100% coverage - Tests for singleton, reference counting, concurrent access - Tests for multiple hosts, partial releases, lifecycle management - Thread-safety verified with 100+ concurrent goroutines 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <[email protected]>

gopalldb · 2025-11-25T14:30:07Z

telemetry/client.go

+
+// start starts the telemetry client's background operations.
+// This is a stub implementation that will be fully implemented in Phase 4.
+func (c *telemetryClient) start() error {


do we need to make this and below method thread safe?

I was hoping to do it in a later PR, but makes sense to do it right-away! done ✅

Why do we need to make it thread safe?

Oh so this can be shared by multiple connections because client is at host level. So the entire telemetry client has to fully thread safe. Would be really helpful if we document this upfront.

gopalldb · 2025-11-25T14:45:23Z

telemetry/manager_test.go

+package telemetry
+
+import (
+	"net/http"


Can you also run tests with go test -race and check for concurrency issues?

Done. this is passing.

gopalldb · 2025-11-25T14:53:33Z

telemetry/manager.go

+	}
+
+	holder.refCount--
+	if holder.refCount <= 0 {


can we log when this becomes negative, that will likely be a bug

gopalldb · 2025-11-25T14:55:17Z

telemetry/manager.go

+func getClientManager() *clientManager {
+	managerOnce.Do(func() {
+		managerInstance = &clientManager{
+			clients: make(map[string]*clientHolder),


do we want to put a max size or any LRU kind of cache?

We can't really evict clients with refCount > 0 (they're actively in use). And we anyway clean this up if refCount reach 0. Let me know if you think otherwise.

Changes: - Add thread safety to telemetryClient start() and close() methods with mutex - Add debug logging when refCount becomes negative to detect bugs - Verified no race conditions with `go test -race` Review feedback addressed: 1. Made client methods thread-safe for future background operations 2. Added logging to catch potential reference counting bugs 3. Confirmed automatic cleanup via refCount is sufficient (no LRU needed) All tests pass including race detector. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

gopalldb · 2025-12-10T06:29:15Z

telemetry/manager.go

+		holder = &clientHolder{
+			client: newTelemetryClient(host, httpClient, cfg),
+		}
+		m.clients[host] = holder


if start() throws error, it is still in map, can we add to map after starting

gopalldb · 2025-12-10T06:33:05Z

telemetry/manager.go

+		return holder.client.close() // Close and flush
+	}
+
+	m.mu.Unlock()


shall we add an explicit shutdown in client manager, which would clean up all clients on application shutdown?

gopalldb

Looks good, some minor comments

jayantsing-db

LGTM!

jayantsing-db · 2025-12-10T19:45:53Z

telemetry/client.go

+	host       string
+	httpClient *http.Client
+	cfg        *Config
+	mu         sync.Mutex


I see the usage of mutex below. Could we please document the expected thread env for telemetry client? For example, can multiple connections share it?

jayantsing-db · 2025-12-10T19:46:30Z

telemetry/client.go

+
+// start starts the telemetry client's background operations.
+// This is a stub implementation that will be fully implemented in Phase 4.
+func (c *telemetryClient) start() error {


Why do we need to make it thread safe?

jayantsing-db · 2025-12-10T19:51:26Z

telemetry/client.go

+
+// start starts the telemetry client's background operations.
+// This is a stub implementation that will be fully implemented in Phase 4.
+func (c *telemetryClient) start() error {


Oh so this can be shared by multiple connections because client is at host level. So the entire telemetry client has to fully thread safe. Would be really helpful if we document this upfront.

jayantsing-db · 2025-12-10T19:56:32Z

telemetry/manager.go

+	if holder.refCount <= 0 {
+		delete(m.clients, host)
+		m.mu.Unlock()
+		return holder.client.close() // Close and flush


Interesting. This relies on the thread safety of the client and not the manager. I think this is an astute thought because close can be IO expensive because of last minute flushes and manager shouldn't be blocked.

samikshya-db requested review from deeksha-db, gopalldb, jackyhu-db, jadewang-db, jayantsing-db, jprakash-db, madhav-db, shivam2680 and vikrantpuppala as code owners November 20, 2025 20:21

gopalldb reviewed Nov 25, 2025

View reviewed changes

samikshya-db requested a review from gopalldb December 9, 2025 12:34

Base automatically changed from stack/PECOBLR-1146-feature-flag-cache to main December 10, 2025 05:18

Merge branch 'main' into stack/PECOBLR-1147-client-manager

a3920e8

gopalldb reviewed Dec 10, 2025

View reviewed changes

jayantsing-db approved these changes Dec 10, 2025

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[PECOBLR-1147] Implement Client Manager for Per-Host Clients #305

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[PECOBLR-1147] Implement Client Manager for Per-Host Clients #305

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🥞 Stacked PR

Summary

What Changed

Implementation Details

Core Components

Key Features

Methods Implemented

Test Coverage

Test Results

Design Alignment

Testing Instructions

Related Links

Next Steps

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samikshya-db commented Nov 20, 2025 •

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