Metrics Endpoint

Thunder exposes runtime and per-function metrics as a JSON object on two endpoints.

Endpoints

Method	Path	Listener
`GET`	`/_internal/metrics`	Admin (port 9000)
`GET`	`/metrics`	Admin (port 9000, alias)

Both endpoints return the same JSON payload.

Caching

Metrics are cached in memory with a 15-second TTL.

Without ?fresh=1, the endpoint returns the cached snapshot if it is less than 15 seconds old.
With ?fresh=1, the endpoint recomputes immediately and refreshes the cache.

curl -s http://localhost:9000/metrics | jq
curl -s 'http://localhost:9000/metrics?fresh=1' | jq '.process_saturation'
curl -s 'http://localhost:9000/_internal/metrics?fresh=1' | jq '.routing'

If admin API key authentication is enabled, /_internal/* routes require the X-API-Key header. The /metrics alias does not use the /_internal prefix.

Top-Level Schema

{
  "function_count": 5,
  "total_requests": 12340,
  "total_errors": 12,
  "total_cold_starts": 50,
  "avg_cold_start_ms": 45,
  "avg_cold_start_us": 45320,
  "avg_cold_start_ms_precise": 45.32,
  "avg_warm_start_ms": 2,
  "avg_warm_start_us": 2150,
  "avg_warm_start_ms_precise": 2.15,
  "total_cold_start_time_us": 2266000,
  "total_warm_start_time_us": 26531000,
  "memory": { ... },
  "process_saturation": { ... },
  "routing": { ... },
  "listener_connection_capacity": { ... },
  "egress_connection_manager": { ... },
  "top10": { ... },
  "functions": [ ... ]
}

Global Aggregates

Field	Type	Description
`function_count`	integer	Number of deployed functions
`total_requests`	integer	Sum of all function requests
`total_errors`	integer	Sum of all function errors
`total_cold_starts`	integer	Sum of all cold starts
`avg_cold_start_ms`	integer	Average cold start time (ms, rounded down)
`avg_cold_start_us`	integer	Average cold start time (microseconds, rounded down)
`avg_cold_start_ms_precise`	float	Average cold start time (ms, precise)
`avg_warm_start_ms`	integer	Average warm request time (ms, rounded down)
`avg_warm_start_us`	integer	Average warm request time (microseconds, rounded down)
`avg_warm_start_ms_precise`	float	Average warm request time (ms, precise)
`total_cold_start_time_us`	integer	Aggregate cold start elapsed time (microseconds)
`total_warm_start_time_us`	integer	Aggregate warm request elapsed time (microseconds)

Averages are 0 (or 0.0) when the divisor is zero.

`memory`

Process and host memory snapshot.

Field	Type	Unit	Description
`process_memory_mb`	float	MB	Current process memory from `sysinfo`
`total_memory_mib`	float	MiB	Host total memory
`available_memory_mib`	float	MiB	Host available memory (max of available and free)
`estimated_per_function_mb`	float	MB	`process_memory_mb / function_count` (`0.0` with no functions)

`process_saturation`

Global autoscaling signal derived from memory, CPU, pool, and file descriptor pressure.

Fields

Field	Type	Description
`score`	float	Global saturation score in `[0.0, 1.0]`
`level`	string	`healthy`, `warning`, or `critical`
`should_scale_out`	boolean	`true` when `score >= 0.75`
`active_signals`	string[]	Components at or above 0.75
`thresholds`	object	`{ warning: 0.75, critical: 0.90 }`
`components`	object	Normalized component values
`debug`	object	Raw memory values for diagnostics

Level Thresholds

Level	Condition
`healthy`	`score < 0.75`
`warning`	`0.75 <= score < 0.90`
`critical`	`score >= 0.90`

Components

Component	Formula
`memory`	`max(memory_process_raw, memory_host_raw * memory_process_raw)`
`cpu`	Ratio of accumulated CPU time to warm request time
`pool_isolates`	`routing.total_isolates / sum(functions[*].pool.max)`
`pool_contexts`	`routing.saturated_contexts / routing.total_contexts`
`pool`	`max(pool_isolates, pool_contexts)`
`fd`	Max of runtime FD pressure and listener clamp pressure

The score is the maximum across all components.

`routing`

Scheduler and capacity state snapshot.

Field	Type	Description
`total_contexts`	integer	Logical contexts currently tracked
`total_isolates`	integer	Isolate count from routing rollups
`global_pool_total_isolates`	integer	Alive isolates from global pool accounting
`global_pool_max_isolates`	integer	Configured global isolate cap
`isolate_accounting_gap`	integer	`global_pool_total_isolates - total_isolates`
`total_active_requests`	integer	Active requests across routing entries
`saturated_rejections`	integer	Total route-target capacity rejections
`saturated_rejections_context_capacity`	integer	Rejections caused by context saturation
`saturated_rejections_scale_blocked`	integer	Rejections where scale-up was blocked
`saturated_rejections_scale_failed`	integer	Rejections where scale-up failed
`burst_scale_batch_last`	integer	Last burst scale batch size
`burst_scale_events_total`	integer	Count of burst scale events
`saturated_contexts`	integer	Contexts at active-request cap
`saturated_isolates`	integer	Fully saturated isolates

`listener_connection_capacity`

Process listener capacity considering the file descriptor budget.

Field	Type	Description
`configuredMaxConnections`	integer	Configured `max_connections` value
`effectiveMaxConnections`	integer	Post-clamp effective listener limit
`softLimit`	integer	Process `RLIMIT_NOFILE` soft limit
`reservedFd`	integer	Reserved FD headroom for system use
`fdBudget`	integer	FD budget available to listener capacity

`egress_connection_manager`

Global outbound lease manager snapshot.

Field	Type	Description
`softLimit`	integer	Process `RLIMIT_NOFILE` soft limit
`openFdCount`	integer	Observed open FD count
`reservedFd`	integer	Reserved FD for non-egress use
`outboundFdBudget`	integer	Estimated FD budget for outbound leases
`adaptiveActiveLimit`	integer	Dynamic upper bound for active leases
`activeLeases`	integer	Currently active leases
`queuedWaiters`	integer	Waiters for lease acquisition
`totalAcquired`	integer	Cumulative successful acquisitions
`totalReleased`	integer	Cumulative releases
`totalRejected`	integer	Cumulative rejections due to backpressure
`totalTimeouts`	integer	Cumulative timed-out waits
`totalReaped`	integer	Cumulative stale leases reaped
`knownTenants`	integer	Tenants currently tracked
`topTenantsByActive`	array	Top tenants by active leases (max 10)
`tokenBucket`	object	Token bucket state: `tokens`, `capacity`, `refillPerSec`

`topTenantsByActive` Entry

Field	Type	Description
`tenant`	string	Tenant identifier
`active`	integer	Active lease count

`tokenBucket`

Field	Type	Description
`tokens`	float	Current token count
`capacity`	float	Bucket capacity
`refillPerSec`	float	Token refill rate per second

`top10`

Ranked views computed from current function data. Each array contains up to 10 entries.

`cold_slowest` / `cold_fastest`

Field	Type
`name`	string
`avg_cold_start_ms`	float
`cold_starts`	integer

`warm_slowest` / `warm_fastest`

Field	Type
`name`	string
`avg_warm_request_ms`	float
`requests`	integer

`cpu_bound`

Field	Type
`name`	string
`cpu_bound_ratio`	float
`avg_cpu_time_ms_per_request`	float
`avg_warm_request_ms`	float

`blocking_cpu`

Field	Type
`name`	string
`avg_cpu_time_ms_per_request`	float
`requests`	integer

`memory_usage`

Field	Type
`name`	string
`peak_heap_used_mb`	float
`current_heap_used_mb`	float
`peak_heap_used_bytes`	integer

`cpu_time_total`

Field	Type
`name`	string
`total_cpu_time_ms`	integer
`requests`	integer

`functions[]`

Array of per-function objects.

Function Info

Field	Type	Description
`name`	string	Function slug
`status`	string	`loading`, `running`, `error`, `shutting_down`
`bundle_format`	string	`eszip` or `snapshot`
`package_v8_version`	string	V8 version in the deploy package
`runtime_v8_version`	string	V8 version in the runtime
`snapshot_compatible_with_runtime`	boolean	V8 versions match
`requires_snapshot_regeneration`	boolean	Snapshot format with V8 mismatch
`stored_eszip_size_bytes`	integer	Size of stored ESZIP fallback
`can_regenerate_snapshot_from_stored_eszip`	boolean	Whether ESZIP is available for rebuild
`created_at`	string	RFC 3339 timestamp
`updated_at`	string	RFC 3339 timestamp
`last_error`	string or null	Last error message
`pool`	object	Isolate pool snapshot
`metrics`	object	Function metrics

`functions[].pool`

Field	Type	Description
`min`	integer	Configured minimum isolates
`max`	integer	Configured maximum isolates
`current`	integer	Currently allocated isolates

`functions[].metrics`

Field	Type	Unit
`total_requests`	integer	—
`active_requests`	integer	—
`total_errors`	integer	—
`total_cpu_time_ms`	integer	ms
`cold_starts`	integer	—
`avg_cold_start_ms`	integer	ms
`total_cold_start_time_ms`	integer	ms
`total_cold_start_time_us`	integer	us
`avg_cold_start_us`	integer	us
`avg_cold_start_ms_precise`	float	ms
`total_warm_start_time_ms`	integer	ms
`total_warm_start_time_us`	integer	us
`avg_warm_request_ms`	integer	ms
`avg_warm_request_us`	integer	us
`avg_warm_request_ms_precise`	float	ms
`current_heap_used_bytes`	integer	bytes
`peak_heap_used_bytes`	integer	bytes
`current_heap_used_mb`	float	MB
`peak_heap_used_mb`	float	MB

Compatibility

The schema is generated in the runtime server router (build_metrics_body).
New fields may be added in minor runtime updates without notice.
Clients should tolerate unknown fields.
For strict integrations, pin the runtime version and validate expected keys.

Metrics Endpoint

Endpoints

Caching

Top-Level Schema

Global Aggregates

memory

process_saturation

Fields

Level Thresholds

Components

routing

listener_connection_capacity

egress_connection_manager

topTenantsByActive Entry

tokenBucket

top10

cold_slowest / cold_fastest

warm_slowest / warm_fastest

cpu_bound

blocking_cpu

memory_usage

cpu_time_total

functions[]