AWS Lambda Managed Instances (LMI)

Runs Lambda functions on EC2 instances in the user's account while AWS manages provisioning, patching, scaling, routing, and load balancing. Combines Lambda's developer experience with EC2's pricing and hardware options.

Works best with the AWS MCP server for sandboxed CLI execution and audit logging. All guidance also works with standard AWS CLI or SAM CLI.

Note: Confirm regional availability, quotas, and instance type offerings against current AWS documentation before production deployment.

Quick Decision: Is LMI Right for This Workload?

Signal	LMI is a strong fit	Standard Lambda is better
Traffic	Steady, predictable, 50M+ req/mo	Bursty, unpredictable, long periods of no traffic
Duration	Long-running asynchronous/ESM jobs that exceed 15 min (up to 90 min on LMI): ETL/data processing, media transcoding, ML inference, financial calc, web scraping	Short invocations; synchronous work needing >15 min (not supported on any Lambda)
Cost	Duration-heavy spend at scale	Low or sporadic invocations
Cold starts	Unacceptable (LMI eliminates for provisioned capacity)	Tolerable
Compute	Latest CPUs, specific families, high network bandwidth, GPU requirements	Standard Lambda memory/CPU sufficient
Isolation	Dedicated EC2 instances in your account, full VPC control	Shared Firecracker micro-VMs acceptable
Scale-to-zero	Does not scale to zero but can create custom schedules with AWS provided solutions	Required (pay nothing when idle)
Code readiness	Thread-safe (Node.js/Java/.NET) or any Python code	Non-thread-safe code, expensive to change

Routing

Read ONLY the single reference file that matches the user's task. Do not preload multiple references.

User need	Action
Cost comparison, pricing analysis, Savings Plans, Reserved Instances	Read cost-comparison.md
Instance types, memory sizing, vCPU ratios, scaling tuning, capacity provider config	Read configuration-guide.md
Thread safety, concurrency model, code review checklist, multi-concurrency readiness	Read thread-safety.md
Before/after code examples, runtime-specific migration, connection pooling	Read migration-patterns.md
IAM roles, VPC setup, CLI commands, SAM template, CDK example	Read infrastructure-setup.md
Errors, throttling, debugging, stuck deployments	Read troubleshooting.md

Troubleshooting quick facts (always mention when diagnosing issues):

• Capacity provider stuck in CREATING → most common cause is private subnets missing a NAT gateway route (instances need outbound internet for image pull and Lambda service communication)
• Function not scaling → check that a version is published (PublishToLatestPublished: true)
• Memory errors → LMI minimum is 2048 MB

Workflow

Step 1: Assess the Workload

Gather these signals before recommending:

1. Traffic pattern: Steady vs bursty? Requests per second?
2. Current costs: Monthly Lambda spend? Existing Savings Plans?
3. Runtime: Node.js, Java, .NET, or Python?
4. Memory/CPU: How much memory? CPU-bound or I/O-bound?
5. Execution duration: Average and P99?
6. Concurrency readiness: Thread safety? Shared /tmp paths? Per-invocation DB connections?
7. VPC: Already in a VPC? Private resource access needed?

Long-running asynchronous/ESM jobs that exceed 15 min are a positive fit — LMI supports a function timeout up to 90 min (5400s) for asynchronous and event-source-mapping invocations (ETL/data processing, media transcoding, ML inference, financial calc, web scraping). Duration is the key differentiator here, not just cost.

When recommending LMI, ALWAYS mention: minimum 3 execution environments for AZ resiliency (cannot go below 3 in production).

Step 2: Build the Cost Comparison

REQUIRED: Present a cost comparison before recommending LMI.

Rule of thumb: LMI becomes cost-competitive at 50-100M+ req/month with steady traffic. Use the LMI Pricing Calculator for accurate comparisons.

Step 3: Configure the Deployment

• Instance families (400+ types, .large and up): C-series (compute), M-series (general), R-series (memory). ARM (Graviton) for best price-performance.
• When using Graviton instances, MUST set Architectures: [arm64] in the function configuration to match.
• Memory-to-vCPU ratios: 2:1 (compute), 4:1 (general, default), 8:1 (memory). Min 2 GB, max 32 GB.
• Multi-concurrency per-vCPU maximums: Node.js 64, Java 32, .NET 32, Python 16. These are system caps — the actual setting is PerExecutionEnvironmentMaxConcurrency (per execution environment, not per vCPU).
• For I/O-bound workloads: use the runtime default or higher PerExecutionEnvironmentMaxConcurrency (e.g., 10 for Node.js) since each request uses minimal CPU while waiting on network.
• For CPU-bound workloads: set PerExecutionEnvironmentMaxConcurrency to 1-2 per vCPU since each request saturates CPU.
• Scaling: MinExecutionEnvironments (default 3), MaxVCpuCount (optional, default 400 — set explicitly as best practice), TargetResourceUtilization.
• Function timeout up to 5400s (90 min) — set via the existing Timeout field (CLI update-function-configuration --timeout 5400, SAM/CFN Timeout: 5400, or console). Uses the existing Timeout field — no separate API, property, tag, or code change is required. Applies to asynchronous and ESM invocations only; synchronous and On-Demand invocations stay capped at 15 min (even if Timeout is higher; GetFunctionConfiguration still reports the configured value). The Init phase is still capped at 15 min. Durable Functions: each step can run up to 90 min; a multi-step workflow up to 1 year.

Step 4: Migrate the Code

Review code for concurrency safety. LMI runs multiple invocations concurrently per execution environment:

• Python: Process-based isolation — globals are NOT shared. No thread-safety changes needed. Focus on /tmp conflicts and memory sizing.
• Node.js: Worker threads — globals shared within a worker. Requires async safety.
• Java/.NET: OS threads/Tasks — handler shared across threads. Requires full thread safety.

Step 5: Set Up Infrastructure

1. Create two IAM roles: execution role (for the function) and operator role (for capacity provider EC2 management)
2. Configure VPC with subnets across 3+ AZs
3. Create capacity provider with VPC config and scaling limits
4. Create or update function with capacity provider attachment
5. Publish a version (triggers instance provisioning)

Step 6: Validate and Cut Over

1. Deploy to a non-production environment first
2. Monitor CloudWatch: CPU utilization, memory, concurrency, throttle rate
3. Gradual traffic shift with weighted aliases (10% → 50% → 100%)
4. Compare costs after 1-2 weeks of production data
5. Decommission standard Lambda once stable

Best Practices

Pricing (always mention when discussing costs)

• Three components: EC2 instance hours + 15% management fee + $0.20/1M requests
• Savings Plans: Compute Savings Plans apply to the EC2 portion (up to 60-72% discount)
• The 15% fee is charged on top of EC2 cost for AWS managing provisioning, patching, scaling, lifecycle

Scaling (always mention when discussing scaling or traffic)

• LMI absorbs a 50% traffic spike immediately and doubles capacity within 5 minutes — if traffic more than doubles faster, requests throttle
• Standard Lambda bursts to 3000 instantly — LMI cannot match this
• Pre-warm with MinExecutionEnvironments before known spikes
• MaxVCpuCount (default 400) — set explicitly as a cost ceiling
• Shape: Reduce MinExecutionEnvironments to lower capacity during off-hours (minimum 3 for AZ resiliency)

Instance Sizing

• 1 vCPU + 1 GB reserved per instance for OS overhead (not available to your function)
• Usable capacity = total - overhead

Configuration

• Start with 4:1 ratio and runtime default concurrency
• Use ARM (Graviton) unless x86 dependencies exist
• Let Lambda choose instance types unless specific hardware needed
• Set MaxVCpuCount to control cost ceiling
• Never set MinExecutionEnvironments below 3 (breaks AZ resiliency)

Migration

• Start with I/O-heavy functions (benefit most from multi-concurrency)
• Review code for concurrency safety before attaching to capacity provider
• Use weighted aliases for gradual traffic shift
• Include request IDs in all log statements
• Initialize DB pools and SDK clients outside the handler

Operations

• Set CloudWatch alarms on throttle rate > 1% and CPU > 80%
• Plan for 14-day instance rotation (automatic)
• Never manually terminate LMI EC2 instances (delete the capacity provider instead)
• Always publish a version — unpublished functions cannot run on LMI

Long-duration invocations (asynchronous/ESM up to 90 min)

• SQS: the queue visibility timeout MUST be ≥ the function timeout (≥ 5400s for a 90-min function). The ESM validates this on create and update — but once the ESM exists, the SQS visibility timeout and the function timeout can each be changed independently (outside the ESM API), which bypasses the check and can reintroduce a mismatch, causing messages to reappear and duplicate invokes.
• Kinesis / DynamoDB Streams: enable partial batch failure reporting (ReportBatchItemFailures) and tune the max batching window and parallelization factor — otherwise one failed record retries the whole batch, re-running up to ~80 min of already-completed work.
• Asynchronous invocations: failed or timed-out invocations follow the retry policy (up to 2 retries by default), then route to the DLQ / on-failure destination.
• Not all ESM sources qualify: Amazon MQ and Amazon DocumentDB ESM remain limited to 15 min; only SQS, Kinesis, and DynamoDB Streams ESM (and asynchronous invocations) get 90 min.
• Observability is unchanged: CloudWatch, CloudTrail (one Invoke event on completion/timeout), and X-Ray (single trace) behave identically. Use X-Ray sub-segments to find slow phases near the 90-min limit.

Long-running networking (functions now run for tens of minutes)

• NAT Gateway idle timeout — send periodic keep-alive packets on long-lived TCP connections through a NAT Gateway.
• Idle connection timeouts (RDS, ElastiCache, external APIs) — add connection health checks / reconnection logic for connections that may go idle mid-computation.
• DNS TTL — re-resolve external hostnames periodically; the AWS SDK does this, but custom HTTP clients may cache beyond TTL.
• Credentials — rely on the execution role's ephemeral credentials (automatically refreshed by the runtime). For non-IAM secrets (DB passwords, API keys), retrieve them from AWS Secrets Manager or SSM Parameter Store with SDK caching and periodic refresh; never embed long-lived credentials in code or environment variables.

Limits Quick Reference

Resource	Limit
Memory	2 GB min, 32 GB max
Asynchronous/ESM invoke timeout	90 min (5400s), via the existing Timeout field
Sync + On-Demand invoke timeout	15 min
Init phase	15 min
ESM sources capped at 15 min	Amazon MQ, Amazon DocumentDB (SQS/Kinesis/DynamoDB Streams get 90 min)
Execution environments	3 minimum (MinExecutionEnvironments, AZ resiliency)
Instance lifespan	14 days (auto-replaced)
Concurrency/vCPU	64 (Node.js), 32 (Java/.NET), 16 (Python)
Runtimes	Node.js 22+, Java 21+, .NET 8+, Python 3.13+, Rust (provided.al2023)
Instance families	C, M, R (.large and up)
Scaling	Burst headroom equals unused capacity from TargetResourceUtilization; new instances launch within minutes

Security Considerations

• Operator role scoping: Add aws:SourceAccount and aws:SourceArn conditions to trust policies to prevent confused deputy attacks.
• VPC egress: Scope security group egress to VPC endpoint security groups or AWS prefix lists rather than 0.0.0.0/0.
• Credentials: Use AWS Secrets Manager or Parameter Store for database credentials — never environment variables for secrets.
• Encryption: Enable SQS SSE, CloudWatch Logs encryption (KMS), and S3 default encryption for any data at rest.
• Logging: Set CloudWatch Log group retention policies. Avoid logging PII or credentials. Enable CloudTrail data events for Lambda.
• Instance rotation: The 14-day automatic rotation ensures security patches are applied without manual intervention.
• References: Lambda Security Best Practices, IAM Best Practices

Files

File	Content
cost-comparison.md	Pricing analysis, break-even calculations, Savings Plans/RI impact
configuration-guide.md	Instance selection, memory ratios, scaling tuning, capacity provider config
thread-safety.md	Concurrency model per runtime, code review checklist, Powertools compatibility
migration-patterns.md	Before/after code by runtime, connection pooling, gradual cutover
infrastructure-setup.md	IAM roles, VPC setup, SAM templates, CLI commands
troubleshooting.md	Common errors, throttling, debugging, stuck deployments