Lambda Logs API - AWS Lambda

Lambda Logs API

Lambda automatically captures runtime logs and streams them to Amazon CloudWatch. This log stream contains the logs that your function code and extensions generate, and also the logs that Lambda generates as part of the function invocation.

Lambda extensions can use the Lambda Runtime Logs API to subscribe to log streams directly from within the Lambda execution environment. Lambda streams the logs to the extension, and the extension can then process, filter, and send the logs to any preferred destination.

      The Extensions API and the Logs API connect Lambda and external extensions.

The Logs API allows extensions to subscribe to three different logs streams:

  • Function logs that the Lambda function generates and writes to stdout or stderr.

  • Extension logs that extension code generates.

  • Lambda platform logs, which record events and errors related to invocations and extensions.


Lambda sends all logs to CloudWatch, even when an extension subscribes to one or more of the log streams.

Subscribing to receive logs

A Lambda extension can subscribe to receive logs by sending a subscription request to the Logs API.

To subscribe to receive logs, you need the extension identifier (Lambda-Extension-Identifier). First register the extension to receive the extension identifier. Then subscribe to the Logs API during initialization. After the initialization phase completes, Lambda does not process subscription requests.


Logs API subscription is idempotent. Duplicate subscribe requests do not result in duplicate subscriptions.

Memory usage

Memory usage increases linearly as the number of subscribers increases. Subscriptions consume memory resources because each subscription opens a new memory buffer to store the logs. To help optimize memory usage, you can adjust the buffering configuration. Buffer memory usage counts towards overall memory consumption in the execution environment.

Destination protocols

You can choose one of the following protocols to receive the logs:

  1. HTTP (recommended) – Lambda delivers logs to a local HTTP endpoint (http://sandbox.localdomain:${PORT}/${PATH}) as an array of records in JSON format. The $PATH parameter is optional. Note that only HTTP is supported, not HTTPS. You can choose to receive logs through PUT or POST.

  2. TCP – Lambda delivers logs to a TCP port in Newline delimited JSON (NDJSON) format.

We recommend using HTTP rather than TCP. With TCP, the Lambda platform cannot acknowledge when it delivers logs to the application layer. Therefore, you might lose logs if your extension crashes. HTTP does not share this limitation.

We also recommend setting up the local HTTP listener or the TCP port before subscribing to receive logs. During setup, note the following:

  • Lambda sends logs only to destinations that are inside the execution environment.

  • Lambda retries the attempt to send the logs (with backoff) if there is no listener, or if the POST or PUT request results in an error. If the log subscriber crashes, it continues to receive logs after Lambda restarts the execution environment.

  • Lambda reserves port 9001. There are no other port number restrictions or recommendations.

Buffering configuration

Lambda can buffer logs and deliver them to the subscriber. You can configure this behavior in the subscription request by specifying the following optional fields. Note that Lambda uses the default value for any field that you do not specify.

  • timeoutMs – The maximum time (in milliseconds) to buffer a batch. Default: 1,000. Minimum: 25. Maximum: 30,000.

  • maxBytes – The maximum size (in bytes) of the logs to buffer in memory. Default: 262,144. Minimum: 262,144. Maximum: 1,048,576.

  • maxItems – The maximum number of events to buffer in memory. Default: 10,000. Minimum: 1,000. Maximum: 10,000.

During buffering configuration, note the following points:

  • Lambda flushes the logs if any of the input streams are closed, for example, if the runtime crashes.

  • Each subscriber can specify a different buffering configuration in their subscription request.

  • Consider the buffer size that you need for reading the data. Expect to receive payloads as large as 2*maxBytes+metadata, where maxBytes is configured in the subscribe request. For example, Lambda adds the following metadata bytes to each record:

    { "time": "2020-08-20T12:31:32.123Z", "type": "function", "record": "Hello World" }
  • If the subscriber cannot process incoming logs quickly enough, Lambda might drop logs to keep memory utilization bounded. To indicate the number of dropped records, Lambda sends a platform.logsDropped log.

Example subscription

The following example shows a request to subscribe to the platform and function logs.

PUT http://${AWS_LAMBDA_RUNTIME_API}/2020-08-15/logs HTTP/1.1 { "schemaVersion": "2020-08-15", "types": [ "platform", "function" ], "buffering": { "maxItems": 1000, "maxBytes": 262144, "timeoutMs": 100 }, "destination": { "protocol": "HTTP", "URI": "http://sandbox.localdomain:8080/lambda_logs" } }

If the request succeeds, the subscriber receives an HTTP 200 success response.

HTTP/1.1 200 OK "OK"

Sample code for Logs API

For sample code showing how to send logs to a custom destination, see Using AWS Lambda extensions to send logs to custom destinations on the AWS Compute Blog.

For Python and Go code examples showing how to develop a basic Lambda extension and subscribe to the Logs API, see AWS Lambda Extensions on the AWS Samples GitHub repository. For more information about building a Lambda extension, see Lambda Extensions API.

Logs API reference

You can retrieve the Logs API endpoint from the AWS_LAMBDA_RUNTIME_API environment variable. To send an API request, use the prefix 2020-08-15/ before the API path. For example:


The OpenAPI specification for the Logs API version 2020-08-15 is available here:


To subscribe to one or more of the log streams available in the Lambda execution environment, extensions send a Subscribe API request.



Body parameters

destination – See Destination protocols. Required: yes. Type: strings.

buffering – See Buffering configuration. Required: no. Type: strings.

types – An array of the types of logs to receive. Required: yes. Type: array of strings. Valid values: "platform", "function", "extension".

schemaVersion – Required: no. Default value: "2020-08-15". Set to "2021-03-18" for the extension to receive platform.runtimeDone messages.

Response parameters

The OpenAPI specifications for the subscription responses version 2020-08-15 are available for the HTTP and TCP protocols:

Response codes

  • 200 – Request completed successfully

  • 202 – Request accepted. Response to a subscription request during local testing.

  • 4XX – Bad Request

  • 500 – Service error

If the request succeeds, the subscriber receives an HTTP 200 success response.

HTTP/1.1 200 OK "OK"

If the request fails, the subscriber receives an error response. For example:

HTTP/1.1 400 OK { "errorType": "Logs.ValidationError", "errorMessage": URI port is not provided; types should not be empty" }

Log messages

The Logs API allows extensions to subscribe to three different logs streams:

  • Function – Logs that the Lambda function generates and writes to stdout or stderr.

  • Extension – Logs that extension code generates.

  • Platform – Logs that the runtime platform generates, which record events and errors related to invocations and extensions.

Function logs

The Lambda function and internal extensions generate function logs and write them to stdout or stderr.

The following example shows the format of a function log message. { "time": "2020-08-20T12:31:32.123Z", "type": "function", "record": "ERROR encountered. Stack trace:\n\my-function (line 10)\n" }

Extension logs

Extensions can generate extension logs. The log format is the same as for a function log.

Platform logs

Lambda generates log messages for platform events such as platform.start, platform.end, and platform.fault.

Optionally, you can subscribe to the 2021-03-18 version of the Logs API schema, which includes the platform.runtimeDone log message.

Example platform log messages

The following example shows the platform start and platform end logs. These logs indicate the invocation start time and invocation end time for the invocation that the requestId specifies.

{ "time": "2020-08-20T12:31:32.123Z", "type": "platform.start", "record": {"requestId": "6f7f0961f83442118a7af6fe80b88d56"} } { "time": "2020-08-20T12:31:32.123Z", "type": "platform.end", "record": {"requestId": "6f7f0961f83442118a7af6fe80b88d56"} }

The platform report log includes metrics about the invocation that the requestId specifies. The initDurationMs field is included in the log only if the invocation included a cold start. If AWS X-Ray tracing is active, the log includes X-Ray metadata. The following example shows a platform report log for an invocation that included a cold start.

{ "time": "2020-08-20T12:31:32.123Z", "type": "", "record": {"requestId": "6f7f0961f83442118a7af6fe80b88d56", "metrics": {"durationMs": 101.51, "billedDurationMs": 300, "memorySizeMB": 512, "maxMemoryUsedMB": 33, "initDurationMs": 116.67 } } }

The platform fault log captures runtime or execution environment errors. The following example shows a platform fault log message.

{ "time": "2020-08-20T12:31:32.123Z", "type": "platform.fault", "record": "RequestId: d783b35e-a91d-4251-af17-035953428a2c Process exited before completing request" }

Lambda generates a platform extension log when an extension registers with the extensions API. The following example shows a platform extension message.

{ "time": "2020-08-20T12:31:32.123Z", "type": "platform.extension", "record": {"name": "", "state": "Ready", "events": ["INVOKE", "SHUTDOWN"] } }

Lambda generates a platform logs subscription log when an extension subscribes to the logs API. The following example shows a logs subscription message.

{ "time": "2020-08-20T12:31:32.123Z", "type": "platform.logsSubscription", "record": {"name": "", "state": "Subscribed", "types": ["function", "platform"], } }

Lambda generates a platform logs dropped log when an extension is not able to process the number of logs that it is receiving. The following example shows a platform.logsDropped log message.

{ "time": "2020-08-20T12:31:32.123Z", "type": "platform.logsDropped", "record": {"reason": "Consumer seems to have fallen behind as it has not acknowledged receipt of logs.", "droppedRecords": 123, "droppedBytes" 12345 } }

Platform runtimeDone messages

If you set the schema version to "2021-03-18" in the subscribe request, Lambda sends a platform.runtimeDone message after the function invocation completes either successfully or with an error. The extension can use this message to stop all the telemetry collection for this function invocation.

The OpenAPI specification for the Log event type in schema version 2021-03-18 is available here:

Lambda generates the platform.runtimeDone log message when the runtime sends a Next or Error runtime API request. The platform.runtimeDone log informs consumers of the Logs API that the function invocation completes. Extensions can use this information to decide when to send all the telemetry collected during that invocation.


Lambda sends the platform.runtimeDone message after the runtime sends the NEXT request when the function invocation completes. The following examples show messages for each of the status values: success, failure, and timeout.

Example success message

{ "time": "2021-02-04T20:00:05.123Z", "type": "platform.runtimeDone", "record": { "requestId":"6f7f0961f83442118a7af6fe80b88", "status": "success" } }

Example failure message

{ "time": "2021-02-04T20:00:05.123Z", "type": "platform.runtimeDone", "record": { "requestId":"6f7f0961f83442118a7af6fe80b88", "status": "failure" } }

Example timeout message

{ "time": "2021-02-04T20:00:05.123Z", "type": "platform.runtimeDone", "record": { "requestId":"6f7f0961f83442118a7af6fe80b88", "status": "timeout" } }