How to specify _HANDLER in AWS Lambda Docker Image - docker

Question

I am trying to create a docker image to run a shell script as aws lambda function. As far as I understood, I need a runtime and a handler.
The default runtime (bootstrap) looks like this:
#!/bin/sh
set -euo pipefail
# Handler format: <script_name>.<bash_function_name>
#
# The script file <script_name>.sh must be located at the root of your
# function's deployment package, alongside this bootstrap executable.
source $(dirname "$0")/"$(echo $_HANDLER | cut -d. -f1).sh"
while true
do
# Request the next event from the Lambda runtime
HEADERS="$(mktemp)"
EVENT_DATA=$(curl -v -sS -LD "$HEADERS" -X GET "http://${AWS_LAMBDA_RUNTIME_API}/2018-06-01/runtime/invocation/next")
INVOCATION_ID=$(grep -Fi Lambda-Runtime-Aws-Request-Id "$HEADERS" | tr -d '[:space:]' | cut -d: -f2)
# Execute the handler function from the script
RESPONSE=$($(echo "$_HANDLER" | cut -d. -f2) "$EVENT_DATA")
# Send the response to Lambda runtime
curl -v -sS -X POST "http://${AWS_LAMBDA_RUNTIME_API}/2018-06-01/runtime/invocation/$INVOCATION_ID/response" -d "$RESPONSE"
done
The Dockerfile:
FROM public.ecr.aws/lambda/provided:al2
COPY bootstrap ${LAMBDA_RUNTIME_DIR}
COPY function.sh ${LAMBDA_TASK_ROOT}
RUN … (install some dependencies)
CMD ["function.handler"]
From what I've read, your supposed to set CMD to the handler. However this does not seem to work as the $_HANDLER variable is not known inside of the container.

Answer 1

I can't figure out what makes the difference, but this combination of Dockerfile and bootstrap works.
#!/bin/sh
set -euo pipefail
source "$LAMBDA_TASK_ROOT"/"$(echo $_HANDLER | cut -d. -f1).sh"
while true
do
# Request the next event from the Lambda runtime
HEADERS="$(mktemp)"
EVENT_DATA=$(curl -sS -LD "$HEADERS" -X GET "http://${AWS_LAMBDA_RUNTIME_API}/2018-06-01/runtime/invocation/next")
INVOCATION_ID=$(grep -Fi Lambda-Runtime-Aws-Request-Id "$HEADERS" | tr -d '[:space:]' | cut -d: -f2)
# Execute the handler function from the script
RESPONSE=$($(echo "$_HANDLER" | cut -d. -f2) "$EVENT_DATA")
# Send the response to Lambda runtime
curl -sS -X POST "http://${AWS_LAMBDA_RUNTIME_API}/2018-06-01/runtime/invocation/$INVOCATION_ID/response" -d "$RESPONSE"
done
FROM public.ecr.aws/lambda/provided:al2
COPY ./bootstrap ${LAMBDA_RUNTIME_DIR}
COPY ./function.sh ${LAMBDA_TASK_ROOT}
RUN chmod 755 ${LAMBDA_RUNTIME_DIR}/bootstrap ${LAMBDA_TASK_ROOT}/function.sh
RUN …
WORKDIR ${LAMBDA_TASK_ROOT}
CMD ["function.handler"]