Functions limits

This page lists the limits that apply to Functions. These limits are enforced at authoring time (when you create or update a function) and at runtime (when the function runs).

Hook execution limits

These timeouts cover the entire hook execution, including all function steps and HTTP calls. If the hook exceeds its timeout, MediaTailor discards all output and proceeds as if no function were attached.

In addition to each hook's own timeout, the PRE_ADS_REQUEST, POST_ADS_RESPONSE, and PRE_MANIFEST_INSERTION hooks that run within a single request share a combined execution budget of 2,000 ms. Time spent by an earlier hook reduces the budget available to later hooks in the same request, and a hook's effective timeout is the smaller of its own timeout and the remaining budget. If the budget is exhausted before a hook starts, MediaTailor skips that hook entirely and continues processing the request. PRE_SESSION_INITIALIZATION runs during session initialization and is not part of the combined budget.

The input payload size limit applies to the serialized JSON input that MediaTailor builds for the POST_ADS_RESPONSE and PRE_MANIFEST_INSERTION hooks. If the input exceeds 128 KB, MediaTailor skips the hook invocation for that request. The output size limit caps the total serialized size that a function can write to the hook's writable namespace; output that exceeds 256 KB is discarded.

Individual function timeouts (such as RequestTimeoutMilliseconds for HTTP_REQUEST functions) must fit within the hook's total timeout. For example, if the hook timeout is 2,000 ms and an HTTP_REQUEST function sets RequestTimeoutMilliseconds to 2,000 ms, the function may time out before completing if any processing occurs before or after the HTTP call.

The player_params total size includes the combined character count of all output key names and values. For example, outputting player_params.deviceType = "mobile" and player_params.region = "us-east-1" counts deviceType, mobile, region, and us-east-1 toward the 1,000-character total.

Expression limits

When an expression exceeds a limit at runtime, MediaTailor stops the expression and records the error. For details on specific error types, see Troubleshooting and monitoring.

Function composition limits

These limits are enforced at authoring time.

HTTP request limits

Expression length limits and header limits are enforced at authoring time, when you create or update the function. The evaluated URL and body size limits, the request timeout, and restricted headers are enforced at runtime, after MediaTailor evaluates the expressions. A URL expression that is accepted at authoring time can still exceed the evaluated URL limit at runtime, in which case the request is not sent.

If a function sets a restricted header, MediaTailor accepts the function configuration at authoring time but drops the header when the HTTP request is sent during execution.

Response limits

These limits are enforced at runtime.

Output limits

These limits are enforced at runtime.

Allowed JSONata functions

For the full list of allowed functions, see JSONata expression reference.

For help diagnosing errors related to these limits, see Troubleshooting and monitoring.