VCL memory overflows | Fastly Documentation

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During VCL execution, exceeding some limits will cause the VCL program to terminate immediately, without invoking vcl_error. In these cases a 503 error is always generated and emitted to the client, and cannot be modified by your service configuration. The most common types of overflows are:

Workspace exhaustion: VCL programs allocate memory greedily and free it when the request ends. Use workspace.bytes_free to understand how much workspace is available.
Header overflows: VCL programs may only define around 90 HTTP headers in total. The exact number available to your program depends on the number of headers used by Fastly during your request.