ExTenant.TenantUtils.retrieve_tenant_from_subdomain

You're seeing just the function retrieve_tenant_from_subdomain, go back to ExTenant.TenantUtils module for more information.
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retrieve_tenant_from_subdomain(subdomain)

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function used in the ExTenant.SubdomainPlug to retrieve the subdomain from the host in the conn it either gets the function to the actual data retrieval from the config settings OR it falls back to use the local get_tenant_from_subdomain_func - which is just used in a test.