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kumo.string.eval_template

kumo.string.eval_template(NAME, SOURCE, CONTEXT, OPT_DIALECT)
Since: Version 2025.03.19-1d3f1f67

The functionality described in this section requires version 2025.03.19-1d3f1f67 of KumoMTA, or a more recent version.

Evaluates a minijinja template.

The parameters are:

  • NAME - the nominal "file name" for the template. No actual file I/O is performed; this parameter tells the template engine what the filename would have been if it were loaded from disk. The filename is used to decide what strategy should be used for automatic output escaping. For example, if the name ends with .json or .html then appropriate json or html entity escaping will automatically be applied to the output.
  • SOURCE - the template source code. This must follow the template syntax.
  • CONTEXT - an object that defines the variables that are available to the template engine.
-- This will print `"John"`
print(
  kumo.string.eval_template('example.json', [[{{ name }}]], { name = 'John' })
)

local kumo = require 'kumo'

local log_record = kumo.serde.json_parse [=[
{
    "type": "Delivery",
    "id": "1d98076abbbc11ed940250ebf67f93bd",
    "sender": "user@sender.example.com",
    "recipient": "user@recipient.example.com",
    "queue": "campaign:tenant@domain",
    "site": "source2->(alt1|alt2|alt3|alt4)?.gmail-smtp-in.l.google.com.",
    "size": 1047,
    "response": {
        "code": 250,
        "enhanced_code": {
            "class": 2,
            "subject": 0,
            "detail": 0
        },
        "content": "OK ids=8a5475ccbbc611eda12250ebf67f93bd",
        "command": "."
    },
    "peer_address": {
        "name": "gmail-smtp-in.l.google.com.",
        "addr": "142.251.2.27"
    },
    "timestamp": 1678069691,
    "created": 1678069691,
    "num_attempts": 0,
    "bounce_classification": "Uncategorized",
    "egress_pool": "pool0",
    "egress_source": "source2",
    "source_address": {
        "address": "10.0.0.1:53210",
        "protocol": "socks5",
        "server": "192.168.1.1:5000"
    },
    "feedback_report": null,
    "meta": {},
    "headers": {},
    "delivery_protocol": "ESMTP",
    "reception_protocol": "ESMTP",
    "bogus": null,
    "nodeid": "557f3ad4-2c8c-11ee-976e-782d7e12e173",
    "tls_cipher": "TLS_AES_256_GCM_SHA384",
    "tls_protocol_version": "TLSv1.3",
    "tls_peer_subject_name": ["C=US","ST=CA","L=SanFrancisco","O=Fort-Funston",
                              "OU=MyOrganizationalUnit","CN=do.havedane.net",
                              "name=EasyRSA","emailAddress=me@myhost.mydomain"],
    "session_id": "9bcd689e-23d9-41b7-a015-63a1382f8b57"
}
]=]

print(kumo.string.eval_template(
  'log.json',
  [[{}
]],
  { log_record = log_record }
))

Template Dialect

Since: Version 2025.12.02-67ee9e96

The functionality described in this section requires version 2025.12.02-67ee9e96 of KumoMTA, or a more recent version.

It is now possible to specify which template engine will be used for template expansion via the OPT_DIALECT parameter. It can have one of the following values:

  • Jinja - this is the implied default. The Mini Jinja template dialect will be parsed and evaluated.
  • Static - The content is treated as a static string and no template expansion will be performed
  • Handlebars - The content will be evaluated by a handlebars compatible template engine.