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Common Expression Language input

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Common Expression Language input

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Use the cel input to read messages from a file path or HTTP API with a variety of payloads using the Common Expression Language (CEL) and the mito CEL extension libraries.

CEL is a non-Turing complete language that can perform evaluation of expression in inputs, which can include file and API endpoints using the mito extension library. The cel input periodically runs a CEL program that is given an execution environment that may be configured by the user, and publishes the set of events that result from the program evaluation. Optionally the CEL program may return cursor states that will be provided to the next execution of the CEL program. The cursor states may be used to control the behaviour of the program.

This input supports:

Auth
- Basic
- Digest
- OAuth2
Retrieval at a configurable interval
Pagination
Retries
Rate limiting
Proxying

Example configurations:

filebeat.inputs:
# Fetch your public IP every minute.
- type: cel
  interval: 1m
  resource.url: https://api.ipify.org/?format=json
  program: |
    bytes(get(state.url).Body).as(body, {
        "events": [body.decode_json()]
    })

or equivalently using the text format from ipify.org

filebeat.inputs:
# Fetch your public IP every minute.
- type: cel
  interval: 1m
  resource.url: https://api.ipify.org/?format=text
  program: |
    {
        "events": [{"ip": string(get(state.url).Body)}]
    }

filebeat.inputs:
- type: cel
  resource.url: http://localhost:9200/_search
  state:
    scroll: 5m
  program: |
    (
        !has(state.cursor) || !has(state.cursor.scroll_id) ?
            post(state.url+"?scroll=5m", "", "")
        :
            post(
                state.url+"/scroll?"+{"scroll_id": [state.cursor.scroll_id]}.format_query(),
                "application/json",
                {"scroll": state.scroll}.encode_json()
            )
    ).as(resp, bytes(resp.Body).decode_json().as(body, {
            "events": body.hits.hits,
            "cursor": {"scroll_id": body._scroll_id},
    }))

Execution

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The execution environment provided for the input includes includes the functions, macros, and global variables provided by the mito library. A single JSON object is provided as an input accessible through a state variable. state contains a string url field and may contain arbitrary other fields configured via the input’s state configuration. If the CEL program saves cursor states between executions of the program, the configured state.cursor value will be replaced by the saved cursor prior to execution.

On start the state is will be something like this:

{
    "url": <resource address>,
    "cursor": { ... },
    ...
}

The state.url field will be present and may be an HTTP end-point or a file path. It is the responsibility of the CEL program to handle removing the scheme from a file URL if it is present. The state.url field may be mutated during execution of the program, but the mutated state will not be persisted between restarts The state.url field must be present in the returned value to ensure that it is available in the next evaluation unless the program has the resource address hard-coded in or it is available from the cursor.

Additional fields may be present at the root of the object and if the program tolerates it, the cursor value may be absent. Only the cursor is persisted over restarts, but all fields in state are retained between iterations of the processing loop except for the produced events array, see below.

If the cursor is present the program should perform and process requests based on its value. If cursor is not present the program must have alternative logic to determine what requests to make.

After completion of a program’s execution it should return a single object with a structure looking like this:

{
    "events": [ 
        {...},
        ...
    ],
    "cursor": [ 
        {...},
        ...
    ],
    "url": <resource address>,
    "status_code": <HTTP request status code if a network request>,
    "header": <HTTP response headers if a network request>,
    "rate_limit": <HTTP rate limit map if required by API>, 
    "want_more": false 
}

	The `events` field must be present, but may be empty or null. If it is not empty, it must only have objects as elements. The field should be an array, but in the case of an error condition in the CEL program it is acceptable to return a single object instead of an array; this will will be wrapped as an array for publication and an error will be logged.
	If `cursor` is present it must be either be a single object or an array with the same length as events; each element i of the `cursor` will be the details for obtaining the events at and beyond event i in the `events` array. If the `cursor` is a single object it is will be the details for obtaining events after the last event in the `events` array and will only be retained on successful publication of all the events in the `events` array.
	If `rate_limit` is present it must be a map with numeric fields `rate` and `burst`. The `rate_limit` field may also have a string `error` field and other fields which will be logged. If it has an `error` field, the `rate` and `burst` will not be used to set rate limit behavior. The Limit, and Okta Rate Limit policy and Draft Rate Limit policy documentation show how to construct this field.
	The evaluation is repeated with the new state, after removing the events field, if the "want_more" field is present and true, and a non-zero events array is returned. If the "want_more" field is present after a failed evaluation, it is set to false.

The status_code, header and rate_limit values may be omitted if the program is not interacting with an HTTP API end-point and so will not be needed to contribute to program control.

Debug state logging

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The CEL input will log the complete state after evaluation when logging at the DEBUG level. This will include any sensitive or secret information kept in the state object, and so DEBUG level logging should not be used in production when sensitive information is retained in the state object. See redact configuration parameters for settings to exclude sensitive fields from DEBUG logs.

CEL extension libraries

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As noted above the cel input provides functions, macros, and global variables to extend the language.

Collections
- Collate
- Drop
- Drop Empty
- Flatten
- Keys
- Max
- Min
- Values
- With
- With Replace
- With Update
- Zip
Crypto
- Base64
- Base64 Raw
- Hex
- MD5
- SHA-1
- SHA-256
- HMAC
- UUID
File
- Dir
- File
HTTP
- HEAD
- GET
- GET Request
- POST
- POST Request
- Request
- Basic Authentication
- Do Request
- Parse URL
- Format URL
- Parse Query
- Format Query
File — the file extension is initialized with MIME handlers for "application/gzip", "application/x-ndjson", "application/zip", "text/csv; header=absent", and "text/csv; header=present".
- Dir
- File
JSON
XML — the XML extension is initialized with XML schema definitions provided via the xsd configuration option.
- Decode XML
Limit — the rate limit extension is initialized with Okta (as "okta") and the Draft Rate Limit (as "draft") policies.
- Rate Limit
MIME — the MIME extension is initialized with MIME handlers for "application/gzip", "application/x-ndjson", "application/zip", "text/csv; header=absent", and "text/csv; header=present".
- MIME
Regexp — the regular expression extension is initialized with the patterns specified in the user input configuration via the regexp field.
Strings
Time
Try
- Try
- Is Error
Debug — the debug handler registers a logger with the name extension cel_debug and calls to the CEL debug function are emitted to that logger.
- Debug

In addition to the extensions provided in the packages listed above, a global variable useragent is also provided which gives the user CEL program access to the filebeat user-agent string.

The CEL environment enables the optional types library using the version defined here.

Additionally, it supports authentication via Basic Authentication, Digest Authentication or OAuth2.

Example configurations with authentication:

filebeat.inputs:
- type: cel
  auth.basic:
    user: user@domain.tld
    password: P@$$W0₹D
  resource.url: http://localhost

filebeat.inputs:
- type: cel
  auth.digest:
    user: user@domain.tld
    password: P@$$W0₹D
  resource.url: http://localhost

filebeat.inputs:
- type: cel
  auth.oauth2:
    client.id: 12345678901234567890abcdef
    client.secret: abcdef12345678901234567890
    token_url: http://localhost/oauth2/token
  resource.url: http://localhost

filebeat.inputs:
- type: cel
  auth.oauth2:
    client.id: 12345678901234567890abcdef
    client.secret: abcdef12345678901234567890
    token_url: http://localhost/oauth2/token
    user: user@domain.tld
    password: P@$$W0₹D
  resource.url: http://localhost

Input state

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The cel input keeps a runtime state between requests. This state can be accessed by the CEL program and may contain arbitrary objects.

The state must contain a url string and may contain any object the user wishes to store in it.

All objects are stored at runtime, except cursor, which has values that are persisted between restarts.

Configuration options

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The cel input supports the following configuration options plus the Common options described later.

`interval`

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Duration between repeated requests. It may make additional pagination requests in response to the initial request if pagination is enabled. Default: 60s.

`program`

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The CEL program that is executed each polling period. This field is required.

`max_executions`

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max_executions is the maximum number of times a CEL program can request to be re-run with a want_more field. This is used to ensure that accidental infinite loops do not halt processing. When the execution budget is exceeded, execution will be restarted at the next interval and a warning will be written into the logs. Default: 1000.

`state`

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state is an optional object that is passed to the CEL program on the first execution. It is available to the executing program as the state variable. It is made available to subsequent executions of the program during the life of input as the returned value of the previous execution, but with the state.events field removed. Except for the state.cursor field, state does not persist over restarts.

`state.cursor`

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The cursor is an object available as state.cursor where arbitrary values may be stored. Cursor state is kept between input restarts and updated after each event of a request has been published. When a cursor is used the CEL program must either create a cursor state for each event that is returned by the program, or a single cursor that reflect the cursor for completion of the full set of events.

filebeat.inputs:
# Fetch your public IP every minute and note when the last request was made.
- type: cel
  interval: 1m
  resource.url: https://api.ipify.org/?format=json
  program: |
    bytes(get(state.url).Body).as(body, {
        "events": [body.decode_json().with({
            "last_requested_at": has(state.cursor) && has(state.cursor.last_requested_at) ?
                state.cursor.last_requested_at
            :
                now
        })],
        "cursor": {"last_requested_at": now}
    })

`regexp`

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A set of named regular expressions that may be used during a CEL program’s execution using the regexp extension library. The syntax used for the regular expressions is RE2.

filebeat.inputs:
- type: cel
  # Define two regular expressions, 'products' and 'solutions' for use during CEL execution.
  regexp:
    products: '(?i)(Elasticsearch|Beats|Logstash|Kibana)'
    solutions: '(?i)(Search|Observability|Security)'

`xsd`

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XML documents may require additional type information to enable correct parsing and ingestion. This information can be provided as an XML Schema Definitions (XSD) for XML documents using the xsd option. The key under which the XSD information is provided is accessed via the decode_xml CEL extension.

filebeat.inputs:
- type: cel
  # Provide an XSD, 'order', for use during CEL execution (truncated for example).
  xsd:
    order: |
       <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
         <xs:element name="order">
           <xs:complexType>
             <xs:sequence>
               <xs:element name="sender" type="xs:string"/>
               <xs:element name="address">
                 <xs:complexType>
                   <xs:sequence>
                     <xs:element name="name" type="xs:string"/>
                     <xs:element name="company" type="xs:string"/>

`auth.basic.enabled`

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When set to false, disables the basic auth configuration. Default: true.

Basic auth settings are disabled if either enabled is set to false or the auth.basic section is missing.

`auth.basic.user`

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The user to authenticate with.

`auth.basic.password`

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The password to use.

`auth.digest.enabled`

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When set to false, disables the digest auth configuration. Default: true.

digest auth settings are disabled if either enabled is set to false or the auth.digest section is missing.

`auth.digest.user`

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The user to authenticate with.

`auth.digest.password`

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The password to use.

`auth.digest.no_reuse`

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When set to true, Digest Authentication challenges are not reused.

`auth.oauth2.enabled`

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When set to false, disables the oauth2 configuration. Default: true.

OAuth2 settings are disabled if either enabled is set to false or the auth.oauth2 section is missing.

`auth.oauth2.provider`

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Used to configure supported oauth2 providers. Each supported provider will require specific settings. It is not set by default. Supported providers are: azure, google, okta.

`auth.oauth2.client.id`

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The client ID used as part of the authentication flow. It is always required except if using google as provider. Required for providers: default, azure, okta.

`auth.oauth2.client.secret`

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The client secret used as part of the authentication flow. It is always required except if using google or okta as provider. Required for providers: default, azure.

`auth.oauth2.user`

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The user used as part of the authentication flow. It is required for authentication - grant type password. It is only available for provider default.

`auth.oauth2.password`

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The password used as part of the authentication flow. It is required for authentication - grant type password. It is only available for provider default.

user and password are required for grant_type password. If user and password is not used then it will automatically use the token_url and client credential method.

`auth.oauth2.scopes`

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A list of scopes that will be requested during the oauth2 flow. It is optional for all providers.

`auth.oauth2.token_url`

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The endpoint that will be used to generate the tokens during the oauth2 flow. It is required if no provider is specified.

For azure provider either token_url or azure.tenant_id is required.

`auth.oauth2.endpoint_params`

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Set of values that will be sent on each request to the token_url. Each param key can have multiple values. Can be set for all providers except google.

- type: cel
  auth.oauth2:
    endpoint_params:
      Param1:
        - ValueA
        - ValueB
      Param2:
        - Value

`auth.oauth2.azure.tenant_id`

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Used for authentication when using azure provider. Since it is used in the process to generate the token_url, it can’t be used in combination with it. It is not required.

For information about where to find it, you can refer to https://docs.microsoft.com/en-us/azure/active-directory/develop/howto-create-service-principal-portal.

`auth.oauth2.azure.resource`

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The accessed WebAPI resource when using azure provider. It is not required.

`auth.oauth2.google.credentials_file`

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The credentials file for Google.

Only one of the credentials settings can be set at once. If none is provided, loading default credentials from the environment will be attempted via ADC. For more information about how to provide Google credentials, please refer to https://cloud.google.com/docs/authentication.

`auth.oauth2.google.credentials_json`

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Your credentials information as raw JSON.

`auth.oauth2.google.jwt_file`

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The JWT Account Key file for Google.

`auth.oauth2.google.jwt_json`

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The JWT Account Key file as raw JSON.

`auth.oauth2.google.delegated_account`

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Email of the delegated account used to create the credentials (usually an admin). Used in combination with auth.oauth2.google.jwt_file or auth.oauth2.google.jwt_json.

`auth.oauth2.okta.jwk_file`

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The RSA JWK Private Key file for your Okta Service App which is used for interacting with Okta Org Auth Server to mint tokens with okta.* scopes.

Only one of the credentials settings can be set at once. For more information please refer to https://developer.okta.com/docs/guides/implement-oauth-for-okta-serviceapp/main/

`auth.oauth2.okta.jwk_json`

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The RSA JWK Private Key JSON for your Okta Service App which is used for interacting with Okta Org Auth Server to mint tokens with okta.* scopes.

Only one of the credentials settings can be set at once. For more information please refer to https://developer.okta.com/docs/guides/implement-oauth-for-okta-serviceapp/main/

`auth.oauth2.okta.jwk_pem`

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The RSA JWK private key PEM block for your Okta Service App which is used for interacting with Okta Org Auth Server to mint tokens with okta.* scopes.

Only one of the credentials settings can be set at once. For more information please refer to https://developer.okta.com/docs/guides/implement-oauth-for-okta-serviceapp/main/

`resource.url`

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The URL of the HTTP API. Required.

The API endpoint may be accessed via unix socket and Windows named pipes by adding +unix or +npipe to the URL scheme, for example, http+unix:///var/socket/.

`resource.timeout`

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Duration before declaring that the HTTP client connection has timed out. Valid time units are ns, us, ms, s, m, h. Default: 30s.

`resource.ssl`

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This specifies SSL/TLS configuration. If the ssl section is missing, the host’s CAs are used for HTTPS connections. See SSL for more information.

`resource.keep_alive.disable`

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This specifies whether to disable keep-alives for HTTP end-points. Default: true.

`resource.keep_alive.max_idle_connections`

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The maximum number of idle connections across all hosts. Zero means no limit. Default: 0.

`resource.keep_alive.max_idle_connections_per_host`

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The maximum idle connections to keep per-host. If zero, defaults to two. Default: 0.

`resource.keep_alive.idle_connection_timeout`

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The maximum amount of time an idle connection will remain idle before closing itself. Valid time units are ns, us, ms, s, m, h. Zero means no limit. Default: 0s.

`resource.retry.max_attempts`

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The maximum number of retries for the HTTP client. Default: 5.

`resource.retry.wait_min`

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The minimum time to wait before a retry is attempted. Default: 1s.

`resource.retry.wait_max`

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The maximum time to wait before a retry is attempted. Default: 60s.

`resource.redirect.forward_headers`

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When set to true request headers are forwarded in case of a redirect. Default: false.

`resource.redirect.headers_ban_list`

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When redirect.forward_headers is set to true, all headers except the ones defined in this list will be forwarded. Default: [].

`resource.redirect.max_redirects`

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The maximum number of redirects to follow for a request. Default: 10.

`resource.rate_limit.limit`

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The value of the response that specifies the maximum overall resource request rate.

`resource.rate_limit.burst`

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The maximum burst size. Burst is the maximum number of resource requests that can be made above the overall rate limit.

`resource.tracer.filename`

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It is possible to log HTTP requests and responses in a CEL program to a local file-system for debugging configurations. This option is enabled by setting the resource.tracer.filename value. Additional options are available to tune log rotation behavior.

To differentiate the trace files generated from different input instances, a placeholder * can be added to the filename and will be replaced with the input instance id. For Example, http-request-trace-*.ndjson.

Enabling this option compromises security and should only be used for debugging.

`resource.tracer.maxsize`

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This value sets the maximum size, in megabytes, the log file will reach before it is rotated. By default logs are allowed to reach 1MB before rotation.

`resource.tracer.maxage`

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This specifies the number days to retain rotated log files. If it is not set, log files are retained indefinitely.

`resource.tracer.maxbackups`

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The number of old logs to retain. If it is not set all old logs are retained subject to the resource.tracer.maxage setting.

`resource.tracer.localtime`

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Whether to use the host’s local time rather that UTC for timestamping rotated log file names.

`resource.tracer.compress`

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This determines whether rotated logs should be gzip compressed.

`redact`

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During debug level logging, the state object and the resulting evaluation result are included in logs. This may result in leaking of secrets. In order to prevent this, fields may be redacted or deleted from the logged state. The redact configuration allows users to configure this field redaction behaviour. For safety reasons if the redact configuration is missing a warning is logged.

In the case of no-required redaction an empty redact.fields configuration should be used to silence the logged warning.

- type: cel
  redact:
    fields: ~

As an example, if a user-constructed Basic Authentication request is used in a CEL program the password can be redacted like so

filebeat.inputs:
- type: cel
  resource.url: http://localhost:9200/_search
  state:
    user: user@domain.tld
    password: P@$$W0₹D
  redact:
    fields:
      - password
    delete: true

Note that fields under the auth configuration hierarchy are not exposed to the state and so do not need to be redacted. For this reason it is preferable to use these for authentication over the request construction shown above where possible.

`redact.fields`

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This specifies fields in the state to be redacted prior to debug logging. Fields listed in this array will be either replaced with a * or deleted entirely from messages sent to debug logs.

`redact.delete`

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This specifies whether fields should be replaced with a * or deleted entirely from messages sent to debug logs. If delete is true, fields will be deleted rather than replaced.

Metrics

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This input exposes metrics under the HTTP monitoring endpoint. These metrics are exposed under the /inputs path. They can be used to observe the activity of the input.

Metric	Description
`resource`	URL or path of the input resource.
`cel_executions`	Number times the CEL program has been executed.
`batches_received_total`	Number of event arrays received.
`events_received_total`	Number of events received.
`batches_published_total`	Number of event arrays published.
`events_published_total`	Number of events published.
`cel_processing_time`	Histogram of the elapsed successful CEL program processing times in nanoseconds.
`batch_processing_time`	Histogram of the elapsed successful batch processing times in nanoseconds (time of receipt to time of ACK for non-empty batches).

Developer tools

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A stand-alone CEL environment that implements the majority of the CEL input’s Comment Expression Language functionality is available in the Elastic Mito repository. This tool may be used to help develop CEL programs to be used by the input. Installation is available from source by running go install github.com/elastic/mito/cmd/mito@latest and requires a Go toolchain.

Common options

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The following configuration options are supported by all inputs.

`enabled`

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Use the enabled option to enable and disable inputs. By default, enabled is set to true.

`tags`

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A list of tags that Filebeat includes in the tags field of each published event. Tags make it easy to select specific events in Kibana or apply conditional filtering in Logstash. These tags will be appended to the list of tags specified in the general configuration.

Example:

filebeat.inputs:
- type: cel
  . . .
  tags: ["json"]

`fields`

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Optional fields that you can specify to add additional information to the output. For example, you might add fields that you can use for filtering log data. Fields can be scalar values, arrays, dictionaries, or any nested combination of these. By default, the fields that you specify here will be grouped under a fields sub-dictionary in the output document. To store the custom fields as top-level fields, set the fields_under_root option to true. If a duplicate field is declared in the general configuration, then its value will be overwritten by the value declared here.

filebeat.inputs:
- type: cel
  . . .
  fields:
    app_id: query_engine_12

`fields_under_root`

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If this option is set to true, the custom fields are stored as top-level fields in the output document instead of being grouped under a fields sub-dictionary. If the custom field names conflict with other field names added by Filebeat, then the custom fields overwrite the other fields.

`processors`

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A list of processors to apply to the input data.

See Processors for information about specifying processors in your config.

`pipeline`

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The ingest pipeline ID to set for the events generated by this input.

The pipeline ID can also be configured in the Elasticsearch output, but this option usually results in simpler configuration files. If the pipeline is configured both in the input and output, the option from the input is used.

`keep_null`

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If this option is set to true, fields with null values will be published in the output document. By default, keep_null is set to false.

`index`

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If present, this formatted string overrides the index for events from this input (for elasticsearch outputs), or sets the raw_index field of the event’s metadata (for other outputs). This string can only refer to the agent name and version and the event timestamp; for access to dynamic fields, use output.elasticsearch.index or a processor.

Example value: "%{[agent.name]}-myindex-%{+yyyy.MM.dd}" might expand to "filebeat-myindex-2019.11.01".

`publisher_pipeline.disable_host`

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By default, all events contain host.name. This option can be set to true to disable the addition of this field to all events. The default value is false.

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