Configuration Variable Implementation

Adding a new configuration variable in records.config requires a number of steps which are mostly documented here.

Before adding a new configuration variable, please discuss it on the mailing list. It will commonly be the case that a better name, or a more general approach to the problem which solves several different issues, may be suggested.

Defining the Variable

To begin, the new configuration variables must be added to RecordsConfig.cc. This contains a long array of configuration variable records. The fields for each record are:

type:RecT

Type of record. The valid values are:

RECT_NULL
Undefined record.
RECT_CONFIG
General configuration variable.
RECT_PROCESS
Process related statistic.
RECT_NODE
Local statistic.
RECT_PLUGIN
Plugin created statistic.

In general, RECT_CONFIG should be used.

name:char const*
The fully qualified name of the configuration variable. Although there appears to be a hierarchial naming scheme, that’s just a convention, and it is not actually used by the code. Nonetheless, new variables should adhere to the hierarchial scheme.
value_type:RecDataT
The data type of the value. It should be one of RECD_INT, RECD_STRING, RECD_FLOAT as appropriate.
default:char const*
The default value for the variable. This is always a string regardless of the value_type.
update:RecUpdateT

Information about how the variable is updated. The valid values are:

RECU_NULL
Behavior is unknown or unspecified.
RECU_DYNAMIC
This can be updated via command line tools.
RECD_RESTART_TS
The traffic_server process must be restarted for a new value to take effect.
RECD_RESTART_TM
The traffic_manager process must be restarted for a new value to take effect.
required:RecordRequiredType
Effectively a boolean that specifies if the record is required to be present, with RR_NULL meaning not required and RR_REQUIRED indicating that it is required. Given that using RR_REQUIRED would be a major incompatibility, RR_NULL is generally the better choice.
check:RecCheckT

Additional type checking. It is unclear if this is actually implemented. The valid values are:

RECC_NULL
No additional checking.
RECC_STR
Verify the value is a string.
RECC_INT
Verify the value is an integer.
RECC_IP
Verify the value is an IP address. Unknown if this checks for IPv6.
pattern:char const*
This provides a regular expressions (PCRE format) for validating the value, beyond the basic type validation performed by RecCheckT. This can be NULL if there is no regular expression to use.
access:RecAccessT

Access control. The valid values are:

RECA_NULL
The value is read / write.
RECA_READ_ONLY
The value is read only.
RECA_NO_ACCESS
No access to the value; only privileged level parts of ATS can access the value.

Variable Infrastructure

The primary effort in defining a configuration variable is handling updates, generally via traffic_ctl config reload. This is handled in a generic way, as described in the next section, or in a more specialized way (built on top of the generic mechanism) for HTTP related configuration variables. This is only needed if the variable is marked as dynamically updateable (RECU_DYNAMIC) although HTTP configuration variables should be dynamic if possible.

Documentation and Defaults

A configuration variable should be documented in records.config. There are many examples in the file already that can be used for guidance. The general format is to use the tag

.. ts:cv:`variable.name.here`

The arguments to this are the same as for the configuration file. The documentation generator will pick out key bits and use them to decorate the entry. In particular if a value is present it will be removed and used as the default value. You can attach some additional options to the variable. These are:

reloadable
The variable can be reloaded via command line on a running Traffic Server.
metric
Specify the units for the value. This is critical for variables that use unexpected or non-obvious metrics, such as minutes instead of seconds, or disk sectors instead of bytes.
deprecated
Mark a variable as deprecated.

Example

:ts:cv:`custom.variable`
reloadable:
units:minutes
deprecated:

If you need to refer to another configuration variable in the documentation, you can use the form

:ts:cv:`the.full.name.of.the.variable`

This will display the name as a link to the full definition.

In general, a new configuration variable should not be present in the default records.config. If it is added, such defaults should be added to the file proxy/config/records.config.default.in. This is used to generate the default records.config. Just add the variable to the file in an appropriate place with a proper default as this will now override whatever default you put in the code for new installs.

Handling Updates

The simplest mechanism for handling updates is the REC_EstablishStaticConfigXXX family of functions. This mechanism will cause the value in the indicated instance to be updated in place when an update to records.config occurs. This is done asynchronously using atomic operations. Use of these variables must keep that in mind.

If a variable requires additional handling when updated a callback can be registered which is called when the variable is updated. This is what the REC_EstablishStaticConfigXXX calls do internally with a callback that simply reads the new value and writes it to storage indicated by the call parameters. The functions used are the link_XXX static functions in RecCore.cc.

To register a configuration variable callback, call RecRegisterConfigUpdateCb with the arguments:

char const* name
The variable name.
callback
A function with the signature <int (char const* name, RecDataT type, RecData data, void* cookie)>. The name value passed is the same as the name passed to the registration function as is the cookie argument. The type and data are the new value for the variable. The return value is currently ignored. For future compatibility return REC_ERR_OKAY.
void* cookie
A value passed to the callback. This is only for the callback, the internals simply store it and pass it on.

callback is called under lock so it should be quick and not block. If that is necessary a continuation should be scheduled to handle the required action.

Note

The callback occurs asynchronously. For HTTP variables as described in the next section, this is handled by the more specialized HTTP update mechanisms. Otherwise it is the implementor’s responsibility to avoid race conditions.

HTTP Configuation Values

Variables used for HTTP processing should be declared as members of the HTTPConfigParams structure (but see Overridable Variables for further details) and use the specialized HTTP update mechanisms which handle synchronization and initialization issues.

The configuration logic maintains two copies of the HTTPConfigParams structure, the master copy and the current copy. The master copy is kept in the m_master member of the HttpConfig singleton. The current copy is kept in the ConfigProcessor. The goal is to provide a (somewhat) atomic update for configuration variables which are loaded individually in to the master copy as updates are received and then bulk copied to a new instance which is then swapped in as the current copy. The HTTP state machine interacts with this mechanism to avoid race conditions.

For each variable, a mapping between the variable name and the appropriate member in the master copy should be established between in the HTTPConfig::startup method. The HttpEstablishStaticConfigXXX functions should be used unless there is a strong, explicit reason to not do so.

The HTTPConfig::reconfigure method handles the current copy of the HTTP configuration variables. Logic should be added here to copy the value from the master copy to the current copy. Generally this will be a simple assignment. If there are dependencies between variables, those should be checked and enforced in this method.

Overridable Variables

HTTP related variables that are changeable per transaction are stored in the OverridableHttpConfigParams structure, an instance of which is the oride member of HTTPConfigParams and therefore the points in the previous section still apply. The only difference for that is the further .oride member specifier in the structure references.

The variable is required to be accessible from the transaction API. In addition to any custom API functions used to access the value, the following items are required for generic access:

  1. Add a value to the TSOverridableConfigKey enumeration in apidefs.h.in.
  2. Augment the TSHttpTxnConfigFind function to return this enumeration value when given the name of the configuration variable. Be sure to count the charaters very carefully.
  3. Augment the _conf_to_memberp function in InkAPI.cc to return a pointer to the appropriate member of OverridableHttpConfigParams and set the type if not a byte value.
  4. Update the testing logic in InkAPITest.cc by adding the string name of the configuration variable to the SDK_Overridable_Configs array.
  5. Update the Lua plugin enumeration TSLuaOverridableConfigKey in ts_lua_http_config.c.
  6. Update the documentation of TSHttpOverridableConfig in TSHttpOverridableConfig.en.rst.